WebGL Programming Guide

Praise for
WebGL Programming Guide
“WebGL provides one of the final features for creating applications that deliver ‘the desk-
top application experience’ in a web browser, and the WebGL Programming Guide leads the
way in creating those applications. Its coverage of all aspects of using WebGL—JavaScript,
OpenGL ES, and fundamental graphics techniques—delivers a thorough education on ev-
erything you need to get going. Web-based applications are the wave of the future, and this
book will get you ahead of the curve!”
Dave Shreiner , Coauthor of The OpenGL Programming Guide, Eighth Edition ; Series Editor,
OpenGL Library (Addison Wesley)
“HTML5 is evolving the Web into a highly capable application platform supporting beauti-
ful, engaging, and fully interactive applications that run portably across many diverse
systems. WebGL is a vital part of HTML5, as it enables web programmers to access the
full power and functionality of state-of-the-art 3D graphics acceleration. WebGL has been
designed to run securely on any web-capable system and will unleash a new wave of devel-
oper innovation in connected 3D web-content, applications, and user interfaces. This book
will enable web developers to fully understand this new wave of web functionality and
leverage the exciting opportunities it creates.”
Neil Trevett , Vice President Mobile Content, NVIDIA; President, The Khronos Group
“With clear explanations supported by beautiful 3D renderings, this book does wonders in
transforming a complex topic into something approachable and appealing. Even without
denying the sophistication of WebGL, it is an accessible resource that beginners should
consider picking up before anything else.”
Evan Burchard , Author, Web Game Developer’s Cookbook (Addison Wesley)
“Both authors have a strong OpenGL background and transfer this knowledge nicely over
to WebGL, resulting in an excellent guide for beginners as well as advanced readers.”
Daniel Haehn , Research Software Developer, Boston Children’s Hospital
“ WebGL Programming Guide provides a straightforward and easy-to-follow look at the me-
chanics of building 3D applications for the Web without relying on bulky libraries or wrap-
pers. A great resource for developers seeking an introduction to 3D development concepts
mixed with cutting-edge web technology.”
Brandon Jones , Software Engineer, Google
“This is more great work from a brilliant researcher. Kouichi Matsuda shows clear and con-
cise steps to bring the novice along the path of understanding WebGL. This is a complex
topic, but he makes it possible for anyone to start using this exciting new web technology.
And he includes basic 3D concepts to lay the foundation for further learning. This will be a
great addition to any web designer’s library.”
Chris Marrin , WebGL Spec. Editor
“ WebGL Programming Guide is a great way to go from a WebGL newbie to a WebGL expert.
WebGL, though simple in concept, requires a lot of 3D math knowledge, and WebGL Pro-
gramming Guide helps you build this knowledge so you’ll be able to understand and apply
it to your programs. Even if you end up using some other WebGL 3D library, the knowl-
edge learned in WebGL Programming Guide will help you understand what those libraries
are doing and therefore allow you to tame them to your application’s specific needs. Heck,
even if you eventually want to program desktop OpenGL and/or DirectX, WebGL Program-
ming Guide is a great start as most 3D books are outdated relative to current 3D technology.
WebGL Programming Guide will give you the foundation for fully understanding modern 3D
graphics.”
Gregg Tavares , An Implementer of WebGL in Chrome
WebGL Programming
Guide
T
he OpenGL graphics system is a software interface to graphics hardware.
(“GL” stands for “Graphics Library”.) It allows you to create interactive programs
that produce color images of moving, three-dimensional objects. With OpenGL,
you can control computer-graphics technology to produce realistic pictures, or
ones that depart from reality in imaginative ways.
The OpenGL Series from Addison-Wesley Professional comprises tutorial and
reference books that help programmers gain a practical understanding of OpenGL
standards, along with the insight needed to unlock OpenGL’s full potential.
Visit informit.com/opengl for a complete list of available products.
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WebGL
Programming
Guide:
Interactive 3D
Graphics Programming
with WebGL
Kouichi Matsuda
Rodger Lea
Many of the designations used by manufacturers and sellers to distinguish their
products are claimed as trademarks. Where those designations appear in this
book, and the publisher was aware of a trademark claim, the designations have
been printed with initial capital letters or in all capitals. OpenGL is a registered
trademark and the OpenGL ES logo is a trademark of Silicon Graphics Inc.
Khronos and WebGL are trademarks of the Khronos Group Inc. Google, Google
Chrome, and Android are trademarks of Google Inc. The Firefox web browser
is a registered trademark of the Mozilla Foundation. Apple, iPhone, Macintosh,
Safari and their logo are trademarks or registered trademarks of Apple Inc.
Microsoft, Microsoft Internet Explorer, Windows, Windows 7, and Windows 8
is a registered trademark of Microsoft Corporation. Nvidia and Nvidia Geforce
are trademarks of NVIDIA Corporation. AMD and Radeon are trademarks of
Advanced Micro Devices, Inc.
The authors and publisher have taken care in the preparation of this book, but
make no expressed or implied warranty of any kind and assume no responsibil-
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tial damages in connection with or arising out of the use of the information or
programs contained herein.
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Copyright © 2013 Pearson Education, Inc.
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ISBN-13: 978-0-321-90292-4
ISBN-10: 0-321-90292-0
Text printed in the United States on recycled paper at Edwards Brothers Malloy
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First printing: June 2013
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Thoughts are filled along with time, the distant days will not return,
and time passed is like a spiral of semiprecious stones...
—Kouichi Matsuda
To my wife, family, and friends—for making life fun.
—Rodger Lea
Contents
Preface xvii
1. Overview of WebGL 1
Advantages of WebGL ....................................................................................................3
You Can Start Developing 3D Graphics Applications Using Only a Text Editor .....3
Publishing Your 3D Graphics Applications Is Easy ...................................................4
You Can Leverage the Full Functionality of the Browser .........................................5
Learning and Using WebGL Is Easy...........................................................................5
Origins of WebGL ...........................................................................................................5
Structure of WebGL Applications ...................................................................................6
Summary ........................................................................................................................7
2. Your First Step with WebGL 9
What Is a Canvas? ..........................................................................................................9
Using the <canvas> Tag ...........................................................................................11
DrawRectangle.js ......................................................................................................13
The World’s Shortest WebGL Program: Clear Drawing Area ......................................16
The HTML File (HelloCanvas.html) .........................................................................17
JavaScript Program (HelloCanvas.js) ........................................................................18
Experimenting with the Sample Program ...............................................................23
Draw a Point (Version 1) ..............................................................................................23
HelloPoint1.html......................................................................................................25
HelloPoint1.js ...........................................................................................................25
What Is a Shader? .....................................................................................................27
The Structure of a WebGL Program that Uses Shaders ...........................................28
Initializing Shaders ...................................................................................................30
Vertex Shader ...........................................................................................................33
Fragment Shader ......................................................................................................35
The Draw Operation ................................................................................................36
The WebGL Coordinate System...............................................................................38
Experimenting with the Sample Program ...............................................................40
Draw a Point (Version 2) ..............................................................................................41
Using Attribute Variables .........................................................................................41
Sample Program (HelloPoint2.js) .............................................................................42
Getting the Storage Location of an Attribute Variable ...........................................44
Assigning a Value to an Attribute Variable .............................................................45
Family Methods of gl.vertexAttrib3f() .....................................................................47
Experimenting with the Sample Program ...............................................................49
Contents
ix
Draw a Point with a Mouse Click .................................................................................50
Sample Program (ClickedPoints.js) ..........................................................................50
Register Event Handlers ...........................................................................................52
Handling Mouse Click Events ..................................................................................53
Experimenting with the Sample Program ...............................................................57
Change the Point Color ................................................................................................58
Sample Program (ColoredPoints.js) .........................................................................59
Uniform Variables ....................................................................................................61
Retrieving the Storage Location of a Uniform Variable ..........................................62
Assigning a Value to a Uniform Variable ................................................................63
Family Methods of gl.uniform4f() ...........................................................................65
Summary ......................................................................................................................66
3. Drawing and Transforming Triangles 67
Drawing Multiple Points...............................................................................................68
Sample Program (MultiPoint.js) ...............................................................................70
Using Buffer Objects ................................................................................................72
Create a Buffer Object (gl.createBuffer()) .................................................................74
Bind a Buffer Object to a Target (gl.bindBuffer()) ...................................................75
Write Data into a Buffer Object (gl.bufferData()) ....................................................76
Typed Arrays .............................................................................................................78
Assign the Buffer Object to an Attribute Variable
(gl.vertexAttribPointer()) ..........................................................................................79
Enable the Assignment to an Attribute Variable (gl.enableVertexAttribArray()) ...81
The Second and Third Parameters of gl.drawArrays() .............................................82
Experimenting with the Sample Program ...............................................................84
Hello Triangle ...............................................................................................................85
Sample Program (HelloTriangle.js) ..........................................................................85
Basic Shapes ..............................................................................................................87
Experimenting with the Sample Program ...............................................................89
Hello Rectangle (HelloQuad) ...................................................................................89
Experimenting with the Sample Program ...............................................................91
Moving, Rotating, and Scaling .....................................................................................91
Translation ...............................................................................................................92
Sample Program (TranslatedTriangle.js) ..................................................................93
Rotation ....................................................................................................................96
Sample Program (RotatedTriangle.js) .......................................................................99
Transformation Matrix: Rotation ..........................................................................102
Transformation Matrix: Translation ......................................................................105
Rotation Matrix, Again ..........................................................................................106
Sample Program (RotatedTriangle_Matrix.js) ........................................................107
WebGL Programming Guide
x
Reusing the Same Approach for Translation .........................................................111
Transformation Matrix: Scaling .............................................................................111
Summary ....................................................................................................................113
4. More Transformations and Basic Animation 115
Translate and Then Rotate .........................................................................................115
Transformation Matrix Library: cuon-matrix.js ....................................................116
Sample Program (RotatedTriangle_Matrix4.js) ......................................................117
Combining Multiple Transformation ....................................................................119
Sample Program (RotatedTranslatedTriangle.js) ....................................................121
Experimenting with the Sample Program .............................................................123
Animation ...................................................................................................................124
The Basics of Animation ........................................................................................125
Sample Program (RotatingTriangle.js) ...................................................................126
Repeatedly Call the Drawing Function (tick())......................................................129
Draw a Triangle with the Specified Rotation Angle (draw()) ................................130
Request to Be Called Again (requestAnimationFrame()) .......................................131
Update the Rotation Angle (animate()) .................................................................133
Experimenting with the Sample Program .............................................................135
Summary ....................................................................................................................136
5. Using Colors and Texture Images 137
Passing Other Types of Information to Vertex Shaders .............................................137
Sample Program (MultiAttributeSize.js) .................................................................139
Create Multiple Buffer Objects ..............................................................................140
The gl.vertexAttribPointer() Stride and Offset Parameters ....................................141
Sample Program (MultiAttributeSize_Interleaved.js) .............................................142
Modifying the Color (Varying Variable)................................................................146
Sample Program (MultiAttributeColor.js) ..............................................................147
Experimenting with the Sample Program .............................................................150
Color Triangle (ColoredTriangle.js) ............................................................................151
Geometric Shape Assembly and Rasterization ......................................................151
Fragment Shader Invocations ................................................................................155
Experimenting with the Sample Program .............................................................156
Functionality of Varying Variables and the Interpolation Process .......................157
Pasting an Image onto a Rectangle ............................................................................160
Texture Coordinates ...............................................................................................162
Pasting Texture Images onto the Geometric Shape ..............................................162
Sample Program (TexturedQuad.js) .......................................................................163
Using Texture Coordinates (initVertexBuffers()) ...................................................166
Setting Up and Loading Images (initTextures()) ...................................................166
Make the Texture Ready to Use in the WebGL System (loadTexture()) ...............170
Contents
xi
Flip an Image’s Y-Axis ............................................................................................170
Making a Texture Unit Active (gl.activeTexture()) ................................................171
Binding a Texture Object to a Target (gl.bindTexture()) .......................................173
Set the Texture Parameters of a Texture Object (gl.texParameteri()) ....................174
Assigning a Texture Image to a Texture Object (gl.texImage2D()) .......................177
Pass the Texture Unit to the Fragment Shader (gl.uniform1i()) ...........................179
Passing Texture Coordinates from the Vertex Shader to the Fragment Shader ...180
Retrieve the Texel Color in a Fragment Shader (texture2D()) ..............................181
Experimenting with the Sample Program .............................................................182
Pasting Multiple Textures to a Shape .........................................................................183
Sample Program (MultiTexture.js) .........................................................................184
Summary ....................................................................................................................189
6. The OpenGL ES Shading Language (GLSL ES) 191
Recap of Basic Shader Programs .................................................................................191
Overview of GLSL ES ..................................................................................................192
Hello Shader! ...............................................................................................................193
Basics ......................................................................................................................193
Order of Execution .................................................................................................193
Comments ..............................................................................................................193
Data (Numerical and Boolean Values) .......................................................................194
Variables ....................................................................................................................194
GLSL ES Is a Type Sensitive Language ........................................................................195
Basic Types ..................................................................................................................195
Assignment and Type Conversion .........................................................................196
Operations ..............................................................................................................197
Vector Types and Matrix Types ..................................................................................198
Assignments and Constructors ..............................................................................199
Access to Components ...........................................................................................201
Operations ..............................................................................................................204
Structures ....................................................................................................................207
Assignments and Constructors ..............................................................................207
Access to Members .................................................................................................207
Operations ..............................................................................................................208
Arrays ....................................................................................................................208
Samplers ....................................................................................................................209
Precedence of Operators .............................................................................................210
Conditional Control Flow and Iteration ....................................................................211
if Statement and if-else Statement .........................................................................211
for Statement ..........................................................................................................211
continue, break, discard Statements ......................................................................212
WebGL Programming Guide
xii
Functions ....................................................................................................................213
Prototype Declarations ...........................................................................................214
Parameter Qualifiers ...............................................................................................214
Built-In Functions .......................................................................................................215
Global Variables and Local Variables .........................................................................216
Storage Qualifiers ........................................................................................................217
const Variables .......................................................................................................217
Attribute Variables..................................................................................................218
Uniform Variables ..................................................................................................218
Varying Variables ...................................................................................................219
Precision Qualifiers .....................................................................................................219
Preprocessor Directives ...............................................................................................221
Summary ....................................................................................................................223
7. Toward the 3D World 225
What’s Good for Triangles Is Good for Cubes ...........................................................225
Specifying the Viewing Direction ...............................................................................226
Eye Point, Look-At Point, and Up Direction .........................................................227
Sample Program (LookAtTriangles.js) ....................................................................229
Comparing LookAtTriangles.js with RotatedTriangle_Matrix4.js .........................232
Looking at Rotated Triangles from a Specified Position .......................................234
Sample Program (LookAtRotatedTriangles.js) .......................................................235
Experimenting with the Sample Program .............................................................236
Changing the Eye Point Using the Keyboard ........................................................238
Sample Program (LookAtTrianglesWithKeys.js) ....................................................238
Missing Parts ..........................................................................................................241
Specifying the Visible Range (Box Type) ....................................................................241
Specify the Viewing Volume ..................................................................................242
Defining a Box-Shaped Viewing Volume ..............................................................243
Sample Program (OrthoView.html) .......................................................................245
Sample Program (OrthoView.js) ............................................................................246
Modifying an HTML Element Using JavaScript ....................................................247
The Processing Flow of the Vertex Shader ............................................................248
Changing Near or Far .............................................................................................250
Restoring the Clipped Parts of the Triangles
(LookAtTrianglesWithKeys_ViewVolume.js) .........................................................251
Experimenting with the Sample Program .............................................................253
Specifying the Visible Range Using a Quadrangular Pyramid ...................................254
Setting the Quadrangular Pyramid Viewing Volume ............................................256
Sample Program (PerspectiveView.js) ....................................................................258
The Role of the Projection Matrix .........................................................................260
Using All the Matrices (Model Matrix, View Matrix, and Projection Matrix) ......262
Contents
xiii
Sample Program (PerspectiveView_mvp.js) ...........................................................263
Experimenting with the Sample Program .............................................................266
Correctly Handling Foreground and Background Objects ........................................267
Hidden Surface Removal ........................................................................................270
Sample Program (DepthBuffer.js) ..........................................................................272
Z Fighting ...............................................................................................................273
Hello Cube ..................................................................................................................275
Drawing the Object with Indices and Vertices Coordinates .................................277
Sample Program (HelloCube.js) .............................................................................278
Writing Vertex Coordinates, Colors, and Indices to the Buffer Object ................281
Adding Color to Each Face of a Cube ....................................................................284
Sample Program (ColoredCube.js) .........................................................................285
Experimenting with the Sample Program .............................................................287
Summary ....................................................................................................................289
8. Lighting Objects 291
Lighting 3D Objects ....................................................................................................291
Types of Light Source .............................................................................................293
Types of Reflected Light .........................................................................................294
Shading Due to Directional Light and Its Diffuse Reflection ...............................296
Calculating Diffuse Reflection Using the Light Direction and the
Orientation of a Surface .........................................................................................297
The Orientation of a Surface: What Is the Normal? .............................................299
Sample Program (LightedCube.js) .........................................................................302
Add Shading Due to Ambient Light ......................................................................307
Sample Program (LightedCube_ambient.js) ..........................................................308
Lighting the Translated-Rotated Object .....................................................................310
The Magic Matrix: Inverse Transpose Matrix ........................................................311
Sample Program (LightedTranslatedRotatedCube.js) ............................................312
Using a Point Light Object .........................................................................................314
Sample Program (PointLightedCube.js) .................................................................315
More Realistic Shading: Calculating the Color per Fragment ...............................319
Sample Program (PointLightedCube_perFragment.js) ..........................................319
Summary ....................................................................................................................321
9. Hierarchical Objects 323
Drawing and Manipulating Objects Composed of Other Objects ............................324
Hierarchical Structure.............................................................................................325
Single Joint Model ..................................................................................................326
Sample Program (JointModel.js) ............................................................................328
Draw the Hierarchical Structure (draw()) ..............................................................332
A Multijoint Model ................................................................................................334
WebGL Programming Guide
xiv
Sample Program (MultiJointModel.js) ...................................................................335
Draw Segments (drawBox()) ...................................................................................339
Draw Segments (drawSegment()) ...........................................................................340
Shader and Program Objects: The Role of initShaders() ............................................344
Create Shader Objects (gl.createShader()) ..............................................................345
Store the Shader Source Code in the Shader Objects (g.shaderSource())..............346
Compile Shader Objects (gl.compileShader()) .......................................................347
Create a Program Object (gl.createProgram()) .......................................................349
Attach the Shader Objects to the Program Object (gl.attachShader()) .................350
Link the Program Object (gl.linkProgram()) ..........................................................351
Tell the WebGL System Which Program Object to Use (gl.useProgram()) ...........353
The Program Flow of initShaders() ........................................................................353
Summary ....................................................................................................................356
10. Advanced Techniques 357
Rotate an Object with the Mouse ...............................................................................357
How to Implement Object Rotation ......................................................................358
Sample Program (RotateObject.js) .........................................................................358
Select an Object ..........................................................................................................360
How to Implement Object Selection .....................................................................361
Sample Program (PickObject.js) .............................................................................362
Select the Face of the Object ..................................................................................365
Sample Program (PickFace.js) .................................................................................366
HUD (Head Up Display) .............................................................................................368
How to Implement a HUD.....................................................................................369
Sample Program (HUD.html) .................................................................................369
Sample Program (HUD.js) ......................................................................................370
Display a 3D Object on a Web Page (3DoverWeb) ...............................................372
Fog (Atmospheric Effect) ............................................................................................372
How to Implement Fog ..........................................................................................373
Sample Program (Fog.js) .........................................................................................374
Use the w Value (Fog_w.js) ....................................................................................376
Make a Rounded Point ...............................................................................................377
How to Implement a Rounded Point ....................................................................377
Sample Program (RoundedPoints.js) ......................................................................378
Alpha Blending ...........................................................................................................380
How to Implement Alpha Blending ......................................................................380
Sample Program (LookAtBlendedTriangles.js) .......................................................381
Blending Function ..................................................................................................382
Alpha Blend 3D Objects (BlendedCube.js) ............................................................384
How to Draw When Alpha Values Coexist ...........................................................385
Contents
xv
Switching Shaders .......................................................................................................386
How to Implement Switching Shaders ..................................................................387
Sample Program (ProgramObject.js) ......................................................................387
Use What You’ve Drawn as a Texture Image .............................................................392
Framebuffer Object and Renderbuffer Object .......................................................392
How to Implement Using a Drawn Object as a Texture .......................................394
Sample Program (FramebufferObjectj.js) ...............................................................395
Create Frame Buffer Object (gl.createFramebuffer()) .............................................397
Create Texture Object and Set Its Size and Parameters .........................................397
Create Renderbuffer Object (gl.createRenderbuffer()) ...........................................398
Bind Renderbuffer Object to Target and Set Size (gl.bindRenderbuffer(),
gl.renderbufferStorage()) ........................................................................................399
Set Texture Object to Framebuffer Object (gl.bindFramebuffer(),
gl.framebufferTexture2D()) ....................................................................................400
Set Renderbuffer Object to Framebuffer Object
(gl.framebufferRenderbuffer()) ...............................................................................401
Check Configuration of Framebuffer Object (gl.checkFramebufferStatus()) ........402
Draw Using the Framebuffer Object ......................................................................403
Display Shadows .........................................................................................................405
How to Implement Shadows .................................................................................405
Sample Program (Shadow.js) ..................................................................................406
Increasing Precision ...............................................................................................412
Sample Program (Shadow_highp.js) ......................................................................413
Load and Display 3D Models .....................................................................................414
The OBJ File Format ...............................................................................................417
The MTL File Format ..............................................................................................418
Sample Program (OBJViewer.js) .............................................................................419
User-Defined Object ...............................................................................................422
Sample Program (Parser Code in OBJViewer.js) ....................................................423
Handling Lost Context ...............................................................................................430
How to Implement Handling Lost Context ..........................................................431
Sample Program (RotatingTriangle_contextLost.js) ..............................................432
Summary ....................................................................................................................434
A. No Need to Swap Buffers in WebGL 437
B. Built-in Functions of GLSL ES 1.0 441
Angle and Trigonometry Functions ...........................................................................441
Exponential Functions ................................................................................................443
Common Functions ....................................................................................................444
Geometric Functions ..................................................................................................447
WebGL Programming Guide
xvi
Matrix Functions .........................................................................................................448
Vector Functions .........................................................................................................449
Texture Lookup Functions ..........................................................................................451
C. Projection Matrices 453
Orthogonal Projection Matrix ....................................................................................453
Perspective Projection Matrix .....................................................................................453
D. WebGL/OpenGL: Left or Right Handed? 455
Sample Program CoordinateSystem.js ........................................................................456
Hidden Surface Removal and the Clip Coordinate System .......................................459
The Clip Coordinate System and the Viewing Volume.............................................460
What Is Correct? .........................................................................................................462
Summary ....................................................................................................................464
E. The Inverse Transpose Matrix 465
F. Load Shader Programs from Files 471
G. World Coordinate System Versus Local Coordinate System 473
The Local Coordinate System .....................................................................................474
The World Coordinate System ...................................................................................475
Transformations and the Coordinate Systems ...........................................................477
H. Web Browser Settings for WebGL 479
Glossary 481
References 485
Index 487
xvii
Preface
Preface
WebGL is a technology that enables drawing, displaying, and interacting with sophis-
ticated interactive three-dimensional computer graphics (“3D graphics”) from within
web browsers. Traditionally, 3D graphics has been restricted to high-end computers or
dedicated game consoles and required complex programming. However, as both personal
computers and, more importantly, web browsers have become more sophisticated, it has
become possible to create and display 3D graphics using accessible and well-known web
technologies. This book provides a comprehensive overview of WebGL and takes the
reader, step by step, through the basics of creating WebGL applications. Unlike other
3D graphics technologies such as OpenGL and Direct3D, WebGL applications can be
constructed as web pages so they can be directly executed in the browsers without install-
ing any special plug-ins or libraries. Therefore, you can quickly develop and try out a
sample program with a standard PC environment; because everything is web based, you
can easily publish the programs you have constructed on the web. One of the promises
of WebGL is that, because WebGL applications are constructed as web pages, the same
program can be run across a range of devices, such as smart phones, tablets, and game
consoles, through the browser. This powerful model means that WebGL will have a signif-
icant impact on the developer community and will become one of the preferred tools for
graphics programming.
Who the Book Is For
We had two main audiences in mind when we wrote this book: web developers looking
to add 3D graphics to their web pages and applications, and 3D graphics programmers
wishing to understand how to apply their knowledge to the web environment. For web
developers who are familiar with standard web technologies such as HTML and JavaScript
and who are looking to incorporate 3D graphics into their web pages or web applica-
tions, WebGL offers a simple yet powerful solution. It can be used to add 3D graphics to
enhance web pages, to improve the user interface (UI) for a web application by using a 3D
interface, and even to develop more complex 3D applications and games that run in web
browsers.
The second target audience is programmers who have worked with one of the main 3D
application programming interfaces (APIs), such as Direct3D or OpenGL, and who are
interested in understanding how to apply their knowledge to the web environment. We
would expect these programmers to be interested in the more complex 3D applications
that can be developed in modern web browsers.
However, the book has been designed to be accessible to a wide audience using a step-by-
step approach to introduce features of WebGL, and it assumes no background in 2D or 3D
graphics. As such, we expect it also to be of interest to the following:
WebGL Programming Guide
xviii
• General programmers seeking an understanding of how web technologies are evolv-
ing in the graphics area
• Students studying 2D and 3D graphics because it offers a simple way to begin to ex-
periment with graphics via a web browser rather than setting up a full programming
environment
• Web developers exploring the “bleeding edge” of what is possible on mobile devices
such as Android or iPhone using the latest mobile web browsers
What the Book Covers
This book covers the WebGL 1.0 API along with all related JavaScript functions. You will
learn how HTML, JavaScript, and WebGL are related, how to set up and run WebGL appli-
cations, and how to incorporate sophisticated 3D program “shaders” under the control
of JavaScript. The book details how to write vertex and fragment shaders, how to imple-
ment advanced rendering techniques such as per-pixel lighting and shadowing, and basic
interaction techniques such as selecting 3D objects. Each chapter develops a number of
working, fully functional WebGL applications and explains key WebGL features through
these examples. After finishing the book, you will be ready to write WebGL applications
that fully harness the programmable power of web browsers and the underlying graphics
hardware.
How the Book Is Structured
This book is organized to cover the API and related web APIs in a step-by-step fashion,
building up your knowledge of WebGL as you go.
Chapter 1—Overview of WebGL
This chapter briefly introduces you to WebGL, outlines some of the key features and
advantages of WebGL, and discusses its origins. It finishes by explaining the relationship
of WebGL to HTML5 and JavaScript and which web browsers you can use to get started
with your exploration of WebGL.
Chapter 2—Your First Step with WebGL
This chapter explains the <canvas> element and the core functions of WebGL by taking
you, step-by-step, through the construction of several example programs. Each example
is written in JavaScript and uses WebGL to display and interact with a simple shape on
a web page. The example WebGL programs will highlight some key points, including:
(1) how WebGL uses the <canvas> element object and how to draw on it; (2) the linkage
between HTML and WebGL using JavaScript; (3) simple WebGL drawing functions; and
(4) the role of shader programs within WebGL.
xix
Preface
Chapter 3—Drawing and Transforming Triangles
This chapter builds on those basics by exploring how to draw more complex shapes and
how to manipulate those shapes in 3D space. This chapter looks at: (1) the critical role of
triangles in 3D graphics and WebGL’s support for drawing triangles; (2) using multiple
triangles to draw other basic shapes; (3) basic transformations that move, rotate, and
scale triangles using simple equations; and (4) how matrix operations make transforma-
tions simple.
Chapter 4—More Transformations and Basic Animation
In this chapter, you explore further transformations and begin to combine transformations
into animations. You: (1) are introduced to a matrix transformation library that hides the
mathematical details of matrix operations; (2) use the library to quickly and easily combine
multiple transformations; and (3) explore animation and how the library helps you
animate simple shapes. These techniques provide the basics to construct quite complex
WebGL programs and will be used in the sample programs in the following chapters.
Chapter 5—Using Colors and Texture Images
Building on the basics described in previous chapters, you now delve a little further into
WebGL by exploring the following three subjects: (1) besides passing vertex coordinates,
how to pass other data such as color information to the vertex shader; (2) the conver-
sion from a shape to fragments that takes place between the vertex shader and the frag-
ment shader, which is known as the rasterization process; and (3) how to map images (or
textures) onto the surfaces of a shape or object. This chapter is the final chapter focusing
on the key functionalities of WebGL.
Chapter 6—The OpenGL ES Shading Language (GLSL ES)
This chapter takes a break from examining WebGL sample programs and explains the core
features of the OpenGL ES Shading Language (GLSL ES) in detail. You will cover: (1) data,
variables, and variable types; (2) vector, matrix, structure, array, and sampler; (3) opera-
tors, control flow, and functions; (4) attributes, uniforms, and varyings; (5) precision
qualifier; and (6) preprocessor and directives. By the end of this chapter you will have a
good understanding of GLSL ES and how it can be used to write a variety of shaders.
Chapter 7—Toward the 3D World
This chapter takes the first step into the 3D world and explores the implications of
moving from 2D to 3D. In particular, you will explore: (1) representing the user’s view
into the 3D world; (2) how to control the volume of 3D space that is viewed; (3) clipping;
(4) foreground and background objects; and (5) drawing a 3D object—a cube. All these
issues have a significant impact on how the 3D scene is drawn and presented to viewers.
A mastery of them is critical to building compelling 3D scenes.
WebGL Programming Guide
xx
Chapter 8—Lighting Objects
This chapter focuses on lighting objects, looking at different light sources and their effects
on the 3D scene. Lighting is essential if you want to create realistic 3D scenes because it
helps to give the scene a sense of depth.
The following key points are discussed in this chapter: (1) shading, shadows, and different
types of light sources including point, directional, and ambient; (2) reflection of light in
the 3D scene and the two main types: diffuse and ambient reflection; and (3) the details of
shading and how to implement the effect of light to make objects look three-dimensional.
Chapter 9—Hierarchical Objects
This chapter is the final chapter describing the core features and how to program with
WebGL. Once completed, you will have mastered the basics of WebGL and will have
enough knowledge to be able to create realistic and interactive 3D scenes. This chapter
focuses on hierarchical objects, which are important because they allow you to progress
beyond single objects like cubes or blocks to more complex objects that you can use for
game characters, robots, and even modeling humans.
Chapter 10—Advanced Techniques
This chapter touches on a variety of important techniques that use what you have learned
so far and provide you with an essential toolkit for building interactive, compelling 3D
graphics. Each technique is introduced through a complete example, which you can reuse
when building your own WebGL applications.
Appendix A—No Need to Swap Buffers in WebGL
This appendix explains why WebGL programs don’t need to swap buffers.
Appendix B—Built-In Functions of GLSL ES 1.0
This appendix provides a reference for all the built-in functions available in the OpenGL
ES Shading Language.
Appendix C—Projection Matrices
This appendix provides the projection matrices generated by Matrix4.setOrtho() and
Matrix4.setPerspective() .
Appendix D—WebGL/OpenGL: Left or Right Handed?
This appendix explains how WebGL and OpenGL deal internally with the coordi-
nate system and clarify that technically, both WebGL and OpenGL are agnostic as to
handedness.
xxi
Preface
Appendix E—The Inverse Transpose Matrix
This appendix explains how the inverse transpose matrix of the model matrix can deal
with the transformation of normal vectors.
Appendix F—Loading Shader Programs from Files
This appendix explains how to load the shader programs from files.
Appendix G—World Coordinate System Versus Local Coordinate System
This appendix explains the different coordinate systems and how they are used in 3D
graphics.
Appendix H—Web Browser Settings for WebGL
This appendix explains how to use advanced web browser settings to ensure that WebGL
is displayed correctly, and what to do if it isn’t.
WebGL-Enabled Browsers
At the time of writing, WebGL is supported by Chrome, Firefox, Safari, and Opera. Sadly,
some browsers, such as IE9 (Microsoft Internet Explorer), don’t yet support WebGL. In
this book, we use the Chrome browser released by Google, which, in addition to WebGL
supports a number of useful features such as a console function for debugging. We have
checked the sample programs in this book using the following environment ( Table P.1 )
but would expect them to work with any browser supporting WebGL.
Table P.1 PC Environment
Browser Chrome (25.0.1364.152 m)
OS Windows 7 and 8
Graphics boards NVIDIA Quadro FX 380, NVIDIA GT X 580, NVIDIA GeForce GTS 450,
Mobile Intel 4 Series Express Chipset Family, AMD Radeon HD
6970
Refer to the www.khronos.org/webgl/wiki/BlacklistsAndWhitelists for an updated list of
which hardware cards are known to cause problems.
To confirm that you are up and running, download Chrome (or use your preferred
browser) and point it to the companion website for this book at https://sites.google.com/
site/webglbook/
Navigate to Chapter 3 and click the link to the sample file HelloTriangle.html . If you can
see a red triangle as shown in Figure P.1 in the browser, WebGL is working.
WebGL Programming Guide
xxii
Figure P.1 Loading HelloTriangle results in a red triangle
If you don’t see the red triangle shown in the figure, take a look at Appendix H , which
explains how to change your browser settings to load WebGL.
Sample Programs and Related Links
All sample programs in this book and related links are available on the compan-
ion websites. The official site hosted by the publisher is www.informit.com/
title/9780321902924 and the author site is hosted at https://sites.google.com/site/
webglbook/ .
The latter site contains the links to each sample program in this book. You can run each
one directly by clicking the links.
If you want to modify the sample programs, you can download the zip file of all the
samples, available on both sites, to your local disk. In this case, you should note that
the sample program consists of both the HTML file and the associated JavaScript file
in the same folder. For example, for the sample program HelloTriangle , you need
both HelloTriangle.html and HelloTriangle.js . To run HelloTriangle , double-click
HelloTriangle.html .
xxiii
Preface
Style Conventions
These style conventions are used in this book:
• Bold —First occurrences of key terms and important words
• Italic —Parameter names and names of references
• Monospace —Code examples, methods, functions, variables, command options,
JavaScript object names, fi lenames, and HTML tags
WebGL Programming Guide
xxiv
Acknowledgments
We have been fortunate to receive help and guidance from many talented individuals
during the process of creating this book, both with the initial Japanese version and the
subsequent English one.
Takafumi Kanda helped by providing numerous code samples for our support librar-
ies and sample programs; without him, this book could not have been realized. Yasuko
Kikuchi, Chie Onuma, and Yuichi Nishizawa provided valuable feedback on early versions
of the book. Of particular note, one insightful comment by Ms. Kikuchi literally stopped
the writing, causing a reevaluation of several sections and leading to a much stronger
book. Hiroyuki Tanaka and Kazsuhira Oonishi (iLinx) gave excellent support with the
sample programs, and Teruhisa Kamachi and Tetsuo Yoshitani supported the writing of
sections on HTML5 and JavaScript. The WebGL working group, especially Ken Russell
(Google), Chris Marin (Apple), and Dan Ginsburg (AMD), have answered many techni-
cal questions. We have been privileged to receive an endorsement from the president
of the Khronos Group, Neil Trevett, and appreciate the help of Hitoshi Kasai (Principal,
MIACIS Associates) who provided the connection to Mr. Trevett and the WebGL working
group. In addition, thank you to Xavier Michel and Makoto Sato (Sophia University), who
greatly helped with the translation of the original text and issues that arose during the
translation. For the English version, Jeff Gilbert, Rick Rafey, and Daniel Haehn reviewed
this book carefully and gave us excellent technical comments and feedback that greatly
improved the book. Our thanks also to Laura Lewin and Olivia Basegio from Pearson, who
have helped with organizing the publication and ensuring the whole process has been as
smooth and as painless as possible.
We both owe a debt of gratitude to the authors of the “Red Book” (OpenGL Programming
Guide) and the “Gold Book” (OpenGL ES 2.0 Programming Guide) both published by
Pearson, without which this book would not have been possible. We hope, in some small
way, that this book repays some of that debt.
xxv
About the Authors
About the Authors
Dr. Kouichi Matsuda has a broad background in user interface and user experience design
and its application to novel multimedia products. His work has taken him from product
development, through research, and back to development, having spent time at NEC,
Sony Corporate Research, and Sony Computer Science Laboratories. He is currently a chief
distinguished researcher focused on user experience and human computer interaction
across a range of consumer electronics. He was the designer of the social 3D virtual world
called “PAW” (personal agent-oriented virtual world), was involved in the development
of the VRML97 (ISO/IEC 14772-1:1997) standard from the start, and has remained active
in both VRML and X3D communities (precursors to WebGL). He has written 15 books on
computer technologies and translated a further 25 into Japanese. His expertise covers user
experiences, user interface, human computer interaction, natural language understanding,
entertainment-oriented network services, and interface agent systems. Always on
the lookout for new and exciting possibilities in the technology space, he combines his
professional life with a love of hot springs, sea in summer, wines, and MANGA (at
which he dabbles in drawing and illustrations). He received his Ph.D. (Engineering)
from the Graduate School of Engineering, University of Tokyo, and can be reached via
WebGL.prog.guide@gmail.com .
Dr. Rodger Lea is an adjunct professor with the Media and Graphics Interdisciplinary
Centre at the University of British Columbia, with an interest in systems aspects of multi-
media and distributed computing. With more than 20 years of experience leading research
groups in both academic and industrial settings, he has worked on early versions of shared
3D worlds, helped define VRML97, developed multimedia operating systems, prototyped
interactive digital TV, and led developments on multimedia home networking standards.
He has published more than 60 research papers and three books, and he holds 12 patents.
His current research explores the growing “Internet of Things,” but he retains a passion
for all things media and graphics.
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Chapter 1
Overview of WebGL
WebGL is a technology that enables drawing, displaying, and interacting with sophisticated
interactive three-dimensional computer graphics (“3D graphics”) from within web browsers.
Traditionally, 3D graphics has been restricted to high-end computers or dedicated game consoles
and has required complex programming. However, as both personal computers and, more impor-
tantly, web browsers, have become more sophisticated, it has become possible to create and
display 3D graphics using accessible and well-known web technologies. WebGL, when combined
with HTML5 and JavaScript, makes 3D graphics accessible to web developers and will play an
important role in the development of next generation, easy-to-use and intuitive user interfaces
and web content. Some examples of this are shown in Figure 1.1 . Over the next few years, you
can expect to see WebGL used on a range of devices from standard PCs to consumer electronics,
smart phones, and tablets.
CHAPTER 1 Overview of WebGL
2
Figure 1.1 Complex 3D graphics within a browser. © 2011 Hiromasa Horie (left), 2012
Kouichi Matsuda (right)
HTML5, the latest evolution of the HTML standard, expands traditional HTML with
features covering 2D graphics, networking, and local storage access. With the advent of
HTML5, browsers are rapidly evolving from simple presentation engines to sophisticated
application platforms. With this evolution comes a need for interface and graphics capa-
bilities beyond 2D. WebGL has been designed for that central role of creating the visual
layer for new browser-based 3D applications and experiences.
Traditionally, creating compelling 3D graphics required you to create a stand-alone appli-
cation using a programming language such as C or C++ along with dedicated computer
graphics libraries such as OpenGL and Direct3D. However, with WebGL, you can now
realize 3D graphics as part of a standard web page using familiar HTML and JavaScript—
with a little extra code for the 3D graphics.
Importantly, because WebGL is supported as the browser’s default built-in technology for
rendering 3D graphics, you can use WebGL directly without having to install special plug-
ins or libraries. Better still, because it’s all browser based, you can run the same WebGL
applications on various platforms, from sophisticated PCs down to consumer electronics,
tablets, and smart phones.
This chapter briefly introduces you to WebGL, outlines some of the key features and
advantages of WebGL, and discusses its origins. It also explains the relationship of WebGL
to HTML5 and JavaScript and the structure of WebGL programs.
Advantages of WebGL
3
Advantages of WebGL
As HTML has evolved, web developers have been able to create increasingly sophisticated
web-based applications. Originally, HTML offered only static content, but the introduc-
tion of scripting support like JavaScript enabled more complex interactions and dynamic
content. HTML5 introduced further sophistication, including support for 2D graphics via
the canvas tag. This allowed a variety of graphical elements on a web page, ranging from
dancing cartoon characters to map animations that respond to user input by updating the
maps in real time.
WebGL takes this one step further, enabling the display and manipulation of 3D graphics
on web pages by using JavaScript. Using WebGL, it becomes possible to create rich user
interfaces and 3D games and to use 3D to visualize and manipulate a variety of informa-
tion from the Internet. Although the technical capabilities of WebGL are impressive, it is
perhaps the ease of use and accessibility that differentiate it from other technologies and
that will ensure its impact. In particular:
• You can start developing 3D graphics applications using only a text editor and
browser.
• You can easily publish the 3D graphics applications using standard web technolo-
gies, making them available to your friends or other web users.
• You can leverage the full functionality of the browser.
• Learning and using WebGL is easy because a lot of material is already available for
study and development.
You Can Start Developing 3D Graphics Applications Using Only
a Text Editor
One handy and convenient point in developing applications using WebGL is that you
don’t need to set up an application developing environment for WebGL. As explained
earlier, because WebGL is built into the browser, there is no need for special applica-
tion development tools such as compilers and linkers to create 3D graphics applications.
As a minimum, to view the sample programs explained in this book, you only need a
WebGL-enabled browser. If you want to edit them or create your own, a standard text
editor (for example, Notepad or TextEdit) is enough. In Figure 1.2 , you can see a WebGL
application running in Chrome and the HTML file opened in Notepad. The JavaScript file
( RotateObject.js ) that uses WebGL is loaded by the HTML file and could also be edited
using a simple text editor.
CHAPTER 1 Overview of WebGL
4
Browser (Chrome) Notepad
Figure 1.2 The only tools needed for developing 3D graphics applications using WebGL
Publishing Your 3D Graphics Applications Is Easy
Traditionally, 3D graphics applications have been developed using a programming
language such as C or C++ and then compiled into an executable binary for a specific plat-
form. This meant, for example, the version for a Macintosh wouldn’t work on Windows or
Linux. Additionally, users often needed to install not only the applications themselves but
also libraries required by the applications to run, which meant another level of complexity
when you wanted to share your work.
In contrast, because WebGL applications are composed of HTML and JavaScript files,
they can be easily shared by simply putting them on a web server just like standard web
pages or distributing the HTML and JavaScript files via email. For example, Figure 1.3
shows some sample WebGL applications published by Google and available at http://code.
google.com/p/webglsamples/ .
Figure 1.3 WebGL sample applications published by Google (with the permission of Gregg
Tavares, Google)
Origins of WebGL
5
You Can Leverage the Full Functionality of the Browser
Because WebGL applications are created as part of a web page, you can utilize the full
functionality of the browser such as arranging buttons, displaying dialog boxes, drawing
text, playing video or audio, and communicating with web servers. These advanced
features come for free, whereas in traditional 3D graphics applications they would need to
be programmed explicitly.
Learning and Using WebGL Is Easy
The specification of WebGL is based on the royalty-free open standard, OpenGL, which
has been widely used in graphics, video games, and CAD applications for many years. In
one sense, WebGL is “OpenGL for web browsers.” Because OpenGL has been used in a
variety of platforms over the past 20 years, there are many reference books, materials, and
sample programs using OpenGL, which can be used to better understand WebGL.
Origins of WebGL
Two of the most widely used technologies for displaying 3D graphics on personal comput-
ers are Direct3D and OpenGL. Direct3D, which is part of Microsoft’s DirectX technologies,
is the 3D graphics technology primarily used on Windows platforms and is a proprietary
application programming interface (API) that Microsoft controls. An alternative, OpenGL
has been widely used on various platforms due to its open and royalty-free nature.
OpenGL is available for Macintosh, Linux, and a variety of devices such as smart phones,
tablet computers, and game consoles (PlayStation and Nintendo). It is also well supported
on Windows and provides an alternative to Direct3D.
OpenGL was originally developed by Silicon Graphics Inc. and published as an open
standard in 1992. OpenGL has evolved through several versions since 1992 and has had a
profound effect on the development of 3D graphics, software product development, and
even film production over the years. The latest version of OpenGL at the time of writing
is version 4.3 for desktop PCs. Although WebGL has its roots in OpenGL, it is actually
derived from a version of OpenGL designed specifically for embedded computers such as
smart phones and video game consoles. This version, known as OpenGL ES (for Embedded
Systems), was originally developed in 2003–2004 and was updated in 2007 (ES 2.0) and
again in 2012 (ES 3.0). WebGL is based on the ES 2.0 version. In recent years, the number
of devices and processors that support the specification has rapidly increased and includes
smart phones (iPhone and Android), tablet computers, and game consoles. Part of the
reason for this successful adoption has been that OpenGL ES added new features but also
removed many unnecessary or old-fashioned features from OpenGL, resulting in a light-
weight specification that still had enough visual expressive power to realize attractive 3D
graphics.
Figure 1.4 shows the relationship among OpenGL, OpenGL ES 1.1/2.0/3.0, and WebGL.
Because OpenGL itself has continued to evolve from 1.5, to 2.0, to 4.3, OpenGL ES
have been standardized as a subset of specific versions of OpenGL (OpenGL 1.5 and
OpenGL 2.0).
CHAPTER 1 Overview of WebGL
6
5 . 1 L G n e p O 0 . 2 L G n e p O
S E L G n e p O
1 . 1
S E L G n e p O
0 . 2
3 . 3 L G n e p O
0 . 1 L G b e W
S E L G n e p O
0 . 3
3 . 4 L G n e p O
Figure 1.4 Relationship among OpenGL, OpenGL ES 1.1/2.0/3.0, and WebGL
As shown in Figure 1.4 , with the move to OpenGL 2.0, a significant new capability,
programmable shader functions , was introduced. This capability has been carried
through to OpenGL ES 2.0 and is a core part of the WebGL 1.0 specification.
Shader functions or shaders are computer programs that make it possible to program
sophisticated visual effects by using a special programming language similar to C. This
book explains shader functions in a step-by-step manner, allowing you to quickly master
the power of WebGL. The programming language that is used to create shaders is called
a shading language . The shading language used in OpenGL ES 2.0 is based on the
OpenGL shading language (GLSL) and referred to as OpenGL ES shading language
(GLSL ES). Because WebGL is based on OpenGL ES 2.0, it also uses GLSL ES for creating
shaders.
The Khronos Group (a non-profit industry consortium created to develop, publish, and
promote various open standards) is responsible for the evolution and standardization of
OpenGL. In 2009, Khronos established the WebGL working group and then started the
standardization process of WebGL based on OpenGL ES 2.0, releasing the first version of
WebGL in 2011. This book is written based primarily on that specification and, where
needed, the latest specification of WebGL published as an Editor’s Draft. For more infor-
mation, please refer to the specification.
1
Structure of WebGL Applications
In HTML, dynamic web pages can be created by using a combination of HTML and
JavaScript. With the introduction of WebGL, the shader language GLSL ES needs to
be added to the mix, meaning that web pages using WebGL are created by using three
1 WebGL 1.0: www.khronos.org/registry/webgl/specs/1.0/ and Editor’s draft: www.khronos.org/
registry/webgl/specs/latest/
Summary
7
languages: HTML5 (as a Hypertext Markup Language), JavaScript, and GLSL ES. Figure 1.5
shows the software architecture of traditional dynamic web pages (left side) and web pages
using WebGL (right side).
5 L M T H t p i r c S a v a J S E L S L G
L G b e W g n i s u e g a p b e W
L M T H t p i r c S a v a J
e g a p b e W
L M T H
L G b e W
S E L G n e p O / L G n e p O
e n i g n E g n i r e d n e R
HTML L M T H
e n i g n E g n i r e d n e R
Figure 1.5 The software architecture of dynamic web pages (left) and web pages using
WebGL (right)
However, because GLSL ES is generally written within JavaScript, only HTML and
JavaScript files are actually necessary for WebGL applications. So, although WebGL does
add complexity to the JavaScript, it retains the same structure as standard dynamic web
pages, only using HTML and JavaScript files.
Summary
This chapter briefly overviewed WebGL, explained some key features, and outlined
the software architecture of WebGL applications. In summary, the key takeaway from
this chapter is that WebGL applications are developed using three languages: HTML5,
JavaScript, and GLSL ES—however, because the shader code (GLSL ES) is generally embed-
ded in the JavaScript, you have exactly the same file structure as a traditional web page.
The next chapter explains how to create applications using WebGL, taking you step by
step through a set of simple WebGL examples.
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Chapter 2
Your First Step with WebGL
As explained in Chapter 1 , “Overview of WebGL,” WebGL applications use both HTML and
JavaScript to create and draw 3D graphics on the screen. To do this, WebGL utilizes the new
<canvas> element, introduced in HTML5, which defines a drawing area on a web page. Without
WebGL, the <canvas> element only allows you to draw two-dimensional graphics using
JavaScript. With WebGL, you can use the same element for drawing three-dimensional graphics.
This chapter explains the <canvas> element and the core functions of WebGL by taking you,
step-by-step, through the construction of several example programs. Each example is written in
JavaScript and uses WebGL to display and interact with a simple shape on a web page. Because of
this, these JavaScript programs are referred to as WebGL applications .
The example WebGL applications will highlight some key points, including:
• How WebGL uses the <canvas> element and how to draw on it
• The linkage between HTML and WebGL using JavaScript
• Simple WebGL drawing functions
• The role of shader programs within WebGL
By the end of this chapter, you will understand how to write and execute basic WebGL applica-
tions and how to draw simple 2D shapes. You will use this knowledge to explore further the
basics of WebGL in Chapters 3 , “Drawing and Transforming Triangles,” 4 , “More Transformations
and Basic Animation,” and 5 , “Using Colors and Texture Images.”
What Is a Canvas?
Before HTML5, if you wanted to display an image in a web page, the only native HTML approach
was to use the <img> tag. This tag, although a convenient tool, is restricted to still images and
CHAPTER 2 Your First Step with WebGL
10
doesn’t allow you to dynamically draw and display the image on the fly. This is one of the
reasons that non-native solutions such as Flash Player have been used.
However, HTML5, by introducing the <canvas> tag, has changed all that, offering a conve-
nient way to draw computer graphics dynamically using JavaScript.
In a similar manner to the way artists use paint canvases, the <canvas> tag defines a
drawing area on a web page. Then, rather than using brush and paints, you can use
JavaScript to draw anything you want in the area. You can draw points, lines, rectangles,
circles, and so on by using JavaScript methods provided for <canvas> . Figure 2.1 shows an
example of a drawing tool that uses the <canvas> tag.
Figure 2.1 A drawing tool using the <canvas> element ( http://caimansys.com/painter/ )
This drawing tool runs within a web page and allows you to interactively draw lines, rect-
angles, and circles and even change their colors.
Although you won’t be creating anything as sophisticated just yet, let’s look at the core
functions of <canvas> by using a sample program, DrawRectangle , which draws a filled
blue rectangle on a web page. Figure 2.2 shows DrawRectangle when it’s loaded into a
browser.
What Is a Canvas?
11
Figure 2.2 DrawRectangle
Using the <canvas> Tag
Let’s look at how DrawRectangle works and explain how the <canvas> tag is used in the
HTML file. Listing 2.1 shows DrawingTriangle.html . Note that all HTML files in this book
are written in HTML5.
Listing 2.1 DrawRectangle.html
1 <!DOCTYPE html>
2 <html lang="en">
3 <head>
4 <meta charset="utf-8" />
5 <title>Draw a blue rectangle (canvas version)</title>
6 </head>
7
8 <body οnlοad="main()">
9 <canvas id="example" width="400" height="400">
10 Please use a browser that supports "canvas"
11 </canvas>
12 <script src="DrawRectangle.js"></script>
13 </body>
14 </html>
CHAPTER 2 Your First Step with WebGL
12
The <canvas> tag is defined at line 9. This defines the drawing area as a 400 × 400 pixel
region on a web page using the width and height attributes in the tag. The canvas is given
an identifier using the id attribute, which will be used later:
<canvas id="example" width="400" height="400"></canvas>
By default, the canvas is invisible (actually transparent) until you draw something into it,
which we’ll do with JavaScript in a moment. That’s all you need to do in the HTML file to
prepare a <canvas> that the WebGL program can use. However, one thing to note is that
this line only works in a <canvas> -enabled browser. However, browsers that don’t support
the <canvas> tag will ignore this line, and nothing will be displayed on the screen. To
handle this, you can display an error message by adding the message into the tag as
follows:
9 <canvas id="example" width="400" height="400">
10 Please use a browser that supports "canvas"
11 </canvas>
To draw into the canvas, you need some associated JavaScript code that performs the
drawing operations. You can include that JavaScript code in the HTML or write it as a
separate JavaScript file. In our examples, we use the second approach because it makes the
code easier to read. Whichever approach you take, you need to tell the browser where the
JavaScript code starts. Line 8 does that by telling the browser that when it loads the sepa-
rate JavaScript code it should use the function main() as the entry point for the JavaScript
program. This is specified for the <body> element using its onload attribute that tells the
browser to execute the JavaScript function main() after it loads the <body> element:
8 <body οnlοad="main()">
Line 12 tells the browser to import the JavaScript file DrawRectangle.js in which the
function main() is defined:
12 <script src="DrawRectangle.js"></script>
For clarity, all sample programs in this book use the same filename for both the HTML file
and the associated JavaScript file, which is imported in the HTML file (see Figure 2.3 ).
What Is a Canvas?
13
> l m t h E P Y T C O D ! <
> " n e “ = g n a l l m t h <
> d a e h <
> / " 8 - f t u " = t e s r a h c a t e m <
> e l t i t / < ) n o i s r e v s a v n a c ( e l g n a t c e r e u l b a w a r D > e l t i t <
> d a e h / <
> " ) ( n i a m " = d a o l n o y d o b <
> " 0 5 1 " = t h g i e h " 0 0 2 " = h t d i w " e l p m a x e " = d i s a v n a c <
” s a v n a c “ s t r o p p u s t a h t r e s w o r b a e s u e s a e l P
> s a v n a c / <
> t p i r c s / < > " s j . e l g n a t c e R w a r D " = c r s t p i r c s <
> y d o b / <
> l m t h / <
s j . e l g n a t c e R w a r D /
{ ) ( n i a m n o i t c n u f
t n e m e l e > s a v n a c < e h t e v e i r t e R / /
; ) ' e l p m a x e ' ( d I y B t n e m e l E t e g . t n e m u c o d = s a v n a c r a v
{ ) s a v n a c ! ( f i
; ) ' t n e m e l e > s a v n a c < e h t e v e i r t e r o t d e l i a F ( g o l . e l o s n o c
; e s l a f n r u t e r
}
G C D 2 r o f t x e t n o c g n i r e d n e r e h t t e G / /
; ) ' d 2 ' ( t x e t n o C t e g . s a v n a c = x t c r a v
e l g n a t c e r e u l b a w a r D / /
r o l o c e u l b a t e S / / ; ' ) 0 . 1 , 5 5 2
,
0 , 0 ( a b g r ' = e l y t S l l i f . x t c
r o l o c e h t h t i w e l g n a t c e r a l l i F / / ; ) 0 5 1 , 0 5 1 , 0 1 , 0 2 1 ( t c e R l l i f . x t c
}
l m t h . e l g n a t c e R w a r D
s j . e l g n a t c e R w a r D
Figure 2.3 DrawRectangle.html and DrawRectangle.js
DrawRectangle.js
DrawRectangle.js is a JavaScript program that draws a blue rectangle on the drawing area
defined by the <canvas> element (see Listing 2.2 ). It has only 16 lines, which consist of
the three steps required to draw two-dimensional computer graphics (2D graphics) on the
canvas:
1. Retrieve the <canvas> element.
2. Request the rendering “context” for the 2D graphics from the element.
3. Draw the 2D graphics using the methods that the context supports.
These three steps are the same whether you are drawing a 2D or a 3D graphic; here, you
are drawing a simple 2D rectangle using standard JavaScript. If you were drawing a 3D
graphic using WebGL, then the rendering context in step (2) at line 11 would be for a 3D
rendering context; however, the high-level process would be the same.
Listing 2.2 DrawRectangle.js
1 // DrawRectangle.js
2 function main() {
3 // Retrieve <canvas> element <- (1)
4 var canvas = document.getElementById('example');
5 if (!canvas) {
6 console.log('Failed to retrieve the <canvas> element');
7 return;
8 }
9
10 // Get the rendering context for 2DCG <- (2)
11 var ctx = canvas.getContext('2d');
CHAPTER 2 Your First Step with WebGL
14
12
13 // Draw a blue rectangle <- (3)
14 ctx.fillStyle = 'rgba(0, 0, 255, 1.0)'; // Set a blue color
15 ctx.fillRect(120, 10, 150, 150); // Fill a rectangle with the color
16 }
Let us look at each step in order.
Retrieve the <canvas> Element
To draw something on a <canvas> , you must first retrieve the <canvas> element from the
HTML file in the JavaScript program. You can get the element by using the method docu-
ment.getElementById() , as shown at line 4. This method has a single parameter, which is
the string specified in the attribute id in the <canvas> tag in our HTML file. In this case,
the string is 'example' and it was defined back in DrawRectangle.html at line 9 (refer to
Listing 2.1 ).
If the return value of this method is not null , you have successfully retrieved the element.
However, if it is null , you have failed to retrieve the element. You can check for this
condition using a simple if statement like that shown at line 5. In case of error, line 6 is
executed. It uses the method console.log() to display the parameter as a string in the
browser’s console.
Note In Chrome, you can show the console by going to Tools, JavaScript Console or
pressing Ctrl+Shift+J (see Figure 2.4 ); in Firefox, you can show it by going to Tools, Web
Developer, Web Console or pressing Ctrl+Shift+K.
Console
Figure 2.4 Console in Chrome
What Is a Canvas?
15
Get the Rendering Context for the 2D Graphics by Using the Element
Because the <canvas> is designed to be flexible and supports both 2D and 3D, it does not
provide drawing methods directly and instead provides a mechanism called a context ,
which supports the actual drawing features. Line 11 gets that context:
11 var ctx = canvas.getContext('2d');
The method canvas.getContext() has a parameter that specifies which type of drawing
features you want to use. In this example you want to draw a 2D shape, so you must
specify 2d (case sensitive).
The result of this call, the context, is stored in the variable ctx ready for use. Note, for
brevity we haven’t checked error conditions, which is something you should always do in
your own programs.
Draw the 2D Graphics Using the Methods Supported by the Context
Now that we have a drawing context, let’s look at the code for drawing a blue rectangle,
which is a two-step process. First, set the color to be used when drawing. Second, draw (or
fill) a rectangle with the color.
Lines 14 and 15 handle these steps:
13 // Draw a blue rectangle <- (3)
14 ctx.fillStyle = 'rgba(0, 0, 255, 1.0)'; // Set color to blue
15 ctx.fillRect(120, 10, 150, 150); // Fill a rectangle with the color
The rgba in the string rgba(0, 0, 255, 1.0) on line 14 indicate r (red), g (green), b
(blue), and a (alpha: transparency), with each RGB parameter taking a value from 0
(minimum value) to 255 (maximum value) and the alpha parameter from 0.0 (transpar-
ent) to 1.0 (opaque). In general, computer systems represent a color by using a combina-
tion of red, green, and blue (light’s three primary colors), which is referred to as RGB
format . When alpha (transparency) is added, the format is called RGBA format .
Line 15 then uses the fillStyle property to specify the fill color when drawing the rect-
angle. However, before going into the details of the arguments on line 15, let’s look at the
coordinate system used by the <canvas> element (see Figure 2.5 ).
CHAPTER 2 Your First Step with WebGL
16
x
y
> s a v n a c <
a e r a g n i w a r d
) 0 , 0 (
) 0 0 4 , 0 0 4 (
) 0 1 , 0 2 1 (
Figure 2.5 The coordinate system of <canvas>
As you can see in the figure, the coordinate system of the <canvas> element has the hori-
zontal direction as the x-axis (right-direction is positive) and the vertical direction as the
y-axis (down-direction is positive). Note that the origin is located at the upper-left corner
and the down direction of the y-axis is positive. The rectangle drawn with a dashed line
is the original <canvas> element in our HTML file (refer to Listing 2.1 ), which we speci-
fied as being 400 by 400 pixels. The dotted line is the rectangle that the sample program
draws.
When we use ctx.fillRect() to draw a rectangle, the first and second parameters of this
method are the position of the upper-left corner of the rectangle within the <canvas> , and
the third and fourth parameters are the width and height of the rectangle (in pixels):
15 ctx.fillRects(120, 10, 150, 150);// Fill a rectangle with the color
After loading DrawRectangle.html into your browser, you will see the rectangle that was
shown in Figure 2.2 .
So far, we’ve only looked at 2D graphics. However, WebGL also utilizes the same <canvas>
element to draw 3D graphics on a web page, so let’s now enter into the WebGL world.
The World’s Shortest WebGL Program:
Clear Drawing Area
Let’s start by constructing the world’s shortest WebGL program, HelloCanvas , which
simply clears the drawing area specified by a <canvas> tag. Figure 2.6 shows the result of
loading the program, which clears (by filling with black) the rectangular area defined by a
<canvas> .
The World’s Shortest WebGL Program: Clear Drawing Area
17
Figure 2.6 HelloCanvas
The HTML File (HelloCanvas.html)
Take a look at HelloCanvas.html , as shown in Figure 2.7 ). Its structure is simple and starts
by defining the drawing area using the <canvas> element at line 9 and then importing
HelloCanvas.js (the WebGL program) at line 16.
Lines 13 to 15 import several other JavaScript files, which provide useful convenience
functions that help WebGL programming. These will be explained in more detail later. For
now, just think of them as libraries.
CHAPTER 2 Your First Step with WebGL
18
> l m t h E P Y T C O D ! < 1
> ” n e ” = g n a l l m t h < 2
> d a e h < 3
> / " 8 - f t u " = t e s r a h c a t e m < 4
> e l t i t / < s a v n a c r a e l C > e l t i t < 5
> d a e h / < 6
7
> " ) ( n i a m " = d a o l n o y d o b < 8
> " 0 0 4 " = t h g i e h " 0 0 4 " = h t d i w " l g b e w " = d i s a v n a c < 9
" s a v n a c " g n i t r o p p u s r e s w o r b e h t e s u e s a e l P 0 1
> s a v n a c / < 1 1
2 1
> t p i r c s / < > " s j . s l i t u - l g b e w / b i l / . . " = c r s t p i r c s < 3 1
> t p i r c s / < > " s j . g u b e d - l g b e w / b i l / . . " = c r s t p i r c s < 4 1
> t p i r c s / < > " s j . s l i t u - n o u c / b i l / . . " = c r s t p i r c s < 5 1
> t p i r c s / < > " s j . s a v n a C o l e H " = c r s t p i r c s < 6 1
> y d o b / < 7 1
> l m t h / < 8 1
h c i h w o t n i > s a v n a c <
s e p a h s s w a r d L G b e W
s e l i f t p i r c S a v a J
t n e i n e v n o c g n i n i a t n o c
L G b e W r o f s n o i t c n u f
g n i w a r d e l i f t p i r c S a v a J
> s a v n a c < e h t o t n i s e p a h s
Figure 2.7 HelloCanvas.html
You’ve set up the canvas (line 9) and then imported the HelloCanvas JavaScript file (line
16), which actually uses WebGL commands to access the canvas and draw your first 3D
program. Let us look at the WebGL program defined in HelloCanvas.js .
JavaScript Program (HelloCanvas.js)
HelloCanvas.js (see Listing 2.3 ) has only 18 lines, including comments and error
handling, and follows the same steps as explained for 2D graphics: retrieve the <canvas>
element, get its rendering context, and then begin drawing.
Listing 2.3 HelloCanvas.js
1 // HelloCanvas.js
2 function main() {
3 // Retrieve <canvas> element
4 var canvas = document.getElementById('webgl');
5
6 // Get the rendering context for WebGL
7 var gl = getWebGLContext(canvas);
8 if (!gl) {
9 console.log('Failed to get the rendering context for WebGL');
10 return;
11 }
The World’s Shortest WebGL Program: Clear Drawing Area
19
12
13 // Specify the color for clearing <canvas>
14 gl.clearColor(0.0, 0.0, 0.0, 1.0);
15
16 // Clear <canvas>
17 gl.clear(gl.COLOR_BUFFER_BIT);
18 }
As in the previous example, there is only one function, main() , which is the link between
the HTML and the JavaScript and set at <body> element using its onload attribute (line 8)
in HelloCanvas.html (refer to Figure 2.7 ).
Figure 2.8 shows the processing flow of the main() function of our WebGL program and
consists of four steps, which are discussed individually next.
L G b e W r o f t x e t n o c g n i r e d n e r e h t t e G
> s a v n a c < r a e l C
t n e m e l e > s a v n a c < e h t e v e i r t e R
> s a v n a c < g n i r a e l c r o l o c e h t t e S
Figure 2.8 The processing flow of the main() function
Retrieve the <canvas> Element
First, main() retrieves the <canvas> element from the HTML file. As explained in
DrawRectangle.js , it uses the document.getElementById() method specifying webgl as the
argument. Looking back at HelloCanvas.html (refer to Figure 2.7 ), you can see that attri-
bute id is set at the <canvas> tag at line 9:
9 <canvas id="webgl" width="400" height="400">
The return value of this method is stored in the canvas variable.
CHAPTER 2 Your First Step with WebGL
20
Get the Rendering Context for WebGL
In the next step, the program uses the variable canvas to get the rendering context
for WebGL. Normally, we would use canvas.getContext() as described earlier to
get the rendering context for WebGL. However, because the argument specified in
canvas.getContext() varies between browsers,
1 we have written a special function
get WebGLContext() to hide the differences between the browsers:
7 var gl = getWebGLContext(canvas);
This is one of the convenience functions mentioned earlier that was written specially for
this book and is defined in cuon-utils.js , which is imported at line 15 in HelloCanvas.
html . The functions defined in the file become available by specifying the path to the file
in the attribute src in the <script> tag and loading the file. The following is the specifica-
tion of getWebGLContext() .
getWebGLContext(element [, debug])
Get the rendering context for WebGL, set the debug setting for WebGL, and display any
error message in the browser console in case of error.
Parameters element Specifies <canvas> element to be queried.
debug (optional) Default is true . When set to true , JavaScript errors
are displayed in the console. Note: Turn off after
debugging; otherwise, performance is affected.
Return value non-null The rendering context for WebGL.
null WebGL is not available.
The processing flow to retrieve the <canvas> element and use the element to get the
rendering context is the same as in DrawRectangle.js shown earlier, where the rendering
context was used to draw 2D graphics on the <canvas> .
In a similar way, WebGL uses the rendering context returned by getWebGLContext() to
draw on the <canvas> . However, now the context is for 3D rather than 2D, so 3D (that is,
WebGL) methods are available. The program stores the context in the variable gl at line
7. You can use any name for the variable. We have intentionally used gl throughout this
book, because it aligns the names of the WebGL-related methods to that of OpenGL ES
2.0, which is the base specification of WebGL. For example, gl.clearColor() at line 14
corresponds to glClearColor() in OpenGL ES 2.0 or OpenGL:
14 gl.clearColor(0.0, 0.0, 0.0, 1.0);
1 Although most browsers are settling on “experimental-webgl” for this argument, not all do.
Additionally, over time, this will evolve to plain ‘webgl,’ so we have chosen to hide this.
The World’s Shortest WebGL Program: Clear Drawing Area
21
This book explains all WebGL-related methods assuming that the rendering context is
held in the variable gl .
Once you have the rendering context for WebGL, the next step is to use the context to set
the color for clearing the drawing area specified by the <canvas> .
Set the Color for Clearing the <canvas>
In the previous section, DrawRectangle.js set the drawing color before drawing the rect-
angle. In a similar way, with WebGL you need to set the color before actually clearing the
drawing area. Line 14 uses gl.clearColor() to set the color in RGBA format.
gl.clearColor (red, green, blue, alpha)
Specify the clear color for a drawing area:
Parameters red Specifies the red value (from 0.0 to 1.0).
green Specifies the green value (from 0.0 to 1.0).
blue Specifies the blue value (from 0.0 to 1.0).
alpha Specifies an alpha (transparency) value (from 0.0 to 1.0).
0.0 means transparent and 1.0 means opaque.
If any of the values of these parameters is less than 0.0 or more than 1.0, it
is truncated into 0.0 or 1.0, respectively.
Return value None
Errors
2
None
The sample program calls gl.clearColor (0.0, 0.0, 0.0, 1.0) at line 14, so black is specified
as the clear color. The followings are examples that specify other colors:
(1.0, 0.0, 0.0, 1.0) red
(0.0, 1.0, 0.0, 1.0) green
(0.0, 0.0, 1.0, 1.0) blue
(1.0, 1.0, 0.0, 1.0) yellow
(1.0, 0.0, 1.0, 1.0) purple
(0.0, 1.0, 1.0, 1.0) light blue
(1.0, 1.0, 1.0, 1.0) white
2 In this book, the item “errors” is shown for all specifi cations of WebGL-related methods. This
indicates errors that cannot be represented by the return value of the method when the method will
result in as error. By default, the errors are not displayed, but they can be displayed in a JavaScript
console by specifying true as the second argument of getWebGLContext() .
CHAPTER 2 Your First Step with WebGL
22
You might have noticed that in our 2D programming example in this chapter,
DrawRectangle , each value for color is specified from 0 to 255. However, because WebGL
is based on OpenGL, it uses the traditional OpenGL values from 0.0 to 1.0. The higher the
value is, the more intense the color becomes. Similarly, for the alpha parameter (fourth
parameter), the higher the value, the less transparent the color.
Once you specify the clear color, the color is retained in the WebGL system and not
changed until another color is specified by a call to gl.clearColor() . This means you
don’t need to specify the clear color again if at some point in the future you want to clear
the area again using the same color.
Clear <canvas>
Finally, you can use gl.clear() to clear the drawing area with the specified clear color:
17 gl.clear(gl.COLOR_BUFFER_BIT);
Note that the argument of this method is gl.COLOR_BUFFER_BIT , not, as you might expect,
the <canvas> element that defines the drawing area to be cleared. This is because the
WebGL method gl.clear() is actually relying on OpenGL, which uses a more sophisti-
cated model than simple canvases, instead using multiple underlying buffers. One such
buffer , the color buffer, is used in this example. By using gl.COLOR_BUFFER_BIT , you are
telling WebGL to use the color buffer when clearing the canvas. WebGL uses a number of
buffers in addition to the color buffer, including a depth buffer and a stencil buffer. The
color buffer will be covered in detail later in this chapter, and you’ll see the depth buffer
in action in Chapter 7 , “Toward the 3D World.” The stencil buffer will not be covered in
this book because it is seldom used.
Clearing the color buffer will actually cause WebGL to clear the <canvas> area on the web
page.
gl.clear(buffer)
Clear the specified buffer to preset values. In the case of a color buffer, the value (color)
specified by gl.clearColor() is used.
Parameters buffer Specifies the buffer to be cleared. Bitwise OR ( | ) operators are used
to specify multiple buffers.
gl.COLOR_BUFFER_BIT Specifies the color buffer.
gl.DEPTH_BUFFER_BIT Specifies the depth buffer.
gl.STENCIL_BUFFER_BIT Specifies the stencil buffer.
Return value None
Errors INVALID_VALUE buffer is none of the preceding three values.
Draw a Point (Version 1)
23
If no color has been specified (that is, you haven’t made a call to gl.clearColor() ), then
the following default value is used (see Table 2.1 ).
Table 2.1 Default Values to Clear Each Buffer and Associated Methods
Buffer Name Default Value Setting Method
Color buffer (0.0, 0.0, 0.0, 0.0) gl.clearColor(red, green, blue, alpha)
Depth buffer 1.0 gl.clearDepth(depth)
Stencil buffer 0 gl.clearStencil(s)
Now that you’ve read through and understand this simple WebGL example, you should
load HelloCanvas into your browser to check that the drawing area is cleared to black.
Remember, you can run all the examples in the book directly from the companion
website. However, if you want to experiment with any, you need to download the exam-
ples from the book’s website to a location on your local disk. If you’ve done that, to load
the example, navigate to that location on your disk and load HelloCanvas.html into your
browser.
Experimenting with the Sample Program
Let’s experiment a little with the sample program to become familiar with the way you
specify colors in WebGL by trying some other colors for the clear operation. Using your
favorite editor, rewrite Line 14 of HelloCanvas.js as follows and save your modification
back to the original file:
14 gl.clearColor(0.0, 0.0, 1.0, 1.0);
After reloading HelloCanvas.html into your browser, HelloCanvas.js is also reloaded,
and then main() is executed to clear the drawing area to blue. Try to use other colors and
check the result. For example, gl.clearColor(0.5, 0.5, 0.5, 1.0) clears the area to
gray.
Draw a Point (Version 1)
In the previous section, you saw how to initialize WebGL and use some simple WebGL-
related methods. In this section, you are going to go one step further and construct a
sample program to draw the simplest shape of all: a point. The program will draw a red
point using 10 pixels at (0.0, 0.0, 0.0). Because WebGL deals with three-dimensional
graphics, three coordinates are necessary to specify the position of the point. You’ll be
introduced to coordinates later, but for now simply accept that a point drawn at (0.0, 0.0,
0.0) is displayed at the center of the <canvas> area.
CHAPTER 2 Your First Step with WebGL
24
The sample program name is HelloPoint1 and, as shown in Figure 2.9 , it draws a red
point (rectangle) at the center of the <canvas> , which has been cleared to black.
3 You will
actually be using a filled rectangle as a point instead of a filled circle because a rectangle
can be drawn faster than a circle. (We will deal with how to draw a rounded point in
Chapter 9 , “Hierarchical Objects.”)
Figure 2.9 HelloPoint1
Just like clearing the color in the previous section, the color of a point must be specified
in RGBA. For red, the value of R is 1.0, G is 0.0, B is 0.0, and A is 1.0. You will remember
that DrawRectangle.js earlier in the chapter specifies the drawing color and then draws a
rectangle as follows:
ctx.fillStyle='rgba(0, 0, 255, 1.0)';
ctx.fillRect(120, 10, 150, 150);
So you are probably thinking that WebGL would do something similar, perhaps some-
thing like this:
gl.drawColor(1.0, 0.0, 0.0, 1.0);
gl.drawPoint(0, 0, 0, 10); // The position of center and the size of point
3 The sample programs in Chapter 2 are written in the simplest way possible so the reader can focus
on understanding the functionality of shaders. In particular, they don’t use “buffer objects” (see
Chapter 3 ), which are generally used in WebGL. Although this helps by simplifying the explanation,
some browsers (especially Firefox) expect buffer objects and may fail to display correctly any
examples without them. In later chapters, and in actual application development, this will not
cause problems because you will be using “buffer objects.” However, if you are having problems, try
another browser. You can switch back in the next chapter.
Draw a Point (Version 1)
25
Unfortunately, this is not possible. WebGL relies on a new type of drawing mechanism
called a shader , which offers a flexible and powerful mechanism for drawing 2D and 3D
objects and must be used by all WebGL applications. Shaders, although powerful, are
more complex, and you can’t just specify a simple draw command.
Because the shader is a critical core mechanism in WebGL programming that you will use
throughout this book, let’s examine it one step at a time so that you can understand it
easily.
HelloPoint1.html
Listing 2.4 shows HelloPoint1.html , which is functionally equivalent to HelloCanvas.
html (refer to Figure 2.7 ). The title of the web page and the JavaScript filename were
changed (lines 5 and 16), but everything else remains the same. From now on, unless
the HTML file is different from this example, we’ll skip showing the HTML files for the
examples.
Listing 2.4 HelloPoint1.html
1 <!DOCTYPE html>
2 <html lang="en">
3 <head>
4 <meta charset="utf-8" />
5 <title>Draw a point (1)</title>
6 </head>
7
8 <body οnlοad="main()">
9 <canvas id="webgl" width="400" height="400">
10 Please use the browser supporting "canvas".
11 </canvas>
12
13 <script src="../libs/webgl-utils.js"></script>
14 <script src="../libs/webgl-debug.js"></script>
15 <script src="../libs/cuon-utils.js"></script>
16 <script src="HelloPoint1.js"></script>
17 </body>
18 </html>
HelloPoint1.js
Listing 2.5 shows HelloPoint1.js . As you can see from the comments in the listing,
two “shader programs” are prepended to the JavaScript (lines 2 to 13). Glance through
the shader programs, and then go to the next section, where you’ll see more detailed
explanations.
CHAPTER 2 Your First Step with WebGL
26
Listing 2.5 HelloPoint1.js
1 // HelloPoint1.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'void main() {\n' +
5 ' gl_Position = vec4(0.0, 0.0, 0.0, 1.0);\n' + // Coordinates
6 ' gl_PointSize = 10.0;\n' + // Set the point size
7 '}\n';
8
9 // Fragment shader program
10 var FSHADER_SOURCE =
11 'void main() {\n' +
12 ' gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);\n' + // Set the color
13 '}\n';
14
15 function main() {
16 // Retrieve <canvas> element
17 var canvas = document.getElementById('webgl');
18
19 // Get the rendering context for WebGL
20 var gl = getWebGLContext(canvas);
21 if (!gl) {
22 console.log('Failed to get the rendering context for WebGL');
23 return;
24 }
25
26 // Initialize shaders
27 if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
28 console.log('Failed to initialize shaders.');
29 return;
30 }
31
32 // Set the color for clearing <canvas>
33 gl.clearColor(0.0, 0.0, 0.0, 1.0);
34
35 // Clear <canvas>
36 gl.clear(gl.COLOR_BUFFER_BIT);
37
38 // Draw a point
39 gl.drawArrays(gl.POINTS, 0, 1);
40 }
Draw a Point (Version 1)
27
What Is a Shader?
HelloPoint1.js is our first WebGL program that uses shaders. As mentioned earlier,
shader programs are necessary when you want to draw something on the screen in
WebGL. Essentially, shader programs are “embedded” in the JavaScript file and, in this
case, set up at the start. This seems at first sight to be complicated, but let’s take it one
step at a time.
WebGL needs the following two types of shaders, which you saw at line 2 and line 9:
• Vertex shader: Vertex shaders are programs that describe the traits (position, colors,
and so on) of a vertex. The vertex is a point in 2D/3D space, such as the corner or
intersection of a 2D/3D shape.
• Fragment shader: A program that deals with per-fragment processing such as light-
ing (see Chapter 8 , “Lighting Objects”). The fragment is a WebGL term that you can
consider as a kind of pixel (picture element).
You’ll explore shaders in more detail later, but simply put, in a 3D scene, it’s not enough
just to draw graphics. You have to also account for how they are viewed as light sources
hit them or the viewer’s perspective changes. Shading does this with a high degree of flex-
ibility and is part of the reason that today’s 3D graphics are so realistic, allowing them to
use new rendering effects to achieve stunning results.
The shaders are read from the JavaScript and stored in the WebGL system ready to be used
for drawing. Figure 2.10 shows the basic processing flow from a JavaScript program into
the WebGL system, which applies the shader programs to draw shapes that the browser
finally displays.
m e t s y S L G b e W
r e d a h S x e t r e V t n e m g a r F
r e d a h S
t p i r c S a v a J
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
; ) … ( s r e d a h S t i n i
…
}
n o i t a r e p o t n e m g a r f - r e P n o i t a r e p o x e t r e v - r e P
r e s w o r B
x
y
: n o i t i s o P
) 0 . 1 , 0 . 0 , 0 . 0 , 0 . 0 (
0 . 0 1 : e z i S
: r o l o C
) 0 . 1 , 0 . 0 , 0 . 0 , 0 . 1 (
) d a o l n o ( d e t u c e x e s i m a r g o r p t p i r c S a v a J
r e f u b r o l o c e h t o t r e d n e R
d e t u c e x e e r a s d o h t e m d e t a l e r - L G b e W
r e f u B r o l o C
y a l p s i D
Figure 2.10 The processing flow from executing a JavaScript program to displaying the result
in a browser
CHAPTER 2 Your First Step with WebGL
28
You can see two browser windows on the left side of the figure. These are the same; the
upper one shows the browser before executing the JavaScript program, and the lower one
shows the browser after execution. Once the WebGL-related methods are called from the
JavaScript program, the vertex shader in the WebGL system is executed, and the fragment
shader is executed to draw the result into the color buffer. This is the clear part—that is,
step 4 in Figure 2.8 , described in the original HelloCanvas example. Then the content in
the color buffer is automatically displayed on the drawing area specified by the <canvas>
in the browser.
You’ll be seeing this figure frequently in the rest of this book. So we’ll use a simplified
version to save space (see Figure 2.11 ). Note that the flow is left to right and the right-
most component is a color buffer, not a browser, because the color buffer is automatically
displayed in the browser.
m e t s y S L G b e W
r e d a h S x e t r e V t n e m g a r F
r e d a h S
n o i t a r e p o t n e m g a r f - r e P n o i t a r e p o x e t r e v - r e P
x
y
: n o i t i s o P
) 0 . 1 , 0 . 0 , 0 . 0 , 0 . 0 (
0 . 0 1 : e z i S
: r o l o C
) 0 . 1 , 0 . 0 , 0 . 0 , 0 . 1 (
r e f f u B r o l o C
t p i r c S a v a J
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
; ) … ( s r e d a h S t i n i
…
}
Figure 2.11 The simplified version of Figure 2.9
Getting back to our example, the goal is to draw a 10-pixel point on the screen. The two
shaders are used as follows:
• The vertex shader specifies the position of a point and its size. In this sample
program, the position is (0.0, 0.0, 0.0), and the size is 10.0.
• The fragment shader specifies the color of fragments displaying the point. In this
sample program, the color is red (1.0, 0.0, 0.0, 1.0).
The Structure of a WebGL Program that Uses Shaders
Based on what you’ve learned so far, let’s look at HelloPoint1.js again (refer to Listing
2.5 ). This program has 40 lines and is a little more complex than HelloCanvas.js (18
lines). It consists of three parts, as shown in Figure 2.12 . The main() function in JavaScript
starts from line 15, and shader programs are located from lines 2 to 13.
Draw a Point (Version 1)
29
s j . 1 t n i P o l e H / 1
m a r g o r p r e d a h s x e t r e V / 2
= E C R U O S _ R E D A H S V r a v 3
+ ' n \ { ) ( n i a m d i o v ‘ 4
+ ' n \ ; ) 0 . 1 , 0 . 0 , 0 . 0 , 0 . 0 ( 4 c e v = n o i t i s o P _ l g ' 5
+ ' n \ ; 0 . 0 1 = e z i S t n i o P _ l g ' 6
; ' n \ } ' 7
8
m a r g o r p r e d a h s t n e m g a r F / 9
= E C R U O S _ R E D A H S F r a v 0 1
+ ' n \ { ) ( n i a m
d i
o v ' 1 1
+ ' n \ ; ) 0 . 1 , 0 . 0 , 0 . 0 , 0 . 1 ( 4 c e v = r o l o C g a r F _ l g ' 2 1
; ' n \ } ' 3 1
4 1
{ ) ( n i a m n o i t c n u f 5 1
t n e m e l e > s a v n a c < e v e i r t e R / 6 1
; ) ' l g b e w ' ( d I y B t n e m e l E t e g . t n e m u c o d = s a v n a c r a v 7 1
8 1
L G b e W r o f t x e t n o c g n i r e d n e r e h t t e G / 9 1
; ) s a v n a c ( t x e t n o C L G b e W t e g = l g r a v 0 2
…
s r e d a h s e z i l a i t i n I / 6 2
… , E C R U O S _ R E D A H S V , l g ( s r e d a h S t i n i ! ( f i 7 2
…
> s a v n a c < g n i r a e l c r o f r o l o c e h t t e S / 2 3
; ) 0 . 1 , 0 . 0 , 0 . 0 , 0 . 0 ( r o l o C r a e l c . l g 3 3
…
> s a v n a c < r a e l C / 5 3
; ) T I B _ R E F F U B _ R O L O C . l g ( r a e l c . l g 6 3
7 3
t n i o p a w a r D / 8 3
; ) 1 , 0 , S T N I O P . l g ( s y a r A w a r d . l g 9 3
} 0 4
Vertex shader program
(Language: GLSL ES)
Fragment shader program
(Language: GLSL ES)
Main program
(Language: JavaScript)
Figure 2.12 The basic structure of a WebGL program with embedded shader programs
The vertex shader program is located in lines 4 to 7, and the fragment shader is located in
lines 11 to 13. These programs are actually the following shader language programs but
written as a JavaScript string to make it possible to pass the shaders to the WebGL system:
// Vertex shader program
void main() {
gl_Position = vec4(0.0, 0.0, 0.0, 1.0);
gl_PointSize = 10.0;
}
// Fragment shader program
void main() {
gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
}
CHAPTER 2 Your First Step with WebGL
30
As you learned in Chapter 1 , shader programs must be written in the OpenGL ES shading
language (GLSL ES) , which is similar to the C language. Finally, GLSL ES comes onto the
stage! You will get to see the details of GLSL ES in Chapter 6 , “The OpenGL ES Shading
Language (GLSL ES),” but in these early examples, the code is simple and should be under-
standable by anybody with a basic understanding of C or JavaScript.
Because these programs must be treated as a single string, each line of the shader is
concatenated using the + operator into a single string. Each line has \n at the end because
the line number is displayed when an error occurs in the shader. The line number is
helpful to check the source of the problem in the codes. However, the \n is not manda-
tory, and you could write the shader without it.
At lines 3 and 10, each shader program is stored in the variables VSHADER_SOURCE and
FSHADER_SOURCE as a string:
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'void main() {\n' +
5 ' gl_Position = vec4(0.0, 0.0, 0.0, 1.0);\n' +
6 ' gl_PointSize = 10.0;\n' +
7 '}\n';
8
9 // Fragment shader program
10 var FSHADER_SOURCE =
11 'void main() {\n' +
12 ' gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);\n' +
13 '}\n';
If you are interested in loading the shader programs from files, refer to Appendix F ,
“Loading Shader Programs from Files.”
Initializing Shaders
Before looking at the details of each shader, let’s examine the processing flow of main()
that is defined from line 15 in the JavaScript (see Figure 2.13 ). This flow, shown in Figure
2.13 , is the basic processing flow of most WebGL applications. You will see the same flow
throughout this book.
Draw a Point (Version 1)
31
L G b e W r o f t x e t n o c g n i r e d n e r e h t t e G
> s a v n a c < r a e l C
s r e d a h s e z i l a i t i n I
w a r D
t n e m e l e > s a v n a c < e h t e v e i r t e R
> s a v n a c < g n i r a e l c r o f r o l o c e h t t e S
Figure 2.13 The processing flow of a WebGL program
This flow is similar to that shown in Figure 2.8 except that a third step (“Initialize
Shaders”) and a sixth step (“Draw”) are added.
The third step “Initialize Shaders” initializes and sets up the shaders that are defined at
line 3 and line 10 within the WebGL system. This step is done using the convenience
function initShaders() that is defined in cuon-util.js . Again, this is one of those special
functions we have provided for this book.
initShaders(gl, vshader, fshader)
Initialize shaders and set them up in the WebGL system ready for use:
Parameters gl Specifies a rendering context.
vshader Specifies a vertex shader program (string).
fshader Specifies a fragment shader program (string).
Return value true Shaders successfully initialized.
false Failed to initialize shaders.
Figure 2.14 shows how the convenience function initShaders() processes the shaders.
You will examine in detail what this function is doing in Chapter 8 . For now, you just
need to understand that it sets up the shaders in the WebGL system and makes them
ready for use.
CHAPTER 2 Your First Step with WebGL
32
m e t s y S L G b e W
m e t s y S L G b e W
r e d a h S x e
t r
e V t n e m g a r F
r e d a h S
n o i t a r e p o t n e m g a r f - r e P n o i t a r e p o x e t r e v - r e P r e
f
u B r o l o C
t p i r c S a v a J
r e d a h S x e t r e V t n e m g a r F
r e d a h S
gl_Position gl_PointSize
gl_FragColor
{ ) ( n i a m d i o v
; ) . . . ( 4 c e v = n o i t i s o P _ l g
; 0 . 0 1 = e z i S t n i o P _ l g
}
{ ) ( n i a m d i o v
; ) . . . ( 4 c e v = r o l o C g a r F _ l g
}
t p i r c S a v a J
{ ) ( n i a m d i o v
; ) . . . ( 4 c e v = n o i t i s o P _ l g
; 0 . 0 1 = e z i S t n i o P _ l g
}
{ ) ( n i a m d i o v
; ) . . . ( 4 c e v = r o l o C g a r F _ l g
}
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
; ) … ( s r e d a h S t i n i
…
}
Figure 2.14 Behavior of initShaders()
As you can see in the upper figure in Figure 2.14 , the WebGL system has two containers:
one for a vertex shader and one for a fragment shader. This is actually a simplification,
but helpful at this stage. We return to the details in Chapter 10 . By default, the contents
of these containers are empty. To make the shader programs, written as JavaScript strings
and ready for use in the WebGL system, we need something to pass these strings to the
system and then set them up in the appropriate containers; initShaders() performs this
operation. Note that the shader programs are executed within the WebGL system, not the
JavaScript program.
The lower portion in Figure 2.14 shows that after executing initShaders() , the shader
programs that are passed as a string to the parameters of initShaders() are set up in the
containers in the WebGL system and then made ready for use. The lower figure schemati-
cally illustrates that a vertex shader is passing gl_Position and gl_PointSize to a frag-
ment shader and that just after assigning values to these variables in the vertex shader, the
fragment shader is executed. In actuality, the fragments that are generated after process-
ing these values are passed to the fragment shader. Chapter 5 explains this mechanism in
detail, but for now you can consider the attributes to be passed.
Draw a Point (Version 1)
33
The important point here is that WebGL applications consist of a JavaScript program executed
by the browser and shader programs that are executed within the WebGL system .
Now, having completed the explanation of the second step “Initialize Shaders” in Figure
2.13 , you are ready to see how the shaders are actually used to draw a simple point. As
mentioned, we need three items of information for the point: its position, size, and color,
which are used as follows:
• The vertex shader specifies the position of a point and its size. In this sample
program, the position is (0.0, 0.0, 0.0), and the size is 10.0.
• The fragment shader specifies the color of the fragments displaying the point. In this
sample program, they are red (1.0, 0.0, 0.0, 1.0).
Vertex Shader
Now, let us start by examining the vertex shader program listed in HelloPoint1.js (refer
to Listing 2.5 ), which sets the position and size of the point:
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'void main() {\n' +
5 ' gl_Position = vec4(0.0, 0.0, 0.0, 1.0);\n' +
6 ' gl_PointSize = 10.0;\n' +
7 '}\n';
The vertex shader program itself starts from line 4 and must contain a single main() func-
tion in a similar fashion to languages such as C. The keyword void in front of main()
indicates that this function does not return a value. You cannot specify other arguments
to main() .
Just like JavaScript, we can use the = operator to assign a value to a variable in a shader.
Line 5 assigns the position of the point to the variable gl_Position , and line 6 assigns its
size to the variable gl_PointSize . These two variables are built-in variables available only
in a vertex shader and have a special meaning: gl_Position specifies a position of a vertex
(in this case, the position of the point), and gl_PointSize specifies the size of the point
(see Table 2.2 ).
Table 2.2 Built-In Variables Available in a Vertex Shader
Type and Variable Name Description
vec4 gl_Position Specifies the position of a vertex
float gl_PointSize Specifies the size of a point (in pixels)
CHAPTER 2 Your First Step with WebGL
34
Note that gl_Position should always be written. If you don’t specify it, the shader’s
behavior is implementation dependent and may not work as expected. In contrast, gl_
PointSize is only required when drawing points and defaults to a point size of 1.0 if you
don’t specify anything.
For those of you mostly familiar with JavaScript, you may be a little surprised when you
see “type” specified in Table 2.2 . Unlike JavaScript, GLSL ES is a “typed” programming
language; that is, it requires the programmer to specify what type of data a variable holds.
C and Java are examples of typed languages. By specifying “type” for a variable, the
system can easily understand what type of data the variable holds, and then it can opti-
mize its processing based on that information. Table 2.3 summarizes the “type” in GLSL
ES used in the shaders in this section.
Table 2.3 Data Types in GLSL ES
Type Description
float Indicates a floating point number
vec4 Indicates a vector of four floating point numbers
float float Float float
Note that an error will occur when the type of data that is assigned to the variable is
different from the type of the variable. For example, the type of gl_PointSize is float, and
you must assign a floating point number to it. So, if you change line 6 from
gl_PointSize = 10.0;
to
gl_PointSize = 10;
it will generate an error simply because 10 is interpreted as an integer number, whereas
10.0 is a floating point number in GLSL ES.
The type of the variable gl_Position , the built-in variable for specifying the position of
a point, is vec4 ; vec4 is a vector made up of three floats. However, you only have three
floats (0.0, 0.0, 0.0) representing X, Y, and Z. So you need to convert these to a vec4
somehow. Fortunately, there is a built-in function, vec4() , that will do this for you and
return a value of type vec4 –, which is just what you need!
Draw a Point (Version 1)
35
vec4 vec4(v0, v1, v2, v3)
Construct a vec4 object from v0 , v1 , v2 , and v3 .
Parameters v0, v1, v2, v3 Specifies floating point numbers.
Return value A vec4 object made from v0 , v1 , v2 , and v3.
In this sample program, vec4() is used at line 5 as follows:
gl_Position = vec4(0.0, 0.0, 0.0, 1.0);
Note that the value that is assigned to gl_Position has 1.0 added as a fourth component.
This four-component coordinate is called a homogeneous coordinate (see the boxed
article below) and is often used in 3D graphics for processing three-dimensional informa-
tion efficiently. Although the homogeneous coordinate is a four-dimensional coordinate,
if the last component of the homogeneous coordinate is 1.0, the coordinate indicates the
same position as a three-dimensional one. So, you can supply 1.0 as the last component if
you need to specify four components as a vertex coordinate.
Homogeneous Coordinates
The homogeneous coordinates use the following coordinate notation: (x, y, z, w). The
homogeneous coordinate (x, y, z, w) is equivalent to the three-dimensional coordinate
(x/w, y/w, z/w). So, if you set w to 1.0, you can utilize the homogeneous coordinate
as a three-dimensional coordinate. The value of w must be greater than or equal to
0. If w approaches zero, the coordinates approach infinity. So we can represent the
concept of infinity in the homogeneous coordinate system. Homogeneous coordinates
make it possible to represent vertex transformations described in the next chapter as a
multiplication of a matrix and the coordinates. These coordinates are often used as an
internal representation of a vertex in 3D graphics systems.
Fragment Shader
After specifying the position and size of a point, you need to specify its color using a frag-
ment shader. As explained earlier, a fragment is a pixel displayed on the screen, although
technically the fragment is a pixel along with its position, color, and other information.
The fragment shader is a program that processes this information in preparation for
displaying the fragment on the screen. Looking again at the fragment shader listed in
HelloPoint1.js (refer to Listing 2.5 ), you can see that just like a vertex shader, a fragment
shader is executed from main() :
CHAPTER 2 Your First Step with WebGL
36
9 // Fragment shader program
10 var FSHADER_SOURCE =
11 'void main() {\n' +
12 ' gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);\n' +
13 '}\n';
The job of the shader is to set the color of the point as its per-fragment operation, which
is carried out at line 12. gl_FragColor is a built-in variable only available in a fragment
shader; it controls the color of a fragment, as shown in Table 2.4 .
Table 2.4 The Built-In Value Available in a Fragment Shader
Type and Variable Name Description
vec4 gl_FragColor Specify the color of a fragment (in RGBA)
When we assign a color value to the built-in variable, the fragment is displayed using
that color. Just like the position in the vertex shader, the color value is a vec4 data type
consisting of four floating point numbers representing the RGBA values. In this sample
program, a red point will be displayed because you assign (1.0, 0.0, 0.0, 1.0) to the
variable.
The Draw Operation
Once you set up the shaders, the remaining task is to draw the shape, or in our case, a
point. As before, you need to clear the drawing area in a similar way to that described
in HelloCanvas.js . Once the drawing area is cleared, you can draw the point using gl.
drawArrays() , as in line 39:
39 gl.drawArrays(gl.POINTS, 0, 1);
gl.drawArrays() is a powerful function that is capable of drawing a variety of basic
shapes, as detailed in the following box.
Draw a Point (Version 1)
37
gl.drawArrays(mode, first, count)
Execute a vertex shader to draw shapes specified by the mode parameter.
Parameters mode Specifies the type of shape to be drawn. The following symbolic
constants are accepted: gl.POINTS , gl.LINES , gl.LINE_STRIP ,
gl.LINE_LOOP , gl.TRIANGLES , gl.TRIANGLE_STRIP , and gl.
TRIANGLE_FAN.
first Specifies which vertex to start drawing from (integer).
count Specifies the number of vertices to be used (integer).
Return value None
Errors INVALID_ENUM mode is none of the preceding values.
INVALID_VALUE first is negative or count is negative.
In this sample program, because you are drawing a point, you specify gl.POINTS as the
mode in the first parameter. The second parameter is set to 0 because you are starting from
the first vertex. The third parameter, count , is 1 because you are only drawing 1 point in
this sample program.
Now, when the program makes a call to gl.drawArrays() , the vertex shader is executed
count times, each time working with the next vertex. In this sample program, the shader
is executed once ( count is set to 1) because we only have one vertex: our point. When
the shader is executed, the function main() in the shader is called, and then each line
in the function is executed sequentially, resulting in (0.0, 0.0, 0.0, 1.0) being assigned to
gl_Position (line 5) and then 10.0 assigned to gl_PointSize (line 6).
Once the vertex shader executes, the fragment shader is executed by calling its main()
function which, in this example, assigns the color value (red) to gl_FragColor (line 12).
As a result, a red point of 10 pixels is drawn at (0.0, 0.0, 0.0, 1.0), or the center of the
drawing area (see Figure 2.15 ).
CHAPTER 2 Your First Step with WebGL
38
m e t s y S L G b e W
r e d a h S x e t r e V t n e m g a r F
r e d a h S
n o i t a r e p o t n e m g a r f - r e P n o i t a r e p o x e t r e v - r e P r e f f u B r o l o C
gl_PointSize
{ ) ( n i a m d i o v
= n o i t i s o P _ l g
; ) 0 . 1 , 0 . 0 , 0 . 0 , 0 . 0 ( 4 c e v
; 0 . 0 1 = e z i S t n i o P _ l g
}
gl_FragColor
t p i r c S a v a J
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
; ) … ( s r e d a h S t i n i
…
; ) … ( s y a r r A w a r d . l g
}
gl_Position
{ ) ( n i a m d i o v
= r o l o C g a r F _ l g
; ) 0 . 1 , 0 . 0 , 0 . 0 , 0 . 1 ( 4 c e v
}
Figure 2.15 The behavior of shaders
At this stage, you should have a rough understanding of the role of a vertex shader and a
fragment shader and how they work. In the rest of this chapter, you’ll build on this basic
understanding through a series of examples, allowing you to become more accustomed to
WebGL and shaders. However, before that, let’s quickly look at how WebGL describes the
position of shapes using its coordinate system.
The WebGL Coordinate System
Because WebGL deals with 3D graphics, it uses a three-dimensional coordinate system
along the x-, y-, and z-axis. This coordinate system is easy to understand because our
world has the same three dimensions: width, height, and depth. In any coordinate system,
the direction of each axis is important. Generally, in WebGL, when you face the computer
screen, the horizontal direction is the x-axis (right direction is positive), the vertical direc-
tion is the y-axis (up direction is positive), and the direction from the screen to the viewer
is the z-axis (the left side of Figure 2.16 ). The viewer’s eye is located at the origin (0.0, 0.0,
0.0), and the line of sight travels along the negative direction of the z-axis, or from you
into the screen (see the right side of Figure 2.16 ). This coordinate system is also called the
right-handed coordinate system because it can be expressed using the right hand (see
Figure 2.17 ) and is the one normally associated with WebGL. Throughout this book, we’ll
use the right-handed coordinate system as the default for WebGL. However, you should
note that it’s actually more complex than this. In fact, WebGL is neither left handed nor
right handed. This is explained in detail in Appendix D , “WebGL/OpenGL: Left or Right
Handed?,” but it’s safe to treat WebGL as right handed for now.
x
y
z
n e e r c S
x
y
z
t h g i s f o e n i L
Figure 2.16 WebGL coordinate system
Draw a Point (Version 1)
39
x
z
y
Figure 2.17 The right-handed coordinate system
As you have already seen, the drawing area specified for a <canvas> element in JavaScript
is different from WebGL’s coordinate system, so a mapping is needed between the two.
By default, as you see in Figure 2.18 , WebGL maps the coordinate system to the area as
follows:
• The center position of a <canvas> : (0.0, 0.0, 0.0)
• The two edges of the x-axis of the <canvas> : (–1.0, 0.0, 0.0) and (1.0, 0.0, 0.0)
• The two edges of the y-axis of the <canvas> : (0.0, –1.0, 0.0) and (0.0, 1.0, 0.0)
x
y
(-1.0, 0.0, 0.0) (1.0, 0.0, 0.0)
(0.0, 0.0, 0.0)
(0.0, -1.0, 0.0)
<canvas>
drawing area
Figure 2.18 The <canvas> drawing area and WebGL coordinate system
As previously discussed, this is the default. It’s possible to use another coordinate
system, which we’ll discuss later, but for now this default coordinate system will be used.
Additionally, to help you stay focused on the core functionality of WebGL, the example
programs will mainly use the x and y coordinates and not use the z or depth coordinate.
Therefore, until Chapter 7 , the z-axis value will generally be specified as 0.0.
CHAPTER 2 Your First Step with WebGL
40
Experimenting with the Sample Program
First, you can modify line 5 to change the position of the point and gain a better under-
standing of the WebGL coordinate system. For example, let’s change the x coordinate
from 0.0 to 0.5 as follows:
5 ' gl_Position = vec4( 0.5 , 0.0, 0.0, 1.0);\n' +
Save the modified HelloPoint1.js and click the Reload button on your browser to reload
it. You will see that the point has moved and is now displayed on the right side of the
<canvas> area (see Figure 2.19 , left side).
Now change the y coordinate to move the point toward the top of the <canvas> as
follows:
5 ' gl_Position = vec4(0.0, 0.5 , 0.0, 1.0);\n' +
Again, save the modified HelloPoint1.js and reload it. This time, you can see the point
has moved and is displayed in the upper part of the canvas (see Figure 2.19 , right side).
Figure 2.19 Modifying the position of the point
As another experiment, let’s try changing the color of the point from red to green by
modifying line 12, as follows:
12 ' gl_FragColor = vec4(0.0, 1.0 , 0.0, 1.0);\n' +
Let’s conclude this section with a quick recap. You’ve been introduced to the two basic
shaders we use in WebGL—the vertex shader and the fragment shader—and seen how,
although they use their own language, they can be executed from within JavaScript.
You’ve also seen that the basic processing flow of a WebGL application using shaders is
the same as in other types of WebGL applications. A key lesson from this section is that
a WebGL program consists of a JavaScript program executing in conjunction with shader
programs.
Draw a Point (Version 2)
41
For those of you with experience in using OpenGL, you may feel that something is
missing; there is no code to swap color buffers. One of the significant features of WebGL
is that it does not need to do that. For more information, see Appendix A , “No Need to
Swap Buffers in WebGL.”
Draw a Point (Version 2)
In the previous section, you explored drawing a point and the related core functions of
shaders. Now that you understand the fundamental behavior of a WebGL program, let’s
examine how to pass data between JavaScript and the shaders. HelloPoint1 always draws
a point at the same position because its position is directly written (“hard-coded”) in the
vertex shader. This makes the example easy to understand, but it lacks flexibility. In this
section, you’ll see how a WebGL program can pass a vertex position from JavaScript to
the vertex shader and then draw a point at that position. The name of the program is
HelloPoint2 , and although the result of the program is the same as HelloPoint1 , it’s a
flexible technique you will use in future examples.
Using Attribute Variables
Our goal is to pass a position from the JavaScript program to the vertex shader. There
are two ways to pass data to a vertex shader: attribute variable and uniform variable (see
Figure 2.20 ). The one you use depends on the nature of the data. The attribute variable
passes data that differs for each vertex, whereas the uniform variable passes data that is
the same (or uniform) in each vertex. In this program, you will use the attribute variable
because each vertex generally has different coordinates.
r e d a h S x e t r e V t n e m g a r F
r e d a h S
gl_Position gl_PointSize
gl_FragColor
e l b a i r a v e t u b i r t t a e l b a i r a v m r o f i n u
t p i r c S a v a J
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
; ) … ( s r e d a h S t i n i
…
}
r e f f u B r o l o C
m e t s y S L G b e W
Figure 2.20 Two ways to pass data to a vertex shader
The attribute variable is a GLSL ES variable which is used to pass data from the world
outside a vertex shader into the shader and is only available to vertex shaders.
CHAPTER 2 Your First Step with WebGL
42
To use the attribute variable, the sample program involves the following three steps:
1. Prepare the attribute variable for the vertex position in the vertex shader.
2. Assign the attribute variable to the gl_Position variable.
3. Pass the data to the attribute variable.
Let’s look at the sample program in more detail to see how to carry out these steps.
Sample Program (HelloPoint2.js)
In HelloPoint2 (see Listing 2.6 ), you draw a point at a position the JavaScript program
specifies.
Listing 2.6 HelloPoint2.js
1 // HelloPoint2.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'void main() {\n' +
6 ' gl_Position = a_Position;\n' +
7 ' gl_PointSize = 10.0;\n' +
8 '}\n';
9
10 // Fragment shader program
... snipped because it is the same as HelloPoint1.js
15
16 function main() {
17 // Retrieve <canvas> element
18 var canvas = document.getElementById('webgl');
19
20 // Get the rendering context for WebGL
21 var gl = getWebGLContext(canvas);
...
26
27 // Initialize shaders
28 if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
...
31 }
32
33 // Get the storage location of attribute variable
34 var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
35 if (a_Position < 0) {
36 console.log('Failed to get the storage location of a_Position');
37 return;
Draw a Point (Version 2)
43
38 }
39
40 // Pass vertex position to attribute variable
41 gl.vertexAttrib3f(a_Position, 0.0, 0.0, 0.0);
42
43 // Set the color for clearing <canvas>
44 gl.clearColor(0.0, 0.0, 0.0, 1.0);
45
46 // Clear <canvas>
47 gl.clear(gl.COLOR_BUFFER_BIT);
48
49 // Draw a point
50 gl.drawArrays(gl.POINTS, 0, 1);
51 }
As you can see, the attribute variable is prepared within the shader on line 4:
4 'attribute vec4 a_Position;\n' +
In this line, the keyword attribute is called a storage qualifier , and it indicates that the
following variable (in this case, a_Position ) is an attribute variable. This variable must
be declared as a global variable because data is passed to it from outside the shader. The
variable must be declared following a standard pattern <Storage Qualifier> <Type>
<Variable Name> , as shown in Figure 2.21 .
; n o i t i s o P _ a 4 c e v e t u b i r t a
e m a N e l b a i r a V e p y T r e i f i l a u Q e g a r o t S
Figure 2.21 The declaration of the attribute variable
In line 4, you declare a_Position as an attribute variable with data type vec4 because,
as you saw in Table 2.2 , it will be assigned to gl_Position , which always requires a vec4
type.
Note that throughout this book, we have adopted a programming convention in which
all attribute variables have the prefix a_ , and all uniform variables have the prefix u_ to
easily determine the type of variables from their names. Obviously, you can use your own
convention when writing your own programs, but we find this one simple and clear.
Once a_Position is declared, it is assigned to gl_Position at line 6:
6 ' gl_Position = a_Position;\n' +
At this point, you have completed the preparation in the shader for receiving data from
the outside. The next step is to pass the data to the attribute variable from the JavaScript
program.
CHAPTER 2 Your First Step with WebGL
44
Getting the Storage Location of an Attribute Variable
As you saw previously, the vertex shader program is set up in the WebGL system using
the convenience function initShaders() . When the vertex shader is passed to the
WebGL system, the system parses the shader, recognizes it has an attribute variable, and
then prepares the location of its attribute variable so that it can store data values when
required. When you want to pass data to a_Position in the vertex shader, you need to
ask the WebGL system to give you the location it has prepared, which can be done using
gl.getAttribLocation() , as shown in line 34:
33 // Get the location of attribute variable
34 var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
35 if (a_Position < 0) {
36 console.log('Fail to get the storage location of a_Position');
37 return;
38 }
The first argument of this method specifies a program object that holds the vertex shader
and the fragment shader. You will examine the program object in Chapter 8 , but for
now, you can just specify gl.program as the argument here. Note that you should use
gl.program only after initShaders() has been called because initShaders() assigns the
program object to the variable. The second parameter specifies the attribute variable name
(in this case a_Position ) whose location you want to know.
The return value of this method is the storage location of the specified attribute variable.
This location is then stored in the JavaScript variable, a_Position , at line 34 for later use.
Again, for ease of understanding, this book uses JavaScript variable names for attribute
variables, which are the same as the GLSL ES attribute variable name. You can, of course,
use any variable name.
The specification of gl.getAttribLocation() is as follows:
gl.getAttribLocation(program, name)
Retrieve the storage location of the attribute variable specified by the name parameter.
Parameters program Specifies the program object that holds a vertex
shader and a fragment shader.
name Specifies the name of the attribute variable
whose location is to be retrieved.
Return value greater than or equal to 0 The location of the specified attribute variable.
-1 The specified attribute variable does not exist or
its name starts with the reserved prefix gl_ or
webgl_ .
Draw a Point (Version 2)
45
Errors INVALID_OPERATION program has not been successfully linked (See
Chapter 9 .)
INVALID_VALUE The length of name is more than the maximum
length (256 by default) of an attribute variable
name.
Assigning a Value to an Attribute Variable
Once you have the attribute variable location, you need to set the value using the
a_Position variable. This is performed at line 41 using the gl.vertexAttrib3f() method.
40 // Set vertex position to attribute variable
41 gl.vertexAttrib3f(a_Position, 0.0, 0.0, 0.0);
The following is the specification of gl.vertexAtrrib3f() .
gl.vertexAttrib3f(location, v0, v1, v2)
Assign the data ( v0 , v1 , and v2 ) to the attribute variable specified by location .
Parameters location Specifies the storage location of an attribute variable to be modified.
v0 Specifies the value to be used as the first element for the attribute
variable.
v1 Specifies the value to be used as the second element for the attri-
bute variable.
v2 Specifies the value to be used as the third element for the attribute
variable.
Return value None
Errors INVALID_OPERATION There is no current program object.
INVALID_VALUE location is greater than or equal to the maximum
number of attribute variables (8, by default).
The first argument of the method call specifies the location returned by gl.getAttrib-
Location() at line 34. The second, third, and fourth arguments specify the floating point
number to be passed to a_Position representing the x, y, and z coordinates of the point.
After calling the method, these three values are passed as a group to a_Position , which
was prepared at line 4 in the vertex shader. Figure 2.22 shows the processing flow of
getting the location of the attribute variable and then writing a value to it.
CHAPTER 2 Your First Step with WebGL
46
m e t s y S L G b e W
gl_Position gl_PointSize
t p i r c S a v a J
r e d a h S x e t r e V
{ ) ( n i a m d i o v
; n o i t i s o P _ a = n o i t i s o P _ l g
; 0 . 0 1 = e z i S t n i o P _ l g
}
{ ) ( n i a m n o i t c n u f
; ) ( t x e t n o C L G b e W t e g = l g r a v
…
; ) … , E C R U O S _ R E D A H S V , l g ( s r e d a h S t i n i
= n o i t i s o P _ a
; ) ’ n o i t i s o P _ a ‘ , … ( n o i t a c o L e t u b i r t A t e g . l g
…
P _ a ( f 3 e t u b i r t A x e t r e v . l g ; ) 0 . 0 , 0 . 0 , 0 . 0 , n o i t i s o
…
}
m e t s y S L G b e W
gl_Position gl_PointSize
t p i r c S a v a J
r e d a h S x e t r e V
{ ) ( n i a m d i o v
; n o i t i s o P _ a = n o i t i s o P _ l g
; 0 . 0 1 = e z i S t n i o P
_ l
g
}
{ ) ( n i a m n o i t c n u f
; ) ( t x e t n o C L G b e W t e g = l g r a v
…
; ) … , E C R U O S _ R E D A H S V , l g ( s r e d a h S t i n i
= n o i t i s o P _ a
; ) ’ n o i t i s o P _ a ‘ , … ( n o i t a c o L e t u b i r t A t e g . l g
…
P _ a ( f 3 e t u b i r t A x e t r e v . l g ; ) 0 . 0 , 0 . 0 , 0 . 0 , n o i t i s o
…
}
n o i t i s o P _ a
n o i t i s o P _ a
Figure 2.22 Getting the storage location of an attribute variable and then writing a value to
the variable
a_Position is then assigned to gl_Position at line 6 in the vertex shader, in effect passing
the x, y, and z coordinates from your JavaScript, via the attribute variable into the shader,
where it’s written to gl_Position . So the program has the same effect as HelloPoint1 ,
where gl_Position is used as the position of a point. However, gl_Position has now been
set dynamically from JavaScript rather than statically in the vertex shader:
4 'attribute vec4 a_Position;\n' +
5 'void main() {\n' +
6 ' gl_Position = a_Position;\n' +
7 ' gl_PointSize = 10.0;\n' +
8 '}\n';
Finally, you clear the <canvas> using gl.clear() (line 47) and draw the point using
gl.drawArrays() (line 50) in the same way as in HelloPoint1.js .
Draw a Point (Version 2)
47
As a final note, you can see a_Position is prepared as vec4 at line 4 in the vertex shader.
However, gl.vertexAttrib3f() at line 41 specifies only three values (x, y, and z), not
four. Although you may think that one value is missing, this method automatically
supplies the value 1.0 as the fourth value (see Figure 2.23 ). As you saw earlier, a default
fourth value of 1.0 for a color ensures it is fully opaque, and a default fourth value of 1.0
for a homogeneous coordinate maps a 3D coordinate into a homogenous coordinate, so
essentially the method is supplying a “safe” fourth value.
m e t s y S L G b e W
gl_Position gl_PointSize
t p i r c S a v a J
r e d a h S x e t r e V
{ ) ( n i a m d i o v
; n o i t i s o P _ a = n o i t i s o P _ l g
; 0 . 0 1 = e z i S t n i o P _ l g
}
{ ) ( n i a m n o i t c n fu
; ) ( t x e t n o C L G b e W t e g = l g r a v
…
; ) … , E C R U O S _ R E D A H S V , l g ( s r e d a h S t i n i
= n o i t i s o P _ a
; ) ’ n o i t i s o P _ a ‘ , … ( n o i t a c o L e t u b i r t A t e g . l g
…
P _ a ( f 3 e t u b i r t A x e t r e v . l g ; ) 0 . 0 , 0 . 0 , 0 . 0 , n o i t i s o
…
}
0 . 0 0 . 0 0 . 0
n o i t i s o P _ a
0 . 0 0 . 0 0 . 0 0 . 1
Figure 2.23 The missing data is automatically supplied
Family Methods of gl.vertexAttrib3f()
gl.vertexAttrib3f() is part of a family of methods that allow you to set some or all of
the components of the attribute variable. gl.vertexAttrib1f() is used to assign a single
value (v0), glvertexAttrib2f() assigns two values (v0 and v1), and gl.vertexAttrib4f()
assigns four values (v0, v1, v2, and v3).
gl.vertexAttrib1f(location, v0)
gl.vertexAttrib2f(location, v0, v1)
gl.vertexAttrib3f(location, v0, v1, v2)
gl.vertexAttrib4f(location, v0, v1, v2, v3)
Assign data to the attribute variable specified by location . gl.vertexAttrib1f() indicates
that only one value is passed, and it will be used to modify the first component of the
attribute variable. The second and third components will be set to 0.0, and the fourth
component will be set to 1.0. Similarly, gl.vertexAttrib2f() indicates that values are
provided for the first two components, the third component will be set to 0.0, and the
fourth component will be set to 1.0. gl.vertexAttrib3f() indicates that values are
provided for the first three components, and the fourth component will be set to 1.0,
whereas gl.vertexAttrib4f() indicates that values are provided for all four components.
CHAPTER 2 Your First Step with WebGL
48
Parameters location Specifies the storage location of the attribute variable.
v0, v1, v2, v3 Specifies the values to be assigned to the first, second,
third, and fourth components of the attribute variable.
Return value None
Errors INVALID_VALUE location is greater than or equal to the maximum number
of attribute variables (8 by default).
The vector versions of these methods are also available. Their name contains “v” (vector),
and they take a typed array (see Chapter 4 ) as a parameter. The number in the method
name indicates the number of elements in the array. For example,
var position = new Float32Array([1.0, 2.0, 3.0, 1.0]);
gl.vertexAttrib4fv(a_Position, position);
In this case, 4 in the method name indicates that the length of the array is 4.
The Naming Rules for WebGL-Related Methods
You may be wondering what 3f in gl.vertexAttrib3f() actually means. WebGL bases
its method names on the function names in OpenGL ES 2.0, which as you now know,
is the base specification of WebGL. The function names in OpenGL comprise the three
components: <base function name> <number of parameters> < parameter type> .
The name of each WebGL method also uses the same components: <base method name>
<number of parameters> <parameter type> as shown in Figure 2.24 .
. l g b i r t A x e t r e v 3f( 2 v , 1 v , 0 v , n o i t a c o l )
e m a n d o h t e m e s a b
e p y t r e t e m a r a p :
“ f d n a s r e b m u n t n i o p g n i t a o l f s e t a c i d n i ”
“ i s r e b m u n r e g e t n i s e t a c i d n i ”
s r e t e m a r a p f o r e b m u n
Figure 2.24 The naming rules of WebGL-related methods
As you can see in the example, in the case of gl.vertexAttrib3f() , the base method
name is vertexAttrib , the number of parameters is 3, and the parameter type is f (that
is, float or floating point number). This method is a WebGL version of the function
glVertexAttrib3f() in OpenGL. Another character for the parameter type is i, which
indicates integer. You can use the following notation to represent all methods from gl.
vertexAttrib1f() to gl.vertexAttrib4f() : gl.vertexAttrib[1234]f() .
Where [] indicates that one of the numbers in it can be selected.
Draw a Point (Version 2)
49
When v is appended to the name, the methods take an array as a parameter. In this case,
the number in the method name indicates the number of elements in the array.
var positions = new Float32Array([1.0, 2.0, 3.0, 1.0]);
gl.vertexAttrib4fv(a_Position, positions);
Experimenting with the Sample Program
Now that you have the program to pass the position of a point from a JavaScript program
to a vertex shader, let’s change that position. For example, if you wanted to display a
point at (0.5, 0.0, 0.0), you would modify the program as follows:
33 gl.vertexAttrib3f(a_Position, 0.5 , 0.0, 0.0);
You could use other family methods of gl.vertexAttrib3f() to perform the same task in
the following way:
gl.vertexAttrib1f(a_Position, 0.5);
gl.vertexAttrib2f(a_Position, 0.5, 0.0);
gl.vertexAttrib4f(a_Position, 0.5, 0.0, 0.0, 1.0);
Now that you are comfortable using attribute variables, let’s use the same approach to
change the size of the point from within your JavaScript program. In this case, you will
need a new attribute variable for passing the size to the vertex shader. Following the
naming rule used in this book, let’s use a_PointSize . As you saw in Table 2.2 , the type of
gl_PointSize is float , so you need to prepare the variable using the same type as follows:
attribute float a_PointSize;
So, the vertex shader becomes:
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'attribute float a_PointSize; \n' +
6 'void main() {\n' +
7 ' gl_Position = a_Position;\n' +
8 ' gl_PointSize = a_PointSize;\n' +
9 '}\n';
Then, after you get the storage location of a_PointSize , to pass the size of the point to the
variable, you can use gl.vertexAttrib1f() . Because the type of a_PointSize is float , you
can use gl.vertexAttrib1f() as follows:
33 // Get the storage location of attribute variable
34 var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
...
CHAPTER 2 Your First Step with WebGL
50
39 var a_PointSize = gl.getAttribLocation(gl.program, 'a_PointSize');
40 // Set vertex position to attribute variable
41 gl.vertexAttrib3f(a_Position, 0.0, 0.0, 0.0);
42 gl.vertexAttrib1f(a_PointSize, 5.0);
At this stage, you might want to experiment a little with the program and make sure you
understand how to use these attribute variables and how they work.
Draw a Point with a Mouse Click
The previous program HelloPoint2 can pass the position of a point to a vertex shader
from a JavaScript program. However, the position is still hard-coded, so it is not so differ-
ent from HelloPoint1 , in which the position was also directly written in the shader.
In this section, you add a little more flexibility, exploiting the ability to pass the position
from a JavaScript program to a vertex shader, to draw a point at the position where the
mouse is clicked. Figure 2.25 shows the screen shot of ClickedPoint .
4
Figure 2.25 ClickedPoint
This program uses an event handler to handle mouse-related events, which will be famil-
iar to those of you who have written JavaScript programs.
Sample Program (ClickedPoints.js)
Listing 2.7 shows ClickedPoints.js . For brevity, we have removed code sections that are
the same as the previous example and replaced these with ....
4 © 2012 Marisuke Kunnya
Draw a Point with a Mouse Click
51
Listing 2.7 ClickedPoints.js
1 // ClickedPoints.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'void main() {\n' +
6 ' gl_Position = a_Position;\n' +
7 ' gl_PointSize = 10.0;\n' +
8 '}\n';
9
10 // Fragment shader program
...
16 function main() {
17 // Retrieve <canvas> element
18 var canvas = document.getElementById('webgl');
19
20 // Get the rendering context for WebGL
21 var gl = getWebGLContext(canvas);
...
27 // Initialize shaders
28 if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)){
...
31 }
32
33 // Get the storage location of a_Position variable
34 var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
...
40 // Register function (event handler) to be called on a mouse press
41 canvas.onmousedown = function(ev) { click(ev, gl, canvas, a_Position); };
...
47 gl.clear(gl.COLOR_BUFFER_BIT);
48 }
49
50 var g_points = []; // The array for a mouse press
51 function click(ev, gl, canvas, a_Position) {
52 var x = ev.clientX; // x coordinate of a mouse pointer
53 var y = ev.clientY; // y coordinate of a mouse pointer
54 var rect = ev.target.getBoundingClientRect();
55
56 x = ((x - rect.left) - canvas.height/2)/(canvas.height/2);
57 y = (canvas.width/2 - (y - rect.top))/(canvas.width/2);
58 // Store the coordinates to g_points array
59 g_points.push(x); g_points.push(y);
60
CHAPTER 2 Your First Step with WebGL
52
61 // Clear <canvas>
62 gl.clear(gl.COLOR_BUFFER_BIT);
63
64 var len = g_points.length;
65 for(var i = 0; i < len; i+=2) {
66 // Pass the position of a point to a_Position variable
67 gl.vertexAttrib3f(a_Position, g_points[i], g_points[i+1], 0.0);
68
69 // Draw a point
70 gl.drawArrays(gl.POINTS, 0, 1);
71 }
72 }
Register Event Handlers
The processing flow from lines 17 to 39 is the same as HelloPoint2.js . These lines get the
WebGL context, initialize the shaders, and then retrieve the location of the attribute vari-
able. The main differences from HelloPoint2.js are the registration of an event handler
(line 41) and the definition of the function click() as the handler (from line 51).
An event handler is an asynchronous callback function that handles input on a web page
from a user, such as mouse clicks or key presses. This allows you to create a dynamic web
page that can change the content that’s displayed according to the user’s input. To use an
event handler, you need to register the event handler (that is, tell the system what code to
run when the event occurs). The <canvas> supports special properties for registering event
handlers for a specific user input, which you will use here.
For example, when you want to handle mouse clicks, you can use the onmousedown prop-
erty of the <canvas> to specify the event handler for a mouse click as follows:
40 // Register function (event handler) to be called on a mouse press
41 canvas.onmousedown = function(ev) { click(ev, gl, canvas, a_Position); };
Line 41 uses the statement function(){ ... } to register the handler:
function(ev){ click(ev, gl, canvas, a_Position); }
This mechanism is called an anonymous function , which, as its name suggests, is a
convenient mechanism when you define a function that does not need to have a name.
For those of you unfamiliar with this type of function, let’s explain the mechanism using
an example. Next, we define the variable thanks as follows using the form:
var thanks = function () { alert(' Thanks a million!'); }
We can execute the variable as a function as follows:
thanks(); // 'Thanks a million!' is displayed
Draw a Point with a Mouse Click
53
You can see that the variable thanks can be operated as a function. These lines can be
rewritten as follows:
function thanks() { alert('Thanks a million!'); }
thanks(); // 'Thanks a million!' is displayed
So why do you need to use an anonymous function? Well, when you draw a point, you
need the following three variables: gl , canvas , and a_Position . These variables are local
variables that are prepared in the function main() in the JavaScript program. However,
when a mouse click occurs, the browser will automatically call the function that is regis-
tered to the <canvas> ’s onmousedown property with a predefined single parameter (that is,
an event object that contains information about the mouse press. Therefore, usually, you
will register the event handler and define the function for it as follows:
canvas.onmousedown = mousedown; // Register "mousedown" as event handler
...
function mousedown(ev) { // Event handler: It takes one parameter "ev"
...
}
However, if you define the function in this way, it cannot access the local variables gl ,
canvas , and a_Position to draw a point. The anonymous function at line 41 provides the
solution to make it possible to access them:
41 canvas.onmousedown = function(ev) { click(ev, gl, canvas, a_Position); };
In this code, when a mouse click occurs, the anonymous function function(ev) is called
first. Then it calls click(ev, gl, canvas, a_Position) with the local variables defined
in main() . Although this may seem a little complicated, it is actually quite flexible and
avoids the use of global variables, which is always good. Take a moment to make sure
you understand this approach, because you will often see this way of registering event
handlers in this book.
Handling Mouse Click Events
Let’s look at what the function click() is doing. The processing flow follows:
1. Retrieve the position of the mouse click and then store it in an array.
2. Clear <canvas>.
3. For each position stored in the array, draw a point.
50 var g_points = []; // The array for mouse click positions
51 function click(ev, gl, canvas, a_Position) {
52 var x = ev.clientX; // x coordinate of a mouse pointer
53 var y = ev.clientY; // y coordinate of a mouse pointer
54 var rect = ev.target.getBoundingClientRect();
CHAPTER 2 Your First Step with WebGL
54
55
56 x = ((x - rect.left) - canvas.height/2)/(canvas.height/2);
57 y = (canvas.width/2 - (y - rect.top))/(canvas.width/2);
58 // Store the coordinates to a_points array <- (1)
59 g_points.push(x); g_points.push(y);
60
61 // Clear <canvas>
62 gl.clear(gl.COLOR_BUFFER_BIT); <- (2)
63
64 var len = g_points.length;
65 for(var i = 0; i < len; i+=2) {
66 // Pass the position of a point to a_Position variable <- (3)
67 gl.vertexAttrib3f(a_Position, g_points[i], g_points[i+1], 0.0);
68
69 // Draw a point
70 gl.drawArrays(gl.POINTS, 0, 1);
71 }
72 }
The information about the position of a mouse click is stored as an event object and
passed by the browser using the argument ev to the function click() . ev holds the posi-
tion information, and you can get the coordinates by using ev.clientX and ev.clientY ,
as shown in lines 52 and 53. However, you cannot use the coordinates directly in this
sample program for two reasons:
1. The coordinate is the position in the “client area” in the browser, not in the
<canvas> (see Figure 2.26 ).
x
y
) 0 , 0 (
> s a v n a c < f o a e r a g n i w a r d e h T
> s a v n a c < n i g i r o e h T
Y t n e i l c . v e d n a X t n e i l c . v e
e h t n i n o i t i s o p e h t d l o h
a e r a t n e i l c
r e s w o r b a f o a e r a t n e i l c e h T
X t n e i l c . v e
Y t n e i l c . v e
a e r a t n e i l c f o n i g i r o e h T
! k c i l C
Figure 2.26 The coordinate system of a browser’s client area and the position of the
<canvas>
2. The coordinate system of the <canvas> is different from that of WebGL (see Figure
2.27 ) in terms of their origin and the direction of the y-axis.
Draw a Point with a Mouse Click
55
x ) 0 , 0 (
y
) 0 . 0 , 0 . 1 , 0 . 0 (
> s a v n a c <
a e r a g n i w a r d
) 0 . 0 , 0 . 0 , 0 . 1 ( ) 0 . 0 , 0 . 0 , 0 . 1 - (
) 0 . 0 , 0 . 1 - , 0 . 0 (
) 0 . 0 , 0 . 0 , 0 . 0 (
x
y
) 0 0 4 , 0 0 4 (
> s a v n a c <
a e r a g n i w a r d
Figure 2.27 The coordinate system of <canvas> (left) and that of WebGL on <canvas> (right)
First you need to transform the coordinates from the browser area to the canvas, and then
you need to transform them into coordinates that WebGL understands. Let’s look at how
to do that.
These transformations are performed at lines 56 and 57 in the sample program:
52 var x = ev.clientX;
53 var y = ev.clientY;
54 var rect = ev.target.getBoundingClientRect();
55 ...
56 x = ((x - rect.left) - canvas.width/2)/(canvas.width/2);
57 y = (canvas.height/2 - (y - rect.top))/(canvas.height/2);
Line 54 gets the position of the <canvas> in the client area. The rect.left and rec.top
indicate the position of the origin of the <canvas> in the browser’s client area (refer to
Figure 2.26 ). So, (x – rect.left) at line 56 and (y – rect.top) at line 57 slide the posi-
tion (x, y) in the client area to the correct position on the <canvas> element.
Next, you need to transform the <canvas> position into the WebGL coordinate system
shown in Figure 2.27 . To perform the transformation, you need to know the center posi-
tion of the <canvas> . You can get the size of the <canvas> by canvas.height (in this case,
400) and canvas.width (in this case, 400). So, the center of the element will be (canvas.
height/2, canvas.width/2) .
Next, you can implement this transformation by sliding the origin of the <canvas>
into the origin of the WebGL coordinate system located at the center of <canvas> . The
((x - rect.left) - canvas.width/2) and (canvas.height/2 - (y -rect.top)) perform
the transformation.
Finally, as shown in Figure 2.27 , the range of the x-axis in the <canvas> goes from 0 to
canvas.width (400), and that of the y-axis goes from 0 to canvas.height (400). Because
CHAPTER 2 Your First Step with WebGL
56
the axes in WebGL range from –1.0 to 1.0, the last step is to map the range of the
<canvas> coordinate system to that of the WebGL coordinate system by dividing the x
coordinate by canvas.width/2 and the y coordinate by canvas.height/2 . You can see this
division in lines 56 and 57.
Line 59 stores the resulting position of the mouse click in the array g_points using the
push() method, which appends the data to the end of the array:
59 g_points.push(x); g_points.push(y);
So every time a mouse click occurs, the position of the click is appended to the array, as
shown in Figure 2.28 . (The length of the array is automatically stretched.) Note that the
index of an array starts from 0, so the first position is g_points[0] .
x e t a n i d r o o c
t s 1 e h t f o
t n i o p d e k c i l c
y e t a n i d r o o c
t s 1 e h t f o
t n i o p d e k c i l c
x e t a n i d r o o c
d n 2 e h t f o
t n i o p d e k c i l c
y e t a n i d r o o c
d n 2 e h t f o
t n i o p d e k c i l c
x e t a n i d r o o c
d r 3 e h t f o
t n i o p d e k c i l c
y e t a n i d r o o c
d r 3 e h t f o
t n i o p d e k c i l c
…
] [ s t n i o p _ g f o s t n e t n o c e h t
] 2 [ s t n i o p _ g ] 1 [ s t n i o p _ g ] 0 [ s t n i o p _ g ] 5 [ s t n i o p _ g ] 4 [ s t n i o p _ g ] 3 [ s t n i o p _ g
Figure 2.28 The content of g_points
You may be wondering why you need to store all the positions rather than just the most
recent mouse click. This is because of the way WebGL uses the color buffer. You will
remember from Figure 2.10 that in the WebGL system, first the drawing operation is
performed to the color buffer, and then the system displays its content to the screen. After
that, the color buffer is reinitialized and its content is lost. (This is the default behavior,
which you’ll investigate in the next section.) Therefore, it is necessary to save all the posi-
tions of the clicked points in the array, so that on each mouse click, the program can
draw all the previous points as well as the latest. For example, the first point is drawn at
the first mouse click. The first and second points are drawn at the second mouse click. The
first, second, and third points are drawn on the third click, and so on.
Returning to the program, the <canvas> is cleared at line 62. After that, the for statement
at line 65 passes each position of the point stored in g_points to a_Position in the vertex
shader. Then gl.drawArrays() draws the point at that position:
65 for(var i = 0; i < len; i+=2) {
66 // Pass the position of a point to a_Position variable
67 gl.vertexAttrib3f(a_Position, g_points[i], g_points[i+1], 0.0);
Just like in HelloPoint2.js , you will use gl.vertexAttrib3f() to pass the point position
to the attribute variable a_Position , which was passed as the fourth parameter of click()
at line 51.
Draw a Point with a Mouse Click
57
The array g_points holds the x coordinate and y coordinate of the clicked points, as
shown in Figure 2.28 . Therefore, g_points[i] holds the x coordinate, and g_points[i+1]
holds the y coordinate, so the loop index i in the for statement at line 65 is incremented
by 2 using the convenient + operator.
Now that you have completed the preparations for drawing the point, the remaining task
is just to draw it using gl.drawArrays() :
69 // Draw a point
70 gl.drawArrays(gl.POINTS, 0, 1);
Although it’s a little complicated, you can see that the use of event handlers combined
with attribute variable provides a flexible and generic means for user input to change what
WebGL draws.
Experimenting with the Sample Program
Now let’s experiment a little with this sample program ClickedPoints . After loading
ClickedPoints.html in your browser, every time you click, it draws a point at the clicked
position, as shown in Figure 2.26 .
Let’s examine what will happen when you stop clearing the <canvas> at line 62.
Comment out the line as follows, and then reload the modified file into your browser.
61 // Clear <canvas>
62 // gl.clear(gl.COLOR_BUFFER_BIT);
63
64 var len = g_points.length;
65 for(var i = 0; i < len; i+=2) {
66 // Pass the position of a point to a_Position variable
67 gl.vertexAttrib3f(a_Position, g_points[i], g_points[i+1], 0.0);
68
69 // Draw a point
70 gl.drawArrays(gl.POINTS, 0, 1);
71 }
72 }
After running the program, you initially see a black background, but at the first click,
you see a red point on a white background. This is because WebGL reinitializes the color
buffer to the default value (0.0, 0.0, 0.0, 0.0) after drawing the point (refer to Table 2.1 ).
The alpha component of the default value is 0.0, which means the color is transparent.
Therefore, the color of the <canvas> becomes transparent, so you see the background color
of the web page (white, in this case) through the <canvas> element. If you don’t want this
behavior, you should use gl.clearColor() to specify the clear color and then always call
gl.clear() before drawing something.
CHAPTER 2 Your First Step with WebGL
58
Another interesting experiment is to look at how to simplify the code. In ClickedPoints.
js , the x and y coordinates are stored separately in the g_points array. However, you can
store the x and y coordinates as a group into the array as follows:
58 // Store the coordinates to a_points array <- (1)
59 g_points.push([x, y]);
In this case, the new two-element array [x, y] is stored as an element in the array
g_points . Conveniently, JavaScript can store an array in an array.
You can retrieve the x and y coordinates from the array as follows. First, you retrieve an
element from the array by specifying its index (line 66). Because the element itself is an
array containing (x, y), you can retrieve the first element and the second element as the
x and y coordinate of each point from the array (line 67):
65 for(var i = 0; i < len; i++) {
66 var xy = g_points[i];
67 gl.vertexAttrib3f(a_Position, xy[0], xy[1], 0.0);
...
71 }
In this way, you can deal with the x and y coordinates as a group, simplifying your
program and increasing the readability.
Change the Point Color
By now you should have a good feel for how the shaders work and how to pass data to
them from your JavaScript program. Let’s build on your understanding by constructing a
more complex program that draws points whose colors vary depending on their position
on the <canvas> .
You already studied how to change the color of a point when you looked at HelloPoint1 .
In that example, you modified the fragment shader program directly to embed the color
value into the shader. In this section, let’s construct the program so that you can specify
the color of a point from JavaScript. This is similar to HelloPoint2 earlier, where you
passed the position of a point from a JavaScript program to the vertex shader. However,
in this sample program, you need to pass the data to a “fragment shader,” not to a vertex
shader.
The name of the sample program is ColoredPoints . If you load the example into your
browser, the result is the same as ClickedPoints except that each point’s color varies
depending on its position (see Figure 2.29 ).
Change the Point Color
59
Figure 2.29 ColoredPoints
To pass data to a fragment shader, you can use a uniform variable and follow the same
steps that you used when working with attribute variables. However, this time the target is
a fragment shader, not a vertex shader:
1. Prepare the uniform variable for the color in the fragment shader.
2. Assign the uniform variable to the gl_FragColor variable.
3. Pass the color data to the uniform variable from the JavaScript program.
Let’s look at the sample program and see how these steps are programmed.
Sample Program (ColoredPoints.js)
The vertex shader of this sample program is the same as in ClickedPoints.js . However,
this time, the fragment shader plays a more important role because the program changes
the color of the point dynamically and, as you will remember, fragment shaders handle
colors. Listing 2.8 shows ColoredPoints.js .
Listing 2.8 ColoredPoints.js
1 // ColoredPoints.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'void main() {\n' +
6 ' gl_Position = a_Position;\n' +
7 ' gl_PointSize = 10.0;\n' +
CHAPTER 2 Your First Step with WebGL
60
8 '}\n';
9
10 // Fragment shader program
11 var FSHADER_SOURCE =
12 'precision mediump float;\n' +
13 'uniform vec4 u_FragColor;\n' + // uniform variable <- (1)
14 'void main() {\n' +
15 ' gl_FragColor = u_FragColor;\n' + <- (2)
16 '}\n';
17
18 function main() {
...
29 // Initialize shaders
30 if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
...
33 }
34
35 // Get the storage location of a_Position variable
36 var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
...
42 // Get the storage location of u_FragColor variable
43 var u_FragColor = gl.getUniformLocation(gl.program, 'u_FragColor');
...
49 // Register function (event handler) to be called on a mouse press
50 canvas.onmousedown = function(ev){ click(ev, gl, canvas, a_Position,
➥u_FragColor) };
...
56 gl.clear(gl.COLOR_BUFFER_BIT);
57 }
58
59 var g_points = []; // The array for a mouse press
60 var g_colors = []; // The array to store the color of a point
61 function click(ev, gl, canvas, a_Position, u_FragColor) {
62 var x = ev.clientX; // x coordinate of a mouse pointer
63 var y = ev.clientY; // y coordinate of a mouse pointer
64 var rect = ev.target.getBoundingClientRect();
65
66 x = ((x - rect.left) - canvas.width/2)/(canvas.width/2);
67 y = (canvas.height/2 - (y - rect.top))/(canvas.height/2);
68
69 // Store the coordinates to g_points array
70 g_points.push([x, y]);
71 // Store the color to g_colors array
72 if(x >= 0.0 && y >= 0.0) { // First quadrant
73 g_colors.push([1.0, 0.0, 0.0, 1.0]); // Red
Change the Point Color
61
74 } else if(x < 0.0 && y < 0.0) { // Third quadrant
75 g_colors.push([0.0, 1.0, 0.0, 1.0]); // Green
76 } else { // Others
77 g_colors.push([1.0, 1.0, 1.0, 1.0]); // White
78 }
79
80 // Clear <canvas>
81 gl.clear(gl.COLOR_BUFFER_BIT);
82
83 var len = g_points.length;
84 for(var i = 0; i < len; i++) {
85 var xy = g_points[i];
86 var rgba = g_colors[i];
87
88 // Pass the position of a point to a_Position variable
89 gl.vertexAttrib3f(a_Position, xy[0], xy[1], 0.0);
90 // Pass the color of a point to u_FragColor variable
91 gl.uniform4f(u_FragColor, rgba[0],rgba[1],rgba[2],rgba[3]); <-(3)
92 // Draw a point
93 gl.drawArrays(gl.POINTS, 0, 1);
94 }
95 }
Uniform Variables
You likely remember how to use an attribute variable to pass data from a JavaScript
program to a vertex shader. Unfortunately, the attribute variable is only available in a
vertex shader, and when using a fragment shader, you need to use a uniform variable.
There is an alternative mechanism, a varying variable (bottom of Figure 2.30 ); however,
it is more complex, so you won’t use it until Chapter 5 .
r e d a h S x e t r e V t n e m g a r F
r e d a h S
gl_Position gl_PointSize
gl_FragColor
e l b a i r a v e t u b i r t a
e l b a i r a v m r o f i n u
t p i r c S a v a J
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
; ) … ( s r e d a h S t i n i
…
}
e l b a i r a v g n i y r a v
r e f u B r o l o C
e l b a i r a v g n i y r a v
Figure 2.30 Two ways of passing a data to a fragment shader
CHAPTER 2 Your First Step with WebGL
62
When you were introduced to attribute variables, you saw that a uniform variable is a
variable for passing “uniform” (nonvariable) data from a JavaScript program to all vertices
or fragments in the shader. Let’s now use that property.
Before using a uniform variable, you need to declare the variable using the form <Storage
Qualifier> <Type> <Variable name> in the same way (see Figure 2.31 ) as declaring an
attribute variable. (See the section “Sample Program (HelloPoint2.js).”)
5
; r o l o C g a r F _ u 4 c e v m r o f i n u
e m a N e l b a i r a V e p y T r e i f i l a u Q e g a r o t S
Figure 2.31 The declaration of the uniform variable
In this sample program, the uniform variable u_FragColor is named after the variable
gl_FragColor because you will assign the uniform variable to the gl_FragColor later.
The u_ in u_FragColor is part of our programming convention and indicates that it is a
uniform variable. You need to specify the same data type as gl_FragColor for u_FragColor
because you can only assign the same type of variable to gl_FragColor . Therefore, you
prepare u_FragColor at line 13, as follows:
10 // Fragment shader program
11 var FSHADER_SOURCE =
12 'precision mediump float;\n' +
13 'uniform vec4 u_FragColor;\n' + // uniform variable
14 'void main() {\n' +
15 ' gl_FragColor = u_FragColor;\n' +
16 '}\n';
Note that line 12 specifies the range and precision of variables by using a precision quali-
fier , in this case medium precision, which is covered in detail in Chapter 5 .
Line 15 assigns the color in the uniform variable u_FragColor to gl_FragColor , which
causes the point to be drawn in whatever color is passed. Passing the color through the
uniform variable is similar to using an attribute variable; you need to retrieve the storage
location of the variable and then write the data to the location within the JavaScript
program.
Retrieving the Storage Location of a Uniform Variable
You can use the following method to retrieve the storage location of a uniform variable.
5 In GLSL ES, you can only specify float data types for an attribute variable; however, you can
specify any type for a uniform variable. (See Chapter 6 for more details.)
Change the Point Color
63
gl.getUniformLocation(program, name)
Retrieve the storage location of the uniform variable specified by the name parameter.
Parameters program Specifies the program object that holds a vertex
shader and a fragment shader.
name Specifies the name of the uniform variable whose loca-
tion is to be retrieved.
Return value non-null The location of the specified uniform variable.
null The specified uniform variable does not exist or its
name starts with the reserved prefix gl_ or webgl_ .
Errors INVALID_OPERATION program has not been successfully linked
(See Chapter 9 .)
INVALID_VALUE The length of name is more than the maximum length
(256 by default) of a uniform variable.
The functionality and parameters of this method are the same as in gl.getAttribLoca-
tion() . However, the return value of this method is null , not –1, if the uniform variable
does not exist or its name starts with a reserved prefix. For this reason, unlike attribute
variables, you need to check whether the return value is null . You can see this error
checking line 44. In JavaScript, null can be treated as false when checking the condition
of an if statement, so you can use the ! operator to check the result:
42 // Get the storage location of uniform variable
43 var u_FragColor = gl.getUniformLocation(gl.program, 'u_FragColor');
44 if (!u_FragColor) {
45 console.log('Failed to get u_FragColor variable');
46 return;
47 }
Assigning a Value to a Uniform Variable
Once you have the location of the uniform variable and the WebGL method, gl.
uniform4f() is used to write data to it. It has the same functionality and parameters as
those of gl.vertexAttrib[1234]f() .
CHAPTER 2 Your First Step with WebGL
64
gl.uniform4f (location, v0, v1, v2, v3)
Assign the data specified by v0 , v1 , v2 , and v3 to the uniform variable specified by
location .
Parameters location Specifies the storage location of a uniform variable to
be modified.
v0 Specifies the value to be used as the first element of
the uniform variable.
v1 Specifies the value to be used as the second element
of the uniform variable.
v2 Specifies the value to be used as the third element of
the uniform variable.
v3 Specifies the value to be used as the fourth element of
the uniform variable.
Return value None
Errors INVALID_OPERATION There is no current program object.
location is an invalid uniform variable location.
Let’s look at the portion of the sample program where the gl.uniform4f() method is used
to assign the data (line 91). As you can see, several processing steps are needed in advance:
71 // Store the color to g_colors array
72 if(x >= 0.0 && y >= 0.0) { // First quadrant
73 g_colors.push([1.0, 0.0, 0.0, 1.0]); // Red
74 } else if(x < 0.0 && y < 0.0) { // Third quadrant
75 g_colors.push([0.0, 1.0, 0.0, 1.0]); // Green
76 } else { // Other quadrants
77 g_colors.push([1.0, 1.0, 1.0, 1.0]); // White
78 }
...
83 var len = g_points.length;
84 for(var i = 0; i < len; i++) {
85 var xy = g_points[i];
86 var rgba = g_colors[i];
...
91 gl.uniform4f(u_FragColor, rgba[0], rgba[1], rgba[2], rgba[3]);
To understand this program logic, let’s return to the goal of this program, which is to vary
the color of a point according to where on the <canvas> the mouse is clicked. In the first
Change the Point Color
65
quadrant, the color is set to red; in the third quadrant, the color is green; in the other
quadrants, the color is white (see Figure 2.32 ).
x
y
t n a r d a u q d n o c e S
) e t i h w (
t n a r d a u q t s r i F
) d e r (
t n a r d a u q d r i h T
) n e e r g (
t n a r d a u q h t r u o F
) e t i h w (
Figure 2.32 The name of each quadrant in a coordinate system and its drawing colors
The code from lines 72 to 78 simply determines which quadrant the mouse click was in
and then, based on that, writes the appropriate color into the array g_colors . Finally, at
line 84, the program loops through the points, passing the correct color to the uniform
variable u_FragColor at line 91. This causes WebGL to write all the points stored so far to
the color buffer, which then results in the browser display being updated.
Before finishing this chapter, let’s take a quick look at the rest of the family methods for
gl.uniform[1234]f() .
Family Methods of gl.uniform4f()
gl.uniform4f() also has a family of methods. gl.uniform1f() is a method to assign a
single value (v0), gl.uniform2f() assigns two values (v0 and v1), and gl.uniform3f()
assigns three values (v0, v1, and v2).
gl.uniform1f(location, v0)
gl.uniform2f(location, v0, v1)
gl.uniform3f(location, v0, v1, v2)
gl.uniform4f(location, v0, v1, v2, v3)
Assign data to the uniform variable specified by location . gl.uniform1f() indicates
that only one value is passed, and it will be used to modify the first component of
the uniform variable. The second and third components will be set to 0.0, and the
fourth component will be set to 1.0. Similarly, gl.uniform2f() indicates that values are
provided for the first two components, the third component will be set to 0.0, and the
fourth component will be set to 1.0. gl.uniform3f() indicates that values are provided
for the first three components and the fourth component will be set to 1.0, whereas
gl.unifrom4f() indicates that values are provided for all four components.
CHAPTER 2 Your First Step with WebGL
66
Parameters location Specifies the storage location of a uniform variable.
v0, v1, v2, v3 Specifies the values to be assigned to the first,
second, third, and fourth component of the uniform
variable.
Return value None
Errors INVALID_OPERATION There is no current program object.
location is an invalid uniform variable location.
Summary
In this chapter, you saw the core functions of WebGL and how to use them. In particu-
lar, you learned about shaders, which are the main mechanism used in WebGL to draw
graphics. Based on this, you constructed several sample programs starting with a simple
program to draw a red point, and then you added complexity by changing its position
based on a mouse click and changed its color. In both cases, these examples helped you
understand how to pass data from a JavaScript program to the shaders, which will be criti-
cal for future examples.
The shapes found in this chapter were still just two-dimensional points. However, you can
apply the same knowledge of the use of the core WebGL functions and shaders to more
complex shapes and to three-dimensional objects.
A key learning point in this chapter is this: A vertex shader performs per-vertex opera-
tions, and a fragment shader performs per-fragment operations. The following chapters
explain some of the other functions of WebGL, while slowly increasing the complexity of
the shapes that you manipulate and display on the screen.
Chapter 3
Drawing and Transforming Triangles
Chapter 2 , “Your First Step with WebGL,” explained the basic approach to drawing WebGL
graphics. You saw how to retrieve the WebGL context and clear a <canvas> in preparation for
drawing your 2D/3DCG. You then explored the roles and features of the vertex and fragment
shaders and how to actually draw graphics with them. With this basic structure in mind, you
then constructed several sample programs that drew simple shapes composed of points on the
screen.
This chapter builds on those basics by exploring how to draw more complex shapes and how to
manipulate those shapes in 3D space. In particular, this chapter looks at
• The critical role of triangles in 3DCG and WebGL’s support for drawing triangles
• Using multiple triangles to draw other basic shapes
• Basic transformations that move, rotate, and scale triangles using simple equations
• How matrix operations make transformations simple
By the end of this chapter, you will have a comprehensive understanding of WebGL’s support for
drawing basic shapes and how to use matrix operations to manipulate those shapes. Chapter 4 ,
“More Transformations and Basic Animation,” then builds on this knowledge to explore simple
animations.
CHAPTER 3 Drawing and Transforming Triangles
68
Drawing Multiple Points
As you are probably aware , 3D models are actually made from a simple building block:
the humble triangle. For example, looking at the frog in Figure 3.1 , the figure on the right
side shows the triangles used to make up the shape, and in particular the three vertices
that make up one triangle of the head. So, although this game character has a complex
shape, its basic components are the same as a simple one, except of course for many more
triangles and their associated vertices. By using smaller and smaller triangles, and there-
fore more and more vertices, you can create more complex or smoother objects. Typically,
a complex shape or game character will consist of tens of thousands of triangles and
their associated vertices. Thus, multiple vertices used to make up triangles are pivotal for
drawing 3D objects.
Figure 3.1 Complex characters are also constructed from multiple triangles
In this section, you explore the process of drawing shapes using multiple vertices.
However, to keep things simple, you’ll continue to use 2D shapes, because the technique
to deal with multiple vertices for a 2D shape is the same as dealing with them for a 3D
object. Essentially, if you can master these techniques for 2D shapes, you can easily under-
stand the examples in the rest of this book that use the same techniques for 3D objects.
As an example of handling multiple vertices, let’s create a program, MultiPoint , that
draws three red points on the screen; remember, three points or vertices make up the
triangle. Figure 3.2 shows a screenshot from Multipoint .
Drawing Multiple Points
69
Figure 3.2 MultiPoint
In the previous chapter, you created a sample program, ClickedPoints , that drew multi-
ple points based on mouse clicks. ClickedPoints stored the position of the points in a
JavaScript array ( g_points[] ) and used the gl.drawArrays() method to draw each point
( Listing 3.1 ). To draw multiple points, you used a loop that iterated through the array,
drawing each point in turn by passing one vertex at a time to the shader.
Listing 3.1 Drawing Multiple Points as Shown in ClickedPoints.js ( Chapter 2 )
65 for(var i = 0; i<len; i+=2) {
66 // Pass the position of a point to a_Position variable
67 gl.vertexAttrib3f(a_Position, g_points[i], g_points[i+1], 0.0);
68
69 // Draw a point
70 gl.drawArrays(gl.POINTS, 0, 1);
71 }
Obviously, this method is useful only for single points. For shapes that use multiple verti-
ces, you need a way to simultaneously pass multiple vertices to the vertex shader so that
you can draw shapes constructed from multiple vertices, such as triangles, rectangles, and
cubes.
WebGL provides a convenient way to pass multiple vertices and uses something called a
buffer object to do so. A buffer object is a memory area that can store multiple vertices in
the WebGL system. It is used both as a staging area for the vertex data and a way to simul-
taneously pass the vertices to a vertex shader.
Let’s examine a sample program before explaining the buffer object so you can get a feel
for the processing flow.
CHAPTER 3 Drawing and Transforming Triangles
70
Sample Program (MultiPoint.js)
The processing flowchart for MultiPoint.js (see Figure 3.3 ) is basically the same as for
ClickedPoints.js ( Listing 2.7 ) and ColoredPoints.js ( Listing 2.8 ), which you saw in
Chapter 2 . The only difference is a new step, setting up the positions of vertices, which is
added to the previous flow.
L G b e W r o f t x e t n o c g n i r e d n e r e h t t e G
> s a v n a c < r a e l C
s r e d a h s e z i l a i t i n I
w a r D
> s a v n a c < g n i r a e l c r o f r o l o c e h t t e S
s e c i t r e v f o s n o i t i s o p e h t t e S
Figure 3.3 Processing flowchart for MultiPoints.js
This step is implemented at line 34, the function initVertexBuffers() , in Listing 3.2 .
Listing 3.2 MultiPoint. js
1 // MultiPoint.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'void main() {\n' +
6 ' gl_Position = a_Position;\n' +
7 ' gl_PointSize = 10.0;\n' +
8 '}\n';
9
10 // Fragment shader program
...
15
16 function main() {
...
Drawing Multiple Points
71
20 // Get the rendering context for WebGL
21 var gl = getWebGLContext(canvas);
...
27 // Initialize shaders
28 if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
...
31 }
32
33 // Set the positions of vertices
34 var n = initVertexBuffers(gl);
35 if (n < 0) {
36 console.log('Failed to set the positions of the vertices');
37 return;
38 }
39
40 // Set the color for clearing <canvas>
...
43 // Clear <canvas>
...
46 // Draw three points
47 gl.drawArrays(gl.POINTS, 0, n); // n is 3
48 }
49
50 function initVertexBuffers(gl) {
51 var vertices = new Float32Array([
52 0.0, 0.5, -0.5, -0.5, 0.5, -0.5
53 ]);
54 var n = 3; // The number of vertices
55
56 // Create a buffer object
57 var vertexBuffer = gl.createBuffer();
58 if (!vertexBuffer) {
59 console.log('Failed to create the buffer object ');
60 return -1;
61 }
62
63 // Bind the buffer object to target
64 gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
65 // Write date into the buffer object
66 gl.bufferData(gl.ARRAY_BUFFER, vertices, gl.STATIC_DRAW);
67
68 var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
...
73 // Assign the buffer object to a_Position variable
74 gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 0, 0);
CHAPTER 3 Drawing and Transforming Triangles
72
75
76 // Enable the assignment to a_Position variable
77 gl.enableVertexAttribArray(a_Position);
78
79 return n;
80 }
The new function initVertexBuffers() is defined at line 50 and used at line 34 to set
up the vertex buffer object. The function stores multiple vertices in the buffer object and
then completes the preparations for passing it to a vertex shader:
33 // Set the positions of vertices
34 var n = initVertexBuffers(gl);
The return value of this function is the number of vertices being drawn, stored in the vari-
able n . Note that in case of error, n is negative.
As in the previous examples, the drawing operation is carried out using a single call to
gl.drawArrays() at Line 48. This is similar to ClickedPoints.js except that n is passed as
the third argument of gl.drawArrays() rather than the value 1:
46 // Draw three points
47 gl.drawArrays(gl.POINTS, 0, n); // n is 3
Because you are using a buffer object to pass multiple vertices to a vertex shader in init-
VertexBuffers() , you need to specify the number of vertices in the object as the third
parameter of gl.drawArrays() so that WebGL then knows to draw a shape using all the
vertices in the buffer object.
Using Buffer Objects
As indicated earlier, a buffer object is a mechanism provided by the WebGL system that
provides a memory area allocated in the system (see Figure 3.4 ) that holds the vertices you
want to draw. By creating a buffer object and then writing the vertices to the object, you
can pass multiple vertices to a vertex shader through one of its attribute variables.
r e d a h S x e t r e V
attribute variable
{ ) ( n i a m n o i t c n u f
= s e c i t r e v r a v
[ ( y a r A 2 3 t a o l F w e n
, 5 . 0 , 0 . 0
, 5 . 0 - , 5 . 0 -
5 . 0 - , 5 . 0
; ) ]
; ) , s e c i t r e v , . . ( a t a r e f u b . l g
L G b e W
m e t s y S
, 5 . 0 , 0 . 0
, 5 . 0 - , 5 . 0 -
5 . 0 - , 5 . 0
t c e j b O r e f u B
5 . 0 , 0 . 0
5 . 0 - , 5 . 0 -
5 . 0 - , 5 . 0
t p i r c S a v a J
Figure 3.4 Passing multiple vertices to a vertex shader by using a buffer object
Drawing Multiple Points
73
In the sample program, the data (vertex coordinates) written into a buffer object is defined
as a special JavaScript array ( Float32Array ) as follows. We will explain this special array in
detail later, but for now you can think of it as a normal array:
51 var vertices = new Float32Array([
52 0.0, 0.5, -0.5, -0.5, 0.5, -0.5
53 ]);
There are five steps needed to pass multiple data values to a vertex shader through a buffer
object. Because WebGL uses a similar approach when dealing with other objects such as
texture objects ( Chapter 4 ) and framebuffer objects ( Chapter 8 , “Lighting Objects”), let’s
explore these in detail so you will be able to apply the knowledge later:
1. Create a buffer object ( gl.createBuffer() ).
2. Bind the buffer object to a target ( gl.bindBuffer() ).
3. Write data into the buffer object ( gl.bufferData() ).
4. Assign the buffer object to an attribute variable ( gl.vertexAttribPointer() ).
5. Enable assignment ( gl.enableVertexAttribArray() ).
Figure 3.5 illustrates the five steps.
{ ) ( n i a m n o i t c n u f
= s e c i t r e v r a v
[ ( y a r A 2 3 t a o l F w e n
, 5 . 0 , 0 . 0
, 5 . 0 - , 5 . 0 -
5 . 0 - , 5 . 0
; ) ]
; ) , s e c i t r e v , . . ( a t a r e f u b . l g
, 5 . 0 , 0 . 0
, 5 . 0 - , 5 . 0 -
5 . 0 - , 5 . 0
t c e j b O r e f u B
5 . 0 , 0 . 0
5 . 0 - , 5 . 0 -
5 . 0 - , 5 . 0
(1)(2)
(4)
(5)
r e d a h S x e t r e V
t p i r c S a v a J
L G b e W
m e t s y S
(3)
attribute variable
Figure 3.5 The five steps to pass multiple data values to a vertex shader using a buffer object
The code performing the steps in the sample program in Listing 3.2 is as follows:
56 // Create a buffer object <- (1)
57 var vertexBuffer = gl.createBuffer();
58 if (!vertexBuffer) {
59 console.log('Failed to create a buffer object');
60 return -1;
61 }
62
63 // Bind the buffer object to a target <- (2)
CHAPTER 3 Drawing and Transforming Triangles
74
64 gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
65 // Write date into the buffer object <- (3)
66 gl.bufferData(gl.ARRAY_BUFFER, vertices, gl.STATIC_DRAW);
67
68 var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
...
73 // Assign the buffer object to a_Position variable <- (4)
74 gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 0, 0);
75
76 // Enable the assignment to a_Position variable <- (5)
77 gl.enableVertexAttribArray(a_Position);
Let’s start with the first three steps (1–3), from creating a buffer object to writing data
(vertex coordinates in this example) to the buffer, explaining the methods used within
each step.
Create a Buffer Object (gl.createBuffer())
Before you can use a buffer object, you obviously need to create the buffer object. This is
the first step, and it’s carried out at line 57:
57 var vertexBuffer = gl.createBuffer();
You use the gl.createBuffer() method to create a buffer object within the WebGL
system. Figure 3.6 shows the internal state of the WebGL system. The upper part of the
figure shows the state before executing the method, and the lower part is after execution.
As you can see, when the method is executed, it results in a single buffer object being
created in the WebGL system. The keywords gl.ARRAY_BUFFER and gl.ELEMENT_ARRAY_
BUFFER in the figure will be explained in the next section, so you can ignore them for
now.
r e d a h S x e t r e V
t c e j b O r e f u B
R E F F U B _ Y A R R A . l g
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
; ) … ( r e f u B e t a e r c . l g
r e d a h S x e t r e V
R E F F U B _ Y A R R A . l g
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
R E F F U B _ Y A R R A _ T N E M E L E . l g
R E F F U B _ Y A R R A _ T N E M E L E . l g
t p i r c S a v a J
t p i r c S a v a J
attribute variable
attribute variable
Figure 3.6 Create a buffer object
Drawing Multiple Points
75
The following shows the specification of gl.createBuffer() .
gl.createBuffer ()
Create a buffer object.
Return value non-null The newly created buffer object.
null Failed to create a buffer object.
Errors None
The corresponding method gl.deleteBuffer() deletes the buffer object created by
gl.createBuffer() .
gl.deleteBuffer (buffer)
Delete the buffer object specified by buffer.
Parameters buffer Specifies the buffer object to be deleted.
Return Value None
Errors None
Bind a Buffer Object to a Target (gl.bindBuffer())
After creating a buffer object, the second step is to bind it to a “target.” The target tells
WebGL what type of data the buffer object contains, allowing it to deal with the contents
correctly. This binding process is carried out at line 64 as follows:
64 gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
The specification of gl.bindBuffer() is as follows.
gl.bindBuffer(target, buffer)
Enable the buffer object specified by buffer and bind it to the target.
Parameters Target can be one of the following:
gl.ARRAY_BUFFER Specifies that the buffer object contains vertex data.
CHAPTER 3 Drawing and Transforming Triangles
76
gl.ELEMENT_
ARRAY_BUFFER
Specifies that the buffer object contains index values
pointing to vertex data. (See Chapter 6 , “The OpenGL
ES Shading Language [GLSL ES].)
buffer Specifies the buffer object created by a previous call to
gl.createBuffer() .
When null is specified, binding to the target is
disabled.
Return Value None
Errors INVALID_ENUM target is none of the above values. In this case, the
current binding is maintained.
In the sample program in this section, gl.ARRAY_BUFFER is specified as the target to store
vertex data (positions) in the buffer object. After executing line 64, the internal state in
the WebGL system changes, as shown in Figure 3.7 .
r e d a h S x e t r e V
t c e j b O r e f u B
R E F F U B _ Y A R R A . l g
t p i r c S a v a J
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
; ) … ( r e f u B d n i b . l g
R E F F U B _ Y A R R A _ T N E M E L E . l g
attribute variable
Figure 3.7 Bind a buffer object to a target
The next step is to write data into the buffer object. Note that because you won’t be using
the gl.ELEMENT_ARRAY_BUFFER until Chapter 6 , it’ll be removed from the following figures
for clarity.
Write Data into a Buffer Object (gl.bufferData())
Step 3 allocates storage and writes data to the buffer. You use gl.bufferData() to do this,
as shown at line 66:
66 gl.bufferData(gl.ARRAY_BUFFER, vertices, gl.STATIC_DRAW);
This method writes the data specified by the second parameter ( vertices ) into the buffer
object bound to the first parameter ( gl.ARRAY_BUFFER ). After executing line 66, the inter-
nal state of the WebGL system changes, as shown in Figure 3.8 .
Drawing Multiple Points
77
r e d a h S x e t r e V { ) ( n i a m n o i t c n u f
= s e c i t r e v r a v
[ ( y a r A 2 3 t a o l F w e n
, 5 . 0 , 0 . 0
, 5 . 0 - , 5 . 0 -
5 . 0 - , 5 . 0
; ) ]
; ) , s e c i t r e v , . . (
t c e j b O r e f u B
5 . 0 , 0 . 0
5 . 0 - , 5 . 0 -
5 . 0 - , 5 . 0
. a t a r e f u b l g D
t p i r c S a v a J
L G b e W
m e t s y S
attribute variable
Figure 3.8 Allocate storage and write data into a buffer object
You can see in this figure that the vertex data defined in your JavaScript program is
written to the buffer object bound to gl.ARRAY_BUFFER . The following table shows the
specification of gl.bufferData() .
gl.bufferData(target, data, usage)
Allocate storage and write the data specified by data to the buffer object bound to target.
Parameters target Specifies gl.ARRAY_BUFFER or gl.ELEMENT_ARRAY_BUFFER.
data Specifies the data to be written to the buffer object (typed
array; see the next section).
usage Specifies a hint about how the program is going to use
the data stored in the buffer object. This hint helps WebGL
optimize performance but will not stop your program from
working if you get it wrong.
gl.STATIC_
DRAW
The buffer object data will be specified once
and used many times to draw shapes.
gl.STREAM_
DRAW
The buffer object data will be specified once
and used a few times to draw shapes.
gl.DYNAMIC_
DRAW
The buffer object data will be specified repeat-
edly and used many times to draw shapes.
Return value None
Errors INVALID_ENUM target is none of the preceding constants
Now, let us examine what data is passed to the buffer object using gl.bufferData() .
This method uses the special array vertices mentioned earlier to pass data to the vertex
shader. The array is created at line 51 using the new operator with the data arranged as
<x coordinate and y coordinate of the first vertex>, <x coordinate and y coordinate of the
second vertex>, and so on:
CHAPTER 3 Drawing and Transforming Triangles
78
51 var vertices = new Float32Array([
52 0.0, 0.5, -0.5, -0.5, 0.5, -0.5
53 ]);
54 var n = 3; // The number of vertices
As you can see in the preceding code snippet, you are using the Float32Array object
instead of the more usual JavaScript Array object to store the data. This is because the
standard array in JavaScript is a general-purpose data structure able to hold both numeric
data and strings but isn’t optimized for large quantities of data of the same type, such as
vertices . To address this issue, the typed array, of which one example is Float32Array ,
has been introduced.
Typed Arrays
WebGL often deals with large quantities of data of the same type, such as vertex coordi-
nates and colors, for drawing 3D objects. For optimization purposes, a special type of array
( typed array ) has been introduced for each data type. Because the type of data in the
array is known in advance, it can be handled efficiently.
Float32Array at line 51 is an example of a typed array and is generally used to store
vertex coordinates or colors. It’s important to remember that a typed array is expected
by WebGL and is needed for many operations, such as the second parameter data of
gl.bufferData() .
Table 3.1 shows the different typed arrays available. The third column shows the corre-
sponding data type in C as a reference for those of you familiar with the C language.
Table 3.1 Typed Arrays Used in WebGL
Typed Array Number of Bytes per Element Description (C Types)
Int8Array 1 8-bit signed integer (signed char)
Uint8Array 1 8-bit unsigned integer (unsigned char)
Int16Array 2 16-bit signed integer (signed short)
Uint16Array 2 16-bit unsigned integer (unsigned short)
Int32Array 4 32-bit signed integer (signed int)
Uint32Array 4 32-bit unsigned integer (unsigned int)
Float32Array 4 32-bit floating point number (float)
Float64Array 8 64-bit floating point number (double)
Like JavaScript, these typed arrays have a set of methods, a property, and a constant avail-
able that are shown in Table 3.2 . Note that, unlike the standard Array object in JavaScript,
the methods push() and pop() are not supported.
Drawing Multiple Points
79
Table 3.2 Methods, Property, Constant of Typed Arrays
Methods, Properties, and Constants Description
get(index) Get the index -th element
set(index, value) Set value to the index -th element
set(array, offset) Set the elements of array from offset -th element
length The length of the array
BYTES_PER_ELEMENT The number of bytes per element in the array
Just like standard arrays, the new operator creates a typed array and is passed the array
data. For example, to create Float32Array vertices, you could pass the array [0.0, 0.5,
-0.5, -0.5, 0.5, -0.5] , which represents a set of vertices. Note that the only way to
create a typed array is by using the new operator. Unlike the Array object, the [] operator
is not supported:
51 var vertices = new Float32Array([
52 0.0, 0.5, -0.5, -0.5, 0.5, -0.5
53 ]);
In addition, just like a normal JavaScript array, an empty typed array can be created by
specifying the number of elements of the array as an argument. For example:
var vertices = new Float32Array(4);
With that, you’ve completed the first three steps of the process to set up and use a buffer
(that is, creating a buffer object in the WebGL system, binding the buffer object to a
target, and then writing data into the buffer object). Let’s now look at how to actually use
the buffer, which takes place in steps 4 and 5 of the process.
Assign the Buffer Object to an Attribute Variable
(gl.vertexAttribPointer())
As explained in Chapter 2 , you can use gl.vertexAttrib[1234]f() to assign data to an
attribute variable. However, these methods can only be used to assign a single data value
to an attribute variable. What you need here is a way to assign an array of values—the
vertices in this case—to an attribute variable.
gl.vertexAttribPointer() solves this problem and can be used to assign a buffer object
(actually a reference or handle to the buffer object) to an attribute variable. This can be
seen at line 74 when you assign a buffer object to the attribute variable a_Position :
74 gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 0, 0);
The specification of gl.vertexAttribPointer() is as follows.
CHAPTER 3 Drawing and Transforming Triangles
80
gl.vertexAttribPointer(location, size, type, normalized, stride,
offset)
Assign the buffer object bound to gl.ARRAY_BUFFER to the attribute variable specified by
location .
Parameters location Specifies the storage location of an attribute variable.
size Specifies the number of components per vertex in the buffer object
(valid values are 1 to 4). If size is less than the number of compo-
nents required by the attribute variable, the missing components
are automatically supplied just like gl.vertexAttrib[1234]f() .
For example, if size is 1, the second and third components will be
set to 0, and the fourth component will be set to 1.
type Specifies the data format using one of the following:
gl.UNSIGNED_BYTE unsigned byte for Uint8Array
gl.SHORT signed short integer for Int16Array
gl.UNSIGNED_SHORT unsigned short integer for Uint16Array
gl.INT signed integer for Int32Array
gl.UNSIGNED_INT unsigned integer for Uint32Array
gl.FLOAT floating point number for Float32Array
normalized Either true or false to indicate whether nonfloating data should
be normalized to [0, 1] or [–1, 1].
stride Specifies the number of bytes between different vertex data
elements, or zero for default stride (see Chapter 4 ).
offset Specifies the offset (in bytes) in a buffer object to indicate what
number-th byte the vertex data is stored from. If the data is stored
from the beginning, offset is 0.
Return value None
Errors INVALID_OPERATION There is no current program object.
INVALID_VALUE location is greater than or equal to the maximum
number of attribute variables (8, by default). stride or
offset is a negative value.
Drawing Multiple Points
81
So, after executing this fourth step, the preparations are nearly completed in the WebGL
system for using the buffer object at the attribute variable specified by location . As you can
see in Figure 3.9 , although the buffer object has been assigned to the attribute variable,
WebGL requires a final step to “enable” the assignment and make the final connection.
r e d a h S x e t r e V
t p i r c S a v a J
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
; ) … ( r e t n i o P b i r t A x e t r e v . l g
…
t c e j b O r e f u B
R E F F U B _ Y A R R A . l g
5 . 0 , 0 . 0
5 . 0 - , 5 . 0 -
attribute variable
Figure 3.9 Assign a buffer object to an attribute variable
The fifth and final step is to enable the assignment of the buffer object to the attribute
variable.
Enable the Assignment to an Attribute Variable
(gl.enableVertexAttribArray())
To make it possible to access a buffer object in a vertex shader, we need to enable the
assignment of the buffer object to an attribute variable by using gl.enableVertexAttrib-
Array() as shown in line 77:
77 gl.enableVertexAttribArray(a_Position);
The following shows the specification of gl.enableVertexAttribArray() . Note that we are
using the method to handle a buffer even though the method name suggests it’s only for
use with “vertex arrays.” This is not a problem and is simply a legacy from OpenGL.
gl.enableVertexAttribArray(location)
Enable the assignment of a buffer object to the attribute variable specified by location .
Parameters location Specifies the storage location of an attribute variable.
Return value None
Errors INVALID_VALUE location is greater than or equal to the maximum number
of attribute variables (8 by default).
When you execute gl.enableVertexAttribArray() specifying an attribute variable that
has been assigned a buffer object, the assignment is enabled, and the unconnected line is
connected as shown in Figure 3.10 .
CHAPTER 3 Drawing and Transforming Triangles
82
r e d a h S x e t r e V { ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
…
t c e j b O r e f u B
R E F F U B _ Y A R R A . l g
5 . 0 , 0 . 0
5 . 0 - , 5 . 0 -
t p i r c S a v a J
gl.vertexAttribArray(...);
attribute variable
Figure 3.10 Enable the assignment of a buffer object to an attribute variable
You can also break this assignment (disable it) using the method
gl.disableVertexAttribArray() .
gl.disableVertexAttribArray(location)
Disable the assignment of a buffer object to the attribute variable specified by location .
Parameters location Specifies the storage location of an attribute variable.
Return Value None
Errors INVALID_VALUE location is greater than or equal to the maximum number
of attribute variables (8 by default).
Now, everything is set! All you need to do is run the vertex shader, which draws the
points using the vertex coordinates specified in the buffer object. As in Chapter 2 , you
will use the method gl.drawArrays , but because you are drawing multiple points, you will
actually use the second and third parameters of gl.drawArrays() .
Note that after enabling the assignment, you can no longer use gl.vertexAttrib[1234]
f() to assign data to the attribute variable. You have to explicitly disable the assignment
of a buffer object. You can’t use both methods simultaneously.
The Second and Third Parameters of gl.drawArrays()
Before entering into a detailed explanation of these parameters, let’s take a look at the
specification of gl.drawArrays() that was introduced in Chapter 2 . Following is a recap of
the method with only the relevant parts of the specification shown.
Drawing Multiple Points
83
gl.drawArrays(mode, first, count)
Execute a vertex shader to draw shapes specified by the mode parameter.
Parameters mode Specifies the type of shape to be drawn. The following symbolic
constants are accepted: gl.POINTS , gl.LINES , gl.LINE_STRIP , gl.
LINE_LOOP , gl.TRIANGLES , gl.TRIANGLE_STRIP , and gl.TRIANGLE_
FAN .
first Specifies what number-th vertex is used to draw from (integer).
count Specifies the number of vertices to be used (integer).
In the sample program this method is used as follows:
47 gl.drawArrays(gl.POINTS, 0, n); // n is 3
As in the previous examples, because you are simply drawing three points, the first param-
eter is still gl.POINTS . The second parameter first is set to 0 because you want to draw
from the first coordinate in the buffer. The third parameter count is set to 3 because you
want to draw three points (in line 47, n is 3).
When your program runs line 47, it actually causes the vertex shader to be executed count
(three) times, sequentially passing the vertex coordinates stored in the buffer object via
the attribute variable into the shader ( Figure 3.11 ).
Note that for each execution of the vertex shader, 0.0 and 1.0 are automatically supplied
to the z and w components of a_Position because a_Position requires four components
( vec4 ) and you are supplying only two.
Remember that at line 74, the second parameter size of g l.vertexAttribPointer() is set
to 2. As just discussed, the second parameter indicates how many coordinates per vertex
are specified in the buffer object and, because you are only specifying the x and y coordi-
nates in the buffer, you set the size value to 2:
74 gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 0, 0);
After drawing all points, the content of the color buffer is automatically displayed in the
browser (bottom of Figure 3.11 ), resulting in our three red points, as shown in Figure 3.2 .
CHAPTER 3 Drawing and Transforming Triangles
84
Experimenting with the Sample Program
Let’s experiment with the sample program to better understand how gl.drawArrays()
works by modifying the second and third parameters. First, let’s specify 1 as the third
argument for count at line 47 instead of our variable n (set to 3) as follows:
47 gl.drawArrays(gl.POINTS, 0, 1 );
In this case, the vertex shader is executed only once, and a single point is drawn using the
first vertex in the buffer object.
If you now specify 1 as the second argument, only the second vertex is used to draw
a point. This is because you are telling WebGL that you want to start drawing from
the second vertex and you only want to draw one vertex. So again, you will see only a
single point, although this time it is the second vertex coordinates that are shown in the
browser:
47 gl.drawArrays(gl.POINTS, 1 , 1);
Vertex Shader
JavaScript
function main() {
…
var gl=getWebGL…
…
gl.drawArrays( …, 0, n);
gl.ARRAY_BUFFER
attribute a_Position;
Buffer Object
0.0, 0.5
-0.5, -0.5
0.5, -0.5
Fragment
Shader
gl_Position = a_Position;
(0.0, 0.5, 0.0, 1.0)
gl_FragColor =
vec4(...);
Vertex Shader
attribute a_Position;
JavaScript
function main() {
…
var gl=getWebGL…
…
gl.drawArrays( …, 0, n);
Buffer Object
gl.ARRAY_BUFFER
0.0, 0.5
-0.5, -0.5
0.5, -0.5
Fragment
Shader
gl_Position = a_Position;
(-0.5, -0.5, 0.0, 1.0)
gl_FragColor =
vec4(...);
Vertex Shader
attribute a_Position;
JavaScript
function main() {
…
var gl=getWebGL…
…
gl.drawArrays(…, 0, n);
Buffer Object
gl.ARRAY_BUFFER
0.0, 0.5
-0.5, -0.5
0.5, -0.5
Fragment
Shader
gl_Position = a_Position;
(0.5, -0.5, 0.0, 1.0)
gl_FragColor =
vec4(...);
First Execution
Second Execution
Third Execution
Color Buffer
Color Buffer
Color Buffer
Browser
Figure 3.11 How the data in a buffer object is passed to a vertex shader during execution
Hello Triangle
85
This gives you a quick feel for the role of the parameters first and count . However, what
will be happen if you change the first parameter mode ? The next section explores the first
parameter in more detail.
Hello Triangle
Now that you’ve learned the basic techniques to pass multiple vertex coordinates to a
vertex shader, let’s try to draw other shapes using multiple vertex coordinates. This section
uses a sample program HelloTriangle , which draws a single 2D triangle. Figure 3.12
shows a screenshot of HelloTriangle .
Figure 3.12 HelloTriangle
Sample Program (HelloTriangle.js)
Listing 3.3 shows HelloTriangle.js , which is almost identical to MultiPoint.js used in
the previous section with two critical differences.
Listing 3.3 HelloTriangle.js
1 // HelloTriangle.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'void main() {\n' +
6 ' gl_Position = a_Position;\n' +
7 '}\n';
8
CHAPTER 3 Drawing and Transforming Triangles
86
9 // Fragment shader program
10 var FSHADER_SOURCE =
11 'void main() {\n' +
12 ' gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);\n' +
13 '}\n';
14
15 function main() {
...
19 // Get the rendering context for WebGL
20 var gl = getWebGLContext(canvas);
...
26 // Initialize shaders
27 if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
...
30 }
31
32 // Set the positions of vertices
33 var n = initVertexBuffers(gl);
...
39 // Set the color for clearing <canvas>
...
45 // Draw a triangle
46 gl.drawArrays(gl.TRIANGLES, 0, n);
47 }
48
49 function initVertexBuffers(gl) {
50 var vertices = new Float32Array([
51 0.0, 0.5, -0.5, -0.5, 0.5, -0.5
52 ]);
53 var n = 3; // The number of vertices
...
78 return n;
79 }
The two differences from MultiPoint.js are
• The line to specify the size of a point gl_PointSize = 10.0 ; has been removed from
the vertex shader. This line only has an effect when you are drawing a point.
• The first parameter of gl.drawArrays() has been changed from gl.POINTS to
gl.TRIANGLES at line 46.
The first parameter, mode, of gl.drawArrays() is powerful and provides the ability to draw
various shapes. Let’s take a look.
Hello Triangle
87
Basic Shapes
By changing the argument we use for the first parameter, mode, of gl.drawArrays() , we
can change the meaning of line 46 into “execute the vertex shader three times ( n is 3),
and draw a triangle using the three vertices in the buffer, starting from the first vertex
coordinate”:
46 gl.drawArrays(gl.TRIANGLES, 0, n);
In this case, the three vertices in the buffer object are no longer individual points, but
become three vertices of a triangle.
The WebGL method gl.drawArrays() is both powerful and flexible, allowing you to
specify seven different types of basic shapes as the first argument. These are explained in
more detail in Table 3.3 . Note that v0, v1, v2 ... indicates the vertices specified in a buffer
object. The order of vertices affects the drawing of the shape.
The shapes in the table are the only ones that WebGL can draw directly, but they are the
basics needed to construct complex 3D graphics. (Remember the frog at the start of this
chapter.)
Table 3.3 Basic Shapes Available in WebGL
Basic
Shape Mode Description
Points gl.POINTS A series of points. The points are drawn at v0, v1, v2 ...
Line
segments
gl.LINES A series of unconnected line segments. The individual lines
are drawn between vertices given by (v0, v1), (v2, v3), (v4,
v5)... If the number of vertices is odd, the last one is ignored.
Line strips gl.LINE_STRIP A series of connected line segments. The line segments are
drawn between vertices given by (v0, v1), (v1, v2), (v2, v3), ...
The first vertex becomes the start point of the first line, the
second vertex becomes the end point of the first line and the
start point of the second line, and so on. The i -th ( i > 1) vertex
becomes the start point of the i -th line and the end point of
the i-1 -th line. (The last vertex becomes the end point of the
last line.)
Line loops gl.LINE_LOOP A series of connected line segments. In addition to the lines
drawn by gl.LINE_STRIP , the line between the last vertex and
the first vertex is drawn. The line segments drawn are (v0, v1),
(v1, v2), ..., and (v n , v0). v n is the last vertex.
Triangles gl.TRIANGLES A series of separate triangles. The triangles given by vertices
(v0, v1, v2), (v3, v4, v5), ... are drawn. If the number of verti-
ces is not a multiple of 3, the remaining vertices are ignored.
CHAPTER 3 Drawing and Transforming Triangles
88
Basic
Shape Mode Description
Triangle
strips
gl.TRIANGLE_
STRIP
A series of connected triangles in strip fashion. The first three
vertices form the first triangle and the second triangle is
formed from the next vertex and one of the sides of the first
triangle. The triangles are drawn given by (v0, v1, v2), (v2, v1,
v3), (v2, v3, v4) ... (Pay attention to the order of vertices.)
Triangle
fans
gl.TRIANGLE_
FAN
A series of connected triangles sharing the first vertex in fan-
like fashion. The first three vertices form the first triangle and
the second triangle is formed from the next vertex, one of the
sides of the first triangle, and the first vertex. The triangles are
drawn given by (v0, v1, v2), (v0, v2, v3), (v0, v3, v4), ...
Figure 3.13 shows these basic shapes.
0 v
1 v
2 v
3 v
4 v
5 v
0 v
1 v
2 v
3 v
4 v
5 v
0 v
1 v
2 v
3 v
4 v
5 v
0 v
1 v
2 v
3 v
4 v
5 v
0 v
1 v
2 v
3 v
4 v
5 v
0 v
1 v
2 v
3 v
5 v
4 v
5 v
4 v
0 v
3 v
1 v
2 v
S T N I O P . l g
P O O L _ E N I L . l g P I R T S _ E N I L . l g S E N I L . l g
N A F _ E L G N A I R T . l g P I R T S _ E L G N A I R T . l g S E L G N A I R T . l g
Figure 3.13 Basic shapes available in WebGL
As you can see from the figure, WebGL can draw only three types of shapes: a point, a
line, and a triangle. However, as explained at the beginning of this chapter, spheres to
cubes to 3D monsters to humanoid characters in a game can be constructed from small
triangles. Therefore, you can use these basic shapes to draw anything.
Hello Triangle
89
Experimenting with the Sample Program
To examine what will happen when using gl.LINES , gl.LINE_STRIP , and gl.LINE_LOOP ,
let’s change the first argument of gl.drawArrays( ) as shown next. The name of each
sample program is HelloTriangle _ LINES , HelloTriangle_LINE_STRIP , and HelloTriangle_
LINE_LOOP , respectively:
46 gl.drawArrays( gl.LINES , 0, n);
46 gl.drawArrays( gl.LINE_STRIP , 0, n);
46 gl.drawArrays( gl.LINE_LOOP , 0, n);
Figure 3.14 shows a screenshot of each program.
Figure 3.14 gl.LINES, gl.LINE_STRIP, and gl.LINE_LOOP
As you can see, gl.LINES draws a line using the first two vertices and does not use the
last vertex, whereas gl.LINE_STRIP draws two lines using the first three vertices. Finally,
gl.LINE_LOOP draws the lines in the same manner as gl.LINE_STRIP but then “loops”
between the last vertex and the first vertex and makes a triangle.
Hello Rectangle (HelloQuad)
Let’s use this basic way of drawing triangles to draw a rectangle. The name of the sample
program is HelloQuad , and Figure 3.15 shows a screenshot when it’s loaded into your
browser.
Figure 3.16 shows the vertices of the rectangle. Of course, the number of vertices is four
because it is a rectangle. As explained in the previous section, WebGL cannot draw a rect-
angle directly, so you need to divide the rectangle into two triangles (v0, v1, v2) and (v2,
v1, v3) and then draw each one using gl.TRIANGLES , gl.TRIANGLE_STRIP , or gl.TRIANGLE_
FAN . In this example, you’ll use gl.TRIANGLE_STRIP because it only requires you to specify
four vertices. If you were to use gl.TRIANGLES , you would need to specify a total of six.
CHAPTER 3 Drawing and Transforming Triangles
90
y
x
3 v
) 0 . 0 , 5 . 0 - , 5 . 0 (
0 v
) 0 . 0 , 5 . 0 , 5 . 0 - (
2 v
) 0 . 0 , 5 . 0 , 5 . 0 (
1 v
) 0 . 0 , 5 . 0 - , 5 . 0 - (
Figure 3.16 The four vertex coordinates of the rectangle
Basing the example on HelloTriangle.js , you need to add an extra vertex coordinate
at line 50. Pay attention to the order of vertices; otherwise, the draw command will not
execute correctly:
50 var vertices = new Float32Array([
51 -0.5, 0.5, -0.5, -0.5, 0.5, 0.5, 0.5, -0.5
52 ]);
Because you’ve added a fourth vertex, you need to change the number of vertices from 3
to 4 at line 53:
53 var n = 4; // The number of vertices
Figure 3.15 HelloQuad
Moving, Rotating, and Scaling
91
Then, by modifying line 46 as follows, your program will draw a rectangle in the browser:
46 gl.drawArrays( gl.TRIANGLE_STRIP , 0, n);
Experimenting with the Sample Program
Now that you have a feel for how to use gl.TRIANGLE_STRIP , let’s change the first
parameter of gl.drawArrays() to gl.TRIANGLE_FAN . The name of the sample program is
HelloQuad_FAN :
46 gl.drawArrays( gl.TRIANGLE_FAN , 0, n);
Figure 3.17 show a screenshot of HelloQuad_FAN . In this case, we can see the ribbon-like
shape on the screen.
0 v
1 v
2 v
3 v
P I R T S _ E L G N A I R T . l g
0 v
1 v
2 v
3 v
N A F _ E L G N A I R T . l g
Figure 3.17 HelloQuad_FAN
Looking at the order of vertices and the triangles drawn by gl.TRIANGLE_FAN shown on
the right side of Figure 3.17 , you can see why the result became a ribbon-like shape.
Essentially, gl.TRIANGLE_FAN causes WebGL to draw a second triangle that shares the first
vertex (v0), and this second triangle overlaps the first, creating the ribbon-like effect.
Moving, Rotating, and Scaling
Now that you understand the basics of drawing shapes like triangles and rectangles, let’s
take another step and try to move (translate), rotate, and scale the triangle and display the
results on the screen. These operations are called transformations (affine transforma-
tions) . This section introduces some math to explain each transformation and help you
to understand how each operation can be realized. However, when you write your own
programs, you don’t need the math; instead, you can use one of several convenient librar-
ies, explained in the next section, that handle the math for you.
CHAPTER 3 Drawing and Transforming Triangles
92
If you find reading this section and in particular the math too much on first read, it’s
okay to skip it and return later. Or, if you already know that transformations can be
written using a matrix, you can skip this section as well.
First, let’s write a sample program, TranslatedTriangle , that moves a triangle 0.5 units
to the right and 0.5 units up. You can use the triangle you drew in the previous section.
The right direction means the positive direction of the x-axis, and the up direction means
the positive direction of the y-axis. (See the coordinate system in Chapter 2 .) Figure 3.18
shows TranslatedTriangle .
Figure 3.18 TranslatedTriangle
Translation
Let us examine what kind of operations you need to apply to each vertex coordinate
of a shape to translate (move) the shape. Essentially, you just need to add a translation
distance for each direction (x and y) to each component of the coordinates. Looking at
Figure 3.19 , the goal is to translate the point p (x, y, z) to the point p' (x', y', z'), so the
translation distance for the x, y, and z direction is Tx , Ty , and Tz , respectively. In this
figure, Tz is 0.
To determine the coordinates of p', you simply add the T values, as shown in
Equation 3.1 .
Equation 3.1
x' = x + Tx
y' = y + Ty
z' = z + Tz
Moving, Rotating, and Scaling
93
These simple equations can be implemented in a WebGL program just by adding each
constant value to each vertex coordinate. You’ve probably realized already that because
they are a per-vertex operation , you need to implement the operations in a vertex
shader. Conversely, they clearly aren’t a per-fragment operation, so you don’t need to
worry about the fragment shader.
Once you understand this explanation, implementation is easy. You need to pass the
translation distances Tx , Ty , and Tz to the vertex shader, apply Equation 3.1 using the
distances, and then assign the result to gl_Position . Let’s look at a sample program that
does this.
Sample Program (TranslatedTriangle.js)
Listing 3.4 shows TranslatedTriangle.js , in which the vertex shader is partially modi-
fied to carry out the translation operation. However, the fragment shader is the same as
in HelloTriangle.js in the previous section. To support the modification to the vertex
shader, some extra code is added to the main() function in the JavaScript.
Listing 3.4 TranslatedTriangle.js
1 // TranslatedTriangle.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'uniform vec4 u_Translation;\n' +
6 'void main() {\n' +
7 ' gl_Position = a_Position + u_Translation;\n' +
8 '}\n';
9
10 // Fragment shader program
...
x
y
y T
x T
) ' z , ' y , ' x ( ' p
) z , y , x ( p
Figure 3.19 Calculating translation distances
CHAPTER 3 Drawing and Transforming Triangles
94
16 // The translation distance for x, y, and z direction
17 var Tx = 0.5, Ty = 0.5, Tz = 0.0;
18
19 function main() {
...
23 // Get the rendering context for WebGL
24 var gl = getWebGLContext(canvas);
...
30 // Initialize shaders
31 if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
...
34 }
35
36 // Set the positions of vertices
37 var n = initVertexBuffers(gl);
...
43 // Pass the translation distance to the vertex shader
44 var u_Translation = gl.getUniformLocation(gl.program, 'u_Translation');
...
49 gl.uniform4f(u_Translation, Tx, Ty, Tz, 0.0);
50
51 // Set the color for clearing <canvas>
...
57 // Draw a triangle
58 gl.drawArrays(gl.TRIANGLES, 0, n);
59 }
60
61 function initVertexBuffers(gl) {
62 var vertices = new Float32Array([
63 0.0.0, 0.5, -0.5, -0.5, 0.5, -0.5
64 ]);
65 var n = 3; // The number of vertices
...
90 return n;
93 }
First, let’s examine main() in JavaScript. Line 17 defines the variables for each translation
distance of Equation 3.1 :
17 var Tx = 0.5, Ty = 0.5, Tz = 0.0;
Because Tx , Ty , and Tz are fixed (uniform) values for all vertices, you use the uniform vari-
able u_Translation to pass them to a vertex shader. Line 44 retrieves the storage location
of the uniform variable, and line 49 assigns the data to the variable:
Moving, Rotating, and Scaling
95
44 var u_Translation = gl.getUniformLocation(gl.program, 'u_Translation');
...
49 gl.uniform4f(u_Translation, Tx, Ty, Tz, 0.0);
Note that gl.uniform4f() requires a homogenous coordinate, so we supply a fourth argu-
ment (w) of 0.0. This will be explained in more detail later in this section.
Now, let’s take a look at the vertex shader that uses this translation data. As you can see,
the uniform variable u_Translation in the shader, to which the translation distances
are passed, is defined as type vec4 at line 5. This is because you want to add the compo-
nents of u_Translation to the vertex coordinates passed to a_Position (as defined by
Equation 3.1 ) and then assign the result to the variable gl_Position , which has type
vec4 . Remember, per Chapter 2 , that the assignment operation in GLSL ES is only allowed
between variables of the same types:
4 'attribute vec4 a_Position;\n' +
5 'uniform vec4 u_Translation;\n' +
6 'void main() {\n' +
7 ' gl_Position = a_Position + u_Translation;\n' +
8 '}\n';
After these preparations have been completed, the rest of tasks are straightforward. To
calculate Equation 3.1 within the vertex shader, you just add each translation distance
(Tx, Ty, Tz) passed in u_Translation to each vertex coordinate (x, y, z) passed in
a_Position .
Because both variables are of type vec4 , you can use the + operator, which will actually
add the four components simultaneously (see Figure 3.20 ). This easy addition of vectors is
a feature of GLSL ES and will be explained in more detail in Chapter 6 .
n o i t i s o P _ a 4 c e v
n o i t a l s n a r T _ u 4 c e v
x1 y1 z1 w1
x1+x2 y1+y2 z1+z2 w1+w2
x2 y2 z2 w2
Figure 3.20 Addition of vec4 variables
Now, we’ll return to the fourth element, (w), of the vector. As explained in Chapter 2 , you
need to specify the homogeneous coordinate to gl_Position , which is a four-dimensional
coordinate. If the last component of the homogeneous coordinate is 1.0, the coordinate
indicates the same position as the three-dimensional coordinate. In this case, because the
last component is w1+w2 to ensure that w1+w2 is 1.0, you need to specify 0.0 to the value of
w (the fourth parameter of gl.uniform4f() ).
CHAPTER 3 Drawing and Transforming Triangles
96
Finally, at line 58, gl.drawArrays(gl.TRIANGLES, 0, n) executes the vertex shader. For
each execution, the following three steps are performed:
1. Each vertex coordinate set is passed to a_Position .
2. u_Translation is added to a_Position .
3. The result is assigned to gl_Position .
Once executed, you’ve achieved your goal because each vertex coordinate set is modi-
fied (translated), and then the translated shape (in this case, a triangle) is displayed on
the screen. If you now load TranslatedTriangle.html into your browser, you will see the
translated triangle.
Now that you’ve mastered translation (moving), the next step is to look at rotation. The
basic approach to realize rotation is the same as translation, requiring you to manipulate
the vertex coordinates in the vertex shader.
Rotation
Rotation is a little more complex than translation because you have to specify multiple
items of information. The following three items are required:
• Rotation axis (the axis the shape will be rotated around)
• Rotation direction (the direction: clockwise or counterclockwise)
• Rotation angle (the number of degrees the shape will be rotated through)
In this section, to simplify the explanation, you can assume that the rotation is performed
around the z -axis, in a counterclockwise direction, and for β degrees. You can use the
same approach to implement other rotations around the x-axis or y-axis.
In the rotation, if β is positive, the rotation is performed in a counterclockwise direction
around the rotation axis looking at the shape toward the negative direction of the z-axis
(see Figure 3.21 ); this is called positive rotation . Just as for the coordinate system, your
hand can define the direction of rotation. If you take your right hand and have your
thumb follow the direction of the rotation axis, your fingers show the direction of rota-
tion. This is called the right-hand-rule rotation . As we discussed in Chapter 2 , it’s the
default we are using for WebGL in this book.
Now let’s find the expression to calculate the rotation in the same way that you did for
translation. As shown in Figure 3.22 , we assume that the point p' (x', y', z') is the β degree
rotated point of p (x, y, z) around the z-axis. Because the rotation is around the z-axis, the
z coordinate does not change, and you can ignore it for now. The explanation is a little
mathematical, so let’s take it a step at a time.
Moving, Rotating, and Scaling
97
x
y
α
β
ｒ
) ' z , ' y , ' x ( ' p
) z , y , x ( p
Figure 3.22 Calculating rotation around the z-axis
In Figure 3.22 , r is the distance from the origin to the point p , and α is the rotation angle
from the x-axis to the point. You can use these items of information to represent the coor-
dinates of p , as shown in Equation 3.2 .
Equation 3.2
x = r cos α
y = r sin α
Similarly, you can find the coordinate of p' by using r , α, and β as follows:
x' = r cos (α + β )
y' = r sin (α + β )
x
y
z
Screen
Figure 3.21 Positive rotation around the z-axis
CHAPTER 3 Drawing and Transforming Triangles
98
Then you can use the addition theorem of trigonometric functions
1 to get the following:
x' = r (cos α cos β – sin α sin β)
y' = r (sin α cos β + cos α sin β)
Finally, you get the following expressions ( Equation 3.3 ) by assigning Equation 3.2 to the
previous expressions and removing r and α.
Equation 3.3
x' = x cos β – y sin β
y' = x sin β + y cos β
z' = z
So by passing the values of sin β and cos β to the vertex shader and then calculating
Equation 3.3 in the shader, you get the coordinates of the rotated point. To calculate sin β
and cos β, you can use the methods of the JavaScript Math object.
Let’s look at a sample program, RotatedTriangle , which rotates a triangle around the
z-axis, in a counterclockwise direction, by 90 degrees. Figure 3.23 shows RotatedTriangle .
Figure 3.23 RotatedTriangle
1 sin(a ± b) = sin a cos b ∓ cos a sin b
cos(a ± b) = cos a cos b ∓ sin a sin b
Moving, Rotating, and Scaling
99
Sample Program (RotatedTriangle.js)
Listing 3.5 shows RotatedTriangle.js which, in a similar manner to
TranslatedTriangle . js , modifies the vertex shader to carry out the rotation operation.
The fragment shader is the same as in TranslatedTriangle.js and, as usual, is not shown.
Again, to support the shader modification, several processing steps are added to main() in
the JavaScript program. Additionally, Equation 3.3 is added in the comments from lines 4
to 6 to remind you of the calculation needed.
Listing 3.5 RotatedTriangle.js
1 // RotatedTriangle.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 // x' = x cos b - y sin b
5 // y' = x sin b + y cos b Equation 3.3
6 // z' = z
7 'attribute vec4 a_Position;\n' +
8 'uniform float u_CosB, u_SinB;\n' +
9 'void main() {\n' +
10 ' gl_Position.x = a_Position.x * u_CosB - a_Position.y *u_SinB;\n'+
11 ' gl_Position.y = a_Position.x * u_SinB + a_Position.y * u_CosB;\n'+
12 ' gl_Position.z = a_Position.z;\n' +
13 ' gl_Position.w = 1.0;\n' +
14 '}\n';
15
16 // Fragment shader program
...
22 // Rotation angle
23 var ANGLE = 90.0;
24
25 function main() {
...
42 // Set the positions of vertices
43 var n = initVertexBuffers(gl);
...
49 // Pass the data required to rotate the shape to the vertex shader
50 var radian = Math.PI * ANGLE / 180.0; // Convert to radians
51 var cosB = Math.cos(radian);
52 var sinB = Math.sin(radian);
53
54 var u_CosB = gl.getUniformLocation(gl.program, 'u_CosB');
55 var u_SinB = gl.getUniformLocation(gl.program, 'u_SinB');
...
CHAPTER 3 Drawing and Transforming Triangles
100
60 gl.uniform1f(u_CosB, cosB);
61 gl.uniform1f(u_SinB, sinB);
62
63 // Set the color for clearing <canvas>
...
69 // Draw a triangle
70 gl.drawArrays(gl.TRIANGLES, 0, n);
71 }
72
73 function initVertexBuffers(gl) {
74 var vertices = new Float32Array([
75 0.0, 0.5, -0.5, -0.5, 0.5, -0.5
76 ]);
77 var n = 3; // The number of vertices
...
105 return n;
106 }
Let’s look at the vertex shader, which is straightforward:
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 // x' = x cos b - y sin b
5 // y' = x sin b + y cos b
6 // z' = z
7 'attribute vec4 a_Position;\n' +
8 'uniform float u_CosB, u_SinB;\n' +
9 'void main() {\n' +
10 ' gl_Position.x = a_Position.x * u_CosB - a_Position.y * u_SinB;\n'+
11 ' gl_Position.y = a_Position.x * u_SinB + a_Position.y * u_CosB;\n'+
12 ' gl_Position.z = a_Position.z;\n' +
13 ' gl_Position.w = 1.0;\n' +
14 '}\n';
Because the goal is to rotate the triangle by 90 degrees, the sine and cosine of 90 need to
be calculated. Line 8 defines two uniform variables for receiving these values, which are
calculated in the JavaScript program and then passed to the vertex shader.
You could pass the rotation angle to the vertex shader and then calculate the values of
sine and cosine in the shader. However, because they are identical for all vertices, it is
more efficient to do it once in the JavaScript.
The name of these uniform variables, u_CosB and u_SinB , are defined following the
naming rule used throughout this book. As you will remember, you use the uniform vari-
able because the values of these variables are uniform (unchanging) per vertex.
Moving, Rotating, and Scaling
101
As in the previous sample programs, x, y, z, and w are passed in a group to the attribute
variable a_Position in the vertex shader. To apply Equation 3.3 to x, y, and z, you need
to access each component in a_Position separately. You can do this easily using the .
operator, such as a_Position.x , a_Position.y , and a_Position.z (see Figure 3.24 and
Chapter 6 ).
n o i t i s o P _ a 4 c e v
w . n o i t i s o P _ a z . n o i t i s o P _ a y . n o i t i s o P _ a x . n o i t i s o P _ a
x y z w
Figure 3.24 Access methods for each component in a vec4
Handily, you can use the same operator to access each component in gl_Position to
which the vertex coordinate is written, so you can calculate x' = x cos β – y sin β from
Equation 3.3 as shown at line 10:
10 ' gl_Position.x = a_Position.x * u_CosB - a_Position.y * u_SinB;\n'+
Similarly, you can calculate y' as follows:
11 ' gl_Position.y = a_Position.x * u_SinB + a_Position.y * u_CosB;\n'+
According to Equation 3.3 , you just need to assign the original z coordinate to z' directly
at line 12. Finally, you need to assign 1.0 to the last component w
2 :
12 ' gl_Position.z = a_Position.z;\n' +
13 ' gl_Position.w = 1.0;\n' +
Now look at main() in the JavaScript code, which starts from line 25. This code is mostly
the same as in TranslatedTriangle.js . The only difference is passing cos β and sin β to
the vertex shader. To calculate the sine and cosine of β, you can use the JavaScript Math.
sin() and Math.cos() methods. However, these methods expect parameters in radians,
not degrees, so you need to convert from degrees to radians by multiplying the number
of degrees by pi and then dividing by 180. You can utilize Math.PI as the value of pi as
shown at line 50, where the variable ANGLE is defined as 90 (degrees) at line 23:
50 var radian = Math.PI * ANGLE / 180.0; // Converts degrees to radians
2 In this program, you can also write gl_Position.w = a_Position.w; because a_Position.w
is 1.0.
CHAPTER 3 Drawing and Transforming Triangles
102
Once you have the angle in radians, lines 51 and 52 calculate cos β and sin β, and then
lines 60 and 61 pass them to the uniform variables in the vertex shader:
51 var cosB = Math.cos(radian);
52 var sinB = Math.sin(radian);
53
54 var u_CosB = gl.getUniformLocation(gl.program, 'u_CosB');
55 var u_SinB = gl.getUniformLocation(gl.program, 'u_SinB');
...
60 gl.uniform1f(u_CosB, cosB);
61 gl.uniform1f(u_SinB, sinB);
When you load this program into your browser, you can see the triangle, rotated through
90 degrees, on the screen. If you specify a negative value to ANGLE , you can rotate the
triangle in the opposite direction (clockwise). You can also use the same equation. For
example, to rotate the triangle in the clockwise direction, you can specify –90 instead of
90 at line 23, and Math.cos() and Math.sin() will deal with the remaining tasks for you.
For those of you concerned with speed and efficiency, the approach taken here (using two
uniform variables to pass the values of cos β and sin β) isn’t optimal. To pass the values as
a group, you can define the uniform variable as follows:
uniform vec2 u_CosBSinB ;
and then pass the values by:
gl.uniform2f( u_CosBSinB,cosB, sinB );
Then in the vertex shader, you can access them using u_CosBSinB.x and u_CosBSinB.y .
Transformation Matrix: Rotation
For simple transformations, you can use mathematical expressions. However, as your
needs become more complex, you’ll quickly find that applying a series of equations
becomes quite complex. For example a “translation after rotation” as shown in Figure 3.25
can be realized by using Equations 3.1 and 3.3 to find the new mathematical expressions
for the transformation and then implementing them in a vertex shader.
x
) 1 (
) 2 (
Figure 3.25 Rotate first and then translate a triangle
Moving, Rotating, and Scaling
103
However, it is time consuming to determine the mathematical expressions every time
you need a new set of transformation and then implement them in a vertex shader.
Fortunately, there is another tool in the mathematical toolbox, the transformation
matrix , which is excellent for manipulating computer graphics.
As shown in Figure 3.26 , a matrix is a rectangular array of numbers arranged in rows (in
the horizontal direction) and columns (in the vertical direction). This notation makes it
easy to write the calculations explained in the previous sections. The brackets indicate
that these numbers are a group.
8 3 0
4 3 6
3 2 6
⎡ ⎤
⎢ ⎥
⎢ ⎥
⎢ ⎥
⎣ ⎦
Figure 3.26 Example of a matrix
Before explaining the details of how to use a transformation matrix to replace the equa-
tions used here, you need to make sure you understand the multiplication of a matrix and
a vector. A vector is an object represented by an n-tuple of numbers, such as the vertex
coordinates (0.0, 0.5, 1.0).
The multiplication of a matrix and a vector can be written as shown in Equation 3.4 .
(Although the multiply operator × is often omitted, we explicitly write the operator in this
book for clarity.) Here, our new vector (on the left) is the result of multiplying a matrix
(in the center) by our original vector (on the right). Note that matrix multiplication is
noncommutative. In other words, A × B is not the same as B × A. We discuss this further
in Chapter 6 .
Equation 3.4
'
'
'
x a b c x
y d e f y
z g h i z
⎡ ⎤ ⎡ ⎤ ⎡ ⎤
⎢ ⎥ ⎢ ⎥ ⎢ ⎥
= ×
⎢ ⎥ ⎢ ⎥ ⎢ ⎥
⎢ ⎥ ⎢ ⎥ ⎢ ⎥
⎣ ⎦ ⎣ ⎦ ⎣ ⎦
This matrix has three rows and three columns and is called a 3×3 matrix. The rightmost
part of the equation is a vector composed of x, y, and z. (In the case of a multiplication
of a matrix and vector, the vector is written vertically, but it has the same meaning as
when it is written horizontally.) This vector has three elements, so it is called a three-
dimensional vector. Again, the brackets on both sides of the array of numbers (vector)
are also just notation for recognizing that these numbers are a group.
In this case, x', y', and z' are defined using the elements of the matrix and the vector,
as shown by Equation 3.5 . Note that the multiplication of a matrix and vector can be
CHAPTER 3 Drawing and Transforming Triangles
104
defined only if the number of columns in a matrix matches the number of rows in a
vector.
Equation 3.5
x' = ax + by + cz
y' = dx + ey + fz
z' = gx + hy + iz
Now, to understand how to use a matrix instead of our original equations, let’s compare
the matrix equations and Equation 3.3 (shown again as Equation 3.6 ).
Equation 3.6
x' = x cos β – y sin β
y' = x sin β + y cos β
z' = z
For example, compare the equation for x':
x' = ax + by + cz
x' = x cos β – y sin β
In this case, if you set a = cos β, b = -sin β, and c = 0, the equations become the same.
Similarly, let us compare the equation for y':
y' = dx + ey + fz
y' = x sin β + y cos β
In this case, if you set d = sin β, e = cos β, and f = 0, you get the same equation. The last
equation about z' is easy. If you set g = 0, h = 0, and i = 1, you get the same equation.
Then, by assigning these results to Equation 3.4 , you get Equation 3.7 .
Equation 3.7
β β
β β
⎡
⎣
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
=
−
⎡
⎣
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
×
⎡
⎣
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
x
y
z
x
y
z
'
'
'
cos sin 0
sin cos 0
0 0 1
This matrix is called a transformation matrix because it “transforms” the right-side
vector (x, y, z) to the left-side vector (x', y', z'). The transformation matrix representing a
rotation is called a rotation matrix .
Moving, Rotating, and Scaling
105
You can see that the elements of the matrix in Equation 3.7 are an array of coefficients in
Equation 3.6 . Once you become accustomed to matrix notation, it is easier to write and
use matrices than to have to deal with a set of transformation equations.
As you would expect, because matrices are used so often in 3DCG, multiplication of a
matrix and a vector is easy to implement in shaders. However, before exploring how, let’s
quickly look at other types of transformation matrices, and then we will start to use them
in shaders.
Transformation Matrix: Translation
Obviously, if we can use a transformation matrix to represent a rotation, we should be
able to use it for other types of transformation, such as translation. For example, let us
compare the equation for x' in Equation 3.1 to that in Equation 3.5 as follows:
x' = ax + by + cz - - - from Equation (3.5)
x' = x + T
x
- - - from Equation (3.1)
Here, the second equation has the constant term T
x , but the first one does not, meaning
that you cannot deal with the second one by using the 3×3 matrix of the first equation.
To solve this problem, you can use a 4×4 matrix and the fourth components of the coor-
dinate, which are set to 1 to introduce the constant terms. That is to say, we assume that
the coordinates of point p are (x, y, z, 1), and the coordinates of the translated point p (p')
are (x', y', z', 1). This gives us Equation 3.8 .
Equation 3.8
⎡
⎣
⎢
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
⎥
=
⎡
⎣
⎢
⎢
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
⎥
⎥
×
⎡
⎣
⎢
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
⎥
x
y
z
a b c d
e f g h
i j k l
m n o p
x
y
z
'
'
'
1 1
This multiplication is defined as follows:
Equation 3.9
x' = ax + by + cz +d
y' = ex + fy + gz + h
z' = ix + jy + kz + l
1 = mx + ny + oz+ p
CHAPTER 3 Drawing and Transforming Triangles
106
From the equation1 = mx + ny + oz + p, it is easy to find that the coefficients are m = 0,
n = 0, o = 0, and p = 1. In addition, these equations have the constant terms d, h, and l,
which look helpful to deal with Equation 3.1 because it also has constant terms. Let us
compare Equation 3.9 and Equation 3.1 (translation), which is reproduced again:
x' = x + T
x
y' = y + T
y
z' = z + T
z
When you compare the x' component of both equations, you can see that a=1, b=0, c=0,
and d=T
x . Similarly, when comparing y' from both equations, you find e = 0, f = 1, g = 0,
and h = T
y ; when comparing z' you see i=0, j=0, k=1, and l =T z . You can use these results
to write a matrix that represents a translation, called a translation matrix , as shown in
Equation 3.10 .
Equation 3.10
⎡
⎣
⎢
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
⎥
=
⎡
⎣
⎢
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
⎥
×
⎡
⎣
⎢
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
⎥
x
y
z
Tx
Ty
Tz
x
y
z
'
'
'
1
1 0 0
0 1 0
0 0 1
0 0 0 1 1
Rotation Matrix, Again
At this stage you have successfully created a rotation and a translation matrix, which are
equivalent to the two equations you used in the example programs earlier. The final step
is to combine these two matrices; however, the rotation matrix (3×3 matrix) and trans-
formation matrix (4×4 matrix) have different numbers of elements. Unfortunately, you
cannot combine matrices of different sizes, so you need a mechanism to make them the
same size.
To do that, you need to change the rotation matrix (3×3 matrix) into a 4×4 matrix. This is
straightforward and requires you to find the coefficient of each equation in Equation 3.9
by comparing it with Equation 3.3 . The following shows both equations:
x' = x cos β – y sin β
y' = x sin β + y cos β
z' = z
Moving, Rotating, and Scaling
107
x' = ax + by + cz + d
y' = ex + fy + gz + h
z' = ix + iy + kz + l
1 = mx + ny + oz + p
For example, when you compare x' = x cos β – y sin β with x' = ax + by + cz +d, you find
a = cos β, b = –sin β, c = 0, and d = 0. In the same way, after comparing in terms of y and
z, you get the rotation matrix shown in Equation 3.11 :
Equation 3.11
β β
β β
⎡
⎣
⎢
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
⎥
=
−
⎡
⎣
⎢
⎢
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
⎥
⎥
×
⎡
⎣
⎢
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
⎥
x
y
z
x
y
z
'
'
'
1
cos sin 0 0
sin cos 0 0
0 0 1 0
0 0 0 1 1
This allows you to represent both a rotation matrix and translation matrix in the same
4×4 matrix, achieving the original goal!
Sample Program (RotatedTriangle_Matrix.js)
Having constructed a 4×4 rotation matrix, let’s go ahead and use this matrix in a WebGL
program by rewriting the sample program RotatedTriangle , which rotates a triangle
90 degrees around the z-axis in a counterclockwise direction, using the rotation matrix.
Listing 3.6 shows RotatedTriangle_Matrix.js , whose output will be the same as Figure
3.23 shown earlier.
Listing 3.6 RotatedTriangle_Matrix.js
1 // RotatedTriangle_Matrix.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'uniform mat4 u_xformMatrix;\n' +
6 'void main() {\n' +
7 ' gl_Position = u_xformMatrix * a_Position;\n' +
8 '}\n';
9
10 // Fragment shader program
...
16 // Rotation angle
17 var ANGLE = 90.0;
CHAPTER 3 Drawing and Transforming Triangles
108
18
19 function main() {
...
36 // Set the positions of vertices
37 var n = initVertexBuffers(gl);
...
43 // Create a rotation matrix
44 var radian = Math.PI * ANGLE / 180.0; // Convert to radians
45 var cosB = Math.cos(radian), sinB = Math.sin(radian);
46
47 // Note: WebGL is column major order
48 var xformMatrix = new Float32Array([
49 cosB, sinB, 0.0, 0.0,
50 -sinB, cosB, 0.0, 0.0,
51 0.0, 0.0, 1.0, 0.0,
52 0.0, 0.0, 0.0, 1.0
53 ]);
54
55 // Pass the rotation matrix to the vertex shader
56 var u_xformMatrix = gl.getUniformLocation(gl.program, 'u_xformMatrix');
...
61 gl.uniformMatrix4fv(u_xformMatrix, false, xformMatrix);
62
63 // Set the color for clearing <canvas>
...
69 // Draw a triangle
70 gl.drawArrays(gl.TRIANGLES, 0, n);
71 }
72
73 function initVertexBuffers(gl) {
74 var vertices = new Float32Array([
75 0.0, 0.5, -0.5, -0.5, 0.5, -0.5
76 ]);
77 var n = 3; // Number of vertices
...
105 return n;
106 }
First, let us examine the vertex shader:
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'uniform mat4 u_xformMatrix;\n' +
Moving, Rotating, and Scaling
109
6 'void main() {\n' +
7 ' gl_Position = u_xformMatrix * a_Position;\n' +
8 '}\n';
At line 7, u_xformMatrix , containing the rotation matrix described in Equation 3.11 , and
a_Position , containing the vertex coordinates (this is the right-side vector in Equation
3.11 ), are multiplied, literally implementing Equation 3.11 .
In the sample program TranslatedTriangle , you were able to implement the addition of
two vectors in one line ( gl_Position = a_Position + u_Translation ). In the same way, a
multiplication of a matrix and vector can be written in one line in GLSL ES. This is conve-
nient, allowing the calculation of the four equations ( Equation 3.9 ) in one line. Again,
this shows how GLSL ES has been designed specifically for 3D computer graphics by
supporting powerful operations like this.
Because the transformation matrix is a 4×4 matrix and GLSL ES requires the data type for
all variables, line 5 declares u_xformMatrix as type mat4 . As you would expect, mat4 is a
data type specifically for holding a 4×4 matrix.
Within the main JavaScript program, the rest of the changes just calculate the rotation
matrix from Equation 3.11 and then pass it to u_xformMatrix . This part starts from
line 44:
43 // Create a rotation matrix
44 var radian = Math.PI * ANGLE / 180.0; // Convert to radians
45 var cosB = Math.cos(radian), sinB = Math.sin(radian);
46
47 // Note: WebGL is column major order
48 var xformMatrix = new Float32Array([
49 cosB, sinB, 0.0, 0.0,
50 -sinB, cosB, 0.0, 0.0,
51 0.0, 0.0, 1.0, 0.0,
52 0.0, 0.0, 0.0, 1.0
53 ]);
54
55 // Pass the rotation matrix to the vertex shader
...
61 gl.uniformMatrix4fv(u_xformMatrix, false, xformMatrix);
Lines 44 and 45 calculate the values of cosine and sine, which are required in the rota-
tion matrix. Then line 48 creates the matrix xformMatrix using a Float32Array . Unlike
GLSL ES, because JavaScript does not have a dedicated object for representing a matrix,
you need to use the Float32Array . One question that arises is in which order you should
store the elements of the matrix (which is arranged in rows and columns) in the elements
of the array (which is arranged in a line). There are two possible orders: row major order
and column major order (see Figure 3.27 ).
CHAPTER 3 Drawing and Transforming Triangles
110
d c b a
h g f e
l k j i
p o n m
d c b a
h g f e
l k j i
p o n m
r o j a m n m u l o c r o j a m w o r
Figure 3.27 Row major order and column major order
WebGL, just like OpenGL, requires you to store the elements of a matrix in the elements
of an array in column major order. So, for example, the matrix shown in Figure 3.27
is stored in an array as follows: [ a, e, i, m, b, f, j, n, c, g, k, o, d, h, l, p ]. In the sample
program, the rotation matrix is stored in the Float32Array in this order in lines 49 to 52.
The array created is then passed to the uniform variable u_xformMatrix by using
gl.uniformMatrix4fv() at line 61. Note that the last letter of this method name is v ,
which indicates that the method can pass multiple data values to the variable.
gl.uniformMatrix4fv (location, transpose, array)
Assign the 4×4 matrix specified by array to the uniform variable specified by location.
Parameters location Specifies the storage location of the uniform variable.
Transpose Must be false in WebGL.
3
array Specifies an array containing a 4×4 matrix in column
major order (typed array).
Return value None
Errors INVALID_OPERATION There is no current program object.
INVALID_VALUE transpose is not false, or the length of array is less
than 16.
If you load and run the sample program in your browser, you’ll see the rotated triangle.
Congratulations! You have successfully learned how to use a transformation matrix to
rotate a triangle.
3 This parameter specifi es whether to transpose the matrix or not. The transpose operation, which
exchanges the column and row elements of the matrix (see Chapter 7 ), is not supported by WebGL’s
implementation of this method and must always be set to false .
Moving, Rotating, and Scaling
111
Reusing the Same Approach for Translation
Now, as you have seen with Equations 3.10 and 3.11 , you can represent both a translation
and a rotation using the same type of 4×4 matrix. Both equations use the matrices in the
form <new coordinates> = <transformation matrix> * <original coordinates> . This
is coded in the vertex shader as follows:
7 ' gl_Position = u_xformMatrix * a_Position;\n' +
This means that if you change the elements of the array xformMatrix from those of a
rotation matrix to those of a translation matrix, you will be able to apply the translation
matrix to the triangle to achieve the same result as shown earlier but which used an equa-
tion ( Figure 3.18 ).
To do that, change line 17 in RotatedTriangle_Matrix.js using the translation distances
from the previous example:
17 varTx = 0.5, Ty = 0.5, Tz = 0.0;
You need to rewrite the code for creating the matrix, remembering that you need to store
the elements of the matrix in column major order. Let’s keep the same name for the array
variable, xformMatrix , even though it’s now being used to hold a translation matrix,
because it reinforces the fact that we are using essentially the same code. Finally, you are
not using the variable ANGLE , so lines 43 to 45 are commented out:
43 // Create a rotation matrix
44 // var radian = Math.PI * ANGLE / 180.0; // Convert to radians
45 // var cosB = Math.cos(radian), sinB = Math.sin(radian);
46
47 // Note: WebGL is column major order
48 var xformMatrix = new Float32Array([
49 1.0, 0.0, 0.0, 0.0,
50 0.0, 1.0, 0.0, 0.0,
51 0.0, 0.0, 1.0, 0.0,
52 Tx, Ty, Tz, 1.0
53 ]);
Once you’ve made the changes, run the modified program, and you will see the same
output as shown in Figure 3.18 . By using a transformation matrix, you can apply various
transformations using the same vertex shader. This is why the transformation matrix is
such a convenient and powerful tool for 3D graphics, and it’s why we’ve covered it in
detail in this chapter.
Transformation Matrix: Scaling
Finally , let’s define the transformation matrix for scaling using the same assumption that
the original point is p and the point after scaling is p' .
CHAPTER 3 Drawing and Transforming Triangles
112
x
y
) ' z , ' y , ' x ( ' p
) z , y , x ( p
Figure 3.28 A scaling transformation
Assuming the scaling factor for the x-axis, y-axis, and z-axis is S
x , S y , and S z respectively,
you obtain the following equations:
x' = S
x × x
y' = S
y × y
z' = S
z × z
The following transformation matrix can be obtained by comparing these equations with
Equation 3.9 .
⎡
⎣
⎢
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
⎥
=
⎡
⎣
⎢
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
⎥
×
⎡
⎣
⎢
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
⎥
x
y
z
Sx
Sy
Sz
x
y
z
'
'
'
1
0 0 0
0 0 0
0 0 0
0 0 0 1 1
As with the previous example, if you store this matrix in xformMatrix , you can scale the
triangle by using the same vertex shader you used in RotatedTriangle_Matrix.js . For
example, the following sample program will scale the triangle by a factor of 1.5 in a verti-
cal direction, as shown in Figure 3.29 :
17 varSx = 1.0, Sy = 1.5, Sz = 1.0;
...
47 // Note: WebGL is column major order
48 var xformMatrix = new Float32Array([
49 Sx, 0.0, 0.0, 0.0,
50 0.0, Sy, 0.0, 0.0,
51 0.0, 0.0, Sz, 0.0,
52 0.0, 0.0, 0.0, 1.0
53 ]
Summary
113
Figure 3.29 Triangle scaled in a vertical direction
Note that if you specify 0.0 to Sx , Sy , or Sz , the scaled size will be 0.0. If you want to keep
the original size, specify 1.0 as the scaling factor.
Summary
In this chapter, you explored the process of passing multiple items of information about
vertices to a vertex shader, the different types of shapes available to be drawn using that
information, and the process of transforming those shapes. The shapes dealt with in this
chapter changed from a point to a triangle, but the method of using shaders remained the
same, as in the examples in the previous chapter. You were also introduced to matrices
and learned how to use transformation matrices to apply translation, rotation, or scaling
to 2D shapes. Although it’s a little complicated, you should now have a good understand-
ing of the math behind calculating the individual transformation matrices.
In the next chapter, you’ll explore more complex transformations but will use a handy
library to hide the details, allowing you to focus on the higher-level tasks.
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Chapter 4
More Transformations and Basic Animation
Chapter 3 , “Drawing and Transforming Triangles,” explained how to use the buffer object to
draw more complex shapes and how to transform them using simple equations. Because this
approach is cumbersome for multiple transformations, you were then introduced to matrices and
their use to simplify transformation operations. In this chapter, you explore further transforma-
tions and begin to combine transformations into animations. In particular, you will
• Be introduced to a matrix transformation library that hides the mathematical details of
matrix operations
• Use the library to quickly and easily combine multiple transformations
• Explore animation and how the library helps you animate simple shapes
These techniques provide the basics to construct complex WebGL programs and will be used in
the sample programs in the following chapters.
Translate and Then Rotate
As you saw in Chapter 3 , although transformations such as translation, rotation, and scaling can
be represented as a 4×4 matrix, it is time consuming to specify each matrix by hand whenever
you write WebGL programs. To simply the task, most WebGL developers use a convenient library
that automates creating these matrices and hides the details. There are several public matrix
libraries for WebGL, but in this section, you’ll use a library created for this book and will see how
the library can be used to combine multiple transformations to achieve results such as “translate
and then rotate a triangle.”
CHAPTER 4 More Transformations and Basic Animation
116
Transformation Matrix Library: cuon-matrix.js
In OpenGL , the parent specification of WebGL, there’s no need to specify by hand each
element of the transformation matrix when you use it. Instead, OpenGL supports a set
of handy functions that make it easy to construct the matrices. For example, glTrans-
latef() , when called with translation distances for the x-, y-, and z-axes as arguments,
will create a translation matrix internally and then set up the matrix needed to translate a
shape (see Figure 4.1 ).
5 0 0 1
0 8 0 1 0
0 3 1 0 0
1 0 0 0
; ) 0 3 , 0 8 , 5 ( f e t a l s n a r T l g
Figure 4.1 An example of glTranslatef() in OpenGL
Unfortunately, WebGL doesn’t provide these functions, so if you wanted to use this
approach you’d need to look for or write them yourself. Because these functions are
useful, we’ve created a JavaScript library for you, cuon-matrix.js , which allows you to
create transformation matrices in a similar way to those specified in OpenGL. Although
this library was written specifically for this book, it is general purpose, and you are free to
use it in your own applications.
As you saw in Chapter 3 , JavaScript provides the sine and cosine function as methods of
the Math object. In the same way, the functions for creating transformation matrices are
provided as methods of the Matrix4 object defined in cuon-matrix.js .
Matrix4 is a new object, defined by the library, and as the name suggests, deals with 4×4
matrices. Internally, these are represented using a typed array, Float32Array , which you
saw in Chapter 3 .
To get a feel for the library, let’s rewrite RotatedTriangle_Matrix using the Matrix4 object
and its methods. The program for this example is called RotatedTriangle_Matrix4 .
Because the Matrix4 object is defined in the library cuon-matrix.js , you need to load it
into your HTML file before using the object. To do this, you simply use the <script> tag,
as shown in Listing 4.1 .
Listing 4.1 RotatedTriangle_Matrix4.html (The Codes for Loading the Library)
13 <script src="../lib/webgl-debug.js"></script>
14 <script src="../lib/cuon-utils.js"></script>
15 <script src="../lib/cuon-matrix.js"></script>
16 <script src="RotatedTriangle_Matrix4.js"></script>
Once loaded, let’s examine how to use it by comparing the previous RotatedTriangle_
Matrix.js with RotatedTriangle_Matrix4.js , which uses the new Matrix4 object.
Translate and Then Rotate
117
Sample Program (RotatedTriangle_Matrix4.js)
This sample program is almost the same as RotatedTriangle_Matrix.js used in Chapter
3 except for the new steps that create the transformation matrix and pass it to u_xformMa-
trix in the vertex shader.
In RotatedTriangle_Matrix.js , you created the transformation matrix as follows:
1 // RotatedTriangle_Matrix.js
...
43 // Create a rotation matrix
44 var radian = Math.PI * ANGLE / 180.0; // Convert to radians
45 varcosB = Math.cos(radian), sinB = Math.sin(radian);
46
47 // Note: WebGL is column major order
48 var xformMatrix = new Float32Array([
49 cosB, sinB, 0.0, 0.0,
50 -sinB, cosB, 0.0, 0.0,
51 0.0, 0.0, 1.0, 0.0,
52 0.0, 0.0, 0.0, 1.0
53 ]);
...
61 gl.uniformMatrix4fv(u_xformMatrix, false, xformMatrix);
In this sample program, you need to rewrite this part using a Matrix4 object and utilize its
method setRotate() to calculate a rotation matrix. The following code snippet shows the
rewritten part from RotatedTriangle_Matrix4.js :
1 // RotatedTriangle_Matrix4.js
...
47 // Create Matrix4 object for a rotation matrix
48 var xformMatrix = new Matrix4();
49 // Set the rotation matrix to xformMatrix
50 xformMatrix.setRotate(ANGLE, 0, 0, 1);
...
56 // Pass the rotation matrix to the vertex shader
57 gl.uniformMatrix4fv(u_xformMatrix, false, xformMatrix.elements );
You can see that the basic processing flow to create a matrix (line 48 and 50) and then
pass it to the uniform variable is the same as in the previous sample. A Matrix4 object is
created using the new operator in the same way that an Array or Date object is created in
JavaScript. Line 48 creates the Matrix4 object, and then line 50 uses setRotate() to calcu-
late the rotation matrix and write it to the xformMatrix object.
The setRotate() method takes four parameters: a rotation angle (specified in degrees, not
radians) and the rotation axis around which the rotation will take place. The rotation axis,
specified by x, y, and z, and associated direction are defined by the line drawn from
CHAPTER 4 More Transformations and Basic Animation
118
(0, 0, 0) to (x, y, z). As you will remember from Chapter 3 ( Figure 3.21 ), the angle is posi-
tive if the rotation is performed in the clockwise direction around the rotation axis. In this
example, because the rotation is performed around the z-axis, the axis is set to (0, 0, 1):
50 xformMatrix.setRotate(ANGLE, 0, 0, 1);
Similarly, for the x-axis, you can specify x = 1, y =0, and z = 0 and for the y-axis, x = 0,
y = 1, and z = 0. Once you have set up the rotation matrix in the variable xformMatrix ,
all you need to do is to pass the matrix to the vertex shader using the same method gl.
uniformMatrix4fv() as before. Note that you cannot pass a Matrix4 object directly as
the last argument of this method because a typed array is required for the parameter.
However, you can retrieve the elements of the Matrix4 object as a typed array by using its
property elements , which you can then pass as shown at line 57:
57 gl.uniformMatrix4fv(u_xformMatrix, false, xformMatrix.elements );
The Matrix4 object supports the methods and properties shown in Table 4.1 .
Table 4.1 The Methods and Properties Supported by Matrix4
Methods and Properties Description
Matrix4.setIdentity() Initialize a matrix (to the identity matrix
* ).
Matrix4.setTranslate(x, y,
z)
Set Matrix4 to the translation matrix, which translates x
units in the direction of the x-axis, y units in the direction of
the y-axis, and z units in the direction of the z-axis.
Matrix4.setRotate(angle,
x, y, z)
Set Matrix4 to the rotation matrix, which rotates angle
degrees around the rotation axis ( x , y , z ). The ( x , y , z ) coor-
dinates do not need to be normalized . (See Chapter 8 ,
“Lighting Objects.”)
Matrix4.setScale(x, y, z) Set Matrix4 to the scaling matrix with scaling factors x , y ,
and z .
Matrix4.translate (x, y,
z)
Multiply the matrix stored in Matrix4 by the translation
matrix, which translates x units in the direction of the x-axis,
y units in the direction of the y-axis, and z units in the direc-
tion of the z-axis, storing the result back into Matrix4 .
Matrix4.rotate(angle, x,
y, z)
Multiply the matrix stored in Matrix4 by the rotation matrix,
which rotates angle degrees around the rotation axis ( x , y ,
z ), storing the results back into Matrix4 . The ( x , y , z ) coor-
dinates do not need to be normalized. (See Chapter 8 .)
Matrix4.scale(x, y, z) Multiply the matrix stored in Matrix4 by the scaling matrix,
with scaling factors x , y , and z , storing the results back into
Matrix4 .
Translate and Then Rotate
119
Methods and Properties Description
Matrix4.set(m) Set the matrix m to Matrix4 . m must be a Matrix4 object.
Matrix4.elements The typed array ( Float32Array ) containing the elements of
the matrix stored in Matrix4 .
* The identity matrix is a matrix that behaves for matrix multiplication like the scalar value 1 does for
scalar multiplication. Multiplying by the identity matrix has no effect on the other matrix. The identity
matrix has 1.0 in its diagonal elements.
As can be seen from the table, there are two types of methods supported by Matrix4 : those
whose names include set , and those that don’t. The set methods calculate the transfor-
mation matrix using their parameters and then set or write the resulting matrix to the
Matrix4 object. In contrast, the methods without set multiply the matrix already stored
in the Matrix4 object by the matrix calculated using the parameters and then store the
result back in the Matrix4 object.
As you can see from the table, these methods are both powerful and flexible. More impor-
tantly, it makes it easy to quickly change transformations. For example, if you wanted to
translate the triangle instead of rotating, you could just rewrite line 50 as follows:
50 xformMatrix.setTranslate( 0.5, 0.5, 0.0);
The example uses the variable name xformMatrix to represent a generic transformation.
Obviously, you can use an appropriate variable name for the transformations within your
applications (such as rotMatrix , as was used in Chapter 3 ).
Combining Multiple Transformation
Now that you are familiar with the basics of the Matrix4 object, let’s see how it can be
used to combine two transformations: a translation followed by a rotation. The sample
program RotatedTranslatedTriangle does just that, resulting in Figure 4.2 . Note that
the example uses a smaller triangle so that you can easily understand the effect of the
transformation.
CHAPTER 4 More Transformations and Basic Animation
120
Figure 4.2 RotatedTranslatedTriangle
Obviously, this example consists of the following two transformations, shown in
Figure 4.3:
1. Translate the triangle along the x-axis.
2. Rotate the triangle translated by (1).
x
y
) 1 (
) 2 (
Figure 4.3 The triangle translated and then rotated
Based on the explanations so far, we can write the equation of the translated triangle from
(1) as follows:
Equation 4.1
〈 〉 =
〈 〉× 〈 〉
translated coordinates
translation matrix original coordinates
" "
Then you just need to rotate the < > translated coordinates " " as follows:
Equation 4.2
〈 〉 =
〈 〉× 〈 〉
translated and then rotated coordinates
rotation matrix translated coordinates
" "
Translate and Then Rotate
121
You can calculate these equations separately, but they can be combined by substituting
Equation 4.1 into Equation 4.2 .
Equation 4.3
( )
〈 〉 =
〈 〉× 〈 〉× 〈 〉
translated and then rotated coordinates
rotation matrix translation matrix original coordinates
”       ”
       
Where
( ) 〈 〉× 〈 〉× 〈 〉 rotation matrix translation matrix original coordinates     
is equal to
( ) 〈 〉× 〈 〉 × 〈 〉 rotation matrix translation matrix original coordinates     
This final step, calculating 〈 〉×〈 〉 rotation matrix translation matrix , can be carried out in a
JavaScript program and the result passed to the vertex shader. The combination of multi-
ple transformations like this is called model transformation (or modeling transforma-
tion ), and the matrix that performs model transformation is called the model matrix .
As a refresher, let’s just look at the multiplication of matrices, which is defined as follows:
=
⎡
⎣
⎢
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
⎥
=
⎡
⎣
⎢
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
⎥
A
a a a
a a a
a a a
B
b b b
b b b
b b b
,
00 01 02
10 11 12
20 21 22
00 01 02
10 11 12
20 21 22
Assuming two 3×3 matrices, A and B as shown, the product of A and B is defined as
follows:
Equation 4.4
× + × + × × + × + × × + × + ×
× + × + × × + × + × × + × + ×
× + × + × × + × + × × + × + ×
⎡
⎣
⎢
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
⎥
a b a b a b a b a b a b a b a b a b
a b a b a b a b a b a b a b a b a b
a b a b a b a b a b a b a b a b a b
00 00 01 10 02 20 00 01 01 11 02 21 00 02 01 12 02 22
10 00 11 10 12 20 10 01 11 11 12 21 10 02 11 12 12 22
20 00 21 10 22 20 20 01 21 11 22 21 20 02 21 12 22 22
We use 3×3 matrices in the example, but the approach scales to the more usual 4×4 matri-
ces. However, note that the multiplication order of matrices is important. The result of
A * B is not equal to that of B * A.
As you would expect, cuon-matrix.js supports a method to carry out matrix multiplica-
tion on Matrix4 objects. Let’s look at how to use that method to combine two matrices to
support a translation followed by a rotation.
Sample Program (RotatedTranslatedTriangle.js)
Listing 4.2 shows RotatedTranslatedTriangle.js . The vertex shader and fragment shader
are the same as in RotatedTriangle_Matrix4.js in the previous section except that the
name of the uniform variable is changed from u_xformMatrix to u_ModelMatrix .
CHAPTER 4 More Transformations and Basic Animation
122
Listing 4.2 RotatedTranslatedTriangle.js
1 // RotatedTranslatedTriangle.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'uniform mat4 u_ModelMatrix;\n' +
6 'void main() {\n' +
7 ' gl_Position = u_ModelMatrix * a_Position;\n' +
8 '}\n';
9 // Fragment shader program
...
16 function main() {
...
33 // Set the positions of vertices
34 var n = initVertexBuffers(gl);
...
40 // Create Matrix4 object for model transformation
41 var modelMatrix = new Matrix4();
42
43 // Calculate a model matrix
44 var ANGLE = 60.0; // Rotation angle
45 varTx = 0.5; // Translation distance
46 modelMatrix.setRotate(ANGLE, 0, 0, 1); // Set rotation matrix
47 modelMatrix.translate(Tx, 0, 0); // Multiply modelMatrix by the calculated
➥translation matrix
48
49 // Pass the model matrix to the vertex shader
50 var u_ModelMatrix = gl.getUniformLocation(gl.program, ' u_ModelMatrix');
...
56 gl.uniformMatrix4fv(u_ModelMatrix, false, modelMatrix.elements);
...
63 // Draw a triangle
64 gl.drawArrays(gl.TRIANGLES, 0, n);
65 }
66
67 function initVertexBuffers(gl) {
68 var vertices = new Float32Array([
69 0.0, 0.3, -0.3, -0.3, 0.3, -0.3
70 ]);
71 var n = 3; // The number of vertices
...
99 return n;
100 }
Translate and Then Rotate
123
The key lines in this listing are lines 46 and 47, which calculate
< > × < > rotation matrix translation matrix   
:
46 modelMatrix.setRotate(ANGLE, 0, 0, 1); // Set rotation matrix
47 modelMatrix.translate(Tx, 0, 0); // Multiply modelMatrix by the calculated
➥translation matrix
Because line 46 uses a method with set ( setRotate()) , the rotation matrix that is calcu-
lated using the parameters is written to the variable modelMatrix . The next line, 47,
uses a method without set ( translate()) , which, as explained earlier, calculates the
translation matrix using the parameters and then multiplies the matrix in modelMa-
trix by the newly calculated translation matrix and writes the result back into model-
Matrix . So, if modelMatrix already contains a rotation matrix, this method calculates
〈 〉× 〈 〉 rotation matrix translation matrix   
and stores the result back into
modelMatrix .
You may have noticed that the order of “translate first and then rotate” is the opposite of
the order of the matrices in the calculation 〈 〉× 〈 〉 rotation matrix translation matrix   
. As shown in
Equation 4.3 , this is because the transformation matrix is multiplied by the original vertex
coordinates of the triangle.
The result of this calculation is passed to u_ModelMatrix in the vertex shader at line
56, and then the drawing operation (line 64) is the same as usual. If you now load this
program into your browser, you can see a red triangle, which has been translated and then
rotated.
Experimenting with the Sample Program
Let’s rewrite the sample program to first rotate the triangle and then translate it. This
simply requires you to exchange the order of the rotation and translation. In this
case, you should note that the translation is performed first by using the set method,
setTranslate() :
46 modelMatrix.setTranslate(Tx, 0, 0);
47 modelMatrix.rotate(ANGLE, 0, 0, 1);
Figure 4.4 shows this sample program.
CHAPTER 4 More Transformations and Basic Animation
124
Figure 4.4 A triangle “rotated first and then translated”
As you can see, by changing the order of a rotation and translation, you get a different
result. This becomes obvious when you examine Figure 4.5 .
e t a l s n a r t n e h t d n a t s r i f e t a t o r
e t a t o r n e h t d n a t s r i f e t a l s n a r t
x
y
) 1 (
) 2 (
x
y
) 1 (
) 2 (
Figure 4.5 The order of transformations will show different results
That concludes the initial explanation of the use of methods defined in cuon-matrix.js
to create transformation matrices. You’ll be using them throughout the rest of this book,
so you’ll have plenty of chance to study them further.
Animation
So far, this chapter has explained how to transform shapes and use the matrix library to
carry out transformation operations. You now have enough knowledge of WebGL to start
on the next step of applying this knowledge to animate shapes.
Animation
125
Let’s start by constructing a sample program, RotatingTriangle , which continually rotates
a triangle at a constant rotation speed (45 degrees/second). Figure 4.6 shows multiple over-
laid screenshots of RotatingTriangle so that you can see the rotation.
Figure 4.6 Multiple overlaid screenshots of RotatingTriangle
The Basics of Animation
To animate a rotating triangle, you simply need to redraw the triangle at a slightly differ-
ent angle each time it draws.
Figure 4.7 shows individual triangles that are drawn at times t0, t1, t2, t3, and t4. Each
triangle is a still image, but you can see that each has a slightly different rotation angle.
When you see a series of these triangles sequentially, your mind interpolates the changes
between them and then puts them together as a smooth flow of animation, just like a flip
book. Of course, you need to clear the previous triangle before drawing a new one. (This is
why you must call gl.clear() before drawing something.) You can apply this animation
method to both 2D shapes and 3D objects.
e m i t
4 t 3 t 2 t 1 t 0 t
Figure 4.7 Draw a slightly different triangle for each drawing
CHAPTER 4 More Transformations and Basic Animation
126
Achieving animation in this way requires two key mechanisms:
Mechanism 1: Repeatedly calls a function to draw a triangle at times t0, t1, t2, t3, and
so on.
Mechanism 2: Clears the previous triangle and then draws a new one with the specified
angle each time the function is called.
The second mechanism is just a simple application of the knowledge you’ve learned so far.
However, the first mechanism is new, so let’s take it step by step by examining the sample
program.
Sample Program (RotatingTriangle.js)
Listing 4.3 shows RotatingTriangle.js . The vertex shader and fragment shader are the
same as in the previous sample program. However, the vertex shader is listed to show the
multiplication of a matrix and vertex coordinates.
The following three points differ from the previous sample program:
• Because the program needs to draw a triangle repeatedly, it’s been modified to
specify the clear color at line 44, not just before the drawing operation. Remember,
the color stays in the WebGL system until it’s overwritten.
• The actual mechanism [Mechanism 1] to repeatedly call a drawing function has been
added (lines 59 to 64).
• [Mechanism 2] The operations to clear and draw a triangle were defined as a func-
tion ( draw() at line 102).
These differences are highlighted in Listing 4.3 (lines 1 to 3). Let’s look at them in more
detail.
Listing 4.3 RotatingTriangle.js
1 // RotatingTriangle.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'uniform mat4 u_ModelMatrix;\n' +
6 'void main() {\n' +
7 ' gl_Position = u_ModelMatrix * a_Position;\n' +
8 '}\n';
9 // Fragment shader program
...
16 // Rotation angle (degrees/second)
17 var ANGLE_STEP = 45.0;
18
Animation
127
19 function main() {
...
36 // Set the positions of vertices
37 var n = initVertexBuffers(gl);
...
43 // Set the color for clearing <canvas> <- (1)
44 gl.clearColor(0.0, 0.0, 0.0, 1.0);
45
46 // Get the storage location of u_ModelMatrix variable
47 var u_ModelMatrix = gl.getUniformLocation(gl.program, ' u_ModelMatrix');
...
53 // Current rotation angle of a triangle
54 varcurrentAngle = 0.0;
55 // Matrix4 object for model transformation
56 var modelMatrix = new Matrix4();
57
58 // Start to draw a triangle <- (2)
59 var tick = function() {
60 currentAngle = animate(currentAngle);// Update the rotation angle
61 draw(gl, n, currentAngle, modelMatrix, u_ModelMatrix);
62 requestAnimationFrame(tick);// Request that the browser calls tick
63 };
64 tick();
65 }
66
67 function initVertexBuffers(gl) {
68 var vertices = new Float32Array ([
69 0.0, 0.5, -0.5, -0.5, 0.5, -0.5
70 ]);
71 var n = 3; // The number of vertices
...
96 return n;
97 }
98
99 function draw(gl,n, currentAngle, modelMatrix, u_ModelMatrix){ <-(3)
100 // Set up rotation matrix
101 modelMatrix.setRotate(currentAngle, 0, 0, 1);
102
103 // Pass the rotation matrix to the vertex shader
104 gl.uniformMatrix4fv( u_ModelMatrix, false, modelMatrix.elements);
105
106 // Clear <canvas>
107 gl.clear(gl.COLOR_BUFFER_BIT);
108
109 // Draw a triangle
CHAPTER 4 More Transformations and Basic Animation
128
110 gl.drawArrays(gl.TRIANGLES, 0, n);
111 }
112
113 // Last time when this function was called
114 var g_last = Date.now();
115 function animate(angle) {
116 // Calculate the elapsed time
117 v ar now = Date.now();
118 var elapsed = now - g_last; // milliseconds
119 g_last = now;
120 // Update the current rotation angle (adjusted by the elapsed time)
121 var newAngle = angle + (ANGLE_STEP * elapsed) / 1000.0;
122 return newAngle %= 360;
126 }
Line 7 in the vertex shader is just a multiplication of a matrix and the vertex coordinates
in the same way as RotatedTranslatedTriangle.js ). u_ModelMatrix is a uniform variable,
and the rotation matrix is passed to the variable from a JavaScript program:
7 ' gl_Position = u_ModelMatrix * a_Position;\n' +
The variable ANGLE_STEP at line 17 defines the rotation angles per second and is set to 45
degrees/second:
17 var ANGLE_STEP = 45.0;
The main() function starts from line 19, but because the code from lines 19 to 37, which
specifies the vertex coordinates, is the same as before, it is omitted.
The first of the three differences is that you specify the clear color once only: at line 44.
Line 47 then retrieves the storage location of u_ModelMatrix in the vertex shader. Because
this location never changes, it’s more efficient to do only this once:
47 var u_ModelMatrix = gl.getUniformLocation(gl.program, 'u_ModelMatrix');
The variable u_ModelMatrix is then used in the draw() function (line 99) that draws the
triangle.
The value of the variable currentAngle starts at 0 degrees and stores how many degrees
the triangle should be rotated from its original position each time it is drawn. As in the
simple rotation examples earlier, it calculates the rotation matrix needed for the trans-
formation. The variable modelMatrix defined at line 56 is a Matrix4 object used to hold
the rotation matrix in draw() . This matrix could be created within draw() ; however, that
would require a new Matrix4 object to be created each time draw() is called, which would
be inefficient. For this reason, the object is created at line 56 and then passed to draw() at
line 61.
Animation
129
Lines 59 to 64 implement Mechanism 1 as the function tick , which is repeatedly called
to draw the triangle. Before you look at how the overall “tick” mechanism actually works,
let’s look at what happens each time it is called:
53 // Current rotation angle of a triangle
54 varcurrentAngle = 0.0;
55 // Matix4 object for model transformation
56 var modelMatrix = new Matrix4();
57
58 // Start to draw a triangle <- (2)
59 var tick = function() {
60 currentAngle = animate(currentAngle); // Update the rotation angle
61 draw(gl, n, currentAngle, modelMatrix, u_ModelMatrix);
62 requestAnimationFrame(tick); // Request that the browser callstick
63 };
64 tick();
Within tick , the call to the function animate() at line 60 updates the current rotation
angle of the triangle, and then the call to draw() at line 61 draws a triangle using gl.
drawArrays() .
draw() is passed the rotation matrix, which rotates the triangle to currentAngle degrees.
In turn, it passes the matrix to the u_ModelMatrix variable in the vertex shader before
calling gl.drawArrays() (lines 104 to 110). This code appears quite complex, so let’s
examine each part in turn.
Repeatedly Call the Drawing Function (tick())
As described earlier, to animate the triangle, you need to perform the following two steps
repeatedly: (1) update the current rotation angle of a triangle ( currentAngle ), and then (2)
call the drawing function with the angle to draw the triangle. Lines 59 to 64 implement
these processing steps.
In this sample program, these tasks are defined by the three operations of line 60, 61,
and 62. These operations are grouped in a single anonymous function using function() ,
and the function is assigned to the variable tick (see Figure 4.8 ). You use an anonymous
function if you want to pass the local variables defined in main() ( gl , n , currentAngle ,
modelMatrix , and u_ModelMatrix ) to draw() as arguments when draw() is called at line 61.
If you need a refresher on anonymous functions, refer to Chapter 2 , “Your First Step with
WebGL,” where you used one to register an event handler.
CHAPTER 4 More Transformations and Basic Animation
130
e l g n a n o i t a t o r t n e r u c e h t e t a d p U
) 0 6 e n i l (
e l g n a e h t g n i s u e l g n a i r t a w a r D
) 1 6 e n i l (
n o i t c n u f s i h t s l l a c r e s w o r b e h t t a h t t s e u q e R
) 2 6 e n i l ( n i a g a ) ) ( k c i t (
Figure 4.8 The operations assigned to “tick”
You can use this basic approach for all types of animation. It is a key technique in 3D
graphics.
When you call requestAnimationFrame() at line 62, you are requesting the browser to call
the function specified as the first parameter at some future time when the three opera-
tions assigned to tick will be executed again. You’ll look at requestAnimationFrame() in a
moment. For now, let’s finish examining the operations executed in tick() .
Draw a Triangle with the Specified Rotation Angle (draw())
The draw() function takes the following five parameters:
• gl: The context in which to draw the triangle
• n: The number of vertices
• currentAngle: The current rotation angle
• modelMatrix: A Matrix4 object to store the rotation matrix calculated using curren-
tAngle
• u_ModelMatrix: The location of the uniform variable to which the modelMatrix is
passed
The actual function code is found in lines 99 to 111:
99 function draw(gl, n, currentAngle, modelMatrix, u_ModelMatrix) {
100 // Set the rotation matrix
101 modelMatrix.setRotate(currentAngle, 0, 0, 1);
102
103 // Pass the rotation matrix to the vertex shader
104 gl.uniformMatrix4fv( u_ModelMatrix, false, modelMatrix.elements);
105
106 // Clear <canvas>
107 gl.clear(gl.COLOR_BUFFER_BIT);
Animation
131
108
109 // Draw the triangle
110 gl.drawArrays(gl.TRIANGLES, 0, n);
111 }
First, line 101 calculates the rotation matrix using the setRotate() method provided by
cuon-matrix.js , writing the resulting matrix to modelMatrix :
101 modelMatrix.setRotate(currentAngle, 0, 0, 1);
Next, line 104 passes the matrix to the vertex shader by using gl.uniformMatrix4fv() :
104 gl.uniformMatrix4fv(u_ModelMatrix, false, modelMatrix.elements);
After that, line 107 clears the <canvas> and then calls gl.drawArrays() at line 110 to
execute the vertex shader to actually draw the triangle. Those steps are the same as used
before.
Now let’s return to the third operation, requestAnimationFrame() , which requests the
browser to call the function tick() at some future time.
Request to Be Called Again (requestAnimationFrame())
Traditionally, if you wanted to repeatedly execute specific tasks (functions) in JavaScript,
you used the method setInterval() .
setInterval (func, delay)
Call the function specified by func multiple times with intervals specified by delay.
Parameters func Specifies the function to be called multiple times.
delay Specifies the intervals (in milliseconds).
Return value Timer id
However, because this JavaScript method was designed before browsers started to support
multiple tabs, it executes regardless of which tab is active. This can lead to performance
problems, so a new method, requestAnimationFrame() , was recently introduced. The
function scheduled using this method is only called when the tab in which it was defined
is active. Because requestAnimationFrame() is a new method and not yet standardized,
it is defined in the library supplied by Google, webgl-utils.js , which handles the differ-
ences among different browsers.
CHAPTER 4 More Transformations and Basic Animation
132
requestAnimationFrame (func)
Requests the function specified by func to be called on redraw (see Figure 4.9 ). This
request needs to be remade after each callback.
Parameters func Specifies the function to be called later. The function takes a
“time” parameter, indicating the timestamp of the callback.
Return value Request id
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{ ) ( k c i t n o i t c n u f
…
r e s w o r b e h t n e h W
, n e e r c s e h t s w a r d e r
s l l a c r e s w o r b e h t
. ) ( k c i t
t p i r c S a v a J
s a v n a c
}
l l a c o t r e s w o r b e h t t s e u q e R
r e s w o r b e h t n e h w ) ( k c i t
. n e e r c s e h t s w a r d e r
Figure 4.9 The requestAnimationFrame () mechanism
By using this method, you avoid animation in inactive tabs and do not increase the load
on the browser. Note, you cannot specify an interval before the function is called; rather,
func (the first parameter) will be called when the browser wants the web page contain-
ing the element (the second parameter) to be painted. In addition, after calling the func-
tion, you need to request the callback again because the previous request is automatically
removed once it’s fulfilled. Line 62 makes that request again once tick is called and
makes it possible to call tick() repeatedly:
62 requestAnimationFrame(tick); // Request the browser to call tick
If you want to cancel the request to call the function, you need to use
cancelAnimationFrame() .
Animation
133
cancelAnimationFrame (requestID)
Cancel the function registered by requestAnimationFrame() .
Parameter requestID Specifies the return value of requestAnimationFrame() .
Return value None
Update the Rotation Angle (animate())
Finally, let’s see how to update the current rotation angle. The program maintains the
current rotation angle of the triangle (that is, how many degrees the triangle has been
rotated from its original position) in the variable currentAngle (defined at line 54). It
calculates the next rotation angle based on this current value.
The update of currentAngle is carried out in the function animate() , which is called at
line 60. This function, defined at line 115, takes one parameter, angle , which represents
the current rotation angle and returns the new rotation angle:
60 currentAngle = animate(currentAngle);// Update the rotation angle
61 draw(gl, n, currentAngle, modelMatrix, u_ModelMatrix);
...
113 // Last time this function was called
114 var g_last = Date.getTime();
115 function animate(angle) {
116 // Calculate the elapsed time
117 var now = Date.getTime();
118 var elapsed = now - g_last;
119 g_last = now;
120 // Update the current rotation angle (adjusted by the elapsed time)
121 var newAngle = angle + (ANGLE_STEP * elapsed) / 1000.0;
122 return newAngle %= 360;
123 }
The process for updating the current rotation angle is slightly complicated. Let’s look at
the reason for that by using Figure 4.10 .
CHAPTER 4 More Transformations and Basic Animation
134
e m i t
4 t 3 t 2 t 1 t 0 t
0 i
i0 1 i o t l a u q e t o n s i 1 i
Figure 4.10 The interval times between each tick() vary
Figure 4.10 illustrates the following:
• tick() is called at t0. It calls draw() to draw the triangle and then reregisters tick() .
• tick() is called at t1. It calls draw() to draw the triangle and then reregisters tick() .
• tick() is called at t2. It calls draw() to draw the triangle and then reregisters tick() .
The problem here is that the interval times between t0 and t1, t1 and t2, and t2 and t3
may be different because of the load on the browser at that time. That is, t1 – t0 could
be different from t2 – t1 .
If the interval time is not constant, then simply adding a fixed amount of angle (degree/
second) to the current rotation angle each time tick() is called will result in an apparent
acceleration or deceleration of the rotation speed.
For this reason, the function animate() needs to be a little more sophisticated and must
determine the new rotation angle based on how long it has been since the function was
last called. To do that, you need to store the time that the function was last called into the
variable g_last and store the current time into the variable now . Then you can calculate
how long it has been since the function was last called by subtraction and store the result
in the variable elapsed (line 118). The amount of rotation is then calculated at line 121
using elapsed as follows.
121 varnewAngle = angle + (ANGLE_STEP * elapsed) / 1000.0;
The variables g_last and now contain the return value from the method now() of a Date
object whose units are a millisecond (1/1000 second). Therefore, if you want to rotate
the triangle by ANGLE_STEP (degree/second), you just need to multiply ANGLE_STEP by
elapsed /1000 to calculate the rotation angle. At line 121, you actually multiply ANGLE_
STEP by elapsed and then divide the result by 1000 because this is slightly more accurate,
but both have the same meaning.
Finally, line 122 ensures the value of newAngle is less than 360 (degrees) and returns the
result.
Animation
135
If you now load RotatedTriangle.html into your browser, you can check that the triangle
rotates at a constant speed. We will reuse this approach for animation in the following
chapters, so it’s worthwhile making sure you have mastered the details.
Experimenting with the Sample Program
In this section , let’s create an animation that consists of multiple transformations.
RotatingTranslatedTriangle translates a triangle 0.35 units in the positive direction of
the x-axis first and then rotates the triangle by 45 degrees/second.
This is easy to achieve if you remember that multiple transformations can be realized by
multiplying each transformation matrix together (refer to Chapter 3 ).
To do this, you just need to insert the translation at line 102. Because the variable model-
Matrix already contains the rotation matrix, you can use translate() , rather than
setTranslate() , to multiply modelMatrix by the translation matrix:
99 function draw(gl, n, currentAngle, modelMatrix, u_ModelMatrix) {
100 // Set a rotation matrix
101 modelMatrix.setRotate(currentAngle, 0, 0, 1);
102 modelMatrix.translate(0.35, 0, 0);
103 // Pass the rotation matrix to the vertex shader
104 gl.uniformMatrix4fv( u_ModelMatrix, false, modelMatrix.elements);
If you load the example, you will see the animation shown in Figure 4.11 .
Figure 4.11 Multiple overlaid screenshots of RotatingTranslatedTriangle
Finally, for those of you wanting a little control, on the companion site for this book is
a sample program, named RotatingTriangle_withButtons , that allows dynamic control
of the rotation speed using buttons (see Figure 4.12 ). You can see the buttons below the
<canvas> .
CHAPTER 4 More Transformations and Basic Animation
136
Figure 4.12 RotatingTriangle_withButtons
Summary
This chapter explored the process of transforming shapes using the transformation matrix
library, combining multiple basic transformations to create a complex transformation, and
animating shapes using the library. There are two key lessons in this chapter: (1) Complex
transformations can be realized by multiplying a series of basic transformation matrices;
(2) You can animate shapes by repeating the transformation and drawing steps.
Chapter 5 , “Using Colors and Texture Images,” is the last chapter that covers basic tech-
niques. It explores colors and textures. Once you master those, you will have enough
knowledge to create your own basic WebGL programs and will be ready to begin exploring
some of the more advanced capabilities of WebGL.
Chapter 5
Using Colors and Texture Images
The previous chapters explained the key concepts underlying the foundations of
WebGL through the use of examples based on 2D shapes. This approach has given you
a good understanding of how to deal with single color geometric shapes in WebGL.
Building on these basics, you now delve a little further into WebGL by exploring the
following three subjects:
• Passing other data such as color information to the vertex shader
• The conversion from a shape to fragments that takes place between the vertex
shader and the fragment shader, which is known as the rasterization process
• Mapping images (or textures) onto the surfaces of a shape or object
This is the final chapter that focuses on the key functionalities of WebGL. After reading
this chapter, you will understand the techniques and mechanism for using colors
and textures in WebGL and will have mastered enough WebGL to allow you to create
sophisticated 3D scenes.
Passing Other Types of Information to Vertex
Shaders
In the previous sample programs, a single buffer object was created first, the vertex
coordinates were stored in it, and then it was passed to the vertex shader. However,
beside coordinates, vertices involved in 3D graphics often need other types of infor-
mation such as color information or point size. For example, let us take a look at a
program you used in Chapter 3 , “Drawing and Transforming Triangles,” which draws
three points: MultiPoint.js . In the shader, in addition to the vertex coordinates, you
provided the point size as extra information. However, the point size was a fixed value
and set in the shader rather than passed from outside:
CHAPTER 5 Using Colors and Texture Images
138
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'void main() {\n' +
6 ' gl_Position = a_Position;\n' +
7 ' gl_PointSize = 10.0;\n' +
8 '}\n'
Line 6 assigns the vertex coordinates to gl_Position , and line 7 assigns a fixed point
size of 10.0 to gl_PointSize . If you now wanted to modify the size of that point from
your JavaScript program, you would need a way to pass the point size with the vertex
coordinates.
Let’s look at an example, MultiAttributeSize , whose goal is to draw three points of
different sizes: 10.0, 20.0, and 30.0, respectively (see Figure 5.1 ).
Figure 5.1 MultiAttributeSize
In the previous chapter, you carried out the following steps to pass the vertex coordinates:
1. Create a buffer object.
2. Bind the buffer object to the target.
3. Write the coordinate data into the buffer object.
4. Assign the buffer object to the attribute variable.
5. Enable the assignment.
If you now wanted to pass several items of vertex information to the vertex shader
through buffer objects, you could just apply the same steps to all the items of information
associated with a vertex. Let’s look at a sample program that uses multiple buffers to do
just that.
Passing Other Types of Information to Vertex Shaders
139
Sample Program (MultiAttributeSize.js)
MultiAttributeSize.js is shown in Listing 5.1 . The fragment shader is basically the same
as in MultiPoint.js , so let’s omit it this time. The vertex shader is also similar, apart from
the fact that you add a new attribute variable that specifies the point size. The numbers 1
through 5 on the right of the listing note the five steps previously outlined.
Listing 5.1 MultiAttributeSize.js
1 // MultiAttributeSize.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'attribute float a_PointSize;\n' +
6 'void main() {\n' +
7 ' gl_Position = a_Position;\n' +
8 ' gl_PointSize = a_PointSize;\n' +
9 '}\n';
...
17 function main() {
...
34 // Set the vertex information
35 var n = initVertexBuffers(gl);
...
47 // Draw three points
48 gl.drawArrays(gl.POINTS, 0, n);
49 }
50
51 function initVertexBuffers(gl) {
52 var vertices = new Float32Array([
53 0.0, 0.5, -0.5, -0.5, 0.5, -0.5
54 ]);
55 var n = 3;
56
57 var sizes = new Float32Array([
58 10.0, 20.0, 30.0 // Point sizes
59 ]);
60
61 // Create a buffer object
62 var vertexBuffer = gl.createBuffer(); <-(1)
63 var sizeBuffer = gl.createBuffer(); <-(1')
...
69 // Write vertex coordinates to the buffer object and enable it
70 gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer); <-(2)
71 gl.bufferData(gl.ARRAY_BUFFER, vertices, gl.STATIC_DRAW); <-(3)
72 var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
CHAPTER 5 Using Colors and Texture Images
140
...
77 gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 0, 0); <-(4)
78 gl.enableVertexAttribArray(a_Position); <-(5)
79
80 // Write point sizes to the buffer object and enable it
81 gl.bindBuffer(gl.ARRAY_BUFFER, sizeBuffer); <-(2')
82 gl.bufferData(gl.ARRAY_BUFFER, sizes, gl.STATIC_DRAW); <-(3')
83 var a_PointSize = gl.getAttribLocation(gl.program, 'a_PointSize');
...
88 gl.vertexAttribPointer(a_PointSize, 1, gl.FLOAT, false, 0, 0); <-(4')
89 gl.enableVertexAttribArray(a_PointSize); <-(5')
...
94 return n;
95 }
First of all, let us examine the vertex shader in Listing 5.1 . As you can see, the attribute
variable a_PointSize , which receives the point size from the JavaScript program, has been
added. This variable, declared at line 5 as a float , is then assigned to gl_PointSize at line
8. No other changes are necessary for the vertex shader, but you will need a slight modi-
fication to the process in initVertexBuffers() so it can handle several buffer objects. Let
us take a more detailed look at it.
Create Multiple Buffer Objects
The function initVertexBuffers() starts at line 51, and the vertex coordinates are
defined from lines 52 to 54. The point sizes are then specified at line 57 using the array
sizes :
57 var sizes = new Float32Array([
58 10.0, 20.0, 30.0 // Point sizes
59 ]);
A buffer object is created at line 62 for the vertex data, and at line 63 another buffer object
( sizeBuffer ) is created for storing the array of “point sizes.”
From lines 70 to 78, the program binds the buffer object for the vertex coordinates, writes
the data, and finally assigns and enables the attribute variables associated with the buffer
object. These tasks are the same as those described in the previous sample programs.
Lines 80 to 89 are new additions for handling the different point sizes. However, the steps
are the same as for a vertex buffer. Bind the buffer object for the point sizes ( sizeBuffer )
to the target (line 81), write the data (line 82), assign the buffer object to the attribute
variable a_PointSize (line 88), and enable it.
Once these steps in initVertexBuffers() are completed, the internal state of the WebGL
system looks like Figure 5.2 . You can see that the two separate buffer objects are created
and then assigned to the two separate attribute variables.
Passing Other Types of Information to Vertex Shaders
141
gl_FragColor
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
; ) … ( s r e d a h S t i n i
…
}
t n e m g a r F
r e d a h S
gl_Position
r e d a h S x e t r e V
n o i t i s o P _ a e t u b i r t a
= r o l o C g a r F _ l g
; ) … ( 4 c e v
= n o i t i s o P _ l g
; n o i t i s o P _ a
= e z i S t n i o P _ l g
; e z i S t n i o P _ a
e z I S t n i o P _ a e t u b i r t a
t c e j b O r e f u B
5 . 0 , 0 . 0
5 . 0 - , 5 . 0 -
5 . 0 - , 5 . 0
r e f u B x e t r e v
r e f u B e z i s
gl_PointSize
r e f u B r o l o C
m e t s y S L G b e W
, 0 . 0 1
, 0 . 0 2
0 . 0 3
t c e j b O r e f u B
t p i r c S a v a J
Figure 5.2 Using two buffer objects to pass data to a vertex shader
In this situation, when gl.drawArrays() at line 48 is executed, all the data stored inside
the buffer objects is sequentially passed to each attribute variable in the order it was stored
inside the buffer objects. By assigning this data to gl_Position at line 7 and gl_PointSize
at line 8, respectively (the vertex shader’s program in Figure 5.2 ), you are now able to
draw different size objects located at different positions.
By creating a buffer object for each type of data in this way and then allocating it to the
attribute variables, you can pass several pieces of information about each vertex to the
vertex shader. Other types of information that can be passed include color, texture coordi-
nates (described in this chapter), and normals ( see Chapter 7 ), as well as point size.
The gl.vertexAttribPointer() Stride and Offset Parameters
Although multiple buffer objects are a great way to handle small amounts of data, in a
complicated 3D object with many thousands of vertices, you can imagine that managing
all the associated vertex data is an extremely difficult task. For example, imagine needing
to manually check each of these arrays when the total count of MultiAttributeSize.js ’s
vertices and sizes reaches 1000.
1 However, WebGL allows the vertex coordinates and the
size to be bundled into a single component and provides mechanisms to access the differ-
ent data types. For example, you can group the vertex and size data in the following way
(refer to Listing 5.2 ), often referred to as interleaving .
Listing 5.2 An Array Containing Multiple Items of Vertex Information
var verticesSizes = new Float32Array([
// Vertex coordinates and size of a point
0.0, 0.5, 10.0, // The 1st point
-0.5, -0.5, 20.0, // The 2nd point
0.5, -0.5, 30.0 // The 3rd point
]);
1 I n practice, because modeling tools that create 3D models actually generate this data, there is no
necessity to either manually input them or visually check their consistency. The use of modeling
tools and the data they generate will be discussed in Chapter 10 .
CHAPTER 5 Using Colors and Texture Images
142
As just described, once you have stored several types of information pertaining to the
vertex in a single buffer object, you need a mechanism to access these different data
elements. You can use the fifth ( stride ) and sixth ( offset ) arguments of gl.vertexAttrib-
Pointer() to do this, as shown in the example that follows.
Sample Program (MultiAttributeSize_Interleaved.js)
Let’s construct a sample program, MultiAttributeSize_Interleaved , which passes multi-
ple data to the vertex shader, just like MultiAttributeSize.js (refer to Listing 5.1 ), except
that it bundles the data into a single array or buffer. Listing 5.3 shows the program in
which the vertex shader and the fragment shader are the same as in MultiAttributeSize.
js .
Listing 5.3 MultiAttributeSize_Interleaved.js
1 // MultiAttributeSize_Interleaved.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'attribute float a_PointSize;\n' +
6 'void main() {\n' +
7 ' gl_Position = a_Position;\n' +
8 ' gl_PointSize = a_PointSize;\n' +
9 '}\n';
...
17 function main() {
...
34 // Set vertex coordinates and point sizes
35 var n = initVertexBuffers(gl);
...
48 gl.drawArrays(gl.POINTS, 0, n);
49 }
50
51 function initVertexBuffers(gl) {
52 var verticesSizes = new Float32Array([
53 // Vertex coordinates and size of a point
54 0.0, 0.5, 10.0, // The 1st vertex
55 -0.5, -0.5, 20.0, // The 2nd vertex
56 0.5, -0.5, 30.0 // The 3rd vertex
57 ]);
58 var n = 3;
59
60 // Create a buffer object
61 var vertexSizeBuffer = gl.createBuffer();
...
Passing Other Types of Information to Vertex Shaders
143
67 // Write vertex coords and point sizes to the buffer and enable it
68 gl.bindBuffer(gl.ARRAY_BUFFER, vertexSizeBuffer);
69 gl.bufferData(gl.ARRAY_BUFFER, verticesSizes, gl.STATIC_DRAW);
70
71 var FSIZE = verticesSizes.BYTES_PER_ELEMENT;
72 // Get the storage location of a_Position, allocate buffer, & enable
73 var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
...
78 gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, FSIZE * 3, 0);
79 gl.enableVertexAttribArray(a_Position); // Enable allocation
80
81 // Get the storage location of a_PointSize, allocate buffer, & enable
82 var a_PointSize = gl.getAttribLocation(gl.program, 'a_PointSize');
...
87 gl.vertexAttribPointer(a_PointSize, 1, gl.FLOAT, false, FSIZE * 3, FSIZE * 2);
88 gl.enableVertexAttribArray(a_PointSize); // Enable buffer allocation
...
93 return n;
94 }
The processing flow of the main() function in JavaScript is the same as
MultiAttributeSize.js , and only the initVertexBuffers() process is modified this time,
so let’s take a look at its content.
First, a typed array is defined at lines 52 to 57, as previously described in Listing 5.2 .
Following the usual processing steps, from line 61 to 69, a buffer object is created (line
61), the object is bound (line 68), and the data is written to the object (line 69). Next,
at line 71, the size (number of bytes) of the element in the verticeSizes array is stored
in the variable FSIZE , which will be needed later on. The size (number of bytes) of each
element of a typed array can be obtained through the property BYTES_PER_ELEMENT .
From line 73 onward, you assign the buffer object to the attribute variable. Retrieving the
storage location of the attribute variable a_Position at line 73 is similar to the previous
example, but the usage of the arguments of gl.vertexAttribPointer() at line 78 is differ-
ent because the buffer now holds two types of data: vertex and point size.
You’ve already looked at the specification of gl.vertexAttribPointer() in Chapter 3 , but
let’s take another look and focus on two parameters: stride and offset .
CHAPTER 5 Using Colors and Texture Images
144
gl.vertexAttribPointer(location, size, type, normalized, stride,
offset)
Assign the buffer object bound to gl.ARRAY_BUFFER to the attribute variable specified by
location . The type and format of the data written in the buffer is also specified.
Parameters location Specifies the storage location of the attribute variable.
size Specifies the number of components per vertex in the buffer
object (valid values are 1 to 4).
type Specifies the data format (in this case, gl.FLOAT )
normalized true or false . Used to indicate whether non- float data should
be normalized to [0, 1] or [–1, 1].
stride Specifies the stride length (in bytes) to get vertex data; that is,
the number of bytes between each vertex element
offset Specifies the offset (in bytes) in a buffer object to indicate where
the vertex data is stored from. If the data is stored from the
beginning. then offset is 0.
The stride specifies the number of bytes used by a group of related vertex data (in this
example, vertex coordinates and point size) inside the buffer object.
In previous examples, where you had only one type of information in the buffer—
vertices—you set the stride to 0. However, in this example, both vertices and point sizes
are laid out in the buffer, as shown in Figure 5.3 .
0 . 0 1
0 . 0 2
0 . 0
5 . 0
5 . 0 -
5 . 0 -
5 . 0
T N E M E L E _ R E P _ S E T Y B . s e z i S s e c i t r e v = E Z I S F
) 3 * E Z I S F ( e d i r t s
f o t e s f o e h T
) 0 ( n o i t i s o P _ a
f o t e s f o e h T
e z i S t n i o P _ a
) 2 * E Z I S F (
) 3 * E Z I S F ( e d i r t s
NOTE : This figure is drawn assuming FSIZE=4
Figure 5.3 Stride and offset
As illustrated in Figure 5.3 , there are three components inside each group of vertex data
(two coordinates, one size), so you need to set the stride equal to three times the size of
each component in the group (that is, three times FSIZE [the number of bytes per element
of the Floats32Array ]).
Passing Other Types of Information to Vertex Shaders
145
The offset parameter indicates the distance to the first element that is being used for this
call. Because you are using the vertex coordinates that are positioned at the head of the
verticesSizes array, the offset is 0. So, at line 78, you specify them as the fifth ( stride ) and
sixth ( offset ) arguments of gl.vertexAttribPointer() :
78 gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, FSIZE * 3, 0);
79 gl.enableVertexAttribArray(a_Position); // Enable allocation
Finally, once the specification of the vertex coordinates has been set up, the assignment to
a_Position is enabled at line 79.
Next, from line 82, you need to do the same for the point size data, so assign a buffer
object to a_PointSize . However, in this case, you are using the same buffer that you used
for vertex data, but you want different data from the buffer. You can make use of the sixth
argument offset to achieve this by setting the offset to the location at which the data (in
this case the point size) to be passed to a_PointSize is positioned in the buffer. The first
two elements of the array are vertex coordinates, so the offset will accordingly be set to
FSIZE * 2 (refer to Figure 5.3 ). Line 87 shows both stride and offset set up correctly:
87 gl.vertexAttribPointer(a_PointSize, 1, gl.FLOAT, false, FSIZE * 3, FSIZE * 2);
88 gl.enableVertexAttribArray(a_PointSize); // Enable buffer allocation
The assignment to a_PointSize is enabled at line 88, and the only remaining task to
perform is the draw operation using gl.drawArrays() .
Each time a vertex shader is invoked, WebGL will extract data from the buffer object using
the values specified in stride and offset and subsequently pass them to the attribute vari-
ables to be used for drawing (see Figure 5.4 ).
t p i r c S a v a J
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
; ) … ( s r e d a h S t i n i
…
}
gl_Position
r e d a h S x e t r e V
= n o i t i s o P _ l g
; n o i t i s o P _ a
= e z i S t n i o P _ l g
; e z i S t n i o P _ a
n o i t i s o P _ a e t u b i r t a
e z i S t n i o P _ a e t u b i r t a
gl_PointSize
gl_FragColor
r e f u B r o l o C
, 5 . 0 , 0 . 0 , 0 . 0 1
, 5 . 0 - , 5 . 0 - , 0 . 0 2
, 5 . 0 - , 5 . 0 , 0 . 0 3
t n e m g a r F
r e d a h S
= r o l o C g a r F _ l g
; ) ( 4 c e v
t c e j b O r e f u B
Figure 5.4 Internal behavior when stride and offset are used
CHAPTER 5 Using Colors and Texture Images
146
Modifying the Color (Varying Variable)
Now that you have seen how to pass several pieces of information to the vertex shader,
let’s use the technique to modify the color of each point. You can achieve this using the
procedure explained previously, substituting color information for point size in the buffer.
After storing the vertex coordinates and the color in the buffer object, you will assign the
color to the attribute variable, which handles the color.
Let’s construct a sample program, MultiAttributeColor , that draws red, blue, and green
points. A screenshot is shown in Figure 5.5. (Because this book is black and white, it might
be difficult to appreciate the difference between the colors, so load and run the code in
your browser.)
Figure 5.5 MultiAttributeColor
As you may remember from Chapter 2 , “Your First Step with WebGL,” the fragment
shader actually handles attributes like color. Up until this point, you’ve set up color stati-
cally in the fragment shader code and not touched it again. However, although you have
learned how to pass the point color information to the vertex shader through the attri-
bute variable, the use of the gl_FragColor variable, which sets the color information, is
restricted to the fragment shader. (Refer to the section “Fragment Shader” in Chapter 2 .)
Therefore, you need to find a way to communicate to the fragment shader the color infor-
mation previously passed to the vertex shader ( Figure 5.6 ).
Passing Other Types of Information to Vertex Shaders
147
gl_FragColor
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
; ) … ( s r e d a h S t i n i
…
}
t n e m g a r F
r e d a h S
gl_Position
r o l o C g a r F _ l g
= n o i t i s o P _ l g
; n o i t i s o P _ a
; n o i t i s o P _ a e t u b i r t a
; r o l o C _ a e t u b i r t a
e w n a c w o H
? a t a d s s a p
r e f u B r o l o C
m e t s y S L G b e W
, 0 . 0 , 0 . 0 , 0 . 1 , 5 . 0 , 0 . 0
, 0 . 0 , 0 . 1 , 0 . 0 , 5 . 0 - , 5 . 0 -
0 . 1 , 0 . 0 , 0 . 0 , 5 . 0 - , 5 . 0
r e d a h S x e t r e V t c e j b O r e f u B
t p i r c S a v a J
Figure 5.6 Passing data from a vertex shader to a fragment shader
In ColoredPoints ( Chapter 2 ), a uniform variable was used to pass the color informa-
tion to the fragment shader; however, because it is a “uniform” variable (not varying),
it cannot be used to pass different colors for each vertex. This is where a new method to
pass data to the fragment shader through the varying variable is needed and relies on a
mechanism that sends data from the vertex shader to the fragment shader: by using the
varying variable. Let’s look at a concrete sample program.
Sample Program (MultiAttributeColor.js)
Listing 5.4 shows the program, which looks similar to the program introduced in the
previous section, MultiAttributeSize_Stride.js , but the part related to the vertex and
fragment shaders is actually slightly different.
Listing 5.4 MultiAttributeColor.js
1 // MultiAttributeColor.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'attribute vec4 a_Color;\n' +
6 'varying vec4 v_Color;\n' + // varying variable
7 'void main() {\n' +
8 ' gl_Position = a_Position;\n' +
9 ' gl_PointSize = 10.0;\n' +
10 ' v_Color = a_Color;\n' + // Pass the data to the fragment shader
11 '}\n';
12
13 // Fragment shader program
14 var FSHADER_SOURCE =
...
18 'varying vec4 v_Color;\n' +
19 'void main() {\n' +
CHAPTER 5 Using Colors and Texture Images
148
20 ' gl_FragColor = v_Color;\n' + // Receive the data from the vertex shader
21 '}\n';
22
23 function main() {
...
40 // Set vertex coordinates and color
41 var n = initVertexBuffers(gl);
...
54 gl.drawArrays(gl.POINTS, 0, n);
55 }
56
57 function initVertexBuffers(gl) {
58 var verticesColors = new Float32Array([
59 // Vertex coordinates and color
60 0.0, 0.5, 1.0, 0.0, 0.0,
61 -0.5, -0.5, 0.0, 1.0, 0.0,
62 0.5, -0.5, 0.0, 0.0, 1.0,
63 ]);
64 var n = 3; // The number of vertices
65
66 // Create a buffer object
67 var vertexColorBuffer = gl.createBuffer();
...
73 // Write the vertex coordinates and colors to the buffer object
74 gl.bindBuffer(gl.ARRAY_BUFFER, vertexColorBuffer);
75 gl.bufferData(gl.ARRAY_BUFFER, verticesColors, gl.STATIC_DRAW);
76
77 var FSIZE = verticesColors.BYTES_PER_ELEMENT;
78 // Get the storage location of a_Position, allocate buffer, & enable
79 var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
...
84 gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, FSIZE * 5, 0);
85 gl.enableVertexAttribArray(a_Position); // Enable buffer assignment
86
87 // Get the storage location of a_Color, assign buffer, and enable
88 var a_Color = gl.getAttribLocation(gl.program, 'a_Color');
...
93 gl.vertexAttribPointer(a_Color, 3, gl.FLOAT, false, FSIZE*5, FSIZE*2);
94 gl.enableVertexAttribArray(a_Color); // Enable buffer allocation
...
96 return n;
97 }
Passing Other Types of Information to Vertex Shaders
149
At line 5 of the vertex shader, an attribute variable a_Color is declared in order to receive
the color data. Next, at line 6, a new varying variable v_Color is declared that will be used
to pass its value to the fragment shader. Please note that you can only use float types (and
related types vec2 , vec3 , vec4 , mat2 , mat3 , and mat4 ) for varying variables:
5 'attribute vec4 a_Color;\n' +
6 'varying vec4 v_Color;\n' +
At line 10, the value of a_Color is assigned to the variable v_Color declared at line 6:
10 ' v_Color = a_Color;\n' +
So how can the fragment shader receive the assigned data? The answer is straightforward.
All that is required is declaring a variable in the fragment shader with the same name and
types as that in the vertex shader:
18 'varying vec4 v_Color;\n' +
In WebGL, when varying variables declared inside the fragment shader have identical
names and types to the ones declared in the vertex shader, the assigned values in the
vertex shader are automatically passed to the fragment shader (see Figure 5.7 ).
gl_FragColor
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
; ) … ( s r e d a h S t i n i
…
}
gl_Position
r = o l o C g a r F _ l g
= n o i t i s o P _ l g
; n o i t i s o P _ a
; n o i t i s o P _ a e t u b i r t a
; r o l o C _ a e t u b i r t a
r e f u B r o l o C
m e t s y S L G b e W
, 0 . 0 , 0 . 0 , 0 . 1 , 5 . 0 , 0 . 0
, 0 . 0 , 0 . 1 , 0 . 0 , 5 . 0 - , 5 . 0 -
0 . 1 , 0 . 0 , 0 . 0 , 5 . 0 - , 5 . 0
r e d a h S x e t r e V
r o l o C _ v g n i y r a v r o l o C _ v g n i y r a v
v_Color;
v_Color = a_Color;
t n e m g a r F
r e d a h S
t c e j b O r e f u B
t p i r c S a v a J
Figure 5.7 The behavior of a varying variable
So, the fragment shader can receive the values assigned to the vertex shader at
line 10 simply by assigning the varying variable v_Color to gl_FragColor at line 20.
As gl_FragColor sets the fragment color, the color of each point will be modified:
20 ' gl_FragColor = v_Color;\n' +
The remaining code is similar to MultiAttributeSize.js . The only differences are that
the name of the typed array for vertex information defined at line 58 is modified to
verticesColors , and the color information such as (1.0, 0.0, 0.0) is added to the data
definition at line 60.
CHAPTER 5 Using Colors and Texture Images
150
As previously explained in Chapter 2 , the color information is specified using the 0.0–1.0
range for each component of the RGBA model. Just like MultiAttributeSize_Stride.
js , you store several different types of data within a single array. The fifth ( stride ) and
sixth ( offset ) arguments of gl.vertexAttribPointer() are modified at lines 84 and 93,
respectively, based on the content of the verticesColors array which, because you have
introduced some color information in addition to the vertex coordinates, means the stride
changes to FSIZE * 5 .
Finally, the draw command at line 54 results in red, blue, and green points being
displayed in the browser.
Experimenting with the Sample Program
Let’s modify the first argument of gl.drawArrays() at line 54 to gl.TRIANGLES and see
what happens upon execution. Alternatively, you can load the ColoredTriangle sample
program from the book’s website:
54 gl.drawArrays(gl.TRIANGLES, 0, n);
The execution output is shown in Figure 5.8 . It might be difficult to grasp the difference
when seen in black and white, but on your screen, notice that a nice smooth-shaded
triangle with red, green, and blue corners is drawn.
Figure 5.8 ColoredTriangle
Color Triangle (ColoredTriangle.js)
151
This significant change from three colored points to a smoothly shaded triangle occurred
just by changing one parameter value. Let’s look at how that came about.
Color Triangle (ColoredTriangle.js)
You already explored the subject of coloring triangles using a single color in Chapter 3 .
This section explains how to specify a different color for each of the triangle’s vertices and
the process within WebGL that results in a smooth color transition between the different
vertices.
To fully comprehend the phenomenon, you need to understand in detail the process
carried out between the vertex and the fragment shaders, as well as the functionality of
the varying variable.
Geometric Shape Assembly and Rasterization
Let’s start the explanation using the example program, HelloTriangle.js , introduced in
Chapter 3 , which simply draws a red triangle. The relevant code snippet necessary for the
explanation is shown in Listing 5.5 .
Listing 5.5 HelloTriangle.js (Code Snippet)
1 // HelloTriangle.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'void main() {\n' +
6 ' gl_Position = a_Position;\n' +
7 '}\n';
8
9 // Fragment shader program
10 var FSHADER_SOURCE =
11 'void main() {\n' +
12 ' gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);\n' +
13 '}\n';
14
15 function main() {
...
32 // Set vertex coordinates
33 var n = initVertexBuffers(gl);
...
45 // Draw a triangle
46 gl.drawArrays(gl.TRIANGLES, 0, n);
47 }
48
49 function initVertexBuffers(gl) {
CHAPTER 5 Using Colors and Texture Images
152
50 var vertices = new Float32Array([
51 0.0, 0.5, -0.5, -0.5, 0.5, -0.5
52 ]);
53 var n = 3; // The number of vertices
...
74 gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 0, 0);
...
81 return n;
82 }
In this program, after writing the vertex coordinates (lines 50 to 52) into the buffer object
in the function initVertexBuffers() , the buffer object is assigned to the attribute variable
a_Position at line 74. Following that, when gl.drawArrays() invokes the vertex shader at
line 46, the three vertex coordinates inside the buffer object are passed to a_Position at
line 4 and assigned to gl_Position at line 6, thus making them available to the fragment
shader. In the fragment shader, the RGBA value (1.0, 0.0, 0.0, 1.0) associated with the red
color is assigned to gl_FragColor , so a red triangle is displayed.
Up until now, you haven’t actually explored how this works, so let’s examine how exactly
a fragment shader performs per-fragment operations when you only give it the triangle’s
three vertex coordinates in gl_Position .
In Figure 5.9 , you can see the problem. The program gives three vertices, but who identi-
fies that the vertex coordinates assigned to gl_Position are the vertices of a triangle? In
addition, to make the triangle look like it is filled with a single color, who decides which
fragments have to be colored? Finally, who is responsible for invoking the fragment
shader and how it handles processing for each of the fragments?
= r o l o C g a r F _ l g
; r o l o C _ v
t n e m g a r F
r e d a h S
Figure 5.9 Vertex coordinate, identification of a triangle from the vertex coordinates,
rasterization, and execution of a fragment shader
Up until now, we have glossed over these details, but there are actually two processes
taking place between the vertex and the fragment shaders, which are shown in Figure
5.10 .
• The geometric shape assembly process: In this stage, the geometric shape is assem-
bled from the specified vertex coordinates. The first argument of gl.drawArray()
specifies which type of shape should be assembled.
• The rasterization process: In this stage, the geometric shape assembled in the
geometric assembly process is converted into fragments.
Color Triangle (ColoredTriangle.js)
153
gl_FragColor
e p a h S
y l b m e s s A
a z i r e t s a R
n o i t -
r e d a h S x e t r e V
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
s y a r A w a r d . l g
; ) n , 0 , S E L G N A I R T . l g (
n o i t i s o P _ a e t u b i r t a
r e f u B
t c e j b O
5 . 0 , 0 . 0
5 . 0 - , 5 . 0 -
5 . 0 - , 5 . 0
= n o i t i s o P _ l g
; n o i t i s o P _ a
= r o l o C g a r F _ l g
; ) . . . ( 4 c e v
gl_Position
m e t s y S L G b e W
t p i r c S a v a J
t n e m g a r F
r e d a h S
Figure 5.10 Assembly and rasterization between a vertex shader and a fragment shader
As you will have realized from Figure 5.10 , gl_Position actually acts as the input to the
geometric shape assembly stage. Note that the geometric shape assembly process is also
called the primitive assembly process because the basic shapes previously shown in
Chapter 2 are also called primitives .
Figure 5.11 shows the processes between the vertex and fragment shaders, which are actu-
ally performed in assembly and rasterization for HelloTriangle.js .
From Listing 5.5 , the third argument n of gl.drawArrays() (line 46) is set to 3, meaning
that the vertex shader is actually invoked three times.
Step 1. The vertex shader is invoked, and then the first coordinate (0.0, 0.5) inside the
buffer object is passed to the attribute variable a_Position . Once this is assigned
to gl_Position , this coordinate is communicated to the geometric shape assem-
bly stage and held there. As you will remember, because only the x and y coordi-
nates are passed to a_Position , the z and w values are supplied, so actually (0.0,
0.5, 0.0,1.0) is held.
Step 2. The vertex shader is once again invoked, and in a similar way the second coordi-
nate (–0.5, –0.5) is passed to the geometric shape assembly stage and held there.
Step 3. The vertex shader is invoked a third time, passing the third coordinate (0.5, –0.5)
to the geometric shape assembly stage and holding it there.
Now the vertex shader processing is complete, and the three coordinates are
readily available for the geometric shape assembly stage.
Step 4. The geometric shape assembly processing starts. Using the three vertices
passed and the information ( gl.TRIANGLES ) contained in the first argument of
gl.drawArrays() , this stage decides how primitives should be assembled. In this
case, a triangle is assembled using the three vertices.
Step 5. Because what is displayed on the screen is a triangle consisting of fragments
(pixels), the geometric shape is converted to fragments. This process is called
rasterization . Here, the fragments that make up the triangle will be generated.
You can see the example of the generated fragments in the box of the rasteriza-
tion stage in Figure 5.11 .
CHAPTER 5 Using Colors and Texture Images
154
gl_FragColor
e p a h S
y l b m e s s A
a z i r e t s a R
n o i t -
r e d a h S x e t r e V
t p i r c S a v a J
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
s y a r A w a r d . l g
; ) n , 0 , S E L G N A I R T . l g (
n o i t i s o P _ a e t u b i r t a
r e f u B
t c e j b O
5 . 0 , 0 . 0
5 . 0 - , 5 . 0 -
5 . 0 - , 5 . 0
t n e m g a r F
r e d a h S
= n o i t i s o P _ l g
; n o i t i s o P _ a
= r o l o C g a r F _ l g
; ) . . . ( 4 c e v
) 1 ( p e t S
gl_FragColor
e p a h S
y l b m e s s A
a z i r e t s a R
n o i t -
n o i t i s o P _ a e t u b i r t a
r e f u B
t c e j b O
5 . 0 , 0 . 0
5 . 0 - , 5 . 0 -
5 . 0 - , 5 . 0
t n e m g a r F
r e d a h S
= n o i t i s o P _ l g
; n o i t i s o P _ a
= r o l o C g a r F _ l g
; ) . . . ( 4 c e v
) 2 ( p e t S
gl_Position gl_Position
gl_FragColor
e p a h S
y l b m e s s A
a z i r e t s a R
n o i t -
t p i r c S a v a J
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
s y a r A w a r d . l g
; ) n , 0 , S E L G N A I R T . l g (
n o i t i s o P _ a e t u b i r t a
5 . 0 , 0 . 0
5 . 0 - , 5 . 0 -
5 . 0 - , 5 . 0
t n e m g a r F
r e d a h S
= n o i t i s o P _ l g
; n o i t i s o P _ a
= r o l o C g a r F _ l g
; ) . . . ( 4 c e v
) 3 ( p e t S
gl_FragColor
e p a h S
y l b m e s s A
a z i r e t s a R
n o i t -
n o i t i s o P _ a e t u b i r t a
5 . 0 , 0 . 0
5 . 0 - , 5 . 0 -
5 . 0 - , 5 . 0
t n e m g a r F
r e d a h S
= n o i t i s o P _ l g
; n o i t i s o P _ a
= r o l o C g a r F _ l g
; ) . . . ( 4 c e v
) 4 ( p e t S
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
s y a r A w a r d . l g
; ) n , 0 , S E L G N A I R T . l g (
t p i r c S a v a J
gl_Position gl_Position
gl_FragColor
e p a h S
y l b m e s s A
a z i r e t s a R
n o i t -
n o i t i s o P _ a e t u b i r t a
5 . 0 , 0 . 0
5 . 0 - , 5 . 0 -
5 . 0 - , 5 . 0
t n e m g a r F
r e d a h S
= n o i t i s o P _ l g
; n o i t i s o P _ a
= r o l o C g a r F _ l g
; ) . . . ( 4 c e v
) 5 ( p e t S
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
s y a r A w a r d . l g
; ) n , 0 , S E L G N A I R T . l g (
t p i r c S a v a J
gl_Position
r e d a h S x e t r e V
r e d a h S x e t r e V
r e d a h S x e t r e V
r e d a h S x e t r e V
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
s y a r A w a r d . l g
; ) n , 0 , S E L G N A I R T . l g (
t p i r c S a v a J
r e f u B
t c e j b O
r e f u B
t c e j b O
r e f u B
t c e j b O
Figure 5.11 The processing flow of geometric shape assembly and rasterization
In this figure, although we show only 10 fragments, the actual number of fragments is
determined according to the area where the triangle is finally displayed on the screen.
If you specify a different geometric shape in the first argument of gl.drawArrays() , the
geometric shape assembled in Step 4 is modified accordingly, as are the number of frag-
ments and their position in Step 5. For example, if you specify gl.LINES , a line will be
assembled out of the first two coordinates, and the remaining one will be discarded. If you
Color Triangle (ColoredTriangle.js)
155
set it to gl.LINE_LOOP , a connected group of line segments will be generated, and only a
transparent (no fill color) triangle will be drawn.
Fragment Shader Invocations
Once the rasterization stage is completed, the fragment shader is invoked to process each
of the generated fragments. So in this example, the fragment shader is invoked 10 times,
as illustrated in Figure 5.12 . To avoid cluttering the figure, we skip the intermediate steps.
All of the fragments are fed one by one to the fragment shader, and for each fragment,
the fragment shader sets the color and writes its output to the color buffer. When the
last fragment shader process is completed at Step 15, the final output is displayed in the
browser.
Shape
Assembly
Rasteriza
-tion
Vertex
Shader
attribute a_Position
Buffer
Object
0.0, 0.5
-0.5, -0.5
0.5, -0.5
Fragment
Shader
gl_Position =
a_Position;
gl_FragColor =
vec4(...);
Step(6)
Shape
Assembly
Rasteriza
-tion
Vertex
Shader
attribute a_Position
Buffer
Object
0.0, 0.5
-0.5, -0.5
0.5, -0.5
Fragment
Shader
gl_Position =
a_Position;
gl_FragColor =
vec4(...);
Step(7)
Shape
Assembly
Rasteriza
-tion
Vertex
Shader
attribute a_Position
Buffer
Object
0.0, 0.5
-0.5, -0.5
0.5, -0.5
Fragment
Shader
gl_Position =
a_Position;
gl_FragColor =
vec4(...);
Step(15)
gl_Position
Color Buffer
Color Buffer
Color Buffer
…
gl_Position gl_Position
gl_FragColor
gl_FragColor gl_FragColor
Browser
Figure 5.12 Fragment shader invocations
The following fragment shader in HelloTriangle.js colors each fragment in red. As a
result, a red filled triangle is written to the color buffer and displayed in the browser.
9 // Fragment shader program
10 var FSHADER_SOURCE =
11 'void main() {\n' +
12 ' gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);\n' +
13 '}\n';
CHAPTER 5 Using Colors and Texture Images
156
Experimenting with the Sample Program
As an experiment, let’s confirm that the fragment shader is called for each fragment by
trying to set the color of each fragment based on its location. Each fragment generated by
the rasterization stage has its coordinates passed to the fragment shader upon invocation.
These coordinates can be accessed through the built-in variables provided inside the frag-
ment shader ( Table 5.1 ).
Table 5.1 Built-In Variables in a Fragment Shade (Input)
Type and Variable Name Description
vec4 gl_FragCoord The first and second component are the coordinates of the frag-
ment in the <canvas> coordinate system (window coordinate
system)
To check that the fragment shader is actually executed for each fragment, you can modify
line 12 in the program, as follows:
1 // HelloTriangle_FragCoord.js
...
9 // Fragment shader program
10 var FSHADER_SOURCE =
11 'precision mediump float;\n' +
12 'uniform float u_Width;\n' +
13 'uniform float u_Height;\n' +
14 'void main() {\n' +
15 ' gl_FragColor = vec4(gl_FragCoord.x/u_Width, 0.0, gl_FragCoord.y/u_Height,
➥1.0);\n' +
16 '}\n';
As you can see, the color components of each fragment, red and blue, are calculated based
on the fragment’s coordinates on the canvas. Note that the canvas’s y-axis is the inverse
direction to the WebGL coordinate system, and because it’s in WebGL, the color value
is expressed in the 0.0 to 1.0 range, you can divide the coordinates by the size of the
<canvas> element (that is, 400 pixels) to get the appropriate color value. As you can see,
the width and height are passed into the shader using the uniform variables u_Width and
u_Height and determined from gl.drawingBufferWidth and gl.drawingBufferHeight .
You can see the execution result in Figure 5.13 , which is a triangle whose fragments are
colored as a function of their position. Running this sample program HelloTriangle_
FragCoord , you will see the transition from the left top to the right bottom.
Color Triangle (ColoredTriangle.js)
157
Drawing area of canvas
(0.0, 0.0)
(400, 400)
x
y
Figure 5.13 Modifying the color per fragment (the figure on the right side shows the <canvas>
coordinate system)
Because you modify the color of each fragment with respect to its coordinates, you will
notice that the color progressively changes according to the coordinates. Again, if this is
not clear from the black-and-white image in Figure 5.13 , run the example from the book’s
website.
Functionality of Varying Variables and the Interpolation Process
At this stage, you have a better understanding of the process flow taking place between
the vertex and the fragment shaders (that is, the geometric shape assembly process and
the subsequent rasterization process), as well as the invocation of the fragment shader for
each of the generated fragments.
Returning to Figure 5.8 , the first ColoredTriangle , let’s use what you’ve learned to under-
stand better why you get such a nicely shaded triangle when you specify a different color
for each the triangle’s vertex. Previously, you saw that the value assigned to the varying
variable inside the vertex shader is passed as a varying variable with identical attributes
(same name, same type) to the fragment shader (see Figure 5.14 ). However, to be more
precise, the value assigned to the varying variable in the vertex shader is interpolated at
the rasterization stage. Consequently, the value passed to the fragment shader actually
differs for each fragment based on that interpolation (see Figure 5.15 ). This is the reason
the varying variable has the name “varying.”
CHAPTER 5 Using Colors and Texture Images
158
gl_FragColor
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
; ) … ( s r e d a h S t i n i
…
}
gl_Position
r = o l o C g a r F _ l g
= n o i t i s o P _ l g
; n o i t i s o P _ a
; n o i t i s o P _ a e t u b i r t a
; r o l o C _ a e t u b i r t a
r e f u B r o l o C
m e t s y S L G b e W
, 0 . 0 , 0 . 0 , 0 . 1 , 5 . 0 , 0 . 0
, 0 . 0 , 0 . 1 , 0 . 0 , 5 . 0 - , 5 . 0 -
0 . 1 , 0 . 0 , 0 . 0 , 5 . 0 - , 5 . 0
r e d a h S x e t r e V
r o l o C _ v g n i y r a v r o l o C _ v g n i y r a v
v_Color;
v_Color = a_Color;
t n e m g a r F
r e d a h S
t c e j b O r e f f u B
t p i r c S a v a J
Figure 5.14 The behavior of a varying variable (reprint of Figure 5.7 )
t n e m g a r F
r e d a h S
gl_Position
; r o l o C _ v g n i y r a v
r e d a h S x e t r e V
= r o l o C g a r F _ l g
; r o l o C _ v
= n o i t i s o P _ l g
; n o i t i s o P _ a
= r o l o C _ v ; r o l o C _ a
; n o i t i s o P _ a e t u b i r t a
; r o l o C _ a e t u b i r t a
, 0 . 0 , 0 . 0 , 0 . 1 , 5 . 0 , 0 . 0
, 0 . 0 , 0 . 1 , 0 . 0 , 5 . 0 - , 5 . 0 -
0 . 1 , 0 . 0 , 0 . 0 , 5 . 0 - , 5 . 0
r e f f u B r o l o C
m e t s y S L G b e W
gl_FragColor
e p a h S
y l b m e s s A
; r o l o C _ v g n i y r a v
a z i r e t s a R
n o i t
a l o p r e t n I
n o i t
t c e j b O r e f f u B
Figure 5.15 Interpolation of a varying variable
More specifically, in ColoredTriangle , because we only assign a different value to the
varying variable for each of the three vertices, each fragment located between vertices
must have its own color interpolated by the WebGL system.
For example, let’s consider the case in which the two end points of a line are specified
with different colors. One of the vertices is red (1.0, 0.0 0.0), whereas the other one is blue
(0.0, 0.0, 1.0). After the colors (red and blue) are assigned to the vertex shader’s v_Color ,
the RGB values for each of the fragments located between those two vertices are calculated
and passed to the fragment shader’s v_Color (see Figure 5.16 ).
d e R
) 0 . 0 , 0 . 0 , 0 . 1 (
e u l B
) 0 . 1 , 0 . 0 , 0 . 0 (
n o i t a z i r e t s a R
a n i r o l o C _ v
r e d a h s x e t r e v
d e t a r e n e g s r o l o c e h T
n o i t a l o p r e t n i y b
) 0 . 0 , 0 . 0 , 0 . 1 ( d e R
) 5 2 . 0 , 0 . 0 , 5 7 . 0 ( r o l o c d e t a l o p r e t n I
) 0 5 . 0 , 0 . 0 , 0 5 . 0 ( r o l o c d e t a l o p r e t n I
) 5 7 . 0 , 0 . 0 , 5 2 . 0 ( r o l o c d e t a l o p r e t n I
) 0 . 1 , 0 . 0 , 0 . 0 ( e u l B
a n i r o l o C _ v
r e d a h s t n e m g a r f
Figure 5.16 Interpolation of color values
Color Triangle (ColoredTriangle.js)
159
In this case, R decreases from 1.0 to 0.0, B increases from 0.0 to 1.0, and all the RGB
values between the two vertices are calculated appropriately—this is called the interpola-
tion process . Once the new color for each of the fragments located between the two verti-
ces is calculated in this way, it is passed to the fragment shader’s v_Color .
We follow an identical procedure in the case of the colored triangle, which is reproduced
in Listing 5.6 . After the three vertices’ colors are assigned to the varying variable v_Color
(line 9), the interpolated color for each fragment is passed to the fragment shader’s
v_Color . Once this is assigned to the gl_FragColor at line 19, a colored triangle is drawn,
as shown in Figure 5.8 . This interpolation process is carried out for each of the varying
variables. If you want to understand more about this process, a good source of informa-
tion is the book Computer Graphics .
Listing 5.6 ColoredTriangle.js
1 // ColoredTriangle.js
2 // Vertex shader program
3 var VSHADER_SOURCE = '\
...
6 varying vec4 v_Color;\
7 void main() {\
8 gl_Position = a_Position;\
9 v_Color = a_Color;\ <- The color at line 59 is assigned to v_Color
10 }';
11
12 // Fragment shader program
13 var FSHADER_SOURCE =
...
17 varying vec4 v_Color;\ <- The interpolated color is passed to v_Color
18 void main() {\
19 gl_FragColor = v_Color;\ <- The color is assigned to gl_FragColor
20 }';
21
22 function main() {
...
53 gl.drawArrays(gl.TRIANGLES, 0, n);
54 }
55
56 function initVertexBuffers(gl) {
57 var verticesColors = new Float32Array([
58 // Vertex coordinates and color
59 0.0, 0.5, 1.0, 0.0, 0.0,
60 -0.5, -0.5, 0.0, 1.0, 0.0,
61 0.5, -0.5, 0.0, 0.0, 1.0,
62 ]);
...
99 }
CHAPTER 5 Using Colors and Texture Images
160
In summary, this section has highlighted the critical rasterization process that takes place
between the vertex and fragment shaders. Rasterization is a key component of 3D graph-
ics and is responsible for taking geometric shapes and building up the fragments that will
draw those shapes. After converting the specified geometric shape into fragments (raster-
ization), it’s possible to set a different color for each of the fragments inside the fragment
shader. This color can be interpolated or set directly by the programmer.
Pasting an Image onto a Rectangle
In the previous section, you explored how to use color when drawing shapes and how
interpolation creates smooth color transitions. Although powerful, this approach is limited
when it comes to reproducing complex visual representations. For example, a problem
arises if you want to create a wall that has the look and feel of the one shown in Figure
5.17 , you would need many triangles, and determining the color and coordinates for each
triangle would prove to be daunting.
Figure 5.17 An example of a complex wall surface
As you’d imagine, in 3D graphics, one of the most important processes is actually solving
this problem. The problem is resolved using a technique called texture mapping , which
can re-create the look of real-world materials. The process is actually straightforward and
consists of pasting an image (like a decal) on the surface of a geometrical shape. By pasting
an image from a real-world photograph on a rectangle made up of two triangles, you can
give the rectangle surface an appearance similar to that of a picture. The image is called a
texture image or a texture .
The role of the texture mapping process is to assign the texture image’s pixel colors to the
fragments generated by the rasterization process introduced in the previous section. The
pixels that make up the texture image are called texels (texture elements), and each texel
codes its color information in the RGB or RGBA format (see Figure 5.18 ).
Pasting an Image onto a Rectangle
161
Enlargement
Each square is a texel
Figure 5.18 Texels
Texture mapping involves the following four steps in WebGL:
1. Prepare the image to be mapped on the geometric shape.
2. Specify the image mapping method for the geometric shape.
3. Load the texture image and configure it for use in WebGL.
4. Extract the texels from the image in the fragment shader, and accordingly set the
corresponding fragment.
To understand the mechanisms involved in texture mapping, let’s examine the sample
program TextureQuad , which “pastes” an image onto a rectangle. If you run it from the
book’s website, you’ll see the result as shown in Figure 5.19 (left).
Note When you want to run the sample programs that use texture images in Chrome
from your local disk, you should add the option --allow-file-access-from-files to
Chrome. This is for security reasons. Chrome, by default, does not allow access to local
files such as ../resources/sky.jpg . For Firefox, the equivalent parameter, set via
about:config , is security.fileuri.strict_origin_policy , which should be set to
false . Remember to set it back when you’re finished because you open a security loop-
hole if local file access is enabled.
sky.jpg
Texture image
Figure 5.19 TextureQuad (left) and the texture image used (right)
Looking in a little more detail at steps (1) to (4) in the following sections, the image
prepared in (1) can be any format that can be displayed in a browser. For now, you can
CHAPTER 5 Using Colors and Texture Images
162
use any pictures you might have taken yourself or alternatively you can use the images
located in the resource folder of the companion website provided with this book.
The mapping method specified in (2) consists of designating “which part of the texture
image” should be pasted to “which part of the geometric shape”. The part of the geomet-
ric shape meant to be covered with the texture is specified using the coordinates of the
vertices that compose a surface. The part of the texture image to be used is specified using
texture coordinates . These are a new form of coordinates so let’s look at how they work.
Texture Coordinates
The texture coordinate system used in WebGL is two-dimensional, as shown in Figure
5.20 . To differentiate the texture coordinates from the widely used x and y axis, WebGL
changes the denomination to the s and t coordinates (st coordinates system).
2
Texture Image
Text at the texture coordinate
(0.7, 0.4)
(0.0, 1.0)
(0.0, 0.0) (1.0, 0.0)
(1.0, 1.0)
t
s
Figure 5.20 WebGL’s Texture coordinate system
As you can see from Figure 5.20 , the coordinates of the four corners are defined as left
bottom corner (0.0, 0.0), right bottom corner (1.0, 0.0), right top corner (1.0, 1.0), and
left top corner (0.0, 1.0). Because these values are not related to the image size, this allows
a common approach to image handling; for example, whether the texture image’s size is
128×128 or 128×256, the right top corner coordinates will always be
(1.0, 1.0).
Pasting Texture Images onto the Geometric Shape
As previously mentioned, in WebGL, by defining the correspondence between the texture
coordinates and the vertex coordinates of the geometric shape, you can specify how the
texture image will be pasted (see Figure 5.21 ).
2 The uv coordinates are often used. However, we are using st coordinates because GLSL ES uses the
component names to access the texture image.
Pasting an Image onto a Rectangle
163
Texture coordinate system
WebGL coordinate system The resulting image produced by WebGL
(0.0, 1.0)
(0.0, 0.0) (1.0, 0.0)
(1.0, 1.0)
t
s
x
y
(-05, 0.5, 0.0) (05, 0.5, 0.0)
(-05, -0.5, 0.0) (05, -0.5, 0.0)
Figure 5.21 Texture coordinates and mapping them to vertices
Here, the texture coordinates (0.0, 1.0) are mapped onto the vertex coordinates (–0.5, 0.5,
0.0), and the texture coordinates (1.0, 1.0) are mapped onto the vertex coordinates (0.5,
0.5, 0.0). By establishing the correspondence for each of the four corners of the texture
image, you obtain the result shown in the right part of Figure 5.21 .
Now, given your understanding of how images can be mapped to shapes, let’s look at the
sample program.
Sample Program (TexturedQuad.js)
In TexturedQuad.js (see Listing 5.7 ), the texture mapping affects both the vertex and the
fragment shaders. This is because it sets the texture coordinates for each vertex and then
applies the corresponding pixel color extracted from the texture image to each fragment.
There are five main parts to the example, each identified by the numbers to the right of
the code.
Listing 5.7 TexturedQuad.js
1 // TexturedQuad.js
2 // Vertex shader program <- (Part1)
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'attribute vec2 a_TexCoord;\n' +
6 'varying vec2 v_TexCoord;\n' +
7 'void main() {\n' +
8 ' gl_Position = a_Position;\n' +
9 ' v_TexCoord = a_TexCoord;\n' +
10 '}\n';
11
12 // Fragment shader program <- (Part2)
13 var FSHADER_SOURCE =
...
17 'uniform sampler2D u_Sampler;\n' +
18 'varying vec2 v_TexCoord;\n' +
19 'void main() {\n' +
CHAPTER 5 Using Colors and Texture Images
164
20 ' gl_FragColor = texture2D(u_Sampler, v_TexCoord);\n' +
21 '}\n';
22
23 function main() {
...
40 // Set the vertices information <- (Part3)
41 var n = initVertexBuffers(gl);
...
50 // Setting the textures
51 if (!initTextures(gl, n)) {
...
54 }
55 }
56
57 function initVertexBuffers(gl) {
58 var verticesTexCoords = new Float32Array([
59 // Vertices coordinates, textures coordinates
60 -0.5, 0.5, 0.0, 1.0,
61 -0.5, -0.5, 0.0, 0.0,
62 0.5, 0.5, 1.0, 1.0,
63 0.5, -0.5, 1.0, 0.0,
64 ]);
65 var n = 4; // The number of vertices
66
67 // Create the buffer object
68 var vertexTexCoordBuffer = gl.createBuffer();
...
74 // Write the vertex coords and textures coords to the object buffer
75 gl.bindBuffer(gl.ARRAY_BUFFER, vertexTexCoordBuffer);
76 gl.bufferData(gl.ARRAY_BUFFER, verticesTexCoords, gl.STATIC_DRAW);
77
78 var FSIZE = verticesTexCoords.BYTES_PER_ELEMENT;
...
85 gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, FSIZE * 4, 0);
86 gl.enableVertexAttribArray(a_Position); // Enable buffer allocation
87
88 // Allocate the texture coordinates to a_TexCoord, and enable it.
89 var a_TexCoord = gl.getAttribLocation(gl.program, 'a_TexCoord');
...
94 gl.vertexAttribPointer(a_TexCoord, 2, gl.FLOAT, false, FSIZE * 4, FSIZE * 2);
95 gl.enableVertexAttribArray(a_TexCoord); // Enable buffer allocation
...
97 return n;
98 }
99
Pasting an Image onto a Rectangle
165
100 function initTextures(gl, n) <- (Part4)
101 var texture = gl.createTexture(); // Create a texture object
...
107 // Get the storage location of the u_Sampler
108 var u_Sampler = gl.getUniformLocation(gl.program, 'u_Sampler');
...
114 var image = new Image(); // Create an image object
...
119 // Register the event handler to be called on loading an image
120 image.onload = function(){ loadTexture(gl, n, texture, u_Sampler, image); };
121 // Tell the browser to load an image
122 image.src = '../resources/sky.jpg';
123
124 return true;
125 }
126
127 function loadTexture(gl, n, texture, u_Sampler, image){ <- (Part5)
128 gl.pixelStorei(gl.UNPACK_FLIP_Y_WEBGL, 1); // Flip the image's y axis
129 // Enable the texture unit 0
130 gl.activeTexture(gl.TEXTURE0);
131 // Bind the texture object to the target
132 gl.bindTexture(gl.TEXTURE_2D, texture);
133
134 // Set the texture parameters
135 gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
136 // Set the texture image
137 gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGB, gl.RGB, gl.UNSIGNED_BYTE, image);
138
139 // Set the texture unit 0 to the sampler
140 gl.uniform1i(u_Sampler, 0);
...
144 gl.drawArrays(gl.TRIANGLE_STRIP, 0, n); // Draw a rectangle
145 }
This program is structured into five main parts:
Part 1: Receive the texture coordinates in the vertex shader and then pass them to the
fragment shader.
Part 2: Paste the texture image onto the geometric shape inside the fragment shader.
Part 3: Set the texture coordinates ( initVertexBuffers() ).
Part 4: Prepare the texture image for loading, and request the browser to read it.
( initTextures() ).
Part 5: Configure the loaded texture so that it can be used in WebGL ( loadTexture() ).
CHAPTER 5 Using Colors and Texture Images
166
Let’s look at the sequence starting from Part 3: the process to set the texture coordinates
using initVertexBuffers() . The shaders are executed after loading the image, so we will
explain them at the end.
Using Texture Coordinates (initVertexBuffers())
You pass texture coordinates to the vertex shader using the same approach you’ve been
using to pass other vertex data to the vertex shader, by combining vertex coordinates and
vertex data into a single buffer. Line 58 defines an array verticesTexCoords containing
pairs of vertex coordinates and their associated texture coordinates:
58 var verticesTexCoords = new Float32Array([
59 // Vertex coordinates and texture coordinates
60 -0.5, 0.5, 0.0, 1.0,
61 -0.5, -0.5, 0.0, 0.0,
62 0.5, 0.5, 1.0, 1.0,
63 0.5, -0.5, 1.0, 0.0,
64 ]);
As you can see, the first vertex (–0.5, 0.5) is mapped to the texture coordinate (0.0, 1.0),
the second vertex (–0.5, –0.5) is mapped to the texture coordinate (0.0, 0.0), the third
vertex (0.5, 0.5) is mapped to the texture coordinate (1.0, 10), and the fourth vertex (0.5,
–0.5) is mapped to the texture coordinate (1.0, 0.0). Figure 5.21 illustrates these mappings.
Lines 75 to 86 then write vertex coordinates and texture coordinates to the buffer object,
assign it to a_Position , and enable the assignment. After that, lines 89 to 94 retrieve the
storage location of the attribute variable a_TexCoord and then assign the buffer object
containing the texture coordinates to the variable. Finally, line 95 enables the assignment
of the buffer object to a_TexCoord :
88 // Assign the texture coordinates to a_TexCoord, and enable it.
89 var a_TexCoord = gl.getAttribLocation(gl.program, 'a_TexCoord');
...
94 gl.vertexAttribPointer(a_TexCoord, 2, gl.FLOAT, false, FSIZE * 4,
➥FSIZE * 2);
95 gl.enableVertexAttribArray(a_TexCoord);
Setting Up and Loading Images (initTextures())
This process is performed from lines 101 to 122 in initTextures() . Line 101 creates
a texture object ( gl.createTexture() ) for managing the texture image in the WebGL
system, and line 108 gets the storage location of a uniform variable ( gl.getUniformLoca-
tion() ) to pass the texture image to the fragment shader () :
101 var texture = gl.createTexture(); // Create a texture object
...
108 var u_Sampler = gl.getUniformLocation(gl.program, 'u_Sampler');
Pasting an Image onto a Rectangle
167
A texture object is created using gl.createTexture() .
gl.createTexture()
Create a texture object to hold a texture image.
Parameters None
Return value non-null The newly created texture object.
null Failed to create a texture object.
Errors None
This call creates the texture object in the WebGL system, as shown in Figure 5.22 .
gl.TEXTURE0 to gl.TEXTURE7 are texture units for managing a texture image, and each has
an associated gl.TEXTURE_2D , which is the texture target for specifying the type of texture.
This will be explained in detail later.
t n e m g a r F
r e d a h S
e l b a i r a v m r o f i n u
e l b a i r a v m r o f i n u
t p i r c S a v a J
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
0 E R U T X E T . lg … 1 E R U T X E T . l g 7 E R U T X E T . l g
…
t n e m g a r F
r e d a h S
e r u t x e T
t c e j b O
t p i r c S a v a J
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
; ) … ( e r u t x e T e t a e r c . l g
… 1 E R U T X E T . l g 0 E R U T X E T . l g 7 E R U T X E T . l g
D 2 _ E R U T X E T . l g
…
D 2 _ E R U T X E T . l g
D 2 _ E R U T X E T . l g D 2 _ E R U T X E T . l g
D 2 _ E R U T X E T . l g
D 2 _ E R U T X E T . l g
Figure 5.22 Create a texture object
The texture object can be deleted using gl.deleteTexture() . Note, if this method is called
with a texture object that has already been deleted, the call has no effect.
gl.deleteTexture(texture)
Delete the texture object specified by texture.
Parameter texture Specifies the texture object to be deleted.
Return value None
Errors None
CHAPTER 5 Using Colors and Texture Images
168
In the next step, it’s necessary to request that the browser load the image that will be
mapped to the rectangle. You need to use an Image object for this purpose:
114 var image = new Image() ; // Create an Image object
...
119 // Register an event handler to be called when image loading completes
120 image.onload = function(){ loadTexture(gl, n, texture, u_Sampler, image); };
121 // Tell the browser to load an image
122 image.src = '../resources/sky.jpg';
This code snippet creates an Image object, registers the event handler ( loadTexture() ) to
be called on loading the image, and tells the browser to load the image.
You need to create an Image object (a special JavaScript object that handles images) using
the new operator, just as you would do for an Array object or Date object. This is done at
line 114.
114 var image = new Image(); // Create an Image object
Because loading of images is performed asynchronously (see the boxed article that
follows), when the browser signals completion of loading, it needs to pass the image
to the WebGL system. Line 120 handles this, telling the browser that, after loading the
image, the anonymous function loadTexture() should be called.
120 image.onload = function(){ loadTexture(gl, n, texture, u_Sampler, image ); };
loadTexture() takes five parameters, with the newly loaded image being passed via
the variable ( Image object) as the last argument image . gl is the rendering context for
WebGL, n is the number of vertices, texture is the texture object created at line 101, and
u_Sampler is the storage location of a uniform variable.
Just like the <img> tag in HTML, we can tell the browser to load the texture image by
setting the image filename to the property src of the Image object (line 122). Note that
WebGL, because of the usual browser security restrictions, is not allowed to use images
located in other domains for texture images:
122 image.src = '../resources/sky.jpg';
After executing line 122, the browser starts to load the image asynchronously, so the
program continues on to the return statement at line 124 and then exits. When the
browser finishes loading the image and wants to pass the image to the WebGL system, the
event handler loadTexture() is called.
Pasting an Image onto a Rectangle
169
Asynchronous Loading Texture Images
Usually, OpenGL applications written in C or C++ load the texture image files straight
from the hard disk where they are stored. However, because WebGL programs are
running inside the browser, it is impossible to load images directly. Instead, it is
necessary to read images indirectly by requesting the browser to do it. (Typically, the
browser sends a request to the web server to obtain the image.) The advantage is that
you can use any kind of image a browser can display, but it also makes the process
more complex because you now have to handle two processes (the browser loading
request, and the actual WebGL loading) that behave “asynchronously” (they run in the
background) and thus do not block execution of the program.
Figure 5.23 shows the substeps between [1] tell the browser to load an image and [7] call
the function loadTexture() after completing loading the texture image.
JavaScript
{ ) ( s e r u t x e T t i n i
…
. e g a m i n a g n i d a o l n o t n e v e e h t r e t s i g e R /
; } ; ) … ( e r u t x e T d a o l { ) ( n o i t c n u f = d a o l n o . e g a m i
. e g a m i n a d a o l o t r e s w o r b e h t l l e T /
; ' g p j . y k s / s e c r u o s e r / . . ' = c r s . e g a m i
…
}
{ ) … ( e r u t x e T d a o l n o i t c n u f
…
}
l l a c o t r e s w o r b e h t l l e T ] 1 [
g n i d a o l r e t f a ) ( e r u t x e T d a o l
. e g a m i n a
. ) ( e r u t x e T d a o l l l a C ] 7 [
o t r e s w o r b e h t l l e T ] 2 [
. e g a m i n a d a o l
b e W
r e v r e s
. e g a m i d a o L ] 4 [
Image file
) g p j . y k s (
] 5 [
e t e l p m o C
n a g n i v e i r t e r
. e g a m i
e h t t s e u q e R ] 3 [
o t r e v r e s b e W
n a e v e i r t e r
. e g a m i
e t e l p m o C ] 6 [
. e g a m i n a g n i d a o l
r e s w o r B
Figure 5.23 Asynchronous loading texture images
In Figure 5.23 , [1] and [2] are executed sequentially, but [2] and [7] are not. After
requesting the browser to load an image in [2], the JavaScript program doesn’t wait for
the image to be loaded, but proceeds to the next stage. (This behavior will be explained
in detail in a moment.) While the JavaScript program is continuing, the browser
sends a request to the web server for the image [3]. When the image loading process is
completed [4] and [5], the browser tells the JavaScript program that the image loading
has completed [6]. This kind of behavior is referred to as asynchronous .
The image loading process is analogous to the way a web page written in HTML displays
images. In HTML, an image is displayed by specifying the file URL to the src attribute of
the <img> tag (below) causing the browser to load the image from the specified URL. This
part corresponds to [2] shown in Figure 5.23 .
<img src="../resources/redflower.jpg">
CHAPTER 5 Using Colors and Texture Images
170
The asynchronous nature of the image loading process can be easily understood by
considering how a web page that includes numerous images is displayed. Typically,
the page text and layout are displayed rapidly, and then images appear slowly as
they are loaded. This is because the image loading and display processes are executed
asynchronously, allowing you to view and interact with the web page without having to
wait for all images to load.
Make the Texture Ready to Use in the WebGL System (loadTexture())
The function loadTexture() is defined as follows:
127 function loadTexture(gl, n, texture, u_Sampler, image) { <- (Part5)
128 gl.pixelStorei(gl.UNPACK_FLIP_Y_WEBGL, 1); // Flip the image's y axis
129 // Enable the texture unit 0
130 gl.activeTexture(gl.TEXTURE0);
131 // Bind the texture object to the target
132 gl.bindTexture(gl.TEXTURE_2D, texture);
133
134 // Set the texture parameters
135 gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
136 // Set the texture image
137 gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGB, gl.RGB, gl.UNSIGNED_BYTE, image);
138
139 // Set the texture unit 0 to the sampler
140 gl.uniform1i(u_Sampler, 0);
...
144 gl.drawArrays(gl.TRIANGLE_STRIP, 0, n); // Draw a rectangle
145 }
Its main purpose is to prepare the image for use by the WebGL system, which it does
using a texture object that is prepared and used in a similar manner to a buffer object.
The following sections explain the code in more detail.
Flip an Image’s Y-Axis
Before using the loaded images as a texture, you need to flip the y-axis:
128 gl.pixelStorei(gl.UNPACK_FLIP_Y_WEBGL, 1);// Flip the image's y-axis
This method flips an image’s Y-axis when it’s loaded. As shown in Figure 5.24 , the t-axis
direction of the WebGL texture coordinate system is the inverse of the y-axis direction of
the coordinate system used by PNG, BMP, JPG, and so on. For this reason, if you flip the
image’s Y-axis, you can map the image to the shape correctly. (You could also flip the t
coordinates by hand instead of flipping the image.)
Pasting an Image onto a Rectangle
171
WebGL coordinate system
t
s
Image coordinate system
y
x
Figure 5.24 The image coordinate system and WebGL texture coordinate system
The following explains gl.pixelStorei() .
gl.pixelStorei(pname, param)
Perform the process defined by pname and param after loading an image.
Parameters pname Specifies any of the following:
gl.UNPACK_FLIP_Y_WEBGL Flips an image’s Y-axis after loading the
image. The default value is false .
gl.UNPACK_PREMULTIPLY_
ALPHA_WEBGL
Multiplies each component of RGB in an
image by A in the image. The default value is
false .
param Specifies none-zero (means true ) or zero
(means false ). It must be specified in the
integer.
Return value None
Errors INVALID_ENUM pname is none of these values.
Making a Texture Unit Active (gl.activeTexture())
WebGL supports multiple texture images (multitexturing) using a mechanism called
a texture unit . A texture unit manages texture images by using a unit number for each
texture. Because of this, even if you only want to use a single texture image, you must
specify and use a texture unit.
CHAPTER 5 Using Colors and Texture Images
172
The number of texture units supported varies according to your hardware and WebGL
implementation, but by default at least eight texture units are supported, and some
systems will support more. The built-in constants, gl.TEXTURE0 , gl.TEXTURE1 , ..., and
gl.TEXTURE7 , represent each texture unit (see Figure 5.25 ).
t n e m g a r F
r e d a h S
e l b a i r a v m r o f i n u
t p i r c S a v a J
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
D 2 _ E R U T X E T . l g
Texture unit
… 1 E R U T X E T . l g 0 E R U T X E T . l g 7 E R U T X E T . l g
D 2 _ E R U T X E T . l g
D 2 _ E R U T X E T . l g
Figure 5.25 Multiple texture units managed by WebGL
Before using a texture unit, it must be made active using a call to gl.activeTexture() (see
Figure 5.26 ):
132 // Make the texture unit 0 active
133 gl.activeTexture(gl.TEXTURE0);
gl.activeTexture(texUnit)
Make the texture unit specified by texUnit active.
Parameters texUnit Specifies the texture unit to be made active: gl.TEXTURE0 ,
gl.TEXTURE1 , ..., or gl.TEXTURE7 . The tailing number indi-
cates the texture unit number.
Return value None
Errors INVALID_ENUM: texUnit is none of these values
t n e m g a r F
r e d a h S
e l b a i r a v m r o f i n u
D 2 _ E R U T X E T . l g
… 1 E R U T X E T . l g 0 E R U T X E T . l g 7 E R U T X E T . l g
D 2 _ E R U T X E T . l g
D 2 _ E R U T X E T . l g
e r u t x e T
t c e j b O
; ) … ( e r u t x e T e v i t c a . l g
t p i r c S a v a J
… L G b e W t e g = l g r a v
…
{ ) ( n i a m n o i t c n u f
Figure 5.26 Activate texture unit (gl.TEXTURE0)
Pasting an Image onto a Rectangle
173
Binding a Texture Object to a Target (gl.bindTexture())
Next, you need to tell the WebGL system what types of texture image is used in the
texture object. You do this by binding the texture object to the target in a similar way to
that of the buffer objects explained in the previous chapter. WebGL supports two types of
textures, as shown in Table 5.2 .
Table 5.2 Types of Textures
Type of Texture Description
gl.TEXTURE_2D Two-dimensional texture
gl.TEXTURE_CUBE_MAP Cube map texture
The sample program uses a two-dimensional image as a texture and specifies gl.
TEXTURE_2D at line 132. The cube map texture is beyond the scope of this book. If you are
interested in more information, please refer to the book OpenGL ES 2.0 Programming Guide :
131 // Bind the texture object to the target
132 gl.bindTexture(gl.TEXTURE_2D, texture);
gl.bindTexture(target, texture)
Enable the texture object specified by texture and bind it to the target . In addition, if a
texture unit was made active by gl.activeTexture() , the texture object is also bound to
the texture unit.
Parameters target Specifies gl.TEXTURE_2D or gl.TEXTURE_CUBE_MAP .
texture Specifies the texture object to be bound.
Return value None
Errors INVALID_ENUM target is none of these values.
Note that this method performs two tasks: enabling the texture object and binding it
to target, and binding it to the texture unit. In this case, because the texture unit 0
( gl.TEXTURE0 ) is active, after executing line 136, the internal state of the WebGL
system is changed, as shown in Figure 5.27 .
CHAPTER 5 Using Colors and Texture Images
174
t n e m g a r F
r e d a h S
e l b a i r a v m r o f i n u
… 1 E R U T X E T . l g 0 E R U T X E T . l g 7 E R U T X E T . l g
D 2 _ E R U T X E T . l g D 2 _ E R U T X E T . l g
e r u t x e T
t c e j b O
t p i r c S a v a J
… L G b e W t e g = l g r a v
…
{ ) ( n i a m n o i t c n u f
; ) … ( e r u t x e T d n i b . l g
D 2 _ E R U T X E T . l g
Figure 5.27 Bind a texture object to the target
At this stage, the program has specified the type of texture that is used in the texture
object ( gl.TEXTURE_2D ) and that will be used to deal with the texture object in the future.
This is important, because in WebGL, you cannot manipulate the texture object directly.
You need to do that through the binding.
Set the Texture Parameters of a Texture Object (gl.texParameteri())
In the next step, you need to set the parameters (texture parameter) that specify how the
texture image will be processed when the texture image is mapped to shapes. The generic
function gl.texParameteri() can be used to set texture parameters.
gl.texParameteri(target, pname, param)
Set the value specified by param to the texture parameter specified by pname in the
texture object bound to target .
Parameters target Specifies gl.TEXTURE_2D or gl.TEXTURE_CUBE_MAP .
pname Specifies the name of the texture parameter ( Table 5.3 ).
param Specifies the value set to the texture parameter pname
( Table 5.4 , Table 5.5 ).
Return value None
Errors INVALID_ENUM target is none of the preceding values
INVALID_OPERATION no texture object is bound to target
There are four texture parameters available, illustrated in Figure 5.28 , which you can
specify to pname :
Pasting an Image onto a Rectangle
175
• Magnification method ( gl.TEXTURE_MAG_FILTER ): The method to magnify a texture
image when you map the texture to a shape whose drawing area is larger than the
size of the texture. For example, when you map a 16×16 pixel image to a 32×32 pixel
shape, the texture should be doubled in size. WebGL needs to fill the gap between
texels due to the magnification, and this parameter specifies the method used to fill
the gap.
• Minification method ( gl.TEXTURE_MIN_FILTER ): The method of minifying a texture
image when you map the texture to a shape whose drawing area is smaller than
the size of the texture. For example, when you map a 32×32 pixel image to a 16×16
pixel shape, the texture should be reduced in size. To do that, the system needs to
cull texels to fit the target size. This parameter specifies the method used to cull
texels.
• Wrapping method on the left and right side ( gl.TEXTURE_WRAP_S ): How to fill the
remaining regions on the left side and the right side of a subregion when you map a
texture image to the subregion of a shape.
• Wrapping method on top and bottom ( gl.TEXTURE_WRAP_T ): Similar to (3), the
method used to fill the remaining regions in the top and bottom of a subregion.
The method to fill this region.
gl.TEXTURE_MAG_FILTER
gl.TEXTURE_WRAP_T
gl.TEXTURE_MIN_FILTER
gl.TEXTURE_WRAP_S
Magnification
Minification
The method to fill this region.
Figure 5.28 Four texture parameters and their effects
Table 5.3 shows each texture parameter and its default value.
CHAPTER 5 Using Colors and Texture Images
176
Table 5.3 Texture Parameters and Their Default Values
Texture Parameter Description Default Value
gl.TEXTURE_MAG_FILTER Texture magnification gl.LINEAR
gl.TEXTURE_MIN_FILTER Texture minification gl.NEAREST_MIPMAP_LINEAR
gl.TEXTURE_WRAP_S Texture wrapping in s-axis gl.REPEAT
gl.TEXTURE_WRAP_T Texture wrapping in t-axis gl.REPEAT
We also show the constant values that can be specified to gl.TEXTURE_MAG_FILTER and
gl.TEXTURE_MIN_FILTER in Table 5.4 and gl.TEXTURE_WRAP_S and gl.TEXTURE_WRAP_T in
Table 5.5 .
Table 5.4 Non-Mipmapped Values, Which Can be Specified to gl.TEXTURE_MAG_FILTER and
gl.TEXTURE_MIN_FILTER
3
Value Description
gl.NEAREST Uses the value of the texel that is nearest (in Manhattan distance) the center
of the pixel being textured.
gl.LINEAR Uses the weighted average of the four texels that are nearest the center
of the pixel being textured. (The quality of the result is clearer than that of
gl.NEAREST , but it takes more time.)
Table 5.5 Values that Can be Specified to gl.TEXTURE_WRAP_S and gl.TEXTURE_WRAP_T
Value Description
gl.REPEAT Use a texture image repeatedly
gl.MIRRORED_REPEAT Use a texture image mirrored-repeatedly
gl.CLAMP_TO_EDGE Use the edge color of a texture image
As shown in Table 5.3 , each parameter has a default value, and you can generally use the
default value as is. However, the default value of gl.TEXTURE_MIN_FILTER is for a special
texture format called MIPMAP . A MIPMAP is a sequence of textures, each of which is
a progressively lower resolution representation of the same image. Because a MIPMAP
texture is not often used, we don’t cover it in this book. For this reason, you set the value
gl.LINEAR to the texture parameter gl.TEXTURE_MIN_FILTER at line 135:
3 Although omitted in this table, other values can be specifi ed for a MIPMAP texture: gl.NEAREST_
MIPMAP_NEAREST , gl.LINEAR_MIPMAP_NEAREST , gl.NEAREST_MIPMAP_LINEAR , and gl.LINEAR_
MIPMAP_LINEAR . See the book OpenGL Programming Guide for these values.
Pasting an Image onto a Rectangle
177
134 // Set the texture parameters
135 gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
After executing line 139, the value is set to the texture object, and then the internal state
of the WebGL system is modified as shown in Figure 5.29 .
t n e m g a r F
r e d a h S
e l b a i r a v m r o f i n u
… 1 E R U T X E T . l g 0 E R U T X E T . l g
7
E R U T X E T . l g
D 2 _ E R U T X E T . l g D 2 _ E R U T X E T . l g
t p i r c S a v a J
… L G b e W t e g = l g r a v
…
{ ) ( n i a m n o i t c n u f
. D 2 _ E R U T X E T l g
t c e j b O e r u t x e T
R E T L I F _ N I M _ E R U T X E T . l g
R A E N I L . l g
; ) … ( i r e t e m a r a P x e t . l g
Figure 5.29 Set texture parameter
The next step is to assign a texture image to the texture object.
Assigning a Texture Image to a Texture Object (gl.texImage2D())
To assign an image to a texture object, you use the method gl.texImage2D() . In addition
to assigning a texture, this method allows you to tell the WebGL system about the image
characteristics.
gl.texImage2D(target, level, internalformat, format, type, image)
Set the image specified by image to the texture object bound to target.
Parameters target Specifies gl.TEXTURE_2D or gl.TEXTURE_CUBE_MAP .
level Specified as 0. (Actually, this parameter is used for a
MIPMAP texture, which is not covered in this book.)
internalformat Specifies the internal format of the image ( Table 5.6 ).
format Specifies the format of the texel data. This must be
specified using the same value as internalformat .
type Specifies the data type of the texel data ( Table 5.7 ).
image Specifies an Image object containing an image to be
used as a texture.
Return value None
Errors INVALID_ENUM target is none of the above values.
INVALID_OPERATION No texture object is bound to target
CHAPTER 5 Using Colors and Texture Images
178
This method is used at line 136 in the sample program:
137 gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, image);
After executing line 144, the texture image loaded into the Image object image in
JavaScript is passed to the WebGL system (see Figure 5.30 ).
e l b a i r a v m r o f i n u
… 1 E R U T X E T . l g 0 E R U T X E T . l g 7 E R U T X E T . l g
D 2 _ E R U T X E T . l g D 2 _ E R U T X E T . l g . D 2 _ E R U T X E T l g
e g a m i
t p i r c S a v a J
… L G b e W t e g = l g r a v
…
{ ) ( n i a m n o i t c n u f
; ) … ( D 2 e g a m I x e t . l g
t c e j b O e r u t x e T
R E T L I F _ N I M _ E R U T X E T . l
R A E N I L . l g
g
t n e m g a r F
r e d a h S
Figure 5.30 Assign an image to the texture object
Let’s take a quick look at each parameter of this method. You must specify 0 to level
because you aren’t using a MIPMAP texture. The format specifies the format of the texel
data, with available formats shown in Table 5.6 . You need to select an appropriate format
for the image used as a texture. The sample program uses gl.RGB format because it uses a
JPG image in which each pixel is composed of RGB components. For other formats, such
as PNG, images are usually specified as gl.RGBA , and BMP images are usually specified as
gl.RGB . gl.LUMINANCE and gl.LUMINANCE_ALPHA are used for a grayscale image and so on.
Table 5.6 The Format of the Texel Data
Format Components in a Texel
gl.RGB Red, green, blue
gl.RGBA Red, green, blue, alpha
gl.ALPHA (0.0, 0.0, 0.0, alpha)
gl.LUMINANCE L, L, L, 1 L: Luminance
gl.LUMINANCE_ALPHA L, L. L. alpha
Here, luminance is the perceived brightness of a surface. It is often calculated as a
weighted average of red, green, and blue color values that gives the perceived brightness
of the surface.
As shown in Figure 5.30 , this method stores the image in the texture object in the WebGL
system. Once stored, you must tell the system about the type of format the image uses
Pasting an Image onto a Rectangle
179
using the internalformat parameter. As mentioned, in WebGL, internalformat must specify
the same value as format .
The type specifies the data type of the texel data (see Table 5.7 ). Usually, we specify gl.
UNSIGNED_BYTE as the data type. Other data types are also available, such as gl.UNSIGNED_
SHORT_5_6_5 (which packs RGB components into 16 bits). These types are used for passing
compressed images to the WebGL system to reduce loading time.
Table 5.7 The Data Type of Texel Data
Type Description
gl.UNSIGNED_BYTE Unsigned byte format. Each color component has 1 byte.
gl.UNSIGNED_SHORT_5_6_5 RGB: Each component has 5, 6, and 5 bits, respectively.
gl.UNSIGNED_SHORT_4_4_4_4 RGBA: Each component has 4, 4, 4, and 4 bits, respectively.
gl.UNSIGNED_SHORT_5_5_5_1 RGBA: Each RGB component has 5 bits, and A has 1 bit.
Pass the Texture Unit to the Fragment Shader (gl.uniform1i())
Once the texture image has been passed to the WebGL system, it must be passed to the
fragment shader to map it to the surface of the shape. As explained before, a uniform vari-
able is used for this purpose because the texture image does not change for each fragment:
13 var FSHADER_SOURCE =
...
17 'uniform sampler2D u_Sampler;\n' +
18 'varying vec2 v_TexCoord;\n' +
19 'void main() {\n' +
20 ' gl_FragColor = texture2D(u_Sampler, v_TexCoord);\n' +
21 '}\n';
This uniform variable must be declared using the special data type for textures shown in
Table 5.8 . The sample program uses a two-dimensional texture ( gl.TEXTURE_2D ), so the
data type is set to sampler2D .
Table 5.8 Special Data Types for Accessing a Texture
Type Description
sampler2D Data type for accessing the texture bound to gl.TEXTURE_2D
samplerCube Data type for accessing the texture bound to gl.TEXTURE_CUBE_MAP
The call to initTextures() (line 100) gets the storage location of this uniform variable
u_Sampler at line 108 and then passes it to loadTexture() as an argument. The uniform
CHAPTER 5 Using Colors and Texture Images
180
variable u_Sampler is set at line 139 by specifying the texture unit number (“n” in
gl.TEXTURE n ) of the texture unit that manages this texture object. In this sample program,
you specify 0 because you are using the texture object bound to gl.TEXTURE0 in the call to
gl.uniformi() :
138 // Set the texture unit 0 to the sampler
139 gl.uniform1i(u_Sampler, 0);
After executing line 139, the WebGL system is modified as shown in Figure 5.31 , thereby
allowing access to the image in the texture object from the fragment shader.
t p i r c S a v a J
… L G b e W t e g = l g r a v
…
{ ) ( n i a m n o i t c n u f
; ) … ( D 2 e g a m I x e t . l g
t c e j b O e r u t x e T
R E T L I F _ N I M _ E R U T X E T . l
R A E N I L . l g
g
t n e m g a r F
r e d a h S
r e l p m a S _ u
… 1 E R U T X E T . l g 0 E R U T X E T . l g 7 E R U T X E T . l g
D 2 _ E R U T X E T . l g D 2 _ E R U T X E T . l g . D 2 _ E R U T X E T l g
e g a m i
Figure 5.31 Set texture unit to uniform variable
Passing Texture Coordinates from the Vertex Shader to the Fragment
Shader
Because the texture coordinates for each vertex are passed to the attribute variable
a_TexCoord , it’s possible to pass the data to the fragment shader through the varying
variable v_TexCoord . Remember that varying variables of the same name and type are
automatically copied between the vertex shader and the fragment shader. The texture
coordinates are interpolated between vertices, so you can use the interpolated texture
coordinates in a fragment shader to specify each texture coordinate for each fragment:
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'attribute vec2 a_TexCoord;\n' +
6 'varying vec2 v_TexCoord;\n' +
7 'void main() {\n' +
8 ' gl_Position = a_Position;\n' +
9 ' v_TexCoord = a_TexCoord;\n' +
10 '}\n';
At this stage, you have completed the preparations for using the texture image in the
WebGL system.
Pasting an Image onto a Rectangle
181
All that’s left is to read the color of the texel located at the corresponding texture coordi-
nates from the texture image and then use it to set the color of the fragment.
Retrieve the Texel Color in a Fragment Shader (texture2D())
Retrieving a color of a texel from the texture image is done at line 20 in the fragment
shader:
20 ' gl_FragColor = texture2D(u_Sampler, v_TexCoord);\n' +
It uses the GLSL ES built-in function texture2D() to read out the texel color from the
shader. texture2D() is an easy-to-use function that can retrieve the texel color at a texture
coordinate by specifying the texture unit number in the first parameter and the texture
coordinates in the second parameter. However, because this function is a built-in function
of GLSL ES, note the data type of the parameters and the return value of the function.
vec4 texture2D(sampler2D sampler, vec2 coord)
Retrieve a texel color at the texture coordinates specified by coord from the texture image
specified by sampler.
Parameters sampler Specifies the texture unit number.
coord Specifies the texture coordinates.
Return value The texel color ( vec4 ) for the coordinates. The color format changes accord-
ing to the internalformat specified by gl.texImage2D() . Table 5.9 shows
the differences. If the texture image is not available for some reason, this
function returns (0.0, 0.0, 0.0, 1.0).
Table 5.9 Return Value of texture2D()
Internalformat Return Value
gl.RGB (R, G, B, 1.0)
gl.RGBA (R, G, B, A)
gl.ALPHA (0.0, 0.0, 0.0, A)
gl.LUMINANCE (L, L, L, 1.0) L indicates luminance
gl.LUMINANCE_ALPHA (L, L, L, A)
The texture magnification and minification parameters determine the return value in cases
where WebGL interpolates the texel. Once this function executes, by assigning the return
value to gl_FragColor , the fragment is displayed using the color. As a result of this opera-
tion, the texture image is mapped to the shape to be drawn (in this case, a rectangle).
CHAPTER 5 Using Colors and Texture Images
182
This is the final step in the process needed for texture mapping. At this stage, your texture
image has been loaded, set up in the WebGL system and mapped to the shape you are
drawing.
As you have seen, texture mapping in WebGL seems a complex process partly because it
must deal with an image and request the browser to load it, and partly because you are
required to use the texture unit even if you use only a single texture. However, once you
master the basic steps, they are the same each time you want to map a texture.
The next section explores the use of textures and will familiarize you with the whole
process.
Experimenting with the Sample Program
To familiarize you with texture mapping, let’s modify the sample program by changing
the texture coordinates. For example, modify the texture coordinates in TexturedQuad as
follows:
var verticesTexCoords = new Float32Array([
// Vertex coordinates and texture coordinates
-0.5, 0.5, -0.3, 1.7,
-0.5, -0.5, -0.3, -0.2,
0.5, 0.5, 1.7, 1.7,
0.5, -0.5, 1.7, -0.2
]);
If you load the modified program TexturedQuad_Repeat , you’ll see an effect like the
screenshot of Figure 5.32 (left side). To understand what’s happening here, take a look at
the figure on the right side, which shows each texture coordinate in the texture coordi-
nate system.
t
(0.0,0.0) (1.0,0.0)
(0.0,1.0) (1.0,1.0)
-0.2 -0.3
1.7
1.7
s
Figure 5.32 Modify the texture coordinate (a screenshot of TexturedQuad_Repeat)
The image isn’t sufficient to cover the larger shape, so as you can see, the texture image is
being repeated. This is driven by the value of gl.TEXTURE_WRAP_S and gl.TEXTURE_WRAP_T ,
Pasting Multiple Textures to a Shape
183
which are set to gl.REPEAT in the sample program, telling the WebGL system to repeat the
texture image to fill the area.
Now let’s modify the texture parameters as follows to see what other effects we can
achieve. The modified program is saved as TexturedQuad_Clamp_Mirror , and Figure 5.33
shows the result when run in your browser:
// Set texture parameters
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR );
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE );
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.MIRRORED_REPEAT );
You can see that the edge color of the texture is repeated in the s-axis (horizontal axis),
and the texture image itself is mirrored-repeated in the t-axis (vertical axis).
Figure 5.33 TexturedQuad_Clamp_Mirror
That concludes the explanation of the basic texture mapping technique available in
WebGL. The next section builds on this basic technique and explores texture mapping
using multiple texture images.
Pasting Multiple Textures to a Shape
Earlier in the chapter, you learned that WebGL can deal with multiple texture images,
which was the reason for the multiple texture units. The examples so far have used
only one texture, and thus one element of the unit. This section will construct a sample
program, MultiTexture , which pastes two texture images to a rectangle, allowing a
better examination of the texture unit mechanism. Figure 5.34 shows a screenshot of
MultiTexture . As you can see, the two texture images are “blended” to create the compos-
ite in the figure.
CHAPTER 5 Using Colors and Texture Images
184
Figure 5.34 MultiTexture
Figure 5.35 shows the two separate texture images used in this sample program. To high-
light WebGL’s ability to deal with various image formats, the sample program intention-
ally uses different image formats for each file.
Figure 5.35 Texture images (sky.jpg on left; circle.gif on right) used in MultiTexture
Essentially, you can map multiple texture images to a shape by repeating the process of
mapping a single texture image to a shape described in the previous section. Let’s examine
the sample program to see how that is done.
Sample Program (MultiTexture.js)
Listing 5.8 shows the basic processing flow of MultiTexture.js , which is similar to
TexturedQuad.js with three key differences: (1) the fragment shader accesses two textures,
(2) the final fragment color is calculated from the two texels from both textures, and (3)
initTextures() creates two texture objects.
Pasting Multiple Textures to a Shape
185
Listing 5.8 MultiTexture.js
1 // TexturedQuad.js
...
13 var FSHADER_SOURCE =
...
17 'uniform sampler2D u_Sampler0;\n' +
18 'uniform sampler2D u_Sampler1;\n' +
19 'varying vec2 v_TexCoord;\n' +
20 'void main() {\n' +
21 ' vec4 color0 = texture2D(u_Sampler0, v_TexCoord);\n' + <-(1)
22 ' vec4 color1 = texture2D(u_Sampler1, v_TexCoord);\n' +
23 ' gl_FragColor = color0 * color1;\n' + <-(2)
24 '}\n';
25
26 function main() {
...
53 // Set textures
54 if (!initTextures(gl, n)) {
...
58 }
59
60 function initVertexBuffers(gl) {
61 var verticesTexCoords = new Float32Array([
62 // Vertex coordinates and texture coordinates
63 -0.5, -0.5, 0.0, 1.0,
64 -0.5, -0.5, 0.0, 0.0,
65 0.5, -0.5, 1.0, 1.0,
66 0.5, -0.5, 1.0, 0.0,
67 ]);
68 var n = 4; // The number of vertices
...
100 return n;
101 }
102
103 function initTextures(gl, n) {
104 // Create a texture object
105 var texture0 = gl.createTexture(); <-(3)
106 var texture1 = gl.createTexture();
...
112 // Get the storage locations of u_Sampler1 and u_Sampler2
113 var u_Sampler0 = gl.getUniformLocation(gl.program, 'u_Sampler0');
114 var u_Sampler1 = gl.getUniformLocation(gl.program, 'u_Sampler1');
...
120 // Create Image objects
CHAPTER 5 Using Colors and Texture Images
186
121 var image0 = new Image();
122 var image1 = new Image();
...
127 // Register the event handler to be called when image loading is completed
128 image0.onload = function(){ loadTexture(gl, n, texture0, u_Sampler0,
➥image0, 0); };
129 image1.onload = function(){ loadTexture(gl, n, texture1, u_Sampler1,
➥image1, 1); };
130 // Tell the browser to load an Image
131 image0.src = '../resources/redflower.jpg';
132 image1.src = '../resources/circle.gif';
133
134 return true;
135 }
136 // Specify whether the texture unit is ready to use
137 var g_texUnit0 = false, g_texUnit1 = false ;
138 function loadTexture(gl, n, texture, u_Sampler, image, texUnit) {
139 gl.pixelStorei(gl.UNPACK_FLIP_Y_WEBGL, 1);// Flip the image's y-axis
140 // Make the texture unit active
141 if (texUnit == 0) {
142 gl.activeTexture(gl.TEXTURE0);
143 g_texUnit0 = true;
144 } else {
145 gl.activeTexture(gl.TEXTURE1);
146 g_texUnit1 = true;
147 }
148 // Bind the texture object to the target
149 gl.bindTexture(gl.TEXTURE_2D, texture);
150
151 // Set texture parameters
152 gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
153 // Set the texture image
154 gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, image);
155 // Set the texture unit number to the sampler
156 gl.uniform1i(u_Sampler, texUnit);
...
161 if (g_texUnit0 && g_texUnit1) {
162 gl.drawArrays(gl.TRIANGLE_STRIP, 0, n); // Draw a rectangle
163 }
164 }
First, let’s examine the fragment shader. In TexturedQuad.js , because the fragment shader
used only one texture image, it prepared a single uniform variable, u_Sampler . However,
this sample program uses two texture images and needs to define two sampler variables as
follows:
Pasting Multiple Textures to a Shape
187
17 'uniform sampler2D u_Sampler0;\n' +
18 'uniform sampler2D u_Sampler1;\n' +
main() in the fragment shader fetches the texel value from each texture image at lines 21
and 22, storing them to the variables color0 and color1 , respectively:
21 ' vec4 color0 = texture2D(u_Sampler0, v_TexCoord);\n' +
22 ' vec4 color1 = texture2D(u_Sampler1, v_TexCoord);\n' +
23 ' gl_FragColor = color0 * color1;\n' +
There are many possible ways to calculate the final fragment color ( gl_FragColor ) using
the texels. This sample program uses a component-wise multiplication of both texel colors
because the result is easy to understand. GLSL ES offers a simple way to write this multipli-
cation in a single line as a multiplication of two vec4 variables at line 23 (see Figure 5.36 ).
0 r o l o c 4 c e v
1 r o l o c 4 c e v
*
)
r1 g1 b1 a1
r2 g2 b2 a2
r1*r2 g1*g2 b1*b2 a1*a2
Figure 5.36 Multiplication of two vec4 variables
Although this sample program uses two texture images, initVertexBuffers() from line 60
is the same as in TexturedQuad.js because it uses the same texture coordinates for both
texture images.
In this sample, initTextures() at line 103 has been modified to repeat the process of
dealing with a texture image twice because now it deals with two images rather than the
single image of the previous example.
Lines 105 and 106 create the two texture objects, one for each texture image. The last
character of each variable name (“0” in texture0 and “1” in texture1 ) indicates which
texture unit (texture unit 0 or texture unit 1) is used. This naming convention of using
the unit number also applies to the variable names for the storage location of uniform
variables (line 113 and 114) and image objects (lines 120 and 121).
Registration of the event handler ( loadTexture() ) is the same as in TexturedQuad.js , with
the last argument set to indicate the different texture units:
128 image0.onload = function() { loadTexture(gl, n, texture0, u_Sampler0,
➥image0, 0); };
129 image1.onload = function() { loadTexture(gl, n, texture1,u_Sampler1,
➥image1, 1); };
The request to load the texture images is in lines 131 and 132:
CHAPTER 5 Using Colors and Texture Images
188
131 image0.src = '../resources/redflower.jpg';
132 image1.src = '../resources/circle.gif';
In this sample program, the function loadTexture() has to be modified to deal with two
textures. The function is defined from line 138 with its core part as follows:
137 var g_texUnit0 = false, g_texUnit1 = false;
138 function loadTexture(gl, n, texture, u_Sampler, image, texUnit) {
139 gl.pixelStorei(gl.UNPACK_FLIP_Y_WEBGL, 1);// Flip the image's y-axis
140 // Make the texture unit active
141 if (texUnit == 0) {
142 gl.activeTexture(gl.TEXTURE0);
143 g_texUnit0 = true;
144 } else {
145 gl.activeTexture(gl.TEXTURE1);
146 g_texUnit1 = true;
147 }
148
149 gl.bindTexture(gl.TEXTURE_2D, texture); // Bind the texture object
150
151 // Set texture parameters
152 gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
153 // Set the texture image
154 gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, image);
155 // Set the texture unit number to the sampler
156 gl.uniform1i(u_Sampler, texUnit);
...
164 if (g_texUnit0 && g_texUnit1) {
165 gl.drawArrays(gl.TRIANGLE_STRIP, 0, n); // Draw a rectangle
166 }
167 }
The important difference in loadTexture() is that you cannot predict which texture
image is loaded first because the browser loads them asynchronously. The sample program
handles this by starting to draw only after loading both textures. To do this, it uses two
global variables ( g_texUnit0 and g_texUnit1 ) at line 137 indicating which textures have
been loaded.
These variables are initialized to false at line 137 and changed to true in the if state-
ment at line 141. This if statement checks the variable texUnit passed as the last param-
eter in loadTexture() . If it is 0, the texture unit 0 is made active and g_texUnit0 is set to
true ; if it is 1, the texture unit 1 is made active and then g_texUnit1 is set to true .
Line 156 sets the texture unit number to the uniform variable. Note that the parameter
texUnit of loadTexture() is passed to gl.uniform1i() . After loading two texture images,
the internal state of the system is changed, as shown in Figure 5.37 .
Pasting Multiple Textures to a Shape
189
… 1 E R U T X E T . l g 0 E R U T X E T . l g
D 2 _ E R U T X E T . l g .
.
D 2 _ E R U T X E T l g
t p i r c S a v a J
… L G b e W t e g = l g r a v
…
{ ) ( n i a m n o i t c n u f
0 r e l p m a S _ u
1 r e l p m a S _ u
; ) … ( i 1 m r o f i n u . l g
; ) … ( i 1 m r o f i n u . l g
t c e j b O e r u t x e T
t n e m g a r F
r e d a h S
t c e j b O e r u t x e T
Figure 5.37 Internal state of WebGL when handling two texture images
Finally, the program invokes the vertex shader after checking whether both texture images
are available at line 165 using g_texUnit0 and g_texUnit1 . The images are then combined
as they are mapped to the shape, resulting in the screenshot in Figure 5.34 .
Summary
In this chapter, you ventured deep into the WebGL world. At this stage you have acquired
all the basic skills needed to use WebGL to deal with 2D geometric shapes and are ready
for the next step: 3D objects. Fortunately, when you deal with 3D objects instead of 2D
shapes, the way you use shaders is surprisingly similar, so you can quickly apply all the
knowledge you’ve learned so far.
The rest of this book focuses mainly on covering the techniques necessary for managing
3D objects. However, before introducing you to the 3D world, the next chapter will take a
brief tour of the OpenGL ES shading language (GLSL ES), covering some features and func-
tionality that have been only touched on in the chapters so far.
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Chapter 6
The OpenGL ES Shading Language (GLSL ES)
This chapter takes a break from examining WebGL sample programs and explains the essential
features of the OpenGL ES Shading Language (GLSL ES) in detail.
As you have seen, shaders are the core mechanism within WebGL for constructing 3DCG
programs, and GLSL ES is the dedicated programming language for writing those shader
programs. This chapter covers:
• Data, variables, and variable types
• Vector, matrix, structure, array, and sampler types
• Operators, control flow, and functions
• Attributes, uniform, and varying variables
• Precision qualifiers
• Preprocessor and directives
By the end of this chapter, you will have a good understanding of GLSL ES and how to use it to
write a variety of shaders. This knowledge will help you tackle the more complex 3D manipula-
tions introduced in Chapters 7 through 9 . Note that language specifications can be quite dry, and
for some of you, this may be more detail than you need. If so, it’s safe to skip this chapter and
use it as a reference when you look at the examples in the rest of the book.
Recap of Basic Shader Programs
As you can see from Listings 6.1 and 6.2 , you can construct shader programs in a similar manner
to constructing programs using the C programming language.
CHAPTER 6 The OpenGL ES Shading Language (GLSL ES)
192
Listing 6.1 Example of a Simple Vertex Shader
// Vertex shader
attribute vec4 a_Position;
attribute vec4 a_Color;
uniform mat4 u_MvpMatrix;
varying vec4 v_Color;
void main() {
gl_Position = u_MvpMatrix * a_Position;
v_Color = a_Color;
}
Variables are declared at the beginning of the code, and then the main() routine defines
the entry point for the program.
Listing 6.2 Example of a Simple Fragment Shader
// Fragment shader
#ifdef GLSL_ES
precision mediump float;
#endif
varying vec4 v_Color;
void main() {
gl_FragColor = v_Color;
}
The version of GLSL ES dealt with in this chapter is 1.00. However, you should note that
WebGL does not support all features defined in GLSL ES 1.00
1 ; rather, it supports a subset
of 1.00 with core features needed for WebGL.
Overview of GLSL ES
The GLSL ES programming language was developed from the OpenGL Shading Language
(GLSL) by reducing or simplifying functionality, assuming that the target platforms were
consumer electronics or embedded devices such as smart phones and game consoles.
A prime goal was to allow hardware manufacturers to simplify the hardware needed to
execute GLSL ES programs. This had two key benefits: reducing power consumption by
devices and, perhaps more importantly, reducing manufacturing costs.
GLSL ES supports a limited (and partially extended) version of the C language syntax.
Therefore, if you are familiar with the C language, you’ll find it easy to understand
GLSL ES. Additionally, the shading language is beginning to be used for general-purpose
processing such as image processing and numerical computation (so called GPGPU),
meaning that GLSL ES has an increasingly wide application domain, thus increasing the
benefits of studying the language.
1 http://www.khronos.org/registry/gles/specs/2.0/GLSL_ES_Specifi cation_1.0.17.pdf
Hello Shader!
193
Hello Shader!
By tradition, most programming books begin with a “Hello World!” example, or in our
case the corresponding shader program. However, because you have already seen several
shader programs in previous chapters, let’s skip that and take a look at the basics of GLSL
ES, using Listing 6.1 and Listing 6.2 shown earlier.
Basics
Like many programming languages, you need to pay attention to the following two items
when you write shader programs using GLSL ES:
• The programs are case-sensitive ( marina and Marina are different).
• A semicolon (;) must be specified at the end of each command.
Order of Execution
Once a JavaScript program is loaded, program lines are executed in the order in which
they were written—sequentially starting from the first program line. However, like C,
shader programs are executed from the function main() and therefore must have one (and
only one) main() function that cannot have any parameters. Looking back, you can see
that each shader program shown in Listing 6.1 and Listing 6.2 defines a single main() .
You must prepend the keyword void to main() , which indicates that the function has
no return value. (See the section “Functions” later in this chapter.) This is different from
JavaScript, where you can define a function using the keyword function , and you don’t
have to worry whether the function returns a value. In GLSL ES, if the function returns a
value, you must specify its data type in front of the function name, or if it doesn’t return
a value, specify void so that the system doesn’t expect a return value.
Comments
As with JavaScript, you can write comments in your shader program, and in fact use the
same syntax as JavaScript. So, the following two types of comment are supported:
• // characters followed by any sequence of characters up to the end of line:
int kp = 496; // kp is a Kaprekar number
• /* characters, followed by any sequence of characters (including new lines), followed
by the */ characters:
/* I have a day off today.
I want to take a day off tomorrow.
*/
CHAPTER 6 The OpenGL ES Shading Language (GLSL ES)
194
Data (Numerical and Boolean Values)
GLSL ES supports only two data types:
• Numerical value: GLSL ES supports integer numbers (for example, 0, 1, 2) and float-
ing point numbers (for example, 3.14, 29.98, 0.23571). Numbers without a decimal
point (.) are treated as integer numbers, and those with a decimal point are treated
as floating point numbers.
• Boolean value: GLSL ES supports true and false as boolean constants.
GLSL ES does not support character strings, which may initially seem strange but makes
sense for a 3D graphics language.
Variables
As you have seen in the previous chapters, you can use any variable names you want as
long as the name follows the basic naming rules:
• The character set for variables names contains only the letters a–z, A–Z, the under-
score (_), and the numbers 0–9.
• Numbers are not allowed to be used as the first character of variable names.
• The keywords shown in Table 6.1 and the reserved keywords shown in Table 6.2 are
not allowed to be used as variable names. However, you can use them as part of the
variable name, so the variable name if will result in error, but iffy will not.
• Variable names starting with gl_ , webgl_ , or _webgl_ are reserved for use by OpenGL
ES. No user-defined variable names may begin with them.
Table 6.1 Keywords Used in GLSL ES
attribute bool break bvec2 bvec3 bvec4
const continue discard do else false
float for highp if in inout
Int invariant ivec2 ivec3 ivec4 lowp
mat2 mat3 mat4 medium out precision
return sampler2D samplerCube struct true uniform
varying vec2 vec3 vec4 void while
Basic Types
195
Table 6.2 Reserved Keywords for Future Version of GLSL ES
asm cast class default
double dvec2 dvec3 dvec4
enum extern external fixed
flat fvec2 fvec3 fvec4
goto half hvec2 hvec3
hvec4 inline input interface
long namespace noinline output
packed public sampler1D sampler1DShadow
sampler2DRect sampler2DRectShadow sampler2DShadow sampler3D
sampler3DRect short sizeof static
superp switch template this
typedef union unsigned using
volatile
GLSL ES Is a Type Sensitive Language
GLSL ES does not require the use of var to declare variables, but it does require you to
specify the type of data a variable will contain. As you have seen in the sample programs,
you declare variables using the form
<data type> <variable name>
such as vec4 a_Position .
As discussed, when you define a function like main() , you must also specify the data type
of the return value of the function. Equally, the type of data on the left side of the assign-
ment operation (=) and that of data on the right side must have the same type; otherwise,
it will result in an error.
For these reasons, GLSL ES is called a type sensitive language , meaning that it belongs to
a class of languages that require you to specify and pay attention to types.
Basic Types
GLSL ES supports the basic data types shown in Table 6.3 .
CHAPTER 6 The OpenGL ES Shading Language (GLSL ES)
196
Table 6.3 GLSL Basic Types
Type Description
float The data type for a single floating point number. It indicates the variable will contain
a single floating point number.
int The data type for a single integer number. It indicates the variable will contain a
single integer number.
bool The data type for a boolean value. It indicates the variable will contain a boolean
value.
Specifying the data type for variables allows the WebGL system to check errors in advance
and process the program efficiently. The following are examples of variable declarations
using basic types.
float klimt; // The variable will contain a single floating number
int utrillo; // The variable will contain a single integer number
bool doga; // The variable will contain a single boolean value
Assignment and Type Conversion
Assignments of values to variables are performed using the assignment operator ( = ). As
mentioned, because GLSL ES is a type-sensitive language, if the data type of the left-side
variable is not equal to that of the assigned data (or variable), it will result in an error:
int i = 8; // OK
float f1 = 8; // Error
float f2 = 8.0; // OK
float f3 = 8.0f; // Error: Expressions like 8.0f used in C are not allowed.
Semantically, 8 and 8.0 are the same values. However, when you assign 8 to a floating
point variable f1 , it will result in an error. In this case, you would see the following error
message:
failed to compile shader: ERROR: 0:11: '=' : cannot convert from 'const mediump int'
to 'float'.
If you want to assign an integer number to a floating point variable, you need to convert
the integer number to a floating point number. This conversion is called type conversion .
To convert an integer into a floating point number, you can use the built-in function
float() , as follows:
int i = 8;
float f1 = float(i); // 8 is converted to 8.0 and assigned to f1
float f2 = float(8); // equivalent to the above operation
Basic Types
197
GLSL ES supports a number of other built-in functions for type conversion, which are
shown in Table 6.4 .
Table 6.4 The Built-In Functions for Type Conversion
Conversion Function Description
To an integer
number
int(float) The fractional part of the floating-point value is
dropped (for example, 3.14 → 3).
int(bool) true is converted to 1, or false is converted to 0.
To a floating
point number
float(int) The integer number is converted to a floating point
number (for example, 8 → 8.0).
float(bool) true is converted to 1.0, or false is converted to
0.0.
To a boolean
value
bool(int) 0 is converted to false , or non-zero values are
converted to true .
bool(float) 0.0 is converted to false , or non-zero values are
converted to true .
Operations
The operators applicable to the basic types are similar to those in JavaScript and are shown
in Table 6.5 .
Table 6.5 The Operators Available for the Basic Types
Operator Operation Applicable Data Type
- Negation (for example,
for specifying a negative
number)
int or float .
* Multiplication int or float . The data type of the
result of the operation is the same
as operands.
/ Division
+ Addition
- Subtraction
++ Increment (postfix and
prefix)
int or float . The data type of the
result of the operation is the same
as operands.
-- Decrement (postfix and
prefix)
= Assignment int , float , or bool
+= -= *= /= Arithmetic assignment int or float .
CHAPTER 6 The OpenGL ES Shading Language (GLSL ES)
198
Operator Operation Applicable Data Type
< > <= >= Comparison int or float .
== != Comparison (equality) int , float , or bool .
! Not bool or an expression that results
in bool [1].
&& Logical and
|| Logical inclusive or
^^ Logical exclusive or [2]
condition?
expression1: expression2
Ternary selection condition is bool or an expression
that results in bool . Data types
other than array can be used in
expression1 and expression2 .
[1] The second operand in a logical and ( && ) operation is evaluated if and only if the first operand
evaluates to true . The second operand in a logical or ( || ) operation is evaluated if and only if the
first operand evaluates to false .
[2] If either the left-side condition or the right-side one is true , the result is true . If both sides are
true , the result is false .
The followings are examples of basic operations:
int i1 = 954, i2 = 459;
int kp = i1 - i2; // 495 is assigned to kp.
float f = float(kp) + 5.5; // 500.5 is assigned to f.
Vector Types and Matrix Types
GLSL ES supports vector and matrix data types which, as you have seen, are useful when
dealing with computer graphics. Both these types contain multiple data elements. A
vector type, which arranges data in a list, is useful for representing vertex coordinates or
color data. A matrix arranges data in an array and is useful for representing transformation
matrices. Figure 6.1 shows an example of both types.
) ( 3 5 1 1 7 3
7 0 4
1 7 3
Figure 6.1 A vector and a matrix
GLSL ES supports a variety of vector or matrix types, as shown in Table 6.6 .
Vector Types and Matrix Types
199
Table 6.6 Vector Types and Matrix Types
Category Types in GLSL ES Description
Vector vec2 , vec3 , vec4 The data types for 2, 3, and 4 component vectors of
floating point numbers
ivec2 , ivec3 , ivec4 The data types for 2, 3, and 4 component vectors of
integer numbers
bvec2 , bvec3 , bvec4 The data types for 2, 3, and 4 component vectors of
boolean values
Matrix mat2 , mat3 , mat4 The data type for 2×2, 3×3, and 4×4 matrix of floating
point numbers (with 4, 9, and 16 elements, respec-
tively)
The following examples show the use of the vector and matrix types:
vec3 position; // variable for 3-component vector of float
// For example: (10.0, 20.0, 30.0)
ivec2 offset; // variable for 2-component vector of integer
// For example: (10, 20)
mat4 mvpMatrix; // the variable for 4×4 matrix of float
Assignments and Constructors
Assignment of data to variables of the type vector or matrix is performed using the =
operator. Remember that the type of data on the left side of the assignment operation and
that of the data/variable on the right side must be the same. In addition, the number of
elements on the left side of the assignment operation must be equal to that of the data/
variable on the right side. To illustrate that, the following example will result in an error:
vec4 position = 1.0; // vec4 variable requires four floating point numbers
In this case, because a vec4 variable requires four floating point numbers, you need to pass
four floating numbers in some way. A solution is to use the built-in functions with the
same name of the data type so; for example, in the case of vec4 , you can use the construc-
tor vec4() . (See Chapter 2 , “Your First Step with WebGL.”) For example, to assign 1.0, 2.0,
3.0, and 4.0 to a variable of type vec4 , you can use vec4() to bundle them into a single
data element as follows:
vec4 position = vec4(1.0, 2.0, 3.0, 4.0);
Functions for making a value of the specified data type are called constructor functions ,
and the name of the constructor is always identical to that of the data type.
CHAPTER 6 The OpenGL ES Shading Language (GLSL ES)
200
Vector Constructors
Vectors are critical in GLSL ES so, as you’d imagine, there are multiple ways to specify
arguments to a vector constructor. For example:
vec3 v3 = vec3(1.0, 0.0, 0.5); // sets v3 to(1.0, 0.0, 0.5)
vec2 v2 = vec2(v3); // sets v2 to (1.0, 0.0) using the 1st and 2nd elements of v3
vec4 v4 = vec4(1.0); // sets v4 to (1.0, 1.0, 1.0, 1.0)
In the second example, the constructor ignores the third element of v3 , and only the
first and second elements of v3 are used to create the new vector. Similarly, in the third
example, if a single value is specified to a vector constructor, the value is used to initialize
all components of the constructed vector. However, if more than one value is specified
to a vector constructor but the number of the values is less than the number of elements
required by the constructor, it will result in an error.
Finally, a vector can be constructed from multiple vectors:
vec4 v4b = vec4(v2, v4); // sets (1.0, 0.0, 1.0, 1.0) to v4b
The rule here is that the vector is filled with values from the first vector (v2), and then
any missing values are supplied by the second vector (v4).
Matrix Constructors
Constructors are also available for matrices and operate in a similar manner to vector
constructors. However, you should make sure the order of elements stored in a matrix is
in a column major order. (See Figure 3.27 for more details of “column-major order.”) The
following examples show different ways of using the matrix constructor:
• If multiple values are specified to a matrix constructor, a matrix is constructed using
them in column major order:
mat4 m4 = mat4 ( 1.0, 2.0, 3.0, 4.0,
5.0, 6.0, 7.0, 8.0,
9.0, 10.0, 11.0, 12.0,
13.0, 14.0, 15.0, 16.0 );
• If multiple vectors are specified to a matrix constructor, a matrix is constructed using
the elements of each vector in column major order:
// two vec2 are used to construct a mat2
vec2 v2_1 = vec2(1.0, 3.0);
vec2 v2_2 = vec2(2.0, 4.0);
mat2 m2_1 = mat2(v2_1, v2_2); // 1.0 2.0
// 3.0 4.0
// vec4 is used to construct mat2
vec4 v4 = vec4(1.0, 3.0, 2.0, 4.0);
⎡
⎣
⎢
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
⎥
1.0 5.0 9.0 13.0
2.0 6.0 10.0 14.0
3.0 7.0 11.0 15.0
4.0 8.0 12.0 16.0
Vector Types and Matrix Types
201
mat2 m2_2 = mat2(v4); // 1.0 2.0
// 3.4 4.0
• If multiple values and multiple vectors are specified to a matrix constructor, a matrix
is constructed using them in column major order:
// Two floating point numbers and vec2 are used to construct a mat2
mat2 m2 = mat2(1.0, 3.0, v2_2); // 1.0 2.0
// 3.0 4.0
• If a single value is specified to a matrix constructor, a matrix is constructed using the
value as its diagonal elements:
mat4 m4 = mat4(1.0); // 1.0 0.0 0.0 0.0
// 0.0 1.0 0.0 0.0
// 0.0 0.0 1.0 0.0
// 0.0 0.0 0.0 1.0
Similar to a vector constructor, if an insufficient number of values is specified to the
constructor (but more than one), it will result in an error.
mat4 m4 = mat4(1.0, 2.0, 3.0); // Error. mat4 requires 16 elements.
Access to Components
To access the components in a vector or matrix, you can use the operators . and [] , as
shown in the following subsections.
The . Operator
An individual component in a vector can be accessed by the variable name followed by
period (.) and then the component name, as shown in Table 6.7 .
Table 6.7 Component Names
Category Description
x , y , z , w Useful for accessing vertex coordinates.
r , g , b , a Useful for accessing colors.
s , t , p , q Useful for accessing texture coordinates. (Note that this book uses only s and
t . p is used instead of r because r is used for colors.)
Because vectors are used for storing various types of data such as vertex coordinates,
colors, and texture coordinates, three types of component names are supported to increase
CHAPTER 6 The OpenGL ES Shading Language (GLSL ES)
202
the readability of programs. However, any of the component names x , r , or s accesses the
first component; any of y , g , or t accesses the second one; and so on, so you can use them
interchangeably if you prefer. For example:
vec3 v3 = vec3(1.0, 2.0, 3.0); // sets v3 to(1.0, 2.0, 3.0)
float f;
f = v3.x; // sets f to 1.0
f = v3.y; // sets f to 2.0
f = v3.z; // sets f to 3.0
f = v3.r; // sets f to 1.0
f = v3.s; // sets f to 1.0
As you can see from the comments of these examples, x , r , and s have different names
but always access the first component. Attempting to access a component beyond the
number of components in the vector will result in an error:
f = v3.w; // w requires access to the fourth element, which doesn't exist.
Multiple components can be selected by appending their names (from the same name set)
after the period (.). This is known as swizzling . In the following example, x , y , z , and w
will be used, but other sets of component names have the same effect:
vec2 v2;
v2 = v3.xy; // sets v2 to (1.0, 2.0)
v2 = v3.yz; // sets v2 to (2.0, 3.0). Any component can be omitted
v2 = v3.xz; // sets v2 to (1.0, 3.0). You can skip any component.
v2 = v3.yx; // sets v2 to (2.0, 1.0). You can reverse the order.
v2 = v3.xx; // sets v2 to (1.0, 1.0). You can repeat any component.
vec3 v3a;
v3a = v3.zyx; // sets v3a to (3.0, 2.0, 1.0). You can use all names.
The component name can also be used in the left-side expression of an assignment opera-
tor ( = ):
vec4 position = vec4(1.0, 2.0, 3.0, 4.0);
position.xw = vec2(5.0, 6.0); // position = (5.0, 2.0, 3.0, 6.0)
Remember, the component names must come from the same set so, for example, v3.was is
not allowed.
Vector Types and Matrix Types
203
The [ ] Operator
In addition to the . operator, the components of a vector or a matrix can be accessed
using the array indexing operator [] . Note that the elements in a matrix are also read out
in column major order. Just like JavaScript, the index starts from 0, so applying [0] to a
matrix selects the first column in the matrix, [1] selects the second one, [2] selects the
third one, and so on. The following shows an example:
mat4 m4 = mat4 ( 1.0, 2.0, 3.0, 4.0,
5.0, 6.0, 7.0, 8.0,
9.0, 10.0, 11.0, 12.0,
13.0, 14.0, 15.0, 16.0);
vec4 v4 = m4[0]; // Retrieve the 1st column from m4: (1.0, 2.0, 3.0, 4.0)
In addition, two [] operators can be used to select a column and then a row of a matrix:
float m23 = m4[1][2]; // sets m23 to the third component of the second
// column of m4 (7.0).
A component name can be used to select a component in conjunction with the [] opera-
tor, as follows:
float m32 = m4[2].y; // sets m32 to the second component of the third
// column of m4 (10.0).
One restriction is that only a constant index can be specified as the index number in the
[] operator. The constant index is defined as
• A integral literal value (for example, 0 or 1)
• A global or local variable qualified as const , excluding function parameters (see the
section “const Variables”)
• Loop indices (see the section “Conditional Control Flow and Iteration”)
• Expressions composed from any of the preceding
The following examples use the type int constant index:
const int index = 0; // "const" keyword specifies the variable is a
// read-only variable.
vec4 v4a = m4[index]; // is the same as m4[0]
The following example uses an expression composed of constant indices as an index.
vec4 v4b = m4[index + 1]; // is the same as m4[1]
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Remember, you cannot use an int variable without the const qualifier as an index
because it is not a constant index (unless it is a loop index):
int index2 = 0;
vec4 v4c = m4[index2]; // Error: because index2 is not a constant index.
Operations
You can apply the operators shown in Table 6.8 to a vector or a matrix. These operators
are similar to the operators for basic types. Note that the only comparative operators avail-
able for a vector and matrix are == and != . The < , > , <= , and >= operators cannot be used
for comparisons of vectors or matrices. In such cases, you can use built-in functions such
as lessThan() . (See Appendix B , “Built-In Functions of GLSL ES 1.0.”)
Table 6.8 The Operators Available for a Vector and a Matrix
Operators Operation Applicable Data Types
* Multiplication vec[234] and mat[234] . The opera-
tions on vec[234] and mat[234] will be
explained below.
The data type of the result of operation is
the same as the operands.
/ Division
+ Addition
- Subtraction
++ Increment (postfix and prefix) vec[234] and mat[234] . The data type of
the result of this operation is the same as
the operands.
-- Decrement (postfix and prefix)
= Assignment vec[234] and mat[234] .
+=, -=,
*=, /=
Arithmetic assignment vec[234] and mat[234] .
==, != Comparison vec[234] and mat[234] . With == , if all
components of the operands are equal,
the result is true . For != , if any of compo-
nents of the operands are not equal, then
the result is true [1].
[1] If you want component-wise equality, you can use the built-in function equal() or
notEqual(). (See Appendix B .)
Note that when an arithmetic operator operates on a vector or a matrix, it is operating
independently on each component of the vector or matrix in component-wise order.
Examples
The following examples show frequently used cases. In the examples, we assume that the
types of variables are defined as follows:
Vector Types and Matrix Types
205
vec3 v3a, v3b, v3c;
mat3 m3a, m3b, m3c;
float f;
Operations on a Vector and Floating Point Number
An example showing the use of the + operator:
// The following example uses the + operator, but the -, *, and /
// operators also have the same effect.
v3b = v3a + f; // v3b.x = v3a.x + f;
// v3b.y = v3a.y + f;
// v3b.z = v3a.z + f;
For example, v3a = vec3(1.0, 2.0, 3.0) and f = 1.0 will result in v3b=(2.0, 3.0,
4.0) .
Operations on Vectors
These operators operate on each component of a vector:
// The following example uses the + operator, but the -, *, and /
// operators also have the same effect.
v3c = v3a + v3b; // v3a.x + v3b.x;
// v3a.y + v3b.y;
// v3a.z + v3b.z;
For example, v3a = vec3(1.0, 2.0, 3.0) and v3b = vec3(4.0, 5.0, 6.0) will result in
v3c=(5.0, 7.0, 9.0) .
Operations on a Matrix and a Floating Point Number
These operators operate on each component of the matrix:
// The following example uses the + operator, but the -, *, and /
// operators also have the same effect.
m3b = m3a * f; // m3b[0].x = m3a[0].x * f; m3b[0].y = m3a[0].y * f;
// m3b[0].z = m3a[0].z * f;
// m3b[1].x = m3a[1].x * f; m3b[1].y = m3a[1].y * f;
// m3b[1].z = m3a[1].z * f;
// m3b[2].x = m3a[2].x * f; m3b[2].y = m3a[2].y * f;
// m3b[2].z = m3a[2].z * f;
Multiplication of a Matrix and a Vector
For multiplication, the result is the sum of products of each element in a matrix and
vector. This result is the same as Equation 3.5 that you saw back in Chapter 3 , “Drawing
and Transforming Triangles”:
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v3b = m3a * v3a; // v3b.x = m3a[0].x * v3a.x + m3a[1].x * v3a.y
// + m3a[2].x * v3a.z;
// v3b.y = m3a[0].y * v3a.x + m3a[1].y * v3a.y
// + m3a[2].y * v3a.z;
// v3b.z = m3a[0].z * v3a.x + m3a[1].z * v3a.y
// + m3a[2].z * v3a.z;
Multiplication of a Vector and a Matrix
Multiplication of a vector and a matrix is possible, as you can see from the following
expressions. Note that this result is different from that when multiplying a matrix by a
vector:
v3b = v3a * m3a; // v3b.x = v3a.x * m3a[0].x + v3a.y * m3a[0].y
// + v3a.z * m3a[0].z;
// v3b.y = v3a.x * m3a[1].x + v3a.y * m3a[1].y
// + v3a.z * m3a[1].z;
// v3b.z = v3a.x * m3a[2].x + v3a.y * m3a[2].y
// + v3a.z * m3a[2].z;
Multiplication of Matrices
This is the same as Equation 4.4 in Chapter 4 , “More Transformations and Basic
Animation”:
m3c = m3a * m3b; // m3c[0].x = m3a[0].x * m3b[0].x + m3a[1].x * m3b[0].y
// + m3a[2].x * m3b[0].z;
// m3c[1].x = m3a[0].x * m3b[1].x + m3a[1].x * m3b[1].y
// + m3a[2].x * m3b[1].z;
// m3c[2].x = m3a[0].x * m3b[2].x + m3a[1].x * m3b[2].y
// + m3a[2].x * m3b[2].z;
// m3c[0].y = m3a[0].y * m3b[0].x + m3a[1].y * m3b[0].y
// + m3a[2].y * m3b[0].z;
// m3c[1].y = m3a[0].y * m3b[1].x + m3a[1].y * m3b[1].y
// + m3a[2].y * m3b[1].z;
// m3c[2].y = m3a[0].y * m3b[2].x + m3a[1].y * m3b[2].y
// + m3a[2].y * m3b[2].z;
// m3c[0].z = m3a[0].z * m3b[0].x + m3a[1].z * m3b[0].y
// + m3a[2].z * m3b[0].z;
// m3c[1].z = m3a[0].z * m3b[1].x + m3a[1].z * m3b[1].y
// + m3a[2].z * m3b[1].z;
// m3c[2].z = m3a[0].z * m3b[2].x + m3a[1].z * m3b[2].y
// + m3a[2].z * m3b[2].z;
Structures
207
Structures
GLSL ES also supports user-defined types, called structure s, which aggregate other already
defined types using the keyword struct . For example:
struct light { // defines the structure "light"
vec4 color;
vec3 position;
}
light l1, l2; // declares variable "l1" and "l2" of the type "light"
This example defines the new structure type light that consists of two members: the vari-
able color and position . Then two variables l1 and l2 of type light are declared after the
definition. Unlike C, the typedef keyword is not necessary because, by default, the name
of the structure becomes the name of the type.
In addition, as a convenience, variables of the new type can be declared with the defini-
tion of the structure, as follows:
struct light { // declares structure and its variable all together
vec4 color; // color of a light
vec3 position; // position of a light
} l1; // variable "l1" of the structure
Assignments and Constructors
Structures support the standard constructor, which has the same name as the structure.
The arguments to the constructor must be in the same order and of the same type as they
were declared in the structure. Figure 6.2 shows an example.
; ) 0 . 0 , 0 . 3 , 0 . 8 ( 3 c e v , ) 0 . 1 , 0 . 0 , 0 . 1 , 0 . 0 ( 4 c e v ( t h g i l = 1 l
r o l o c n o i t i s o p
Figure 6.2 A constructor of structure
Access to Members
Each member of a structure can be accessed by appending the variable name with a period
(.) and then the member name. For example:
vec4 color = l1.color;
vec3 position = l1.position;
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Operations
For each member in the structure, you can use any operators allowed for that member’s
type. However, the operators allowed for the structure itself are only the assignment ( = )
and comparative operators ( == and != ); see Table 6.9.
Table 6.9 The Operators Available for a Structure
Operator Operation Description
= Assignment The assignment and comparison operators are not allowed for
the structures that contain arrays or sampler types
==,!= Comparison
When using the == operator, the result is true if, and only if, all the members are
component-wise equal. When using the !=, the result is false if one of the members is
not component-wise equal.
Arrays
GLSL ES arrays have a similar form to the array in JavaScript, with only one-dimensional
arrays being supported. Unlike arrays in JavaScript, the new operator is not necessary to
create arrays, and methods such as push() and pop() are not supported. The arrays can be
declared by a name followed by brackets ( [] ) enclosing their sizes. For example:
float floatArray[4]; // declares an array consisting of four floats
vec4 vec4Array[2]; // declares an array consisting of two vec4s
The array size must be specified as an integral constant expression greater than zero
where the integral constant expression is defined as follows:
• A numerical value (for example, 0 or 1)
• A global or local variable qualified as const , excluding function parameters (see the
section “const Variables”)
• Expressions composed of both of the above
Therefore, the following will result in an error:
int size = 4;
vec4 vec4Array[size]; // Error. If you declare "const int size = 4;"
// it will not result in an error
Note that arrays cannot be qualified as const .
Samplers
209
Array elements can be accessed using the array indexing operator ([]). Note that, like C,
the index starts from 0. For example, the third element of the float Array defined earlier
can be accessed as follows:
float f = floatArray[2];
Only an integral constant expression or uniform variable (see the section “Uniform
Variables”) can be used as an index of an array. In addition, unlike JavaScript or C, an
array cannot be initialized at declaration time. So each element of the array must be
initialized explicitly as follows:
vec4Array[0] = vec4(4.0, 3.0, 6.0, 1.0);
vec4Array[1] = vec4(3.0, 2.0, 0.0, 1.0);
Arrays support only [] operators. However, elements in an array do support the standard
operators available for their type. For example, the following operator can be applied to
the elements of floatArray or vec4Array :
// multiplies the second element of floatArray by 3.14
float f = floatArray[1] * 3.14;
// multiplies the first element of vec4Array by vec4(1.0, 2.0, 3.0, 4.0);
vec4 v4 = vec4Array[0] * vec4(1.0, 2.0, 3.0, 4.0);
Samplers
GLSL ES supports a dedicated type called sampler for accessing textures. (See Chapter 5 ,
“Using Colors and Texture Images.”) Two types of samplers are available: sampler2D and
samplerCube . Variables of the sampler type can be used only as a uniform variable (see
the section “Uniform Variables”) or an argument of the functions that can access textures
such as texture2D() . (See Appendix B .) For example:
uniform sampler2D u_Sampler;
In addition, the only value that can be assigned to the variable is a texture unit number,
and you must use the WebGL method gl.uniform1i() to set the value. For example,
TexturedQuad.js in Chapter 5 uses gl.uniform1i(u_Sampler, 0) to pass the texture unit
0 to the shader.
Variables of type sampler are not allowed to be operands in any expressions other than = ,
== , and != .
Unlike other types explained in the previous sections, the number of sampler type vari-
ables is limited depending on the shader type (see Table 6.10 ). In the table, the keyword
mediump is a precision qualifier. (This qualifier is explained in detail in the section
“Precision Qualifiers,” toward the end of this chapter.)
CHAPTER 6 The OpenGL ES Shading Language (GLSL ES)
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Table 6.10 Minimum Number of Variables of the Sampler Type
Shaders that Use the
Variable
Built-In Constants Representing the
Maximum Number
Minimum Number
Vertex shader
const mediump int
gl_MaxVertexTextureImageUnits
0
Fragment shader
const mediump int
gl_MaxTextureImageUnits
8
Precedence of Operators
Operator precedence is shown in Table 6.11 . Note the table contains several operators that
are not explained in this book but are included for reference.
Table 6.11 The Precedence of Operators
Precedence Operators
1 parenthetical grouping (())
2 function calls, constructors (()), array indexing ([]), period (.)
3 increment/decrement (++, --), negate (-), inverse(~) , not(!)
4 multiplication (*), division (/), remainder (%)
5 addition (+), subtraction (-)
6 bit-wise shift (<<, >>)
7 comparative operators (<, <=, >=, >)
8 equality (==, !=)
9 bit-wise and (&)
10 bit-wise exclusive or (^)
11 bit-wise or (|)
12 and (&&)
13 exclusive or (^^)
14 or (||)
15 ternary selection (? :)
16 assignment (=), arithmetic assignments (+=, -=, *=, /=, %=,
<<=, >>=, &=, ^=, |= )
17 sequence(,)
Bold font indicates operators reserved for future versions of GLSL.
Conditional Control Flow and Iteration
211
Conditional Control Flow and Iteration
Conditional control flow and iteration in the shading language are almost the same as in
JavaScript or C.
if Statement and if-else Statement
A conditional control flow can use either if or if-else . An if-else statement follows the
pattern shown here:
if ( conditional-expression1 ) {
commands here are executed if conditional-expression1 is true.
} else if ( conditional-expression2 ) {
commands here are executed if conditional-expression1 is false but conditional-
expression2 is true.
} else {
commands here are executed if conditional-expression1 is false and conditional-
expression2 is false.
}
The following shows a code example using the if-else statement:
if(distance < 0.5) {
gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0); // red
} else {
gl_FragColor = vec4(0.0, 1.0, 0.0, 1.0); // green
}
As shown in this example, the conditional expression in the if or if-else statement must
be either a boolean value or an expression that becomes a boolean value. Boolean vector
types, such as bvec2 , are not allowed in the conditional expression.
Switch statement are not allowed, and you should note that usage of the if or if-else
statement will slow down the shaders.
for Statement
The for statement can be used as follows:
for ( for-init-statement ; conditional-expression ; loop-index-expression ) {
the commands which you want to execute repeatedly .
}
For example:
for (int i = 0; i < 3; i++) {
sum += i;
}
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Note that the loop index ( i in the preceding example) of the for statement can be
declared only in the for-init-statement . The conditional-expression can be omitted, and an
empty condition becomes true. The for statement has the following restrictions:
• Only a single loop index is allowed. The loop index must have the type int or
float .
• loop-index-expression must have one of the following forms (supposing that i is a
loop index):
i++ , i-- , i+= constant-expression , i-= constant-expression
• conditional-expression is a comparison between a loop index and an integral constant
expression. (See the section “Array.”)
• Within the body of the loop, the loop index cannot be assigned.
These limitations are in place so that the compiler can perform inline expansion of for
statements.
continue, break, discard Statements
Just like JavaScript or C, continue and break statements are allowed only within a for
statement and are generally used within if statements:
continue skips the remainder of the body of the innermost loop containing the
continue, increases/decreases the loop index, and then moves to the next loop.
break exits the innermost loop containing the break. No further execution of the loop is
done.
The following show examples of the continue statement:
for (int i = 0; i < 10; i++) {
if (i == 8) {
continue; // skips the remainder of the body of the innermost loop
}
// When i == 8, this line is not executed
}
The following shows an example of the break statement:
for (int i = 0; i < 10; i++) {
if (i == 8) {
break; // exits "for" loop
}
// When i >= 8, this line is not executed.
}
// When i == 8, this line is executed.
Functions
213
The discard statement is only allowed in fragment shaders and discards the current frag-
ment, abandoning the operation on the current fragment and skipping to the next frag-
ment. The use of discard will be explained in more detail in the section “Make a Rounded
Point” in Chapter 10 , “Advanced Techniques.”
Functions
In contrast to the way functions are defined in JavaScript, the functions in GLSL ES are
defined in the same manner as in C. For example:
returnType functionName ( type0 arg0 , type1 arg1 , ..., typen argn ) {
do some computation
return returnValue ;
}
Argument type s must use one of the data types explained in this chapter, and like main() ,
functions with no arguments are allowed. When the function returns no value, the return
statement does not need to be included. In this case, returnType must be void . You can
also specify a structure as the returnValue , but the structure returned cannot contain an
array.
The following example shows a function to convert an RGBA value into a luminance
value:
float luma (vec4 color) {
float r = color.r;
float g = color.g;
float b = color.b;
return 0.2126 * r + 0.7162 * g + 0.0722 * b;
// The preceding four lines could be rewritten as follows:
// return 0.2126 * color.r + 0.7162 * color.g + 0.0722 * color.b;
}
You can call the function declared above in the same manner as in JavaScript or C by
using its name followed by a list of arguments in parentheses:
attribute vec4 a_Color; // (r, g, b, a) is passed
void main() {
...
float brightness = luma(a_Color);
...
}
Note that an error will result if, when called, argument types do not match the declared
parameter types. For example, the following will result in an error because the type of the
parameter is float, but the caller passes an integer:
CHAPTER 6 The OpenGL ES Shading Language (GLSL ES)
214
float square(float value) {
return value * value;
}
void main() {
...
float x2 = square(10); // Error: Because 10 is integer. 10.0 is OK.
...
}
As you can see from the previous examples, functions work just like those in JavaScript
or C except that you cannot call the function itself from inside the body of the function
(that is, a recursive call of the function isn’t allowed). For the more technically minded,
this is because the compilers can in-line function calls.
Prototype Declarations
When a function is called before it is defined, it must be declared with a prototype. The
prototype declaration tells WebGL in advance about the types of parameters and the
return value of the function. Note that this is different from JavaScript, which doesn’t
require a prototype. The following is an example of a prototype declaration for luma() ,
which you saw in the previous section:
float luma(vec4); // a prototype declaration
main() {
...
float brightness = luma(color); // luma() is called before it is defined.
...
}
float luma (vec4 color) {
return 0.2126 * color.r + 0.7162 * color.g + 0.0722 * color.b;
}
Parameter Qualifiers
GLSL ES supports qualifiers for parameters that control the roles of parameters within a
function. They can define that a parameter (1) is to be passed into a function, (2) is to be
passed back out of a function, and (3) is to be passed both into and out of a function. (2)
and (3) can be used just like a pointer in C. These are shown in Table 6.12 .
Built-In Functions
215
Table 6.12 Parameter Qualifiers
Qualifiers Roles Description
in Passes a value into the function The parameter is passed by value.
Its value can be referred to and
modified in the function. The caller
cannot refer to the modification.
const in Passes a value into the function The parameter is passed by
constant value. Its value can be
referred to but cannot be modified.
out Passes a value out of the function The parameter is passed by refer-
ence. If its value is modified, the
caller can refer to the modification.
inout Passes a value both into/out of the
function
The parameter is passed by refer-
ence, and its value is copied in the
function. Its value can be referred
to and modified in the function. The
caller can also refer to the modifica-
tion.
<none:
default>
Passes a value into the function Same as in .
For example, luma() can return the result of its calculation using a parameter qualified by
out instead of a return value, as follows:
void luma2 (in vec3 color, out float brightness) {
brightness = 0.2126 * color.r + 0.7162 * color.g + 0.0722 * color.b;
}
Because the function itself no longer returns a value, the return type of this function is
changed from float to void . Additionally, the qualifier in , in front of the first parameter,
can be omitted because in is a default parameter qualifier.
This function can be used as follows:
luma2(color, brightness); // the result is stored into "brightness"
// same as brightness = luma(color)
Built-In Functions
In addition to user-defined functions, GLSL ES supports a number of built-in functions
that perform operations frequently used in computer graphics. Table 6.13 gives an over-
view of the built-in functions in GLSL ES, and you can look at Appendix B for the detailed
definition of each function.
CHAPTER 6 The OpenGL ES Shading Language (GLSL ES)
216
Table 6.13 Built-In Functions in GLSL ES
Category Built-In Functions
Angle functions radians (converts degrees to radians), degrees (converts radians to
degrees)
Trigonometry
functions
sin (sine function), cos (cosine function), tan (tangent function), asin
(arc sine function), acos (arc cosine function), and atan (arc tangent
function)
Exponential
functions
pow (x
y ),
exp (natural exponentiation), log (natural logarithm), exp 2 (2
x ),
log2 (base 2 logarithm), sqrt (square root), and inversesqrt (inverse
of sqrt )
Common functions abs (absolute value), min (minimum value), max (maximum value), mod
(remainder), sign (sign of a value), floor (floor function), ceil (ceil
function), clamp (clamping of a value), mix (linear interpolation), step
(step function), smoothstep (Hermite interpolation), and fract (frac-
tional part of the argument)
Geometric functions length (length of a vector), distance (distance between two points),
dot (inner product), cross (outer product), normalize (vector with
length of 1), reflect (reflection vector), and faceforward (converting
normal when needed to “faceforward”)
Matrix functions matrixCompMult (component-wise multiplication)
Vector relational
functions
lessThan (component-wise “<”), lessThanEqual (component-wise
“<=”), greaterThan (component-wise “>”), greaterThanEqual
(component-wise “>=”), equal (component-wise “==”), notEqual
(component-wise “!=”), any (true if any component is true), all (true if
all components are true), and not (component-wise logical complement)
Texture lookup
functions
texture2D (texture lookup in the 2D texture), textureCube (texture
lookup in the cube map texture), texture2DProj (projective version of
texture2D() ), texture2DLod (level of detail version of texture2D() ),
textureCubeLod (lod version of textureCube() ), and
texture2DProjLod (projective version of texture2DLod() )
Global Variables and Local Variables
Just like JavaScript or C, GLSL ES supports both global variables and local variables. Global
variables can be accessed from anywhere in the program, and local variables can be
accessed only from within a limited portion of the program.
In GLSL ES, in a similar manner to JavaScript or C, variables declared “outside” a function
become global variables, and variables declared “inside” a function become local variables.
The local variables can be accessed only from within the function containing them. For
this reason, the attribute, uniform, and varying variables described in the next section
must be declared as global variables because they are accessed from outside the function.
Storage Qualifiers
217
Storage Qualifiers
As explained in the previous chapters, GLSL ES supports storage qualifiers for attribute,
uniform, and varying variables (see Figure 6.3 ). In addition, a const qualifier is supported
to specify a constant variable to be used in a shader program.
x e t r e V
r e d a h S
t n e m g a r F
r e d a h S
gl_Position
gl_FragColor
e l b a i r a v e t u b i r t a e l b a i r a v m r o f i n u
t p i r c S a v a J
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
; ) … ( s r e d a h S t i n i
…
}
e l b a i r a v g n i y r a v
e l b a i r a v m r o f i n u
e l b a i r a v g n i y r a v
r e f u B r o l o C
Figure 6.3 Attribute, uniform, and varying variables
const Variables
Unlike JavaScript, GLSL ES supports the const qualifier to specify a constant variable, or
one whose value cannot be modified.
The const qualifier is specified in front of the variable type, just like an attribute vari-
able. Variables qualified by const must be initialized at their declaration time; otherwise,
they are unusable because no data can be assigned to them after their declaration. Some
examples include:
const int lightspeed = 299792458; // light speed (m/s)
const vec4 red = vec4(1.0, 0.0, 0.0, 1.0); // red
const mat4 identity = mat4(1.0); // identity matrix
Assigning data to the variable qualified by const will result in an error. For example:
const int lightspeed;
lightspeed = 299792458;
will result in the following error message:
failed to compile shader: ERROR: 0:11: 'lightspeed' : variables
with qualifier 'const' must be initialized
ERROR: 0:12: 'assign': l-value required (can't modify a const variable)
CHAPTER 6 The OpenGL ES Shading Language (GLSL ES)
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Attribute Variables
As you have seen in previous chapters, attribute variables are available only in vertex
shaders. They must be declared as a global variable and are used to pass per-vertex data
to the vertex shader. You should note that it is “per-vertex.” For example, if there are two
vertices, (4.0, 3.0, 6.0) and (8.0, 3.0, 0.0), data for each vertex can be passed to an attribute
variable. However, data for other coordinates, such as (6.0, 3.0, 3.0), which is a halfway
point between the two vertices and not a specified vertex, cannot be passed to the vari-
able. If you want to do that, you need to add the coordinates as a new vertex. Attribute
variables can only be used with the data types float , vec2 , vec3 , vec4 , mat2 , mat3 , and
mat4 . For example:
attribute vec4 a_Color;
attribute float a_PointSize;
There is an implementation-dependent limit on the number of attribute variables avail-
able, but the minimum number is 8. The limits on the number of each type of variable are
shown in Table 6.14 .
Table 6.14 The Limitation on the Number of Attribute, Uniform, and Varying Variables
Types of Variables
The Built-In Constants for the
Maximum Number
Minimum
Number
attribute variables
const mediump int
gl_MaxVertexAttribs
8
uniform variables Vertex shader
const mediump int
gl_MaxVertexUniformVectors
128
Fragment shader
const mediump int
gl_MaxFragmentUniformVectors
16
varying variables
const mediump int
gl_MaxVaryingVectors
8
Uniform Variables
Uniform variables are allowed to be used in both vertex and fragment shaders and must
be declared as global variables. Uniform variables are read-only and can be declared as any
data types other than array and structure. If a uniform variable of the same name and data
type is declared in both a vertex shader and a fragment shader, it is shared between them.
Uniform variables contain “uniform” (common) data, so your JavaScript program must
only use them to pass such data. For example, because transformation matrices contain
the uniform values for all vertices, they can be passed to uniform variables:
uniform mat4 u_ViewMatrix;
uniform vec3 u_LightPosition;
Precision Qualifiers
219
There is an implementation-dependent limit on the number of uniform variables that can
be used ( Table 6.14 ). Note that the limit in a vertex shader is different from that in a frag-
ment shader.
Varying Variables
The last type of qualifier is varying . Varying variables also must be declared as global
variables and are used to pass data from a vertex shader to a fragment shader by declaring
a variable with the same type and name in both shaders. (See v_Color in Listing 6.1 and
Listing 6.2 .) The following are examples of varying variable declarations:
varying vec2 v_TexCoord;
varying vec4 v_Color;
Just like attribute variables, the varying variables can be declared only with the following
data types: float , vec2 , vec3 , vec4 , mat2 , mat3 , and mat4 . As explained in Chapter 5 , the
value of a varying variable written by a vertex shader is not passed to a fragment shader as
is. Rather, the rasterization process between the vertex and fragment shaders interpolates
the value according to the shape to be drawn, and then the interpolated value is passed
per fragment. This interpolation process is the reason for the limitations on the data types
that can be used with a varying variable.
The number of varying variables also has an implementation dependent limit. The
minimum number is 8 (see Table 6.14 ).
Precision Qualifiers
Precision qualifiers were newly introduced in GLSL ES to make it possible to execute
shader programs more efficiently and to reduce their memory size. As the name suggests,
it is a simple mechanism to specify how much precision (the number of bits) each data
type should have. Simply put, specifying higher precision data requires more memory
and computation time, and specifying lower precision requires less. By using these quali-
fiers, you can exercise fine-grained control over aspects of performance and size. However,
precision qualifiers are optional, and a reasonable default compromise can be specified
using the following lines:
#ifdef GL_ES
precision mediump float;
#endif
Because WebGL is based on OpenGL ES 2.0, which was designed for consumer electronics
and embedded systems, WebGL programs may end up executing on a range of hardware
platforms. In some cases, the computation time and memory efficiency could be improved
by using lower precision data types when performing calculations and operations. Perhaps
more importantly, this also enables reduced power consumption and thus extended
battery life on mobile devices.
CHAPTER 6 The OpenGL ES Shading Language (GLSL ES)
220
You should note, however, that just specifying lower precision may lead to incorrect
results within WebGL, so it’s important to balance efficiency and correctness.
As shown in Table 6.15 , WebGL supports three types of precision qualifiers: highp (high
precision), mediump (medium precision), and lowp (lower precision).
Table 6.15 Precision Qualifiers
Precision
Qualifiers
Descriptions
Default Range and Precision
Float int
highp High precision. The minimum precision
required for a vertex shader.
(–2
62 , 2 62 )
Precision: 2
– 16
(–2
16 , 2 16 )
mediump Medium precision. The minimum precision
required for a fragment shader. More than
lowp , and less than highp .
(–2
14 , 2 14 )
Precision: 2
– 10
(–2
10 , 2 10 )
lowp Low precision. Less than mediump , but all
colors can be represented.
(–2, 2)
Precision: 2
– 8
(–2
8 , 2 8 )
There are a couple of things to note. First, fragment shaders may not support highp in
some WebGL implementations; a way to check this is shown later in this section. Second,
the actual range and precision are implementation dependent, which you can check by
using gl.getShaderPrecisionFormat() .
The following are examples of the declaration of variables using the precision qualifiers:
mediump float size; // float of medium precision
highp vec4 position; // vec4 composed of floats of high precision
lowp vec4 color; // vec4 composed of floats of lower precision
Because specifying a precision for all variables is time consuming, a default for each data
type can be set using the keyword precision , which must be specified at the top of a
vertex shader or fragment shader using the following syntax:
precision precision-qualifier name-of-type ;
This sets the precision of the data type specified by name-of-type to the precision specified
by precision-qualifier . In this case, variables declared without a precision qualifier have this
default precision automatically set. For example:
precision mediump float; // All floats have medium precision
precision highp int; // All ints have high precision
Preprocessor Directives
221
This specifies all data types related to float, such as vec2 and mat3 , to have medium preci-
sion, and all integers to have high precision. For example, because vec4 consists of four
float types, each float of the vector is set to medium precision.
You may have noticed that in the examples in previous chapters, you didn’t specify preci-
sion qualifiers to the data types other than float in fragment shaders. This is because
most data types have a default precision value; however, there is no default precision for
float in a fragment shader. See Table 6.16 for details.
Table 6.16 Default Precision of Type
Type of Shader Data Type Default Precision
Vertex shader int highp
float highp
sampler2D lowp
samplerCube lowp
Fragment shader int medium
float None
sampler2D lowp
samplerCube lowp
The fact that there is no default precision for float requires programmers to carefully use
floats in their fragment shaders. So, for example, using a float without specifying the
precision will result in the following error:
failed to compile shader: ERROR: 0:1 : No precision specified for (float).
As mentioned, whether a WebGL implementation supports highp in a fragment shader is
implementation dependent. If it is supported, the built-in macro GL_FRAGMENT_PRECISION_
HIGH is defined (see the next section).
Preprocessor Directives
GLSL ES supports preprocessor directives, which are commands (directives) for the prepro-
cessor stage before actual compilation. They are always preceded by a hash mark ( # ). The
following example was used in ColoredPoints.js :
#ifdef GL_ES
precision mediump float;
#endif
CHAPTER 6 The OpenGL ES Shading Language (GLSL ES)
222
These lines check to see if the macro GL_ES is defined, and if so the lines between #ifde f
and #endif are executed. They are similar to an if statement in JavaScript or C.
The following three preprocessor directives are available in GLSL ES:
#if constant-expression
If the constant-expression is true, this part is executed.
#endif
#ifdef macro
If the macro is defined, this part is executed.
#endif
#ifndef macro
If the macro is not defined, this part is executed.
#endif
The #define is used to define macros. Unlike C, macros in GLSL ES cannot have macro
parameters:
#define macro-name string
You can use #undef to undefine the macro:
#undef macro-name
You can use #else directives just like an if statement in JavaScript or C. For example:
#define NUM 100
#if NUM == 100
If NUM == 100 then this part is executed.
#else
If NUM != 100 then this part is executed.
#endif
Macros can use any name except for the predefined macros names shown in Table 6.17 .
Table 6.17 Predefined Macros
Macro Description
GL_ES Defined and set to 1 in OpenGL ES 2.0
GL_FRAGMENT_PRECISION_HIGH highp is supported in a fragment shader
Summary
223
So you can use the macro with preprocessor directives as follows:
#ifdef GL_ES
#ifdef GL_FRAGMENT_PRECISION_HIGH
precision highp float; // highp is supported. floats have high precision
#else
precision mediump float; // highp is not supported. floats have medium precision
#endif
#endif
You can specify which version of GLSL ES is used in the shader by using the #version
directive:
#version number
Accepted versions include 100 (for GLSL ES 1.00) and 101 (for GLSL ES 1.01). By default,
shaders that do not include a #version directive will be treated as written in GLSL ES
version 1.00. The following example specifies version 1.01:
#version 101
The #version directive must be specified at the top of the shader program and can only be
preceded by comments and white space.
Summary
This chapter explained the core features of the OpenGL ES Shading Language (GLSL ES) in
some detail.
You have seen that the GLSL ES shading language has many similarities to C but has
been specialized for computer graphics and has had unnecessary C features removed. The
specialized computer graphics features include support for vector and matrix data types,
special component names for accessing the components of a vector or matrix, and opera-
tors for a vector or matrix. In addition, GLSL ES supports many built-in functions for oper-
ations frequently used in computer graphics, all designed to allow you to create efficient
shader programs.
Now that you have a better understanding of GLSL ES, the next chapter will return to
WebGL and explore more sophisticated examples using this new knowledge.
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Chapter 7
Toward the 3D World
In the previous chapters, we used examples based on 2D geometrical shapes
to explain how the WebGL system works and how it supports the behavior
of shaders, transformations such as translation and rotation, animation, and
texture mapping. However, the techniques you’ve learned so far can be applied
not only to 2D shapes but also to 3D objects. In this chapter you’ll take the first
step into the 3D world and explore the implications of moving from 2D to 3D.
In particular, you will explore:
• Representing the user’s view into the 3D world
• Controlling the volume of 3D space that is viewed
• Clipping
• Handling foreground and background objects
• Drawing a 3D object (a cube)
All these issues have a significant impact on how the 3D scene is drawn and
presented to viewers, and a mastery of them is critical to building compel-
ling 3D scenes. As usual, we’ll take you step by step so you will quickly master
the basics and be able to move on to the more complex issues of lighting and
performance in the final chapters.
What’s Good for Triangles Is Good for Cubes
So far, you’ve used the humble triangle in many of the explorations and
programs. As previously discussed, you’ve seen how 3D objects are composed
of 2D shapes—in particular the triangle. Figure 7.1 shows a cube that has been
built up from 12 triangles.
CHAPTER 7 Toward the 3D World
226
Figure 7.1 A cube composed of triangles
So when you deal with 3D objects, you just need to apply the techniques you have
learned to each triangle that makes up the objects. The only difference from past exam-
ples, and it’s a significant one, is that you now need to consider the depth information
of the triangles in addition to the x and y coordinates. Let’s begin by exploring how you
specify and control the viewing direction—that is, the view into the 3D scene the user
has—and then look at the visible range that controls how much of the scene the user sees.
The explanations focus on the basic triangle because it simplifies things; however, what’s
true for triangles is true for 3D objects.
Specifying the Viewing Direction
The critical factor when considering 3D objects is that they have depth in a 3D space.
This means you need to take care of several issues that you didn’t have to consider when
using 2D shapes. First, because of the nature of 3D space, you can look at the object from
anywhere in the space; that is, your viewpoint can be anywhere. When describing the way
you view objects, two important points need consideration:
• The viewing direction (where you are looking from, and at which part of the scene
are you looking?)
• The visible range (given the viewing direction, where can you actually see?)
In this first section let’s explore viewing direction and the techniques that allow you to
place the eye point anywhere in 3D space and then look at objects from various direc-
tions. You’ll take a look at the second item and see how to specify the visible range in the
next section.
As introduced in Chapter 2 , “Your First Step with WebGL” (refer to Figure 2.16 ), let’s
assume that, by default, the eye point is placed at the origin (0, 0, 0), and the line of sight
extends along the z-axis in the negative direction (inward toward your computer screen).
In this section, you will move the eye point from the default location to other locations
and then view a triangle from there.
Specifying the Viewing Direction
227
Let’s construct a sample program, LookAtTriangles , that locates the eye point at (0.20,
0.25, 0.25) and then views three triangles from there toward the origin (0, 0, 0). Using
three triangles makes it easy to understand the depth information in the 3D scene. Figure
7.2 shows a screen shot of LookAtTriangles and the color and z coordinate of each
triangle.
Figure 7.2 LookAtTriangles (left), and the color and z coordinate of each triangle (right)
The program uses softer colors because they are easier on the eyes.
Eye Point, Look-At Point, and Up Direction
To specify where you are looking from and which part of the scene you are looking at in
the 3D space, you need two items of position information: the eye point (where you are
looking from) and the look-at point (which part of the scene you are looking at). In addi-
tion, in 3DCG, you need to specify which direction is up in the scene. As such, a total of
three items of information are required to specify the viewing direction (see Figure 7.3 ).
r o t c e v p u
t n i o p t a - k o o l
e n e c s
) Z e y e , Y e y e , X e y e (
) Z p u , Y p u , X p u (
) Z t a , Y t a , X t a (
t n i o p e y e
Figure 7.3 Eye point, look-at point, and up direction
z
-0.75
0.75
x
y
CHAPTER 7 Toward the 3D World
228
Eye point: This is the starting point from which the 3D space is viewed. In the following
sections, the coordinates of this position are referred to as (eyeX, eyeY, eyeZ).
Look-at point: This is the point at which you are looking and which determines the
direction of the line of sight from the eye point. As the name suggests, the eye point is a
point, not a vector, so another point (such as the look-at point) is required to determine
the direction in which you are looking. The look-at point is a point on the line of sight
extending from the eye point. The coordinates of the look-at point are referred to as
(atX, atY, atZ).
Up direction: This determines the up direction in the scene that is being viewed from
the eye point to the look-at point. If only the eye point and the look-at point are deter-
mined, there is freedom to rotate the line of sight from the eye point to the look-at
point. (In Figure 7.4 , inclining the head causes the top and bottom of the scene to shift.)
To define the rotation, you must determine the up direction along the line of sight. The
up direction is specified by three numbers representing the direction. The coordinates for
this direction are referred to as (upX, upY, upZ).
r o t c e v p u
t n i o p t a - k o o l
e n e c s
t n i o p e y e
Figure 7.4 Eye point, look-at point, and up direction
In WebGL, you can specify the position and direction the eye point faces by converting
these three items of information in a matrix and passing the matrix to a vertex shader.
This matrix is called a view transformation matrix or view matrix , because it changes
the view of the scene. In cuon-matrix.js , the method Matrix4.setLookAt() is defined
to calculate the view matrix from the three items of information: eye point, look-at point
and up direction.
Matrix4.setLookAt(eyeX, eyeY, eyeZ, atX, atY, atZ, upX, upY, upZ)
Calculate the view matrix derived from the eye point ( eyeX , eyeY , eyeZ ), the look-at point
( atX , atY , atZ ), and the up direction ( upX , upY , upZ ). This view matrix is set up in the
Matrix4 object. The look-at point is mapped to the center of the <canvas> .
Parameters eyeX, eyeY, eyeZ Specify the position of the eye point .
atX, atY, atZ Specify the position of the look-at point .
Specifying the Viewing Direction
229
upX, upY, upZ Specify the up direction in the scene. If the up direction is
along the positive y-axis, then ( upX , upY , upZ ) is (0, 1, 0).
Return value None
In WebGL, the default settings when using Matrix4.setLookAt() are defined as follows:
• The eye point is placed at (0, 0, 0) (that is, the origin of the coordinate system).
• The look-at point is along the negative z-axis, so a good value is (0, 0, –1) . 1 The up
direction is specified along the positive y-axis, so a good value is (0, 1, 0).
So, for example, if the up direction is specified as (1, 0, 0), the positive x-axis becomes the
up direction; in this case, you will see the scene tilted by 90 degrees.
A view matrix representing the default settings in WebGL can be simply produced as
follows (see Figure 7.5 ).
; ) ( 4 x i r t a M w e n = x i r t a M w e i V l a i t i n i r a v
; ) 0 , 1 , 0 , 1 - , 0 , 0 , 0 , 0 , 0 ( t A k o o L t e s . x i r t a M w e i V l a i t i n i
r o t c e v p u t n i o p t a - k o o l t n i o p e y e
Figure 7.5 An example of setLookAt()
Now you understand how to use the method setLookAt() , so let’s take a look at its use in
an actual sample program.
Sample Program (LookAtTriangles.js)
LookAtTriangles.js , shown in Listing 7.1 , is a program that changes the position of the
eye point and then draws the three triangles shown in Figure 7.2 . Although it is difficult
to see on paper, the three triangles are, in order of proximity, blue, yellow, and green,
respectively, all fading to red in the bottom-right corner.
Listing 7.1 LookAtTriangles.js
1 // LookAtTriangles.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'attribute vec4 a_Color;\n' +
6 'uniform mat4 u_ViewMatrix;\n' +
1 The z component could be any negative value. The value -1 is an example but we could have chosen
any other negative value.
CHAPTER 7 Toward the 3D World
230
7 'varying vec4 v_Color;\n' +
8 'void main() {\n' +
9 ' gl_Position = u_ViewMatrix * a_Position;\n' +
10 ' v_Color = a_Color;\n' +
11 '}\n';
12
13 // Fragment shader program
14 var FSHADER_SOURCE =
...
18 'varying vec4 v_Color;\n' +
19 'void main() {\n' +
20 ' gl_FragColor = v_Color;\n' +
21 '}\n';
22
23 function main() {
...
40 // Set the vertex coordinates and color (blue triangle is in front)
41 var n = initVertexBuffers(gl);
...
50 // Get the storage location of u_ViewMatrix variable
51 var u_ViewMatrix = gl.getUniformLocation(gl.program,'u_ViewMatrix');
...
57 // Set the eye point, look-at point, and up direction
58 var viewMatrix = new Matrix4();
59 viewMatrix.setLookAt(0.20, 0.25, 0.25, 0, 0, 0, 0, 1, 0);
60
61 // Pass the view matrix to u_ViewMatrix variable
62 gl.uniformMatrix4fv(u_ViewMatrix, false, viewMatrix.elements);
...
67 // Draw a triangle
68 gl.drawArrays(gl.TRIANGLES, 0, n);
69 }
70
71 function initVertexBuffers(gl) {
72 var verticesColors = new Float32Array([
73 // vertex coordinates and color
74 0.0, 0.5, -0.4, 0.4 1.0, 0.4, // The back green triangle
75 -0.5, -0.5, -0.4, 0.4 1.0, 0.4,
76 0.5, -0.5, -0.4, 1.0, 0.4 0.4,
77
78 0.5, 0.4, -0.2, 1.0, 0.4 0.4, // The middle yellow triangle
79 -0.5, 0.4, -0.2, 1.0, 1.0, 0.4,
80 0.0, -0.6, -0.2, 1.0, 1.0, 0.4,
81
82 0.0, 0.5, 0.0, 0.4 0.4 1.0, // The front blue triangle
Specifying the Viewing Direction
231
83 -0.5, -0.5, 0.0, 0.4 0.4 1.0,
84 0.5, -0.5, 0.0, 1.0, 0.4 0.4
85 ]);
86 var n = 9;
87
88 // Create a buffer object
89 var vertexColorbuffer = gl.createBuffer();
...
96 gl.bindBuffer(gl.ARRAY_BUFFER, vertexColorbuffer);
97 gl.bufferData(gl.ARRAY_BUFFER, verticesColors, gl.STATIC_DRAW);
...
121 return n;
122 }
This program is based on ColoredTriangle.js in Chapter 5 , “Using Colors and Texture
Images.” The fragment shader, the method of passing the vertex information, and so on,
is the same as in ColoredTriangle.js . The three main differences follow:
• The view matrix is passed to the vertex shader (line 6) and then multiplied by the
vertex coordinates (line 9).
• The vertex coordinates and color values of the three triangles (line 72 to 85) are set
up in initVertexBuffers() , which is called from line 41 of main() in JavaScript.
• The view matrix is calculated at lines 58 and 59 in main() and passed to the uniform
variable u_ViewMatrix in the vertex shader at line 62. You should note that the posi-
tion of the eye point is (0.2, 0.25, 0.25); the position of the look-at point is (0, 0, 0);
the up direction is (0, 1, 0).
Let’s start by looking at the second difference and the function initVertexBuffers() (line
71). The difference between this program and the original program, ColoredTriangle.
js , is that verticesColors at line 72 (which is the array of vertex coordinates and colors
for a single triangle) is modified for the three triangles, and the z coordinates are added
in the array. These coordinates and colors are stored together in the buffer object vertex-
ColorBuffer (lines 96 and 97) created at line 89. Because you are now dealing with three
triangles (each with three vertices), you need to specify 9 as the third argument of gl.
drawArrays() at line 68.
To specify the view matrix (that is, where you are looking and which part of the scene you
are looking at [item 3]), you need to set up and pass the view matrix to the vertex shader.
To do this, a Matrix4 object viewMatrix is created at line 58, and you use setLookAt() to
calculate and store the view matrix to viewMatrix at line 59. This view matrix is passed to
u_ViewMatrix at line 62, which is the uniform variable used in the vertex shader:
57 // Set the eye point, look-at point, and up direction
58 var viewMatrix = new Matrix4();
59 viewMatrix.setLookAt(0.20, 0.25, 0.25, 0, 0, 0, 0, 1, 0);
60
CHAPTER 7 Toward the 3D World
232
61 // Pass the view matrix to the u_ViewMatrix variable
62 gl.uniformMatrix4fv(u_ViewMatrix, false, viewMatrix.elements);
Those are all the changes needed in the JavaScript program. Now let’s examine what is
happening in the vertex shader:
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'attribute vec4 a_Color;\n' +
6 'uniform mat4 u_ViewMatrix;\n' +
7 'varying vec4 v_Color;\n' +
8 'void main() {\n' +
9 ' gl_Position = u_ViewMatrix * a_Position;\n' +
10 ' v_Color = a_Color;\n' +
11 '}\n';
The vertex shader starts from line 4. The only two lines that differ from ColoredTriangle.
js are indicated by boldface: Line 6 defines the uniform variable u_ViewMatrix , and line 9
multiplies the matrix by the vertex coordinates. These modifications seem quite trivial, so
how do they change the position of the eye point?
Comparing LookAtTriangles.js with RotatedTriangle_Matrix4.js
Looking at the vertex shader in this sample program, you may notice a similarity
with that in RotatedTriangle_Matrix4.js , which was explained in Chapter 4 , “More
Transformations and Basic Animation.” That vertex shader created a rotation matrix using
a Matrix4 object and then used the matrix to rotate a triangle. Let’s take a look at that
shader again:
1 // RotatedTriangle_Matrix4.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'uniform mat4 u_rotMatrix;\n' +
6 'void main() {\n' +
7 ' gl_Position = u_rotMatrix * a_Position;\n' +
8 '}\n';
The vertex shader in this section ( LookAtTriangles.js ) is listed as follows:
1 // LookAtTriangles.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'attribute vec4 a_Color;\n' +
6 'uniform mat4 u_ViewMatrix;\n' +
Specifying the Viewing Direction
233
7 'varying vec4 v_Color;\n' +
8 'void main() {\n' +
9 ' gl_Position = u_ViewMatrix * a_Position;\n' +
10 ' v_Color = a_Color;\n' +
11 '}\n';
As you can see, the attribute variable for color values ( a_Color ) and the varying variable
that passes the values to the fragment shader ( v_Color ) were added, and the name of the
uniform variable was changed from u_rotMatrix to u_ViewMatrix . Despite these differ-
ences, the calculation of the value assigned to gl_Position is the same as that in the
vertex shader of RotatedTriangle_Matrix4.js : multiplying a mat4 matrix by a_Position .
(Compare line 7 in RotatedTriangle_Matrix4.js with line 9 in LookAtTriangles.js .)
This tells you that the operation performing “where you are looking and which part of the
scene you are looking at” is actually equivalent to transformations such as translating or
rotating a triangle.
Let’s use an example to explain this. Assume that you are looking at a triangle from the
origin (0, 0, 0) along the negative direction of the z-axis, and then the eye point moves to
the position (0, 0, 1) (the left-side figure of Figure 7.6 ). In this case, the distance between
the eye point and the triangle has increased by 1.0 unit of the z-axis. To achieve the same
effect, you could leave the eye point alone and instead move the triangle 1.0 unit away
(the right-side figure of Figure 7.6 ).
x
y
z
x
y
z
t n i o p e y e
Figure 7.6 Movement of the eye point is identical to that of objects in the scene
This is exactly what happens in our sample program. The setLookAt() method of the
Matrix4 object just calculates the matrix to carry out this transformation using the infor-
mation about the position of the eye point, look-at point, and up direction. So, by then
multiplying the matrix by the vertex coordinates of the objects in the scene, you obtain
the same effect as moving the eye point. Essentially, instead of moving the eye point
in one direction, the objects viewed (that is, the world itself) are moved in the opposite
direction. You can use the same approach to handle rotation of the eye point.
Because moving the eye point is the same type of transformation as rotating or translating
a triangle, you can represent both of them as a transformation matrix. Let’s look at how
CHAPTER 7 Toward the 3D World
234
you calculate the matrix when you want to rotate a triangle and move the position of the
eye point.
Looking at Rotated Triangles from a Specified Position
RotatedTriangle_Matrix4 in Chapter 4 displayed the triangle rotated around the z-axis. In
this section, you modify LookAtTriangles to display the triangles viewed from a specified
eye point along the line of sight. In this case, two matrices are required: a rotation matrix
to rotate the triangles and a view matrix to specify the view of the scene. The first issue to
consider is in which order you should multiply them.
So far, you know that multiplying a matrix by a vertex coordinate will apply the trans-
formation defined by the matrix to the coordinates. That is to say, multiplying a rotation
matrix by a vertex coordinate causes it to be rotated.
Multiplying a view matrix by a vertex coordinate causes the vertex to be transformed to
the correct position as viewed from the eye position. In this sample program, we want to
view the rotated triangles from a specified position, so we need to rotate the triangles and
then look at them from the specified eye position. In other words, we need to rotate the
three vertex coordinates comprising the triangle. Then we need to transform the rotated
vertex coordinates (the rotated triangle) as we look at them from the specified position.
We can achieve this by carrying out a matrix multiplication in the order described in the
previous sentence. Let’s check the equations.
As explained previously, if you want to rotate a shape, you need to multiply a rotation
matrix by the vertex coordinates of the shape as follows:
〈 〉 =
〈 〉× 〈 〉
rotated vertex coordinates
rotation matrix original vertex coordinates
" "    
    
By multiplying a view matrix by the rotated vertex coordinates in the preceding equation,
you can obtain the rotated vertex coordinates that are viewed from the specified position.
〈 〉 =
〈 〉× 〈 〉
rotated vertex coordinates viewed from specified position
view matrix rotated vertex coordinates
" "     "       " 
  " "  
If you substitute the first expression into the second one, you obtain the following:
〈 〉 =
〈 〉× 〈 〉× 〈 〉
rotated vertex coordinates viewed from specified position
view matrix rotation matrix original vertex coordinates
" "     "       "
      
In this expression, you use a rotation matrix, but you can also apply a translation matrix,
a scaling matrix, or a combination of them. Such a matrix is generally called a model
matrix . Using that term, you can rewrite the expression shown in Equation 7.1 .
Equation 7.1
〈 〉× 〈 〉× 〈 〉 view matrix model matrix vertex coordinates     
Specifying the Viewing Direction
235
Now you need to implement this expression in a shader program, but because it is quite
a simple expression, you can implement it as-is in a vertex shader. The sample program
LookAtRotatedTriangles implements the transformation, and a screen shot is shown in
Figure 7.7 . Note, in this figure, that the white dashed line shows the triangle before rota-
tion so that you can easily see the rotation.
Figure 7.7 LookAtRotatedTriangles
Sample Program (LookAtRotatedTriangles.js)
LookAtRotatedTriangles.js is programmed by slightly modifying LookAtTriangles.js .
You just need to add the uniform variable u_ModelMatrix to pass the model matrix to the
shader and then add some processing in JavaScript’s main() function to pass the matrix to
the variable. The relevant code is shown in Listing 7.2 .
Listing 7.2 LookAtRotatedTriangles.js
1 // LookAtRotatedTriangles.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'attribute vec4 a_Color;\n' +
6 'uniform mat4 u_ViewMatrix;\n' +
7 'uniform mat4 u_ModelMatrix;\n' +
8 'varying vec4 v_Color;\n' +
9 'void main() {\n' +
10 ' gl_Position = u_ViewMatrix * u_ModelMatrix * a_Position;\n' +
11 ' v_Color = a_Color;\n' +
12 '}\n';
...
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24 function main() {
...
51 // Get the storage locations of u_ViewMatrix and u_ModelMatrix
52 varu_ViewMatrix = gl.getUniformLocation(gl.program,'u_ViewMatrix');
53 var u_ModelMatrix = gl.getUniformLocation(gl.program, 'u_ModelMatrix');
...
59 // Specify the eye point and line of sight
60 var viewMatrix = new Matrix4();
61 viewMatrix.setLookAt(0.20, 0.25, 0.25, 0, 0, 0, 0, 1, 0);
62
63 // Calculate the rotation matrix
64 var modelMatrix = new Matrix4();
65 modelMatrix.setRotate(-10, 0, 0, 1); // Rotate around z-axis
66
67 // Pass each matrix to each uniform variable
68 gl.uniformMatrix4fv(u_ViewMatrix, false, viewMatrix.elements);
69 gl.uniformMatrix4fv(u_ModelMatrix, false, modelMatrix.elements);
First, let’s examine the vertex shader. You can see that line 10 simply implements
Equation 7.1 as-is by using u_ModelMatrix at line 7, which receives data from the
JavaScript program:
10 ' gl_Position = u_ViewMatrix * u_ModelMatrix * a_Position;\n' +
In the main() function in JavaScript, you already have the code for calculating a view
matrix, so you just need to add the code for calculating the rotation matrix perform-
ing a –10 degree rotation around the z-axis. Line 53 gets the storage location of the
u_ModelMatrix variable, and line 64 creates modelMatrix for the rotation matrix. Then
line 65 calculates the matrix using Matrix4.setRotate() , and line 69 passes it to
u_ModelMatrix .
When you run this sample program, you will see the triangles shown in Figure 7.6 illus-
trating that the matrices multiplied by the vertex coordinates ( a_Position ) have the
desired effect. That is, the vertex coordinates were rotated by u_ModelMatrix , and then
the resulting coordinates were transformed by u_ViewMatrix to the correct position as if
viewed from the specified position.
Experimenting with the Sample Program
In LookAtRotatedTriangles.js , you implemented Equation 7.1 as- is. However, because
the multiplication of the view matrix and model matrix is performed per vertex in the
vertex shader, this implementation is inefficient when processing many vertices. The
result of the matrix multiplication in Equation 7.1 is identical for each vertex, so you can
calculate it in advance and pass the result to the vertex shader. The matrix obtained by
multiplying a view matrix by a model matrix is called a model view matrix . That is,
Specifying the Viewing Direction
237
〈
〉 = 〈 〉× 〈 〉 model view matrix view matrix model matrix         
Then, you can rewrite the expression in Equation 7.1 as shown in Equation 7.2 .
Equation 7.2
〈 〉× 〈 〉 model view matrix vertex coordinates     
If you use Equation 7.2 , you can rewrite the sample program shown in Listing 7.3 . This
sample program is LookAtRotatedTriangles_mvMatrix .
Listing 7.3 LookAtRotatedTriangles_mvMatrix.js
1 // LookAtRotatedTriangles_mvMatrix.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
...
6 'uniform mat4 u_ModelViewMatrix;\n' +
7 'varying vec4 v_Color;\n' +
8 'void main() {\n' +
9 ' gl_Position = u_ModelViewMatrix * a_Position;\n' +
10 ' v_Color = a_Color;\n' +
11 '}\n';
...
23 function main() {
...
50 // Get the storage locations of u_ModelViewMatrix and u_ModelMatrix
51 var u_ModelViewMatrix = gl.getUniformLocation(gl.program, 'u_ModelViewMatrix');
...
59 viewMatrix.setLookAt(0.20, 0.25, 0.25, 0, 0, 0, 0, 1, 0);
...
63 modelMatrix.setRotate(-10, 0, 0, 1); // Calculate rotation matrix
64
65 // Multiply both matrices
66 var modelViewMatrix = viewMatrix.multiply(modelMatrix);
67
68 // Pass the model viewmatrix to u_ModelViewMatrix
69 gl.uniformMatrix4fv(u_ModelViewMatrix, false, modelViewMatrix.elements);
In the vertex shader, the name of the uniform variable was modified to u_
ModelViewMatrix and calculated in line 9. However, the processing steps in the vertex
shader are identical to the original LookAtTriangles.js .
Within the JavaScript program, the method of calculating viewMatrix and modelMatrix
from lines 59 to 63 is identical to that in LookAtRotatedTriangles.js and, when multi-
plied, result in modelViewMatrix (line 66). The multiply() method is used to multiply
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Matrix4 objects. It multiplies the matrix on the right side ( viewMatrix ) by the matrix
specified by the argument ( modelMatrix ) of the method. So this code actually performs
modelViewMatrix= viewMatrix * modelMatrix . Unlike with GLSL ES, you need to use a
method to perform matrix multiplication instead of the * operator.
Having obtained modelViewMatrix , you just need to pass it to the u_ModelViewMatrix
variable at line 69. Once you run the program, you can see the same result as shown in
Figure 7.6 .
As a final point, in this sample program, each matrix was calculated piece by piece at lines
59, 63, and 66 to better show the flow of the calculation. However, this could be rewritten
in one line for efficiency:
var modelViewMatrix = new Matrix4();
modelViewMatrix.setLookAt(0.20, 0.25, 0.25, 0, 0, 0, 0, 1, 0).rotate(-10, 0, 0, 1);
// Pass the model view matrix to the uniform variable
gl.uniformMatrix4fv(u_ModelViewMatrix, false, modelViewMatrix.elements);
Changing the Eye Point Using the Keyboard
Let’s modify LookAtTriangles to change the position of the eye point when the arrow
keys are pressed. LookAtTrianglesWithKeys uses the right arrow key to increase the x coor-
dinate of the eye point by 0.01 and the left arrow key to decrease the coordinate by 0.01.
Figure 7.8 shows a screen shot of the sample program when run. If you hold down the left
arrow key, the scene changes to that seen on the right side of Figure 7.8 .
Figure 7.8 LookAtTrianglesWithKeys
Sample Program (LookAtTrianglesWithKeys.js)
Listing 7.4 shows the sample code. The vertex shader and the fragment shader are the
same as those in LookAtTriangles.js . The basic processing flow of main() in JavaScript
is also the same. The code for registering the event handler called on a key press is added
Specifying the Viewing Direction
239
to the sample program, and the code for drawing the triangles has been moved into the
function draw() .
Listing 7.4 LookAtTrianglesWithKeys.js
1 // LookAtTrianglesWithKeys.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'attribute vec4 a_Color;\n' +
6 'uniform mat4 u_ViewMatrix;\n' +
7 'varying vec4 v_Color;\n' +
8 'void main() {\n' +
9 ' gl_Position = u_ViewMatrix * a_Position;\n' +
10 ' v_Color = a_Color;\n' +
11 '}\n';
...
23 function main() {
...
50 // Get the storage location of the u_ViewMatrix variable
51 varu_ViewMatrix = gl.getUniformLocation(gl.program,'u_ViewMatrix');
...
57 // Create Matrix4 object for a view matrix
58 var viewMatrix = new Matrix4();
59 // Register the event handler to be called on key press
60 document.onkeydown = function(ev){ keydown(ev, gl, n, u_ViewMatrix,
➥viewMatrix); };
61
62 draw(gl, n, u_ViewMatrix, viewMatrix); // Draw a triangle
63 }
...
117 var g_eyeX = 0.20, g_eyeY = 0.25, g_eyeZ = 0.25; // The eye point
118 function keydown(ev, gl, n, u_ViewMatrix, viewMatrix) {
119 if(ev.keyCode == 39) { // The right arrow key was pressed
120 g_eyeX += 0.01;
121 } else
122 if (ev.keyCode == 37) { // The left arrow key was pressed
123 g_eyeX -= 0.01;
124 } else { return; } // Prevent unnecessary drawing
125 draw(gl, n, u_ViewMatrix, viewMatrix);
126 }
127
128 function draw(gl, n, u_ViewMatrix, viewMatrix) {
129 // Set the eye point and line of sight
130 viewMatrix.setLookAt(g_eyeX, g_eyeY, g_eyeZ, 0, 0, 0, 0, 1, 0);
CHAPTER 7 Toward the 3D World
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131
132 // Pass the view matrix to the u_ViewMatrix variable
133 gl.uniformMatrix4fv(u_ViewMatrix, false, viewMatrix.elements);
134
135 gl.clear(gl.COLOR_BUFFER_BIT); // Clear <canvas>
136
137 gl.drawArrays(gl.TRIANGLES, 0, n); // Draw a triangle
138 }
In this sample, you are using the event handler to change the position of the eye point
when the right arrow key or the left arrow key is pressed. Before explaining the event
handler, let’s look at the function draw() that is called from the event handler.
The process performed in draw() is straightforward. Line 130 calculates the view matrix
using the global variables g_eyeX , g_eyeY , and g_eyeZ defined at line 117, which contain
0.2, 0.25, and 0.25, respectively. Then the matrix is passed to the uniform variable
u_ViewMatrix in the vertex shader at line 133. Back in main() , the storage location of
u_ViewMatrix is retrieved at line 51, and a Matrix4 object ( viewMatrix ) is created at line
58. These two operations are carried out in advance because it is redundant if you perform
them for each draw operation, particularly retrieving the storage. After that, line 135 clears
<canvas> , and line 137 draws the triangles.
The variables g_eyeX , g_eyeY , and g_eyeZ specify the eye position and are recalculated
in the event handler whenever a key is pressed. To call the event handler on key press,
you need to register it to the onkeydown property of the document object. In this event
handler, because you need to call draw() to draw the triangles, you must pass all argu-
ments that draw() requires. This is why an anonymous function registers the handler as
follows:
59 // Register the event handler to be called on key press
60 document.onkeydown = function(ev){ keydown(ev, gl, n, u_ViewMatrix,
➥viewMatrix); };
This sets up the event handler keydown() to be called when the key is pressed. Let’s
examine how keydown() is implemented.
118 function keydown(ev, gl, n, u_ViewMatrix, viewMatrix) {
119 if(ev.keyCode == 39) { // The right arrow key was pressed
120 g_eyeX += 0.01;
121 } else
122 if (ev.keyCode == 37) { // The left arrow key was pressed
123 g_eyeX -= 0.01;
124 } else { return ; } // Prevent unnecessary drawing
125 draw(gl, n, u_ViewMatrix, viewMatrix); // Draw a triangle
126 }
Specifying the Visible Range (Box Type)
241
What keydown() is doing is also straightforward. When a key is pressed, keydown()
is called with the information about the event stored in the first parameter, ev , of
keydown() . Then you just need to check which key was pressed by examining the value of
ev.keyCode , modify g_eyeX , and draw the triangles. When the right arrow key is pressed,
the code increases g_eyeX by 0.01, and when the left arrow key is pressed, it decreases
g_eyeX by 0.01.
If you run the sample program, you can see the triangles shift every time you press the
arrow key.
Missing Parts
As you play with the sample program, you may notice that as you shift the eye position to
the extreme right or left, part of the triangle disappears (see Figure 7.9 ).
Figure 7.9 Part of the triangle disappears
This is because you haven’t specified the visible range (the boundaries of what you can
actually see) correctly. As mentioned in the first section of this chapter, WebGL does not
display objects outside the visible range. In the case of Figure 7.9 , part of the triangle went
out of the visible range while pressing the arrow keys.
Specifying the Visible Range (Box Type)
Although WebGL allows you to place 3D objects anywhere in 3D space, it only displays
those that are in the visible range. In WebGL, this is primarily a performance issue; there’s
no point in drawing 3D objects if they are not visible to the viewer. In a way, this mimics
the way human sight works (see Figure 7.10 ); we see objects within the visible range based
CHAPTER 7 Toward the 3D World
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on our line of sight, which is approximately 200 degrees in the horizontal field of view.
WebGL also has a similar limited range and does not display 3D objects outside of that
range.
Figure 7.10 Human visual field
In addition to the up/down, left/right range along the line of sight, WebGL has a depth
range that indicates how far you can see. These ranges are called the viewing volume . In
Figure 7.9 , because the depth range was not sufficient, part of the triangle disappears as it
moves out of the viewing volume.
Specify the Viewing Volume
There are two ways of specifying a viewing volume:
• Using a rectangular parallelepiped, or more informally, a box ( orthographic projec-
tion )
• Using a quadrangular pyramid ( perspective projection )
Perspective projection gives more information about depth and is often easier to view
because you use perspective views in real life. You should use this projection to show
the 3D scene in perspective, such as a character or a battlefield in a 3D shooting game.
Orthographic projection makes it much easier to compare two objects, such as two parts
of a molecule model, because there is no question about how the viewpoint may affect
the perception of distance. You should use the projection to show 3D objects in an ortho-
graphic view like those in technical drawing.
First, we will explain how the viewing volume works based on the box-shaped viewing
volume.
The box-shaped viewing volume is shaped as shown in Figure 7.11 . This viewing volume
is set from the eye point toward the line of sight and occupies the space delimited by the
two planes: the near clipping plane and the far clipping plane . The near clipping plane
is defined by ( right , top , - near ), (- left , top , - near ), (- left , - bottom , - near ), and ( right , - bottom ,
- near ). The far clipping plane is defined by ( right , top , - far ), (- left , top , - far ), (- left , - bottom ,
- far ), and ( right , - bottom , - far ).
Specifying the Visible Range (Box Type)
243
r a e n
r a f
invisible
visible
eye point
left
right
bottom
top
far clipping plane
near clipping plane
invisible
x
y
z
Figure 7.11 Box-shaped viewing volume
The scene viewed from the near clipping plane toward the line of sight is displayed on the
<canvas> . If the aspect ratio of the near clipping plane is different from that of <canvas> ,
the scene is scaled according to the ratio, and the aspect ratio of the geometric shapes or
objects in the scene are distorted. (You will explore this behavior in the last part of this
section.) The range from the near clipping plane to the far clipping plane defines the
viewing volume. Only objects inside this volume are displayed. If the objects lie partially
inside the volume, only the part inside the volume is displayed.
Defining a Box-Shaped Viewing Volume
To set the box-shaped viewing volume, you use the method setOrtho() supported by the
Matrix4 object defined in cuon-matrix.js .
Matrix4.setOrtho(left, right, bottom, top, near, far)
Calculate the matrix (orthographic projection matrix) that defines the viewing volume
specified by its arguments, and store it in Matrix4 . However, left must not be equal to
right , bottom not equal to top , and near not equal to far .
Parameters left, right Specify the distances to the left side and right side of the near
clipping plane.
bottom, top Specify the distances to the bottom and top of the near clipping
plane.
near, far Specify the distances to the near and far clipping planes along
the line of sight.
Return value None
CHAPTER 7 Toward the 3D World
244
Here, you are using a matrix again, which in this case is referred to as the ortho-
graphic projection matrix . The sample program OrthoView will use this type of
projection matrix to set the box-shaped viewing volume and then draw three trian-
gles—as used in LookAtRotatedTriangles —to test the effect of the viewing volume. In
LookAtRotatedTriangles , you placed the eye point at a different location from that of the
origin. However, in this sample program, you’ll use the origin (0, 0, 0) and set the line of
sight along the negative z-axis to make it easy to check the effect of the viewing volume.
The viewing volume is specified as shown in Figure 7.12 , which uses near =0.0, far =0.5,
left =–1.0, right =1.0, bottom =–1.0, and top =1.0 because the triangles lie between 0.0 and –0.4
along the z-axis (refer to Figure 7.2 ).
) 0 . 0 , 0 . 1 , 0 . 1 - (
) 5 . 0 - , 0 . 1 , 0 . 1 - (
) 0 . 0 , 0 . 1 - , 0 . 1 (
) 0 . 0 , 0 . 1 - , 0 . 1 - ( ) 5 . -0 , 0 . 1 - , 0 . 1 - (
z
x
y
. . ) 0 0 , 0 1 , 0 . 1 (
. ) -0.5 , 0 1 , 0 . 1 (
t n i o p e y e
Figure 7.12 The box-shaped viewing volume used in OrthoView
In addition, we add key-press event handlers to change the values of near and far to check
the effect of changing the size of the viewing volume. The following are the active keys
and their mappings.
Arrow Key Action
Right Increases near by 0.01
Left Decreases near by 0.01
Up Increases far by 0.01
Down Decreases far by 0.01
So that you can see the current values of near and far, they are displayed below the
canvas, as shown in Figure 7.13 .
Specifying the Visible Range (Box Type)
245
Figure 7.13 OrthoView
Let’s examine the sample program.
Sample Program (OrthoView.html)
Because this sample program shows the near and far value on the web page and not in
the <canvas> , you need to add something to the HTML file listing as shown in Listing 7.5
( OrthoView.html) .
Listing 7.5 OrthoView.html
1 <!DOCTYPE html>
2 <html>
3 <head lang="ja">
4 <meta charset="utf-8" />
5 <title>Set Box-shaped Viewing Volume</title>
6 </head>
7
8 <body οnlοad="main()">
9 <canvas id="webgl" width="400" height="400">
10 Please use a browser that supports <canvas>
11 </canvas>
12 <p id="nearFar"> The near and far values are displayed here. </p>
13
14 <script src="../lib/webgl-utils.js"></script>
...
18 <script src="OrthoView.js"></script>
CHAPTER 7 Toward the 3D World
246
19 </body>
20 </html>
As you can see, line 12 was added. This line shows “The near and far values are displayed
here” and uses JavaScript to rewrite the contents of nearFar to show the current near and
far values.
Sample Program (OrthoView.js)
Listing 7.6 shows OrthoView.js . This program is almost the same as
LookAtTrianglesWithKeys.js , which changes the position of the eye point by using the
arrow keys.
Listing 7.6 OrthoView.js
1 // OrthoView.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'attribute vec4 a_Color;\n' +
6 'uniform mat4 u_ProjMatrix;\n' +
7 'varying vec4 v_Color;\n' +
8 'void main() {\n' +
9 ' gl_Position = u_ProjMatrix * a_Position;\n' +
10 ' v_Color = a_Color;\n' +
11 '}\n';
...
23 function main() {
24 // Retrieve<canvas> element
25 var canvas = document.getElementById('webgl');
26 // Retrieve the nearFar element
27 var nf = document.getElementById('nearFar');
...
52 // Get the storage location of u_ProjMatrix variable
53 var u_ProjMatrix = gl.getUniformLocation(gl.program,'u_ProjMatrix');
...
59 // Create the matrix to set the eye point and line of sight
60 var projMatrix = new Matrix4();
61 // Register the event handler to be called on key press
62 document.onkeydown = function(ev) { keydown(ev, gl, n, u_ProjMatrix,
➥projMatrix, nf); };
63
64 draw(gl, n, u_ProjMatrix, projMatrix, nf); // Draw triangles
65 }
...
116 // The distances to the near and far clipping plane
Specifying the Visible Range (Box Type)
247
117 var g_near = 0.0, g_far = 0.5;
118 function keydown(ev, gl, n, u_ProjMatrix, projMatrix, nf) {
119 switch(ev.keyCode) {
120 case 39: g_near += 0.01; break; // The right arrow key was pressed
121 case 37: g_near -= 0.01; break; // The left arrow key was pressed
122 case 38: g_far += 0.01; break; // The up arrow key was pressed
123 case 40: g_far -= 0.01; break; // The down arrow key was pressed
124 default: return; // Prevent the unnecessary drawing
125 }
126
127 draw(gl, n, u_ProjMatrix, projMatrix, nf);
128 }
129
130 function draw(gl, n, u_ProjMatrix, projMatrix, nf) {
131 // Set the viewing volume using a matrix
132 projMatrix.setOrtho(-1, 1, -1, 1, g_near, g_far);
133
134 // Set the projection matrix to u_ProjMatrix variable
135 gl.uniformMatrix4fv(u_ProjMatrix, false, projMatrix.elements);
136
137 gl.clear(gl.COLOR_BUFFER_BIT); // Clear <canvas>
138
139 // Display the current near and far values
140 nf.innerHTML = 'near: ' + Math.round(g_near * 100)/100 + ', far: ' +
➥ Math.round(g_far*100)/100;
141
142 gl.drawArrays(gl.TRIANGLES, 0, n); // Draw the triangles
143 }
In a similar way to LookAtTrianglesWithKeys , keydown() , defined at line 118, is called
on key press, and draw() is called at the end of keydown() (line 127). The draw() func-
tion defined at line 130 sets the viewing volume, rewrites the near and far value on the
web page, and then draws the three triangles. The key point in this program is the draw()
function; however, before explaining the function, let’s quickly show how to rewrite
HTML elements using JavaScript.
Modifying an HTML Element Using JavaScript
The method of modifying an HTML element using JavaScript is similar to that of drawing
in a <canvas> with WebGL. That is, after retrieving the HTML element by using getEle-
mentById() and the id of the element, you write a message to the element in JavaScript.
In this sample program, you modify the following <p> element to show the message such
as “near: 0.0, far: 0.5”:
12 <p id="nearFar"> The near and far values are displayed here. </p>
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248
This element is retrieved at line 27 in OrthoView.js using getElementById() as before.
Once you’ve retrieved the element, you need to specify the string ( 'nearFar' ) that was
bound to id at line 12 in the HTML file, as follows:
26 // Retrieve nearFar element
27 var nf = document.getElementById('nearFar');
Once you retrieve the <p> element into the variable nf (actually, nf is a JavaScript object),
you just need to change the content of this element. This is straightforward and uses the
innerHTML property of the object. For example, if you write:
nf.innerHTML = 'Good Morning, Marisuke-san!';
You will see the message “Good Morning, Marisuke-san!” on the web page. You can also
insert HTML tags in the message. For example, ‘Good Morning, <b>Marisuke</b>-san!’
will highlight “Marisuke.”
In OrthoView.js , you use the following equation to display the current near and far
values. These values are stored in the global variables g_near and g_far declared at line
117. When printing them, they are formatted using Math.round() as follows:
139 // Display the current near and far values
140 nf.innerHTML = 'near: ' + Math.round(g_near*100)/100 + ', far: ' +
➥Math.round(g_far*100)/100;
The Processing Flow of the Vertex Shader
As you can see with the following code, the processing flow in the vertex shader is almost
the same as that in LookAtRotatedTriangles.js except that the uniform variable name
( u_ProjMatrix ) at line 6 was changed. This variable holds the matrix used to set the
viewing volume. So you just need to multiply the matrix ( u_ProjMatrix ) by the vertex
coordinates to set the viewing volume at line 9:
2 // Vertex shader program
3 var VSHADER_SOURCE =
...
6 'uniform mat4 u_ProjMatrix;\n' +
7 'varying vec4 v_Color;\n' +
8 'void main() {\n' +
9 ' gl_Position = u_ProjMatrix * a_Position;\n' +
10 ' v_Color = a_Color;\n' +
11 '}\n';
Line 62 registers the event handler for the arrow key press. Note that nf is passed as the
last argument to the handler to allow it to access the <p> element. The event handler use
the key press to determine the contents of the element in draw() , which is called in the
handler:
Specifying the Visible Range (Box Type)
249
61 // Register the event handler to be called on key press
62 document.onkeydown = function(ev) { keydown(ev, gl, n, u_ProjMatrix,
➥projMatrix, nf); };
The keydown() at line 121 identifies which arrow key is pressed and then modifies the
value of g_near and g_far before calling draw() at line 127. Line 117 defines g_near and
g_far , which are used by the setOrtho() method. These are defined as global variables
because they are used in both keydown() and draw() :
116 // The distances to the near and far clipping plane
117 var g_near = 0.0, g_far = 0.5;
118 function keydown(ev, gl, n, u_ProjMatrix, projMatrix, nf) {
119 switch(ev.keyCode) {
120 case 39: g_near += 0.01; break; // The right arrow key was pressed
...
123 case 40: g_far -= 0.01; break; // The down arrow key was pressed
124 default: return; // Prevent the unnecessary drawing
125 }
126
127 draw(gl, n, u_ProjMatrix, projMatrix, nf);
128 }
Let’s examine the function draw() . The processing flow of draw() , defined at line 130, is
the same as in LookAtTrianglesWithKeys.js except for changing the message on the web
page at line 140:
130 function draw(gl, n, u_ProjMatrix, projMatrix, nf) {
131 // Set the viewing volume
132 projMatrix.setOrtho(-1.0, 1.0, -1.0, 1.0, g_near, g_far);
133
134 // Set the projection matrix to u_ProjMatrix variable
135 gl.uniformMatrix4fv(u_ProjMatrix, false, projMatrix.elements);
...
139 // Display the current near and far values
140 nf.innerHTML = 'near: ' + Math.round(g_near * 100)/100 + ', far: ' +
➥Math.round(g_far*100)/100;
141
142 gl.drawArrays(gl.TRIANGLES, 0, n); // Draw the triangles
143 }
Line 132 calculates the matrix for the viewing volume ( projMatrix ) and passes it to
u_ProjMatrix at line 135. Line 140 displays the current near and far value on the web
page. Finally, at line 142, the triangles are drawn.
CHAPTER 7 Toward the 3D World
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Changing Near or Far
When you run this program and increase the near value (right-arrow key), the display will
change, as shown in Figure 7.14 .
Figure 7.14 Increase the near value using the right arrow key
By default, near is 0.0, so all three triangles are displayed. Next, when you increase near
using the right arrow key, the blue triangle (the front triangle) disappears because the
viewing volume moves past it, as shown in Figure 7.15 . This result is shown as the middle
figure in Figure 7.14 .
z
x
y
Figure 7.15 The blue triangle went outside the viewing volume
Again, if you continue to increase near by pressing the right arrow key, when near becomes
larger than 0.2, the near plane moves past the yellow triangle, so it is outside the viewing
volume and disappears. This leaves only the green triangle (the right figure in Figure 7.14 ).
At this point, if you use the left arrow key to decrease near so it becomes less than 0.2, the
yellow triangle becomes visible again. Alternatively, if you keep on increasing near , the
green triangle will also disappear, leaving the black canvas.
Specifying the Visible Range (Box Type)
251
As you can imagine, the behavior when you alter the far value is similar. As shown in
Figure 7.16 , when far becomes less than 0.4, the back triangle (the green one) will disap-
pear. Again, if you keep decreasing far , only the blue triangle will remain.
Figure 7.16 Decrease the far value using the down arrow key
This example should clarify the role of the viewing volume. Essentially, for any object you
want to display, you need to place it inside the viewing volume.
Restoring the Clipped Parts of the Triangles
(LookAtTrianglesWithKeys_ViewVolume.js)
In LookAtTrianglesWithKeys , when you kept pressing the arrow keys, part of the triangle
is clipped, as shown in Figure 7.17 . From the previous discussion, it’s clear this is because
some part went outside the viewing volume. In this section, you will modify the sample
program to display the triangle correctly by setting the appropriate viewing volume.
Figure 7.17 A part of the triangle is clipped.
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As you can see from the figure, the far corner of the triangle from the eye point is clipped.
Obviously, the far clipping plane is too close to the eye point, so you need to move the far
clipping plane farther out than the current one. To achieve this, you can modify the argu-
ments of the viewing volume so that left =–1.0, right=1.0, bottom =–1.0, top =1.0, near =0.0,
and far =2.0.
You will use two matrices in this program: the matrix that sets the viewing volume (the
orthographic projection matrix), and the matrix that sets the eye point and the line of
sight (view matrix). Because setOrtho() sets the viewing volume from the eye point, you
need to set the position of the eye point and then set the viewing volume. Consequently,
you will multiply the view matrix by the vertex coordinates to get the vertex coordi-
nates, which are “viewed from the eye position” first, and then multiply the orthographic
projection matrix by the coordinates. You can calculate them as shown in Equation 7.3 .
Equation 7.3
〈 〉× 〈 〉× 〈 〉 orthographic projection matrix view matrix vertex coordinates       
This can be implemented in the vertex shader, as shown in Listing 7.7 .
Listing 7.7 LookAtTrianglesWithKeys_ViewVolume.js
1 // LookAtTrianglesWithKeys_ViewVolume.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'attribute vec4 a_Color;\n' +
6 'uniform mat4 u_ViewMatrix;\n' +
7 'uniform mat4 u_ProjMatrix;\n' +
8 'varying vec4 v_Color;\n' +
9 'void main() {\n' +
10 ' gl_Position = u_ProjMatrix * u_ViewMatrix * a_Position;\n' +
11 ' v_Color = a_Color;\n' +
12 '}\n';
...
24 function main() {
...
51 // Get the storage locations of u_ViewMatrix and u_ProjMatrix
52 varu_ViewMatrix = gl.getUniformLocation(gl.program,'u_ViewMatrix');
53 var u_ProjMatrix = gl.getUniformLocation(gl.program,'u_ProjMatrix');
...
59 // Create the matrix to specify the view matrix
60 var viewMatrix = new Matrix4();
61 // Register the event handler to be called on key press
62 document.onkeydown = function(ev) { keydown(ev, gl, n, u_ViewMatrix,
➥viewMatrix); };
Specifying the Visible Range (Box Type)
253
63
64 // Create the matrix to specify the viewing volume and pass it to u_ProjMatrix
65 var projMatrix = new Matrix4();
66 projMatrix.setOrtho(-1.0, 1.0, -1.0, 1.0, 0.0, 2.0);
67 gl.uniformMatrix4fv(u_ProjMatrix, false, projMatrix.elements);
68
69 draw(gl, n, u_ViewMatrix, viewMatrix); // Draw the triangles
70 }
Line 66 calculates the orthographic projection matrix ( projMatrix ) by modifying far from
1.0 to 2.0. The result matrix is passed to u_ProjMatrix in the vertex shader at line 67.
A uniform variable is used because the elements in the matrix are uniform for all vertex
coordinates. If you run this sample program and move the eye point as before, you can
see that the triangle no longer gets clipped (see Figure 7.18 ).
Figure 7.18 LookAtTrianglesWithKeys_ViewVolume
Experimenting with the Sample Program
As we explained in the section “Specify the Viewing Volume,” if the aspect ratio of
<canvas> is different from that of the near clipping plane, distorted objects are displayed.
Let’s explore this. First, in OrthoView_halfSize (based on Listing 7.7 ), you reduce the
current size of the near clipping plane to half while keeping its aspect ratio:
projMatrix.setOrtho(-0.5, 0.5, -0.5, 0.5, 0, 0.5);
The result is shown on the left of Figure 7.19 . As you can see, the triangles appear twice
as large as those of the previous sample because the size of <canvas> is the same as before.
Note that the parts of the triangles outside the near clipping plane are clipped.
CHAPTER 7 Toward the 3D World
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Figure 7.19 Modify the size of the near clipping plane
In OrthoView_halfWidth , you reduce only the width of the near clipping plane by chang-
ing the first two arguments in setOrtho() as follows:
projMatrix.setOrtho(-0.3, 0.3, -1.0, 1.0, 0.0, 0.5);
You can see the results on the right side of Figure 7.19 . This is because the near clipping
plane is horizontally reduced and then horizontally extended (and thus distorted) to fit
the square-shaped <canvas> when the plane is displayed.
Specifying the Visible Range Using a Quadrangular
Pyramid
Figure 7.20 shows a tree-lined road scene. In this picture, all the trees on the left and right
sides are approximately of the same height, but the farther back they are, the smaller
they look. Equally, the building in the distance appears smaller than the trees that are
closer to the viewer, even though the building is actually taller than the trees. This effect
of distant objects looking smaller gives the feeling of depth. Although our eyes perceive
reality in this way, it’s interesting to notice that children’s drawings rarely show this kind
of perspective.
Specifying the Visible Range Using a Quadrangular Pyramid
255
Figure 7.20 Tree-lined road
In the case of the box-shaped viewing volume explained in the previous section, identi-
cally sized triangles are drawn the same size, regardless of their distance from the eye
point. To overcome this constraint, you can use the quadrangular pyramid viewing
volume, which allows you to give this sense of depth, as seen in Figure 7.20 .
Here you construct the sample program PerspectiveView , which sets a quadrangular
pyramid viewing volume that points along the negative z-axis from the eye point set at
(0, 0, 5). Figure 7.21 shows a screen shot of PerspectiveView and the location of each
triangle.
z
-0.75
0.75
x
y
1.0
-1.0
0.0
-2.0
-4.0
Figure 7.21 PerspectiveView; location of each triangle
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As can be seen from the figure on the right, three identically sized triangles are positioned
on the right and left sides along the coordinate’s axes, in a way similar to the tree-lined
road. By using a quadrangular pyramid viewing volume, WebGL can automatically display
remote objects as if they are smaller, thus achieving the sense of depth. This is shown in
the left side of the figure.
To really notice the change in size, as in the real world, the objects need to be located
at a substantial distance. For example, when looking at the box, to actually make the
background area looks smaller than the foreground area, this box needs to have consider-
able depth. So this time, you will use a slightly more distant position (0, 0, 0.5) than the
default value (0, 0, 0) for the eye point.
Setting the Quadrangular Pyramid Viewing Volume
The quadrangular pyramid viewing volume is shaped as shown in Figure 7.22 . Just like
the box-shaped configuration, the viewing volume is set at the eye point along the line of
sight, and objects located between the far and near clipping planes are displayed. Objects
positioned outside the viewing volume are not shown, while those straddling the bound-
ary will only have parts located inside the viewing volume visible.
v o f
r a e n
r a f
z
y
eye point
eye point
up
vector
near clipping plane
far clipping plane
aspect
(aspect of near clipping plane)
invisible
invisible
visible
line of sight
Figure 7.22 Quadrangular pyramid viewing volume
Regardless of whether it is a quadrangular pyramid or a box, you set the viewing volume
using matrices, but the arguments differ. The Matrix4 ’s method setPerspective() is used
to configure the quadrangular pyramid viewing volume.
Specifying the Visible Range Using a Quadrangular Pyramid
257
Matrix4.setPerspective(fov, aspect, near, far)
Calculate the matrix (the perspective projection matrix) that defines the viewing volume
specified by its arguments, and store it in Matrix4 . However, the near value must be less
than the far value .
Parameters fov Specifies field of view, angle formed by the top and bottom
planes. It must be greater than 0 .
aspect Specifies the aspect ratio of the near plane (width/height).
near, far Specify the distances to the near and far clipping planes along the
line of sight ( near > 0 and far > 0).
Return value None
The matrix that sets the quadrangular pyramid viewing volume is called the perspective
projection matrix .
Note that the specification for the near plane is different from that of the box type with
the second argument, aspect, representing the near plane aspect ratio. For example, if we
set the height to 100 and the width to 200, the aspect ratio is 0.5.
The positioning of the triangles with regard to the viewing volume we are using is illus-
trated in Figure 7.23 . It is specified by near =1.0, far =100, aspect =1.0 (the same aspect ratio
as the canvas), and fov =30.0.
eye point
(0, 0, 5)
(0, 0, 4)
near = 1
far=100
z
-0.75
0.75
x
y
1.0
-1.0
Figure 7.23 The positions of the triangles with respect to the quadrangular pyramid viewing
volume
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The basic processing flow is similar to that of LookAtTrianglesWithKeys_ViewVolume.js in
the previous section. So let’s take a look at the sample program.
Sample Program (PerspectiveView.js)
The sample program is detailed in Listing 7.8 .
Listing 7.8 PerspectiveView.js
1 // PerspectiveView.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'attribute vec4 a_Color;\n' +
6 'uniform mat4 u_ViewMatrix;\n' +
7 'uniform mat4 u_ProjMatrix;\n' +
8 'varying vec4 v_Color;\n' +
9 'void main() {\n' +
10 ' gl_Position = u_ProjMatrix * u_ViewMatrix * a_Position;\n' +
11 ' v_Color = a_Color;\n' +
12 '}\n';
...
24 function main() {
...
41 // Set the vertex coordinates and color (blue triangle is in front)
42 var n = initVertexBuffers(gl);
...
51 // Get the storage locations of u_ViewMatrix and u_ProjMatrix
52 varu_ViewMatrix = gl.getUniformLocation(gl.program,'u_ViewMatrix');
53 var u_ProjMatrix = gl.getUniformLocation(gl.program,'u_ProjMatrix');
...
59 var viewMatrix = new Matrix4(); // The view matrix
60 var projMatrix = new Matrix4(); // The projection matrix
61
62 // Calculate the view and projection matrix
63 viewMatrix.setLookAt(0, 0, 5, 0, 0, -100, 0, 1, 0);
64 projMatrix.setPerspective(30, canvas.width/canvas.height, 1, 100);
65 // Pass The view matrix and projection matrix to u_ViewMatrix and u_ProjMatrix
66 gl.uniformMatrix4fv(u_ViewMatrix, false, viewMatrix.elements);
67 gl.uniformMatrix4fv(u_ProjMatrix, false, projMatrix.elements);
...
72 // Draw the rectangles
73 gl.drawArrays(gl.TRIANGLES, 0, n);
74 }
75
Specifying the Visible Range Using a Quadrangular Pyramid
259
76 function initVertexBuffers(gl) {
77 var verticesColors = new Float32Array([
78 // Three triangles on the right side
79 0.75, 1.0, -4.0, 0.4, 1.0, 0.4, // The green triangle in back
80 0.25, -1.0, -4.0, 0.4, 1.0, 0.4,
81 1.25, -1.0, -4.0, 1.0, 0.4, 0.4,
82
83 0.75, 1.0, -2.0, 1.0, 1.0, 0.4, // The yellow triangle in middle
84 0.25, -1.0, -2.0, 1.0, 1.0, 0.4,
85 1.25, -1.0, -2.0, 1.0, 0.4, 0.4,
86
87 0.75, 1.0, 0.0, 0.4, 0.4, 1.0, // The blue triangle in front
88 0.25, -1.0, 0.0, 0.4, 0.4, 1.0,
89 1.25, -1.0, 0.0, 1.0, 0.4, 0.4,
90
91 // Three triangles on the left side
92 -0.75, 1.0, -4.0, 0.4, 1.0, 0.4, // The green triangle in back
93 -1.25, -1.0, -4.0, 0.4, 1.0, 0.4,
94 -0.25, -1.0, -4.0, 1.0, 0.4, 0.4,
95
96 -0.75, 1.0, -2.0, 1.0, 1.0, 0.4, // The yellow triangle in middle
97 -1.25, -1.0, -2.0, 1.0, 1.0, 0.4,
98 -0.25, -1.0, -2.0, 1.0, 0.4, 0.4,
99
100 -0.75, 1.0, 0.0, 0.4, 0.4, 1.0, // The blue triangle in front
101 -1.25, -1.0, 0.0, 0.4, 0.4, 1.0,
102 -0.25, -1.0, 0.0, 1.0, 0.4, 0.4,
103 ]);
104 var n = 18; // Three vertices per triangle * 6
...
138 return n;
139 }
The vertex and fragment shaders are completely identical (including the names of the
variables) to the ones used in LookAtTriangles_ViewVolume.js .
The processing flow of main() in JavaScript is also similar. Calling initVertexBuffers()
at line 42 writes the vertex coordinates and colors of the six triangles to be displayed into
the buffer object. In initVertexBuffers() , the vertex coordinates and colors for the six
triangles are specified: three triangles positioned on the right side from line 79 and three
triangles positioned on the left side from line 92. As a result, the number of vertices to be
drawn at line 104 is changed to 18 (3×6=18, to handle six triangles).
At lines 52 and 53 in main() , the locations of the uniform variables that store the view
matrix and perspective projection matrix are retrieved. Then at line 59 and 60, the vari-
ables used to hold the matrices are created.
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260
At line 63, the view matrix is calculated, with the eye point set at (0, 0, 5), the line of
sight set along the z-axis in the negative direction, and the up direction set along the
y-axis in the positive direction. Finally at line 64, the projection matrix is set up using a
quadrangular pyramid viewing volume:
64 projMatrix.setPerspective(30, canvas.width/canvas.height, 1, 100);
The second argument aspect (the horizontal to vertical ratio of the near plane) is derived
from the <canvas> width and height ( width and height property), so any modification of
the <canvas> aspect ratio doesn’t lead to distortion of the objects displayed.
Next, as the view and perspective projection matrices are available, you pass them to the
appropriate uniform variables at lines 66 and 67. Finally, you draw the triangles at line 73,
and upon execution you get a result including perspective similar to that shown in Figure
7.20 .
Finally, one aspect touched on earlier but not fully explained is why matrices are used to
set the viewing volume. Without using mathematics, let’s explore that a little.
The Role of the Projection Matrix
Let’s start by examining the perspective projection matrix. Looking at the screen shot of
PerspectiveView in Figure 7.24 , you can see that, after applying the projection matrix, the
objects in the distance are altered in two ways.
Figure 7.24 PerspectiveView
First, the farther away the triangles are, the smaller they appear. Second, the triangles are
parallel shifted so they look as if they are positioned inward toward the line of sight. In
comparison to the identically sized triangles that are laid out as shown on the left side
of Figure 7.25 , the following two transformations have been applied: (1) triangles farther
Specifying the Visible Range Using a Quadrangular Pyramid
261
from the viewer are scaled down (transformed) in proportion to the distance from the
viewer, and (2) the triangles are then transformed to be shifted toward the line of sight, as
illustrated on the right side of Figure 7.25 . These two transformations, shown on the right
side of Figure 7.25 , enable the effect you see in the photograph scene shown in Figure
7.20 .
z x
y
x
y
point at
infinity
z
Figure 7.25 Conceptual rendering of the perspective projection transformation
This means that the specification of the viewing volume can be represented as a combina-
tion of transformations, such as the scaling or translation of geometric shapes and objects,
in accordance with the shape of the viewing volume. The Matrix4 object’s method
setPerspective() automatically calculates this transformation matrix from the arguments
of the specified viewing volume. The elements of the matrix are discussed in Appendix C ,
“Projection Matrices.” If you are interested in the mathematical explanation of the coordi-
nate transform related to the viewing volume, please refer to the book Computer Graphics .
To put it another way, the transformation associated with the perspective projection trans-
forms the quadrangular pyramid viewing volume into a box-shaped viewing volume (right
part of Figure 7.25 ).
Note that the orthographic projection matrix does not perform all the work needed for
this transformation to generate the required optical effect. Rather, it performs the prelimi-
nary preparation that is required by the post vertex shader processing—where the actual
processing is done. If you are interested in this, please refer to Appendix D , “WebGL/
OpenGL: Left or Right Handed?”
The projection matrix, combined with the model matrix and the view matrix, is able
to handle all the necessary geometric transformations (translation, rotation, scaling) for
achieving the different optical effects. The following section will explore how to combine
these matrices to do that using a simple example.
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262
Using All the Matrices (Model Matrix, View Matrix, and Projection
Matrix)
One of the issues with PerspectiveView.js is the amount of code needed to set up the
vertex coordinates and the color data. Because we only have to deal with six triangles in
this case, it’s still manageable, but it could get messy if the number of triangles increased.
Fortunately, there is an effective drawing technique to handle this problem.
If you take a close look at the triangles, you will notice that the configuration is identical
to that in Figure 7.26 , where the dashed triangles are shifted along the x-axis in the posi-
tive (0.75) and negative (–0.75) directions, respectively.
z
-0.75
0.75
x
y
Figure 7.26 Drawing after translation
Taking advantage of this, it is possible to draw the triangles in PerspectiveView in the
following way:
1. Prepare the vertex coordinates data of the three triangles that are laid out centered
along the z-axis.
2. Translate the original triangles by 0.75 units along the x-axis, and draw them.
3. Translate the original triangles by –0.75 units along the x-axis, and draw them.
Now let’s try to use this approach in some sample code ( PerspectiveView_mvp) .
In the original PerspectiveView program the projection and view matrices were used to
specify the viewer’s viewpoint and viewing volume and PerspectiveView_mvp , the model
matrix, was used to perform the translation of the triangles.
At this point, it’s worthwhile to review the actions these matrices perform. To do that,
let’s refer to LookAtTriangles , which you wrote earlier to allow the viewer to look at a
rotated triangle from a specific location. At that time, you used this expression, which is
identical to Equation 7.1 :
Specifying the Visible Range Using a Quadrangular Pyramid
263
〈 〉× 〈 〉× 〈 〉 view matrix model matrix vertex coordinates     
Building on that, in LookAtTriangles_ViewVolume , which correctly displays the clipped
triangle, you used the following expression, which, when you use projection matrix
to include either orthographic projection or perspective projection, is identical to
Equation 7.3 :
〈 〉× 〈 〉× 〈 〉 projection matrix view matrix vertex coordinates     
You can infer the following from these two expressions:
Equation 7.4
〈 〉× 〈 〉× 〈 〉× 〈 〉 projection matrix view matrix model matrix vertex coordinates       
This expression shows that, in WebGL, you can calculate the final vertex coordinates
by using three types of matrices: the model matrix, the view matrix, and the projection
matrix.
This can be understood by considering that Equation 7.1 is identical to Equation 7.4 , in
which the projection matrix becomes the identity matrix, and Equation 7.3 is identical
to Equation 7.4 , whose model matrix is turned into the identity matrix. As explained in
Chapter 4 , the identity matrix behaves for matrix multiplication like the scalar 1 does with
scalar multiplication. Multiplying by the identity matrix has no effect on the other matrix.
So let’s construct the sample program using Equation 7.4 .
Sample Program (PerspectiveView_mvp.js)
PerspectiveView_mvp.js is shown in Listing 7.9 . The basic processing flow is similar to
that of PerspectiveView.js . The only difference is the modification of the calculation in
the vertex shader (line 11) to implement Equation 7.4 , and the passing of the additional
matrix ( u_ModelMatrix ) used for the calculation.
Listing 7.9 PerspectiveView_mvp.js
1 // PerspectiveView_mvp.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'attribute vec4 a_Color;\n' +
6 'uniform mat4 u_ModelMatrix;\n' +
7 'uniform mat4 u_ViewMatrix;\n' +
8 'uniform mat4 u_ProjMatrix;\n' +
9 'varying vec4 v_Color;\n' +
10 'void main() {\n' +
11 ' gl_Position = u_ProjMatrix * u_ViewMatrix * u_ModelMatrix * a_Position;\n' +
12 ' v_Color = a_Color;\n' +
CHAPTER 7 Toward the 3D World
264
13 '}\n';
...
25 function main() {
...
42 // Set the vertex coordinates and color (blue triangle is in front)
43 var n = initVertexBuffers(gl);
...
52 // Get the storage locations of u_ModelMatrix, u_ViewMatrix, and u_ProjMatrix.
53 var u_ModelMatrix = gl.getUniformLocation(gl.program, 'u_ModelMatrix');
54 var u_ViewMatrix = gl.getUniformLocation(gl.program,'u_ViewMatrix');
55 var u_ProjMatrix = gl.getUniformLocation(gl.program,'u_ProjMatrix');
...
61 var modelMatrix = new Matrix4(); // Model matrix
62 var viewMatrix = new Matrix4(); // View matrix
63 var projMatrix = new Matrix4(); // Projection matrix
64
65 // Calculate the model matrix, view matrix, and projection matrix
66 modelMatrix.setTranslate(0.75, 0, 0); // Translate 0.75 units
67 viewMatrix.setLookAt(0, 0, 5, 0, 0, -100, 0, 1, 0);
68 projMatrix.setPerspective(30, canvas.width/canvas.height, 1, 100);
69 // Pass the model, view, and projection matrix to uniform variables.
70 gl.uniformMatrix4fv(u_ModelMatrix, false, modelMatrix.elements);
71 gl.uniformMatrix4fv(u_ViewMatrix, false, viewMatrix.elements);
72 gl.uniformMatrix4fv(u_ProjMatrix, false, projMatrix.elements);
73
74 gl.clear(gl.COLOR_BUFFER_BIT);// clear <canvas>
75
76 gl.drawArrays(gl.TRIANGLES, 0, n); // Draw triangles on right
77
78 // Prepare the model matrix for another pair of triangles
79 modelMatrix.setTranslate(-0.75, 0, 0); // Translate -0.75
80 // Modify only the model matrix
81 gl.uniformMatrix4fv(u_ModelMatrix, false, modelMatrix.elements);
82
83 gl.drawArrays(gl.TRIANGLES, 0, n);// Draw triangles on left
84 }
85
86 function initVertexBuffers(gl) {
87 var verticesColors = new Float32Array([
88 // Vertex coordinates and color
89 0.0, 1.0, -4.0, 0.4, 1.0, 0.4, // The back green triangle
90 -0.5, -1.0, -4.0, 0.4, 1.0, 0.4,
91 0.5, -1.0, -4.0, 1.0, 0.4, 0.4,
92
93 0.0, 1.0, -2.0, 1.0, 1.0, 0.4, // The middle yellow triangle
Specifying the Visible Range Using a Quadrangular Pyramid
265
94 -0.5, -1.0, -2.0, 1.0, 1.0, 0.4,
95 0.5, -1.0, -2.0, 1.0, 0.4, 0.4,
96
97 0.0, 1.0, 0.0, 0.4, 0.4, 1.0, // The front blue triangle
98 -0.5, -1.0, 0.0, 0.4, 0.4, 1.0,
99 0.5, -1.0, 0.0, 1.0, 0.4, 0.4,
100 ]);
...
135 return n;
136 }
This time, you need to pass the model matrix to the vertex shader, so u_ModelMatrix is
added at line 6. The matrix is used at line 11, which implements Equation 7.5:
11 ' gl_Position = u_ProjMatrix * u_ViewMatrix * u_ModelMatrix * a_Position;\n' +
Next, main() in JavaScript calls initVertexBuffers() at line 43. In this function, the
vertex coordinates of the triangles to be passed to the buffer object are defined (line 87).
This time, you are handling the vertex coordinates of three triangles centered along the
z-axis instead of the six triangles used in PerspectiveView.js . As mentioned before, this is
because you will use the three triangles in conjunction with a translation.
At line 53, the storage location of u_ModelMatrix in the vertex shader is obtained. At
line 61, the arguments for the matrix ( modelMatrix ) passed to the uniform variable are
prepared, and at line 66, the matrix is calculated. First, this matrix will translate the trian-
gles by 0.75 units along the x-axis:
65 // Calculate the view matrix and the projection matrix
66 modelMatrix.setTranslate(0.75, 0, 0); // Translate 0.75
...
70 gl.uniformMatrix4fv(u_ModelMatrix, false, modelMatrix.elements);
...
76 gl.drawArrays(gl.TRIANGLES, 0, n); // Draw a triangle
The matrix calculations, apart from the model matrix at line 66, are the same as in
PerspectiveView.js . The model matrix is passed to u_ModelMatrix at line 70 and used to
draw the right side row of triangles (line 76).
In a similar manner, the row of triangles for the left side is translated by –0.75 units along
the x-axis, and then the model matrix is calculated again at line 79. Because the view
matrix and projection matrix make use of this model matrix, you only need to assign
the model matrix to the uniform variable once (line 81). Once the matrix is set up, you
perform the draw operation at line 83 with gl.drawArrays() :
78 // Prepare the model matrix for another pair of triangles
79 modelMatrix.setTranslate(-0.75, 0, 0); // Translate -0.75
80 // Modify only the model matrix
CHAPTER 7 Toward the 3D World
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81 gl.uniformMatrix4fv(u_ModelMatrix, false, modelMatrix.elements);
82
83 gl.drawArrays(gl.TRIANGLES, 0, n); // Draw triangles on left
As you have seen, this approach allows you to draw two sets of triangles from a single set
of triangle data, which reduces the number of vertices needed but increases the number
of calls to gl.drawArrays() . The choice of which approach to use for better performance
depends on the application and the WebGL implementation.
Experimenting with the Sample Program
In PerspectiveView_mvp , you calculated 〈
〉× 〈 〉× 〈 〉 projection matrix view matrix model matrix     
directly inside the vertex shader. This calculation of is the same for all the vertices, so
there is no need to recalculate it inside the shader for each vertex. It can be computed in
advance inside the JavaScript code, as it was in LookAtRotatedTriangles_mvMatrix earlier
in the chapter, allowing a single matrix to be passed to the vertex shader. This matrix is
called the model view projection matrix , and the name of the variable that passes it is
u_MvpMatrix . The sample program used to show this is ProjectiveView_mvpMatrix , in
which the vertex shader is modified as shown next and, as you can see, is significantly
simpler:
1 // PerspectiveView_mvpMatrix.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'attribute vec4 a_Color;\n' +
6 'uniform mat4 u_MvpMatrix;\n' +
7 'varying vec4 v_Color;\n' +
8 'void main() {\n' +
9 ' gl_Position = u_MvpMatrix * a_Position;\n' +
10 ' v_Color = a_Color;\n' +
11 '}\n';
In JavaScript, main() , the storage location of u_ModelMatrix is retrieved at line 51, and
then the matrix to be stored in the uniform variable is calculated at line 57:
50 // Get the storage location of u_MvpMatrix
51 var u_MvpMatrix = gl.getUniformLocation(gl.program, 'u_MvpMatrix');
...
57 var modelMatrix = new Matrix4(); // The model matrix
58 var viewMatrix = new Matrix4(); // The view matrix
Correctly Handling Foreground and Background Objects
267
59 var projMatrix = new Matrix4(); // The projection matrix
60 var mvpMatrix = new Matrix4(); // The model view projection matrix
61
62 // Calculate the model, view, and projection matrices
63 modelMatrix.setTranslate(0.75, 0, 0);
64 viewMatrix.setLookAt(0, 0, 5, 0, 0, -100, 0, 1, 0);
65 projMatrix.setPerspective(30, canvas.width/canvas.height, 1, 100);
66 // Calculate the model view projection matrix
67 mvpMatrix.set(projMatrix).multiply(viewMatrix).multiply(modelMatrxi);
68 // Pass the model view projection matrix to u_MvpMatrix
69 gl.uniformMatrix4fv(u_MvpMatrix, false, mvpMatrix.elements);
...
73 gl.drawArrays(gl.TRIANGLES, 0, n); // Draw a rectangle
74
75 // Prepare the model matrix for another pair of triangles
76 modelMatrix.setTranslate(-0.75, 0, 0);
77 // Calculate the model view projection matrix
78 mvpMatrix.set(projMatrix).multiply(viewMatrix).multiply(modelMatrxi);
79 // Pass the model view projection matrix to u_MvpMatrix
80 gl.uniformMatrix4fv(u_MvpMatrix, false, mvpMatrix.elements);
81
82 gl.drawArrays(gl.TRIANGLES, 0, n); // Draw a rectangle
83 }
The critical calculation is carried out at line 67. The projection matrix ( projMatrix ) is
assigned to mvpMatrix . Then the view matrix ( viewMatrix ) is multiplied by the model
matrix ( modelMatrix ), and the result is written back to mvpMatrix , using the set version
of the method. This is in turn assigned to u_MvpMatrix at line 69, and the triangles on
the right side are drawn at line 73. Similarly, the calculation of the model view projec-
tion matrix for the triangles on the left side is performed at line 78. It is then passed to
u_MvpMatrix at line 80, and the triangles are drawn at line 82.
With this information, you are now able to write code that moves the eye point, sets the
viewing volume, and allows you to view three-dimensional objects from various angles.
Additionally, you have learned how to deal with clipping that resulted in partially missing
objects. However, one potential problem remains. As you move the eye point to a differ-
ent location, it’s possible for objects in the foreground to be hidden by objects in the
background. Let’s look at how this problem comes about.
Correctly Handling Foreground and Background
Objects
In the real world, if you place two boxes on a desk as shown in Figure 7.27 , the fore-
ground box partially hides the background one.
CHAPTER 7 Toward the 3D World
268
Figure 7.27 The front object partially hides the back object
Looking at the sample programs constructed so far, such as the screen shot of
PerspectiveView (refer to Figure 7.21 ), the green triangle located at the back is partially
hidden by the yellow and blue triangles. It looks as if WebGL, being designed for display-
ing 3D objects, has naturally figured out the correct order.
However, that is unfortunately not the case. By default, WebGL, to accelerate the drawing
process, draws objects in the order of the vertices specified inside the buffer object. Up
until now, you have always arranged the order of the vertices so that the objects located
in the background are drawn first, thus resulting in a natural rendering.
For example, in PerspectiveView_mvpMatrix.js , you specified the coordinates and color
of the triangles in the following order. Note the z coordinates (bold font):
var verticesColors = new Float32Array([
// vertex coordinates and color
0.0, 1.0, -4.0 , 0.4, 1.0, 0.4, // The green one at the back
-0.5, -1.0, -4.0 , 0.4, 1.0, 0.4,
0.5, -1.0, -4.0 , 1.0, 0.4, 0.4,
0.0, 1.0, -2.0 , 1.0, 1.0, 0.4, // The yellow one in the middle
-0.5, -1.0, -2.0 , 1.0, 1.0, 0.4,
0.5, -1.0, -2.0 , 1.0, 0.4, 0.4,
0.0, 1.0, 0.0 , 0.4, 0.4, 1.0, // The blue one in the front
-0.5, -1.0, 0.0 , 0.4, 0.4, 1.0,
0.5, -1.0, 0.0 , 1.0, 0.4, 0.4,
]);
WebGL draws the triangles in the order z in which you specified the vertices (that is,
the green triangle [back], then the yellow triangle [middle], and finally the blue triangle
Correctly Handling Foreground and Background Objects
269
[front]). This ensures that objects closer to the eye point cover those farther away, as seen
in Figure 7.13 .
To verify this, let’s modify the order in which the triangles are specified by first drawing
the blue triangle in the front, then the yellow triangle in the middle, and finally the green
triangle at the back:
var verticesColors = new Float32Array([
// vertex coordinates and color
0.0, 1.0, 0.0, 0.4, 0.4, 1.0, // The blue one in the front
-0.5, -1.0, 0.0, 0.4, 0.4, 1.0,
0.5, -1.0, 0.0, 1.0, 0.4, 0.4
0.0, 1.0, -2.0, 1.0, 1.0, 0.4, // The yellow one in the middle
-0.5, -1.0, -2.0, 1.0, 1.0, 0.4,
0.5, -1.0, -2.0, 1.0, 0.4, 0.4,
0.0, 1.0, -4.0, 0.4, 1.0, 0.4, // The green one at the back
-0.5, -1.0, -4.0, 0.4, 1.0, 0.4,
0.5, -1.0, -4.0, 1.0, 0.4, 0.4,
]);
When you run this, you’ll see the green triangle, which is supposed to be located at the
back, has been drawn at the front (see Figure 7.28 ).
Figure 7.28 The green triangle in the back is displayed at the front
Drawing objects in the specified order, the default behavior in WebGL, can be quite
efficient when the sequence can be determined beforehand and the scene doesn’t
CHAPTER 7 Toward the 3D World
270
subsequently change. However, when you examine the object from various directions by
moving the eye point, it is impossible to decide the drawing order in advance.
Hidden Surface Removal
To cope with this problem, WebGL provides a hidden surface removal function. This
function eliminates surfaces hidden behind foreground objects, allowing you to draw the
scene so that the objects in the back are properly hidden by those in front, regardless of
the specified vertex order. This function is already embedded in WebGL and simply needs
to be enabled.
Enabling hidden surface removal and preparing WebGL to use it requires the following
two steps:
1. Enabling the hidden surface removal function
gl.enable(gl.DEPTH_TEST);
2. Clearing the depth buffer used for the hidden surface removal before drawing
gl.clear(gl.DEPTH_BUFFER_BIT);
The function gl.enable() , used in step 1, actually enables various functions in WebGL.
gl.enable(cap)
Enable the function specified by cap (capability).
Parameters cap Specifies the function to be
enabled.
gl.DEPTH_TEST
2
Hidden surface removal
gl.BLEND Blending (see Chapter 9 ,
“Hierarchical Objects”)
gl.POLYGON_OFFSET_FILL Polygon offset (see the next
section), and so on
3
Return value None
Errors : INVALID_ENUM None of the acceptable values is specified in cap
3 Although not covered in this book, you can also specify gl.CULL_FACE , gl.DITHER , gl.SAMPLE_
ALPHA_TO_COVERAGE , gl.SAMPLE_COVERAGE , gl.SCISSOR_TEST , and gl.STENCIL_TEST . See the
book OpenGL Programming Guide for more information on these.
2 A “DEPTH_TEST” in the hidden surface removal function might sound strange, but actually its name
comes from the fact that it decides which objects to draw in the foreground by verifying (TEST) the
depth (DEPTH) of each object.
Correctly Handling Foreground and Background Objects
271
The depth buffer cleared in the
gl.clear() statement (step 2) is a buffer used internally
to remove hidden surfaces. While WebGL draws objects and geometric shapes in the color
buffer displayed on the <canvas> , hidden surface removal requires the depth (from the
eye point) for each geometrical shape and object. The depth buffer holds this information
(see Figure 7.29 ). The depth direction is the same as the z-axis direction, so it is sometimes
called the z-buffer.
m e t s y S L G b e W
r e d a h S x e t r e V
t p i r c S a v a J
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
; ) … ( s r e d a h S t i n i
…
}
x
y
r e f u B h t p e D
h t p e D
t s e T
l a v o m e R e c a f r u S n e d d i H r e f u B r o l o C
t n e m g a r F
r e d a h S
Figure 7.29 Depth buffer used in hidden surface removal
Because the depth buffer is used whenever a drawing command is issued, it must be
cleared before any drawing operation; otherwise, you will see incorrect results. You specify
the depth buffer using gl.DEPTH_BUFFER_BIT and proceed as follows to clear it:
gl.clear(gl.DEPTH_BUFFER_BIT);
Up until now, you only cleared the color buffer. Because you now need to also clear the
depth buffer, you can clear both buffers simultaneously by taking the bitwise or (|) of
gl.COLOR_BUFFER_BIT (which represents the color buffer) and gl.DEPTH_BUFFER_BIT
(which represents the depth buffer) and specifying it as an argument to gl.clear() :
gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
You can use the bitwise or operation this way whenever you need to clear both buffers at
the same time.
To disable the function you enabled with gl.enable() , you use gl.disable() .
gl.disable(cap)
Disable the function specified by cap (capability).
Parameters cap Same as gl.enable() .
Return value None
Errors INVALID_ENUM None of the acceptable values is specified in cap
CHAPTER 7 Toward the 3D World
272
Sample Program (DepthBuffer.js)
Let’s add the hidden surface removal methods from (1) and (2) to PerspectiveView_
mvpMatrix.js and change the name to DepthBuffer.js . Note that the order of the vertex
coordinates specified inside the buffer object is not changed, so you will draw from
front to back the blue, yellow, and green triangles. The result is identical to that of the
PerspectiveView_mvpMatrix . We detail the program in Listing 7.10 .
Listing 7.10 DepthBuffer.js
1 // DepthBuffer.js
...
23 function main() {
...
41 var n = initVertexBuffers(gl);
...
47 // Specify the color for clearing <canvas>
48 gl.clearColor(0, 0, 0, 1);
49 // Enable the hidden surface removal function
50 gl.enable(gl.DEPTH_TEST);
73 // Clear the color and depth buffer
74 gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
75
76 gl.drawArrays(gl.TRIANGLES, 0, n); // Draw triangles
...
85 gl.drawArrays(gl.TRIANGLES, 0, n); // Draw triangles
86 }
87
88 function initVertexBuffers(gl) {
89 var verticesColors = new Float32Array([
90 // Vertex coordinates and color
91 0.0, 1.0, 0.0, 0.4, 0.4, 1.0, // The blue triangle in front
92 -0.5, -1.0, 0.0, 0.4, 0.4, 1.0,
93 0.5, -1.0, 0.0, 1.0, 0.4, 0.4,
94
95 0.0, 1.0, -2.0, 1.0, 1.0, 0.4, // The yellow triangle in middle
96 -0.5, -1.0, -2.0, 1.0, 1.0, 0.4,
97 0.5, -1.0, -2.0, 1.0, 0.4, 0.4,
98
99 0.0, 1.0, -4.0, 0.4, 1.0, 0.4, // The green triangle in back
100 -0.5, -1.0, -4.0, 0.4, 1.0, 0.4,
101 0.5, -1.0, -4.0, 1.0, 0.4, 0.4,
102 ]);
103 var n = 9;
...
Correctly Handling Foreground and Background Objects
273
137 return n;
138 }
If you run DepthBuffer , you can see that hidden face removal is performed and that
objects placed at the back are hidden by objects located at the front. This demonstrates
that the hidden surface removal function can eliminate the hidden surfaces regardless of
the position of the eye point. Equally, this also shows that in anything but a trivial 3D
scene, you will always need to enable hidden surface removal and systematically clear the
depth buffer before any drawing operation.
You should note that hidden surface removal requires you to correctly set up the viewing
volume. If you fail to do this (use WebGL in its default configuration), you are likely
to see incorrect results. You can specify either a box or a quadrangular pyramid for the
viewing volume.
Z Fighting
Hidden surface removal is a sophisticated and powerful feature of WebGL that correctly
handles most of the cases where surfaces need to be removed. However, it fails when two
geometrical shapes or objects are located at extremely close positions and results in the
display looking a little unnatural. This phenomenon is known as Z fighting and is illus-
trated in Figure 7.30 . Here, we draw two triangles sharing the same z coordinate.
Figure 7.30 Visual artifact generated by Z fighting (the left side)
The Z fighting occurs because of the limited precision of the depth buffer and means the
system is unable to asses which object is in front and which is behind. Technically, when
handling 3D models, you could avoid this by paying thorough attention to the z coordi-
nates’ values at the model creation stage; however, implementing this workaround would
prove to be unrealistic when dealing with the animation of several objects.
CHAPTER 7 Toward the 3D World
274
To help resolve this problem, WebGL provides a feature known as the polygon offset .
This works by automatically adding an offset to the z coordinate, whose value is a func-
tion of each object’s inclination with respect to the viewer’s line of sight. You only need
to add two lines of code to enable this function.
1. Enabling the polygon offset function:
gl.enable(gl.POLYGON_OFFSET_FILL);
2. Specifying the parameter used to calculate the offset (before drawing):
gl.polygonOffset(1.0, 1.0);
The same method that enabled the hidden surface removal function is used, but with a
different parameter. The details for gl.polygonOffset() are shown here.
gl.polygonOffset(factor, units)
Specify the offset to be added to the z coordinate of each vertex drawn afterward.
The offset is calculated with the formula m * factor + r * units , where m represents the
inclination of the triangle with respect to the line of sight, and where r is the smallest
difference between two z coordinates values the hardware can distinguish.
Return value None
Errors None
Let’s look at the program Zfighting , which uses the polygon offset to reduce z fighting
(see Listing 7.11 ).
Listing 7.11 Zfighting.js
1 // Zfighting.js
...
23 function main() {
...
69 // Enable the polygon offset function
70 gl.enable(gl.POLYGON_OFFSET_FILL);
71 // Draw a rectangle
72 gl.drawArrays(gl.TRIANGLES, 0, n/2); // The green triangle
73 gl.polygonOffset(1.0, 1.0); // Set the polygon offset
74 gl.drawArrays(gl.TRIANGLES, n/2, n/2); // The yellow triangle
75 }
76
77 function initVertexBuffers(gl) {
Hello Cube
275
78 var verticesColors = new Float32Array([
79 // Vertex coordinates and color
80 0.0, 2.5, -5.0 , 0.0, 1.0, 0.0, // The green triangle
81 -2.5, -2.5, -5.0 , 0.0, 1.0, 0.0,
82 2.5, -2.5, -5.0 , 1.0, 0.0, 0.0,
83
84 0.0, 3.0, -5.0 , 1.0, 0.0, 0.0, // The yellow triangle
85 -3.0, -3.0, -5.0 , 1.0, 1.0, 0.0,
86 3.0, -3.0, -5.0 , 1.0, 1.0, 0.0,
87 ]);
88 var n = 6;
If you look at the program from line 80, you can see that the z coordinate for each vertex
is set to –5.0, so z fighting should occur.
Within the rest of the code, the polygon offset function is enabled at line 70. After
that, the green and yellow triangles are drawn at lines 72 and 74. For ease of reading,
the program uses only one buffer object, so gl.drawArrays() requires the second and
third arguments to be correctly set. The second argument represents the number of the
vertex to start from, while the third argument gives the number of vertices to be drawn.
Once the green triangle has been drawn, the polygon offset parameter is set using gl.
polygonOffset() . Subsequently, all the vertices drawn will have their z coordinate offset.
If you load this program, you will see the two triangles drawn correctly with no z fight-
ing effects, as in Figure 7.28 (right side). If you now comment out line 73 and reload the
program, you will notice that z fighting occurs and looks similar to the left side of Figure
7.28 .
Hello Cube
So far, the explanation of various WebGL features has been illustrated using simple trian-
gles. You now have enough understanding of the basics to draw 3D objects. Let’s start by
drawing the cube shown in Figure 7.31 . (The coordinates for each vertex are shown on the
right side.) The program used is called HelloCube , in which the eight vertices that define
the cube are specified using the following colors: white, magenta (bright reddish-violet),
red, yellow, green, cyan (bright blue), blue, and black. As was explained in Chapter 5 ,
“Using Colors and Texture Images,” because colors between the vertices are interpolated,
the resulting cube is shaded with an attractive color gradient (actually a “color solid,” an
analog of the two-dimensional “color wheel”).
CHAPTER 7 Toward the 3D World
276
v1(-1,1,1)
v3(1,-1,1)
v4(1,-1,-1)
v0(1,1,1)
v5(1,1,-1)
v2(-1,-1,1)
v7(-1,-1,-1)
v6(-1,1,-1)
white
red
yellow
green
cyan
blue
black
magenta
x
y
z
Figure 7.31 HelloCube and its vertex coordinates
Let’s consider the case where you would like to draw the cube like this with the command
you’ve been relying upon until now: gl.drawArrays() . In this case, you need to draw
using one of the following modes: gl.TRIANGLES , gl.TRIANGLE_STRIP , or gl.TRIANGLE_FAN .
The most simple and straightforward method would consist of drawing each face with
two triangles. In other words, you can draw a face defined by four vertices (v0, v1, v2, v3),
using two triangles defined by the two sets of three vertices (v0, v1, v2) and (v0, v2, v3),
respectively, and repeat the same process for all the other faces. In this case, the vertices
coordinates specified inside the buffer object would be these:
var vertices = new Float32Array([
1.0, 1.0, 1.0, -1.0, 1.0, 1.0, -1.0, -1.0, 1.0, // v0, v1, v2
1.0, 1.0, 1.0, -1.0, -1.0, 1.0, 1.0, -1.0, 1.0, // v0, v2, v3
1.0, 1.0, 1.0, 1.0, -1.0, 1.0, 1.0, -1.0, -1.0, // v0, v3, v4
...
]);
Because one face is made up of two triangles, you need to know the coordinates of six
vertices to define it. There are six faces, so a total of 6×6 = 36 vertices are necessary. After
having specified the coordinates of each of the 36 vertices, write them in the buffer
object and then call gl.drawArrays(gl.TRIANGLES, 0, 36) , which draws the cube. This
approach requires that you specify and handle 36 vertices, although the cube actually only
requires 8 unique vertices because several triangles share common vertices.
You could, however, take a more frugal approach by drawing a single face with gl.
TRIANGLE_FAN . Because gl.TRIANGLE_FAN allows you to draw a face defined by the 4-vertex
set (v0, v1, v2, v3), you end up only having to deal with a total of 4×6=24 vertices
4 .
However, you now need to call gl.drawArrays() separately for each face (six faces). So,
each of these two approaches has both advantages and drawbacks, but neither seems ideal.
4 You can cut down on the number of vertices using this kind of representation. It decreases the
number of necessary vertices to 14, which can be drawn with gl.TRIANGLE_STRIP .
Hello Cube
277
As you would expect, WebGL has a solution: gl.drawElements() . It’s an alternative way to
directly draw a three-dimensional object in WebGL, with a minimum of vertices. To use
this method, you will need the vertex coordinates of the entire object, which you will use
to explicitly describe how you want WebGL to draw the shape (the cube).
If we break our cube (see the right side of Figure 7.31 ) into vertices that constitute trian-
gles, we get the structure shown in Figure 7.32 . Looking at the left side of the figure,
you can see that Cube points to a Faces list, which, as the name implies, shows that the
cube is split into six faces: front, right, left, top, bottom, and back. In turn, each face is
composed of two triangles picked up from the Triangles list. The numbers in the Triangles
list represent the indices assigned to the Coordinate list. The vertex coordinates’ indices
are numbered in order starting from zero.
t h g i r
t n o r f
4
7
6
4
6
5
cube
1, 1, 1
-1, 1, 1
-1, -1, 1
1, -1, 1
1, -1, -1
1, 1, -1
-1, 1, -1
-1, -1, -1
0
1
2
0
2
3
0
3
4
0
4
5
0
1
2
3
4
5
6
7
1, 1, 1
1, 0, 1
1, 0, 0
1, 1, 0
0, 1, 0
0, 1, 1
0, 0, 1
0, 0, 0
v1(-1,1,1)
v3(1,-1,1)
v4(1,-1,-1)
v0(1,1,1)
v5(1,1,-1)
v2(-1,-1,1)
v7(-1,-1,-1)
front
back
down
left
up
right
p u
v6(-1,1,-1)
Figure 7.32 The associations of the faces that make up the cube, triangles, vertex
coordinates, and colors
This approach results in a data structure that describes the way the object (a cube) can be
built from its vertex and color data.
Drawing the Object with Indices and Vertices Coordinates
So far, you have been using gl.drawArrays() to draw vertices. However, WebGL supports
an alternative approach, gl.drawElements() , that looks similar to that of gl.draw
Arrays() . However, it has some advantages that we’ll explain later. First, let’s look at
how to use gl.drawElements() . You need to specify the indices based, not on gl.ARRAY_
BUFFER , but on gl.ELEMENT_ARRAY_BUFFER (introduced in the explanation of the buffer
object in Chapter 4 ). The key difference is that gl.ELEMENT_ARRAY_BUFFER handles data
structured by the indices.
CHAPTER 7 Toward the 3D World
278
gl.drawElements(mode, count, type, offset)
Executes the shader and draws the geometric shape in the specified mode using the
indices specified in the buffer object bound to gl.ELEMENT_ARRAY_BUFFER .
Parameters mode Specifies the type of shape to be drawn (refer to Figure
3.17 ).
The following symbolic constants are accepted:
gl.POINTS, gl.LINE_STRIP, gl.LINE_LOOP, gl.LINES,
gl.TRIANGLE_STRIP, gl.TRIANGLE_FAN , or gl.TRIANGLES
count Number of indices to be drawn (integer).
type Specifies the index data type: gl.UNSIGNED_BYTE or gl.
UNSIGNED_SHORT
5
offset Specifies the offset in bytes in the index array where you
want to start rendering.
Return value None
Errors INVALID_ENUM mode is none of the preceding values.
INVALID_VALUE A negative value is specified for count or offset
Writing indices to the buffer object bound to gl.ELEMENT_ARRAY_BUFFER is done in the
same way you write the vertex information to the buffer object with gl.drawArrays() .
That is to say, you use gl.bindBuffer() and gl.bufferData() , but the only difference is
that the first argument, target , is set to gl.ELEMENT_ARRAY_BUFFER . Let’s take a look at the
sample program.
Sample Program (HelloCube.js)
The sample program is shown in Listing 7.12 . The vertex and fragment shaders set a quad-
rangular pyramid viewing volume and perform a perspective projection transformation
like ProjectiveView_mvpMatrix.js . It’s important to understand that gl.drawElements()
doesn’t do anything special. The vertex shader simply transforms the vertex coordinates,
and the fragment shader sets the color passed by the varying variable to gl_FragColor .
The key difference from the previous programs comes down to the processing of the
buffer object in initVertexBuffers() .
5 Even if type doesn’t correspond to the type ( Uint8Array or Uint16Array ) of the data array specifi ed
in gl.ELEMENT_ARRAY_BUFFER , no error is returned. However, if, for example, you specify the index
with a Uint16Array type, and set type to gl.UNSIGNED_BYTE , in some cases, the object might not
be completely displayed.
Hello Cube
279
Listing 7.12 HelloCube.js
1 // HelloCube.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
...
8 'void main() {\n' +
9 ' gl_Position = u_MvpMatrix * a_Position;\n' +
10 ' v_Color = a_Color;\n' +
11 '}\n';
12
13 // Fragment shader program
14 var FSHADER_SOURCE =
...
19 'void main() {\n' +
20 ' gl_FragColor = v_Color;\n' +
21 '}\n';
22
23 function main() {
...
40 // Set the vertex coordinates and color
41 var n = initVertexBuffers(gl);
...
47 // Set the clear color and enable the hidden surface removal
48 gl.clearColor(0.0, 0.0, 0.0, 1.0);
49 gl.enable(gl.DEPTH_TEST);
...
58 // Set the eye point and the viewing volume
59 var mvpMatrix = new Matrix4();
60 mvpMatrix.setPerspective(30, 1, 1, 100);
61 mvpMatrix.lookAt(3, 3, 7, 0, 0, 0, 0, 1, 0);
62
63 // Pass the model view projection matrix to u_MvpMatrix
64 gl.uniformMatrix4fv(u_MvpMatrix, false, mvpMatrix.elements);
65
66 // Clear the color and depth buffer
67 gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
68
69 // Draw the cube
70 gl.drawElements(gl.TRIANGLES, n, gl.UNSIGNED_BYTE, 0);
71 }
72
73 function initVertexBuffers(gl) {
...
82 var verticesColors = new Float32Array([
CHAPTER 7 Toward the 3D World
280
83 // Vertex coordinates and color
84 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, // v0 White
85 -1.0, 1.0, 1.0, 1.0, 0.0, 1.0, // v1 Magenta
86 -1.0, -1.0, 1.0, 1.0, 0.0, 0.0, // v2 Red
...
91 -1.0, -1.0, -1.0, 0.0, 0.0, 0.0 // v7 Black
92 ]);
93
94 // Indices of the vertices
95 var indices = new Uint8Array([
96 0, 1, 2, 0, 2, 3, // front
97 0, 3, 4, 0, 4, 5, // right
98 0, 5, 6, 0, 6, 1, // up
99 1, 6, 7, 1, 7, 2, // left
100 7, 4, 3, 7, 3, 2, // down
101 4, 7, 6, 4, 6, 5 // back
102 ]);
103
104 // Create a buffer object
105 var vertexColorBuffer = gl.createBuffer();
106 var indexBuffer = gl.createBuffer();
...
111 // Write the vertex coordinates and color to the buffer object
112 gl.bindBuffer(gl.ARRAY_BUFFER, vertexColorBuffer);
113 gl.bufferData(gl.ARRAY_BUFFER, verticesColors, gl.STATIC_DRAW);
114
115 var FSIZE = verticesColors.BYTES_PER_ELEMENT;
116 // Assign the buffer object to a_Position and enable it
117 var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
...
122 gl.vertexAttribPointer(a_Position, 3, gl.FLOAT, false, FSIZE * 6, 0);
123 gl.enableVertexAttribArray(a_Position);
124 // Assign the buffer object to a_Position and enable it
125 var a_Color = gl.getAttribLocation(gl.program, 'a_Color');
...
130 gl.vertexAttribPointer(a_Color, 3, gl.FLOAT, false, FSIZE * 6, FSIZE * 3);
131 gl.enableVertexAttribArray(a_Color);
132
133 // Write the indices to the buffer object
134 gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, indexBuffer);
135 gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, indices, gl.STATIC_DRAW);
136
137 return indices.length;
138 }
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281
The processing flow in the JavaScript main() is the same as in ProjectiveView_mvpMatrix.
js , but let’s quickly review it. After having written the vertex data in the buffer object
through a call to initVertexBuffers() at line 41, you enable the hidden surface removal
function at line 49. This is necessary to allow WebGL to correctly draw the cube, taking
into consideration the relationship between the front and the back faces.
You set the eye point and the viewing volume from line 59 to line 61 and pass the model
view projection matrix to the vertex shader’s uniform variable u_MvpMatrix .
At line 67, you clear the color and depth buffers and then draw the cube using
gl.drawElements() at line 70. The use of gl.drawElements() in this program is the
main difference to ProjectiveView_mvpMatrix.js , so let’s take a look at that.
Writing Vertex Coordinates, Colors, and Indices to the Buffer Object
The method to assign the vertex coordinates and the color information to the attribute
variable using the buffer object in initVertexBuffers() is unchanged. This time, because
you won’t necessarily use the vertex information in the order specified in the object
buffer, you need to additionally specify in which order you will use it. For that you will
use the vertex order specified in verticesColors as indices. In short, the vertex informa-
tion specified first in the buffer object will be set to index 0, the vertex information speci-
fied in second place in the buffer object will be set to index 1, and so on. Here, we show
the part of the program that specifies the indices in initVertexBuffers() :
73 function initVertexBuffers(gl) {
...
82 var verticesColors = new Float32Array([
83 // Vertex coordinates and color
84 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, // v0 White
85 -1.0, 1.0, 1.0, 1.0, 0.0, 1.0, // v1 Magenta
...
91 -1.0, -1.0, -1.0, 0.0, 0.0, 0.0 // v7 Black
92 ]);
93
94 // Indices of the vertex coordinates
95 var indices = new Uint8Array([
96 0, 1, 2, 0, 2, 3, // front
97 0, 3, 4, 0, 4, 5, // right
98 0, 5, 6, 0, 6, 1, // up
99 1, 6, 7, 1, 7, 2, // left
100 7, 4, 3, 7, 3, 2, // down
101 4, 7, 6, 4, 6, 5 // back
102 ]);
103
104 // Create a buffer object
105 var vertexColorBuffer = gl.createBuffer();
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282
106 var indexBuffer = gl.createBuffer();
...
136 // Write the indices to the buffer object
137 gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, indexBuffer);
138 gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, indices, gl.STATIC_DRAW);
139
140 return indices.length;
141 }
As you may have noticed, at line 106, you create the buffer object ( indexBuffer ) in which
to write the indices. These indices are stored in the array indices at line 95. Because the
indices are integers (0, 1, 2, ...), you use an integer typed array Uint8Array (unsigned
8-bit encoded integer). If there are more than 256 indices, use Uint16Array instead. The
content of this array is the triangles list of Figure 7.33 , where each grouping of three
indices points to the three vertex coordinates for that triangle. Generally, this index
doesn’t need to be manually created because 3D modeling tools, introduced in the next
chapter, usually generate it along with the vertices information.
cube
1, 1, 1
-1, 1, 1
-1, -1, 1
1, -1, 1
1, -1, -1
1, 1, -1
-1, 1, -1
-1, -1, -1
0
1
2
0
2
3
0
3
4
0
4
5
0
1
2
3
4
5
6
7
1, 1, 1
1, 0, 1
1, 0, 0
1, 1, 0
0, 1, 0
0, 1, 1
0, 0, 1
0, 0, 0
gl.ELEMENT_ARRAY_BUFFER gl.ARRAY_BUFFER
v6(-1,1,-1)
v1(-1,1,1)
v3(1,-1,1)
v4(1,-1,-1)
v0(1,1,1)
v5(1,1,-1)
v2(-1,-1,1)
v7(-1,-1,-1)
front
back
down
left
up
right
Figure 7.33 Contents of gl.ELEMENT_ARRAY_BUFFER and gl.ARRAY_BUFFER
The setup for the specified indices is performed at lines 134 and 135. This is similar to the
way buffer objects have been written previously, with the difference that the first argu-
ment is modified to gl.ELEMENT_ARRAY_BUFFER . This is to let the WebGL system know that
the contents of the buffer are indices.
Once executed, the internal state of the WebGL system is as detailed in Figure 7.34 .
Hello Cube
283
t n e m g a r F
r e d a h S
t p i r c S a v a J
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
; ) … ( r e f u B d n i b . l g
R E F F U B _ Y A R R A _ T N E M E L E . l g
t c e j b O r e f u B
, 3 , 2 , 0 , 2 , 1 , 0
, 5 , 4 , 0 , 4 , 3 , 0
, 1 , 6 , 0 , 6 , 5 , 0
, 2 , 7 , 1 , 7 , 6 , 1
, 2 , 3 , 7 , 3 , 4 , 7
5 , 6 , 4 , 6 , 7 , 4
t c e j b O r e f u B
, 1 , 1 , 1 , 1 , 1 , 1
, 1 , 0 , 1 , 1 , 1 , 1 -
, 0 , 0 , 1 , 1 , 1 - , 1 -
, 0 , 1 , 1 , 1 , 1 - , 1
, 0 , 1 , 0 , 1 - , 1 - , 1
, 1 , 1 , 0 , 1 - , 1 , 1
, , , , - , , 1 0 0 1 1 1 -
0 , 0 , 0 , 1 - , 1 - , 1 -
R E F F U B _ Y A R R A . l g
e l b a i r a v e t u b i r t a
Figure 7.34 gl.ELEMENT_ARRAY_BUFFER and gl.ARRAY_BUFFER
Once set up, the call to gl.drawElements() at line 70 draws the cube:
69 // Draw the cube
70 gl.drawElements(gl.TRIANGLES, n, gl.UNSIGNED_BYTE, 0);
You should note that the second argument of gl.drawElements() , the number of indices,
represents the number of vertex coordinates involved in the drawing, but it is not identi-
cal to the number of vertices coordinates written to gl.ARRAY_BUFFER .
When you call gl.drawElements() , the indices are extracted from the buffer object
( indexBuffer ) bound to gl.ELEMENT_ARRAY_BUFFER , while the associated vertex informa-
tion is retrieved from the buffer object ( vertexColorBuffer ) bound to gl.ARRAY_BUFFER .
All these pieces of information are then passed to the attribute variable. The process is
repeated for each index, and then the whole cube gets drawn by a single call to
gl.drawElements() . With this approach, because you refer to the vertex information
through indices, you can recycle the vertex information. Although gl.drawElements()
allows you to curb memory usage by sharing the vertex information, the cost is a process
to convert the indices to vertex information (that is, a level of indirection). This means
that the choice between gl.drawElements() and gl.drawArrays() , because they both
have pros and cons, will eventually depend on the system implementation.
At this stage, although it’s clear that gl.drawElements() is an efficient way to draw 3D
shapes, one key feature is missing. There is no way to control color, so it is helpful to draw
a cube using a single solid color, as shown in Figure 7.31 .
For example, let’s consider the case where you would like to modify the color of each face
of the cube, as shown in Figure 7.35 , or map textures to the faces. You need to know the
color or texture information for each face, yet you cannot implement this with the combi-
nation of indices, triangle list, and vertex coordinates shown in Figure 7.33 .
CHAPTER 7 Toward the 3D World
284
Figure 7.35 Cube with differently colored faces
In the following section, we will examine how to address this problem and specify the
color information for each face.
Adding Color to Each Face of a Cube
As discussed before, you can only pass per-vertex information to the vertex shader. This
implies that you need to pass the face’s color and the vertices of the triangles as vertex
information to the vertex shader. For instance, to draw the “front” face in blue, made up
of v0, v1, v2, and v3 ( Figure 7.33 ), you need to specify the same blue color for each of the
vertices.
However, as you may have noticed, v0 is also shared by the “right” and “top” faces as well
as the “front” face. Therefore, if you specify the color blue for the vertices that form the
“front” face, you are then unable to choose a different color for those vertices that also
belong to another face. To cope with this problem, although this might not seem as effi-
cient, you must create duplicate entries for the shared vertices in the vertices coordinates
listing, as illustrated in Figure 7.36 . Doing so, you will have to handle common vertices
with identical coordinates in the face’s triangle list as separate entities.
6
6 If you break down all the faces into triangles and draw using gl.drawArrays() , you have
to process 6 vertices * 6 faces = 36 vertices, so the difference between gl.drawArrays() and
gl.drawElements() in memory usage is negligible. This is because a cube or a cuboid is a special
3D object whose faces are connected vertically; therefore, each vertex needs to have three colors.
However, in the case of complex 3D models, specifying several colors to a single vertex would be rare.
Hello Cube
285
e b u c
0 z , 0 y , 0 x
1 z , 1 y , 1 x
2 z , 2 y , 2 x
3 z , 3 y , 3 x
0 z , 0 y , 0 x
3 z , 3 y , 3 x
4 z , 4 y , 4 x
5 z , 5 y , 5 x
0
1
2
3
4
5
6
7
0 2
1 2
2 2
3 2
4 z , 4 y , 4 x
7 z , 7 y , 7 x
6 z , 6 y , 6 x
5 z , 5 y , 5 x
1 , 0 , 0
1 , 0 , 0
1 , 0 , 0
1 , 0 , 0
0 , 1 , 0
0 , 1 , 0
0 , 1 , 0
0 , 1 , 0
1 , 1 , 0
1 , 1 , 0
1 , 1 , 0
1 , 1 , 0
… …
) e u l b ( t n o r f
) d e r ( p u
) n e e r g ( t h g i r
back
down
left
up
right
front
0
1
2
0
2
3
4
5
6
4
6
7
20
21
22
20
22
23
v6(-1,1,-1)
v1(-1,1,1)
v3(1,-1,1)
v4(1,-1,-1)
v0(1,1,1)
v5(1,1,-1)
v2(-1,-1,1)
v7(-1,-1,-1)
Figure 7.36 The faces that constitute the cube, the triangles, and the relationship between
vertices coordinates (configured so that you can choose a different color for each face)
When opting for such a configuration, the contents of the index list, which consists of
the face’s triangle list, will differ from face to face, thus allowing you to modify the color
for each face. This approach can also be used if you want to map a texture to each face.
You would need to specify the texture coordinates for each vertex, but you can actually
deal with this by rewriting the color list ( Figure 7.36 ) as texture coordinates. The sample
program in the section “Rotate Object” in Chapter 10 covers this approach in more detail.
Let’s take a look at the sample program ColoredCube , which displays a cube with each face
painted a different color. The screen shot of ColoredCube is identical to Figure 7.35 .
Sample Program (ColoredCube.js)
The sample program is shown in Listing 7.13 . Because the only difference from
HelloCube.js is the method of storing vertex information into the buffer object, let’s look
in more detail at the code related to the initVertexBuffers() . The main differences to
HelloCube.js are
• In HelloCube.js , the vertex coordinates and color are stored in a single buffer
object, but because this make the array unwieldy, the program has been modified so
that they are now stored in separate buffer objects.
• The respective contents of the vertex array (which stores the vertex coordinates), the
color array (which stores the color information), and the index array (which stores
the indices) are modified in accordance with the configuration described in Figure
7.36 (lines 83, 92, and 101).
CHAPTER 7 Toward the 3D World
286
• To keep the sample program as compact as possible, the function
initArrayBuffer() is defined, which bundles the buffer object creation, binding,
writing of data, and enabling (lines 116, 119, and 129).
As you examine the program, take note of how the second bullet is implemented to match
the structure shown in Figure 7.36 .
Listing 7.13 ColoredCube.js
1 // ColoredCube.js
...
23 function main() {
...
40 // Set the vertex information
41 var n = initVertexBuffers(gl);
...
69 // Draw the cube
70 gl.drawElements(gl.TRIANGLES, n, gl.UNSIGNED_BYTE, 0);
71 }
72
73 function initVertexBuffers(gl) {
...
83 var vertices = new Float32Array([ // Vertex coordinates
84 1.0, 1.0, 1.0, -1.0, 1.0, 1.0, -1.0,-1.0, 1.0, 1.0,-1.0, 1.0,
85 1.0, 1.0, 1.0, 1.0,-1.0, 1.0, 1.0,-1.0,-1.0, 1.0, 1.0,-1.0,
86 1.0, 1.0, 1.0, 1.0, 1.0,-1.0, -1.0, 1.0,-1.0, -1.0, 1.0, 1.0,
...
89 1.0,-1.0,-1.0, -1.0,-1.0,-1.0, -1.0, 1.0,-1.0, 1.0, 1.0,-1.0
90 ]);
91
92 var colors = new Float32Array([ // Colors
93 0.4, 0.4, 1.0, 0.4, 0.4, 1.0, 0.4, 0.4, 1.0, 0.4, 0.4, 1.0,
94 0.4, 1.0, 0.4, 0.4, 1.0, 0.4, 0.4, 1.0, 0.4, 0.4, 1.0, 0.4,
95 1.0, 0.4, 0.4, 1.0, 0.4, 0.4, 1.0, 0.4, 0.4, 1.0, 0.4, 0.4,
...
98 0.4, 1.0, 1.0, 0.4, 1.0, 1.0, 0.4, 1.0, 1.0, 0.4, 1.0, 1.0
99 ]);
100
101 var indices = new Uint8Array([ // Indices of the vertices
102 0, 1, 2, 0, 2, 3, // front
103 4, 5, 6, 4, 6, 7, // right
104 8, 9,10, 8,10,11, // up
...
107 20,21,22, 20,22,23 // back
108 ]);
Hello Cube
287
109
110 // Create a buffer object
111 var indexBuffer = gl.createBuffer();
...
115 // Write the vertex coordinates and color to the buffer object
116 if (!initArrayBuffer(gl, vertices, 3, gl.FLOAT, 'a_Position'))
117 return -1;
118
119 if (!initArrayBuffer(gl, colors, 3, gl.FLOAT, 'a_Color'))
120 return -1;
...
122 // Write the indices to the buffer object
123 gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, indexBuffer);
124 gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, indices, gl.STATIC_DRAW);
125
126 return indices.length;
127 }
128
129 function initArrayBuffer(gl, data, num, type, attribute) {
130 var buffer = gl.createBuffer(); // Create a buffer object
...
135 // Write date into the buffer object
136 gl.bindBuffer(gl.ARRAY_BUFFER, buffer);
137 gl.bufferData(gl.ARRAY_BUFFER, data, gl.STATIC_DRAW);
138 // Assign the buffer object to the attribute variable
139 var a_attribute = gl.getAttribLocation (gl.program, attribute);
...
144 gl.vertexAttribPointer(a_attribute, num, type, false, 0, 0);
145 // Enable the assignment of the buffer object to the attribute variable
146 gl.enableVertexAttribArray(a_attribute);
147
148 return true;
149 }
Experimenting with the Sample Program
In ColoredCube , you specify a different color for each face. So what happens when you
choose an identical color for all the faces? For example, let’s try to set the color infor-
mation in ColoredCube.js ’s colors array to “white,” as shown next. We will call this
program ColoredCube_singleColor.js :
1 // ColoredCube_singleColor.js
...
92 var colors = new Float32Array([
93 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0,
CHAPTER 7 Toward the 3D World
288
94 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0,
...
98 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1
99 ]);
When you execute the program, you see an output like the screenshot shown in Figure
7.37 . One result of using a single color is that it becomes difficult to actually recognize
the cube. Up until now you could differentiate each face because they were differently
colored; therefore, you could recognize the whole shape as a solid. However, when you
switch to a unique color, you lose this three-dimensional impression.
Figure 7.37 Cube with its faces being identically colored
In contrast, in the real world, when you put a white box on a table, you can identify it as
a solid (see Figure 7.38 ). This is because each face, although the same white color, presents
a slightly different appearance because each is lit slightly differently. In ColoredCube_
singleColor , such an effect is not programmed, so the cube is hard to recognize. We will
explore how to correctly light 3D scenes in the next chapter.
Hello Cube
289
Figure 7.38 White box in the real world
Summary
In this chapter, through the introduction of the depth information, you have examined
setting the viewer’s eye point and viewing volume, looked at how to draw real 3D objects,
and briefly examined the local and world coordinate system. Many of the examples were
similar to those previously explained for the two-dimensional world, except for the intro-
duction of the z-axis to handle depth information.
The next chapter explains how to light 3D scenes and how to draw and manipulate
three-dimensional shapes with complex structures. We will also return to the function
initShaders() , which has hidden a number of complex issues that you now have enough
understanding to explore.
This page intentionally left blank
Chapter 8
Lighting Objects
This chapter focuses on lighting objects, looking at different light sources and
their effects on the 3D scene. Lighting is essential if you want to create realistic
3D scenes because it helps to give the scene a sense of depth.
The following key points are discussed in this chapter:
• Shading, shadows, and different types of light sources including point,
directional, and ambient
• Reflection of light in the 3D scene and the two main types: diffuse and
ambient
• The details of shading and how to implement the effect of light to make
objects, such as the pure white cube in the previous chapter, look three-
dimensional
By the end of this chapter, you will have all the knowledge you need to create
lighted 3D scenes populated with both simple and complex 3D objects.
Lighting 3D Objects
When light hits an object in the real world, part of the light is reflected by the
surface of the object. Only after this reflected light enters your eyes can you see
the object and distinguish its color. For example, a white box reflects white light
which, when it enters your eyes, allows you to tell that the box is white.
CHAPTER 8 Lighting Objects
292
In the real world, two important phenomena occur when light hits an object
(see Figure 8.1 ):
• Depending on the light source and direction, surface color is shaded.
• Depending on the light source and direction, objects “cast” shadows on the ground
or the floor.
Shadowing
Shading
Figure 8.1 Shading and shadowing
In the real world, you usually notice shadows, but you quite often don’t notice shading,
which gives 3D objects their feeling of depth. Shading is subtle but always present. As
shown in Figure 8.1 , even surfaces of a pure white cube are distinguishable because each
surface is shaded differently by light. As you can see, the surfaces hit by more light are
brighter, and the surfaces hit by less light are darker, or more shaded. These differences
allow you to distinguish each surface and ensure that the cube looks cubic.
In 3D graphics, the term shading
1 is used to describe the process that re-creates this
phenomenon where the colors differ from surface to surface due to light. The other
phenomenon, that the shadow of an object falls on the floor or ground, is re-created using
a process called shadowing . This section discusses shading. Shadowing is discussed in
Chapter 10 , which focuses on a set of useful techniques that build on your basic knowl-
edge of WebGL.
1 Shading is so critical to 3D graphics that the core language, GLSL ES, is a shader language, the
OpenGL ES Shading Language. The original purpose of shaders was to re-create the phenomena of
shading.
Lighting 3D Objects
293
When discussing shading, you need to consider two things:
• The type of light source that is emitting light
• How the light is reflected from surfaces of an object and enters the eye
Before we begin to program, let’s look at different types of light sources and how light is
reflected from different surfaces.
Types of Light Source
When light illuminates an object, a light source emits the light. In the real world, light
sources are divided into two main categories: directional light , which is something like
the sun that emits light naturally, and point light , which is something like a light bulb
that emits light artificially. In addition, there is ambient light that represents indirect
light (that is, light emitted from all light sources and reflected by walls or other objects
(see Figure 8.2 ). In 3D graphics, there are additional types of light sources. For example,
there is a spot light representing flashlights, headlamps, and so on. However, in this book,
we don’t address these more specialized light sources. Refer to the book OpenGL ES 2.0
Programming Guide for further information on these specialized light sources.
t h g i l t n e i b m A t h g i l t n i o P t h g i l l a n o i t c e r i D
Figure 8.2 Directional light, point light, and ambient light
Focusing on the three main types of light source covered in this book:
Directional light: A directional light represents a light source whose light rays are paral-
lel. It is a model of light whose source is considered to be at an infinite distance, such
as the sun. Because of the distance travelled, the rays are effectively parallel by the time
they reach the earth. This light source is considered the simplest, and because its rays are
parallel can be specified using only direction and color.
Point light: A point light represents a light source that emits light in all directions from
one single point. It is a model of light that can be used to represent light bulbs, lamps,
CHAPTER 8 Lighting Objects
294
flames, and so on. This light source is specified by its position and color.
2 However, the
light direction is determined from the position of the light source and the position at
which the light strikes a surface. As such, its direction can change considerably within
the scene.
Ambient light: Ambient light (indirect light) is a model of light that is emitted from
the other light source (directional or point), reflected by other objects such as walls, and
reaches objects indirectly. It represents light that illuminates an object from all directions
and has the same intensity.
3 For example, if you open the refrigerator door at night, the
entire kitchen becomes slightly lighter. This is the effect of the ambient light. Ambient
light does not have position and direction and is specified only by its color.
Now that you know the types of light sources that illuminate objects, let’s discuss how
light is reflected by the surface of an object and then enters the eye.
Types of Reflected Light
How light is reflected by the surface of an object and thus what color the surface will
become is determined by two things: the type of the light and the type of surface of the
object. Information about the type of light includes its color and direction. Information
about the surface includes its color and orientation.
When calculating reflection from a surface, there are two main types: diffuse reflection
and environment (or ambient ) reflection . The remainder of this section describes how to
calculate the color due to reflection using the two pieces of information described earlier.
There is a little bit of math to be considered, but it’s not complicated.
Diffuse Reflection
Diffuse reflection is the reflection of light from a directional light or a point light. In
diffuse reflection, the light is reflected (scattered) equally in all directions from where
it hits (see Figure 8.3 ). If a surface is perfectly smooth like a mirror, all incoming light
is reflected; however, most surfaces are rough like paper, rock, or plastic. In such cases,
the light is scattered in random directions from the rough surface. Diffuse reflection is a
model of this phenomenon.
2 This type of light actually attenuates; that is, it is strong near the source and becomes weaker farther
from the source. For the sake of simplicity of the description and sample programs, light is treated
as nonattenuating in this book. For attenuation, please refer to the book OpenGL ES 2.0 Programming
Guide .
3 In fact, ambient light is the combination of light emitted from light sources and refl ected by various
surfaces. It is approximated in this way because it would otherwise need complicated calculations to
take into account all the many light sources and how and where they are refl ected.
Lighting 3D Objects
295
θ
orientation of
the surface
A B C
orientation of
the surface
diffuse reflection
light
direction
light
source
light
source
diffuse reflection (reflection differs by light direction)
Figure 8.3 Diffuse reflection
In diffuse reflection, the color of the surface is determined by the color and the direction
of light and the base color and orientation of the surface. The angle between the light
direction and the orientation of the surface is defined by the angle formed by the light
direction and the direction “perpendicular” to the surface. Calling this angle θ , the surface
color by diffuse reflection is calculated using the following formula.
Equation 8.1
θ
〈 〉 =
〈 〉× 〈 〉×
surface color by diffuse reflection
light color base color of surface
      
        cos 
where < light color > is the color of light emitted from a directional light or a point light.
Multiplication with the <base color of the surface> is performed for each RGB component
of the color. Because light by diffuse reflection is scattered equally in all directions from
where it hits, the intensity of the reflected light at a certain position is the same from
any angle (see Figure 8.4 ).
e c a f r u s e h t f o n o i t a t n e i r o
Figure 8.4 The intensity of light at a given position is the same from any angle
Ambient Reflection
Ambient reflection is the reflection of light from another light source. In ambient reflec-
tion, the light is reflected at the same angle as its incoming angle. Because an ambient
light illuminates an object equally from all directions with the same intensity, its bright-
ness is the same at any position (see Figure 8.5 ). It can be approximated as follows.
CHAPTER 8 Lighting Objects
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Equation 8.2
〈 〉 =
〈 〉× 〈 〉
surface color by ambient reflection
light color base color of surface
      
        
where <light color> is the color of light emitted from other light source.
eflection r t n e i b m a eflection r eflection r t n e i b m a e m a s e h t ( ) n o i t i s o p y n a t a
orientation of the surface
Figure 8.5 Ambient reflection
When both diffuse reflection and ambient reflection are present, the color of the surface is
calculated by adding, as follows.
Equation 8.3
〈 〉 =
〈 〉+ 〈 〉
surface color by diffuse and ambient reflection
surface color by diffuse reflection surface color by ambient reflection
          
              
Note that it is not required to always use both light sources, or use the formulas exactly
as mentioned here. You are free to modify each formula to achieve the effect you require
when showing the object.
Now let’s construct some sample programs that perform shading (shading and coloring
the surfaces of an object by placing a light source at an appropriate position). First let’s try
to implement shading due to directional light and its diffuse reflection.
Shading Due to Directional Light and Its Diffuse Reflection
As described in the previous section, surface color is determined by light direction and the
orientation of the surface it strikes when considering diffuse reflection. The calculation of
the color due to directional light is easy because its direction is constant. The formula for
calculating the color of a surface by diffuse reflection ( Equation 8.1 ) is shown again here:
θ
〈 〉 =
〈 〉× 〈 〉×
surface color by diffuse reflection
light color base color of surface
      
        cos 
The following three pieces of information are used:
• The color of the light source (directional light)
• The base color of the surface
• The angle ( θ ) between the light and the surface
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The color of a light source may be white, such as sunlight, or other colors, such as the
orange of lighting in road tunnels. As you know, it can be represented by RGB. White
light such as sunlight has an RGB value of (1.0, 1.0, 1.0). The base color of a surface
means the color that the surface was originally defined to have, such as red or blue. To
calculate the color of a surface, you need to apply the formula for each of the three RGB
components; the calculation is performed three times.
For example, assume that the light emitted from a light source is white (1.0, 1.0, 1.0), and
the base color of the surface is red (1.0, 0.0, 0.0). From Equation 8.1 , when θ is 0.0 (that is,
when the light hits perpendicularly), cos θ becomes 1.0. Because the R component of the
light source is 1.0, the R component of the base surface color is 1.0, and the cos θ is 1.0,
the R component of the surface color by diffuse reflection is calculated as follows:
R = 1.0 * 1.0 * 1.0 = 1.0
The G and B components are also calculated in the same way, as follows:
G = 1.0 * 0.0 * 1.0 = 0.0
B = 1.0 * 0.0 * 1.0 = 0.0
From these calculations, when white light hits perpendicularly on a red surface, the
surface color by diffuse reflection turns out to be (1.0, 0.0, 0.0), or red. This is consistent
with real-world experience. Conversely, when the color of the light source is red and the
base color of a surface is white, the result is the same.
Let’s now consider the case when θ is 90 degrees, or when the light does not hit the
surface at all. From your real-world experience, you know that in this case the surface will
appear black. Let’s validate this. Because cos θ is 0 when θ is 90 degrees, and anything
multiplied by zero is zero, the result of the formula is 0 for R, G, and B; that is, the surface
color becomes (0.0, 0.0, 0.0), or black, as expected. Equally, when θ is 60 degrees, you’d
expect that a small amount of light falling on a red surface would result in a darker red
color, and because cos θ is 0.5, the surface color is (0.5, 0.0, 0.0), which is dark red, as
expected.
These simple examples have given you a good idea of how to calculate surface color due
to diffuse reflection. To allow you to factor in directional light, let’s transform the preced-
ing formula to make it easy to handle so you can then explore how to draw a cube lit by
directional light.
Calculating Diffuse Reflection Using the Light Direction and the
Orientation of a Surface
In the previous examples, an arbitrary value for θ was chosen. However, typically it is
complicated to get the angle θ between the light direction and the orientation of a surface.
For example, when creating a model, the angle at which light hits each surface cannot
be determined in advance. In contrast, the orientation of each surface can be determined
CHAPTER 8 Lighting Objects
298
regardless of where light hits from. Because the light direction is also determined when
its light source is determined, it seems convenient to try to use these two pieces of
information.
Fortunately, mathematics tells us that cos θ is derived by calculating the dot product of
the light direction and the orientation of a surface. Because the dot product is so often
used, GLSL ES provides a function to calculate it.
4 (More details can be found in Appendix
B , “Built-In Functions of GLSL ES 1.0.”) When representing the dot product by “ · ”, cos θ is
defined as follows:
θ = 〈 〉 〈 〉 light direction orientation of a surface cos        i
From this, Equation 8.1 can be transformed as following Equation 8.4 :
Equation 8.4
( )
〈 〉 =
〈 〉× 〈 〉×
〈 〉 〈 〉
surface color by diffuse reflection
light color base color of surface
light direction orientation of a surface
      
      
       i
Here, there are two points to be considered: the length of the vector and the light direc-
tion. First, the length of vectors that represent light direction and orientation of the
surface, such as (2.0, 2.0, 1.0), must be 1.0,
5 or the color of the surface may become too
dark or bright. Adjusting the components of a vector so that its length becomes 1.0 is
called normalization .
6 GLSL ES provides functions for normalizing vectors that you can
use directly.
The second point to consider concerns the light direction for the reflected light. The light
direction is the opposite direction from that which the light rays travel (see Figure 8.6 ).
4 Mathematically, the dot product of two vectors n and l is written as follows:
n • 1 = | n | x |1| x cos θ
where || means the length of the vector. From this equation, you can see that when the lengths of n
and l are 1.0, the dot product is equal to cos θ . If n is ( n
x , n y , n z ) and l is ( l x , l y , l z ), then n l = n x * l x + n y *
l
y + n z * l z from the law of cosines.
5 If the components of the vector n are (n
x , n y , n z ), its length is as follows:
6 Normalized n is (n
x /m, n y /m, n z /m), where m is the length of n. |n| = sqrt(9) = 3. The vector (2.0, 2.0,
1.0) above is normalized into (2.0/3.0, 2.0/3.0, 1.0/3.0).
= = + + n n n length of n   |n |     x y z
2 2 2
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299
θ
orientation of
the surface
diffuse reflection
light
direction
light
source
Figure 8.6 The light direction is from the reflecting surface to the light source
Because we aren’t using an angle to specify the orientation of the surface, we need another
mechanism to do that. The solution is to use normal vectors.
The Orientation of a Surface: What Is the Normal?
The orientation of a surface is specified by the direction perpendicular to the surface and
is called a normal or a normal vector . This direction is represented by a triple number,
which is the direction of a line from the origin (0, 0, 0) to (n
x , n y , n z ) specified as the
normal. For example, the direction of the normal (1, 0, 0) is the positive direction of the
x-axis, and the direction of the normal (0, 0, 1) is the positive direction of the z-axis.
When considering surfaces and their normals, two properties are important for our
discussion.
A Surface Has Two Normals
Because a surface has a front face and a back face, each side has its own normal; that is,
the surface has two normals. For example, the surface perpendicular to the z-axis has a
front face that is facing toward the positive direction of the z-axis and a back face that is
facing the negative direction of the z-axis, as shown in Figure 8.7 . Their normals are (0, 0,
1) and (0, 0, –1), respectively.
x
y
z
x
y
z
) 1 , 0 , 0 ( l a m r o n e h t , 1) 0 , 0 ( l a m r o n e h t
0 v
1 v
2 v
3 v
Figure 8.7 Normals
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300
In 3D graphics, these two faces are distinguished by the order in which the vertices are
specified when drawing the surface. When you draw a surface specifying vertices in the
order
7 v0, v1, v2, and v3, the front face is the one whose vertices are arranged in a clock-
wise fashion when you look along the direction of the normal of the face (same as the
right-handed rule determining the positive direction of rotation in Chapter 3 , “Drawing
and Transforming Triangles”). So in Figure 8.7 , the front face has the normal (0, 0, –1) as
in the right side of the figure.
The Same Orientation Has the Same Normal
Because a normal just represents direction, surfaces with the same orientation have the
same normal regardless of the position of the surfaces.
If there is more than one surface with the same orientation placed at different positions,
the normals of these surfaces are identical. For example, the normals of a surface perpen-
dicular to the z-axis, whose center is placed at (10, 98, 9), are still (0, 0, 1) and (0, 0, –1).
They are the same as when it is positioned at the origin (see Figure 8.8 ).
x
y
z
x
y
z
) 1 , 0 , 0 ( l a m r o n e h t ) 1 - , 0 , 0 ( l a m r o n e h t
0 v
1 v
2 v
3 v
Figure 8.8 If the orientation of the surface is the same, the normal is identical regardless of
its position
The left side of Figure 8.9 shows the normals that are used in the sample programs in this
section. Normals are labeled using, for example “n(0, 1, 0)” as in this figure.
7 Actually, this surface is composed of two triangles: a triangle drawn in the order v0, v1, and v2, and a
triangle drawn in the order v0, v2, and v3.
Lighting 3D Objects
301
x
y
z
) 1 , 0 , 0 ( n
) 0 , 1 , 0 ( n
) 0 , 0 , 1 ( n
) 0 , 0 , 1 - ( n
) 1 - , 0 , 0 ( n
x
y
z
) 0 , 0 , 1 ( n
) . d e y a l p s i d t o n e r a s l a m r o n l l a (
) 0 , 1 - , 0 ( n
v4(1,-1,-1)
v4(1,-1,-1)
v3(1,-1,1) v3(1,-1,1)
v2(-1,-1,1)
v2(-1,-1,1)
v7(-1,-1,-1)
v0(1, 1, 1)
v1(-1,1,1)
v1(-1,1,1)
v6(-1, 1,-1)
v6(-1, 1,-1)
v5(1, 1,-1)
v5(1, 1,-1)
v0(1, 1, 1)
v7(-1,-1,-1)
) 1 , 0 , ( n 0
) 0 , 1 , 0 ( n
Figure 8.9 Normals of the surfaces of a cube
Once you have calculated the normals for a surface, the next task is to pass that data to
the shader programs. In the previous chapter, you passed color data for a surface to the
shader as “per-vertex data.” You can pass normal data using the same approach: as per-
vertex data stored in a buffer object. In this section, as shown in Figure 8.9 (right side),
the normal data is specified for each vertex, and in this case there are three normals per
vertex, just as there are three color data specified per vertex.
8
Now let’s construct a sample program LightedCube that displays a red cube lit by a white
directional light. The result is shown in Figure 8.10 .
Figure 8.10 LightedCube
8 Cubes or cuboids are simple but special objects whose three surfaces are connected perpendicularly.
They have three different normals per vertex. On the other hand, smooth objects such as game
characters have one normal per vertex.
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302
Sample Program (LightedCube.js)
The sample program is shown in Listing 8.1 . It is based on ColoredCube from the previous
chapter, so the basic processing flow of this program is the same as ColoredCube .
As you can see from Listing 8.1 , the vertex shader has been significantly modified so that
it calculates Equation 8.4 . In addition, the normal data is added in initVertexBuffers()
defined at line 89, so that they can be passed to the variable a_Normal . The fragment
shader is the same as in ColoredCube , and unmodified. It is reproduced so that you can
see that no fragment processing is needed.
Listing 8.1 LightedCube.js
1 // LightedCube.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'attribute vec4 a_Color;\n' +
6 'attribute vec4 a_Normal;\n' + // Normal
7 'uniform mat4 u_MvpMatrix;\n' +
8 'uniform vec3 u_LightColor;\n' + // Light color
9 'uniform vec3 u_LightDirection;\n' + // world coordinate, normalized
10 'varying vec4 v_Color;\n' +
11 'void main() {\n' +
12 ' gl_Position = u_MvpMatrix * a_Position ;\n' +
13 // Make the length of the normal 1.0
14 ' vec3 normal = normalize(vec3(a_Normal));\n' +
15 // Dot product of light direction and orientation of a surface
16 ' float nDotL = max(dot(u_LightDirection, normal), 0.0);\n' +
17 // Calculate the color due to diffuse reflection
18 ' vec3 diffuse = u_LightColor * vec3(a_Color) * nDotL;\n' +
19 ' v_Color = vec4(diffuse, a_Color.a);\n' +
20 '}\n';
21
22 // Fragment shader program
...
28 'void main() {\n' +
29 ' gl_FragColor = v_Color;\n' +
30 '}\n';
31
32 function main() {
...
49 // Set the vertex coordinates, the color, and the normal
50 var n = initVertexBuffers(gl);
...
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303
61 var u_MvpMatrix = gl.getUniformLocation(gl.program, 'u_MvpMatrix');
62 var u_LightColor = gl.getUniformLocation(gl.program, 'u_LightColor');
63 var u_LightDirection = gl.getUniformLocation(gl.program, 'u_LightDirection');
...
69 // Set the light color (white)
70 gl.uniform3f(u_LightColor, 1.0, 1.0, 1.0);
71 // Set the light direction (in the world coordinate)
72 var lightDirection = new Vector3([0.5, 3.0, 4.0]);
73 lightDirection.normalize(); // Normalize
74 gl.uniform3fv(u_LightDirection, lightDirection.elements);
75
76 // Calculate the view projection matrix
77 var mvpMatrix = new Matrix4(); // Model view projection matrix
78 mvpMatrix.setPerspective(30, canvas.width/canvas.height, 1, 100);
79 mvpMatrix.lookAt(3, 3, 7, 0, 0, 0, 0, 1, 0);
80 // Pass the model view projection matrix to the variable u_MvpMatrix
81 gl.uniformMatrix4fv(u_MvpMatrix, false, mvpMatrix.elements);
...
86 gl.drawElements(gl.TRIANGLES, n, gl.UNSIGNED_BYTE, 0);// Draw a cube
87 }
88
89 function initVertexBuffers(gl) {
...
98 var vertices = new Float32Array([ // Vertices
99 1.0, 1.0, 1.0, -1.0, 1.0, 1.0, -1.0,-1.0, 1.0, 1.0,-1.0, 1.0,
100 1.0, 1.0, 1.0, 1.0,-1.0, 1.0, 1.0,-1.0,-1.0, 1.0, 1.0,-1.0,
...
104 1.0,-1.0,-1.0, -1.0,-1.0,-1.0, -1.0, 1.0,-1.0, 1.0, 1.0,-1.0
105 ]);
...
117
118 var normals = new Float32Array([ // Normals
119 0.0, 0.0, 1.0, 0.0, 0.0, 1.0, 0.0, 0.0, 1.0, 0.0, 0.0, 1.0,
120 1.0, 0.0, 0.0, 1.0, 0.0, 0.0, 1.0, 0.0, 0.0, 1.0, 0.0, 0.0,
...
124 0.0, 0.0,-1.0, 0.0, 0.0,-1.0, 0.0, 0.0,-1.0, 0.0, 0.0,-1.0
125 ]);
...
140 if(!initArrayBuffer(gl,'a_Normal', normals, 3, gl.FLOAT)) return -1;
...
154 return indices.length;
155 }
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304
As a reminder, here is the calculation that the vertex shader performs ( Equation 8.4 ):
( )
〈 〉 =
〈 〉× 〈 〉×
〈 〉 〈 〉
surface color by diffuse reflection
light color base color of surface
light direction orientation of a surface
      
      
       i
You can see that four pieces of information are needed to calculate this equation: (1) light
color, (2) a surface base color, (3) light direction, and (4) surface orientation. In addition,
<light direction> and <surface orientation> must be normalized (1.0 in length).
Processing in the Vertex Shader
From the four pieces of information necessary for Equation 8.4 , the base color of a
surface is passed as a_Color at line 5 in the following code, and the surface orientation is
passed as a_Normal at line 6. The light color is passed using u_LightColor at line 8, and
the light direction is passed as u_LightDirection at line 9. You should note that only
u_LightDirection is passed in the world coordinate
9 system and has been normalized in
the JavaScript code for ease of handling. This avoids the overhead of normalizing it every
time it’s used in the vertex shader:
4 'attribute vec4 a_Position;\n' +
5 'attribute vec4 a_Color;\n' + <-(2) surface base color
6 'attribute vec4 a_Normal;\n' + // Normal <-(4) surface orientation
7 'uniform mat4 u_MvpMatrix;\n' +
8 'uniform vec3 u_LightColor;\n' + // Light color <-(1)
9 'uniform vec3 u_LightDirection;\n' + // world coordinate,normalized <-(3)
10 'varying vec4 v_Color;\n' +
11 'void main() {\n' +
12 ' gl_Position = u_MvpMatrix * a_Position ;\n' +
13 // Make the length of the normal 1.0
14 ' vec3 normal = normalize(vec3(a_Normal));\n' +
15 // Dot product of light direction and orientation of a surface
16 ' float nDotL = max(dot(u_LightDirection, normal), 0.0);\n' +
17 // Calculate the color due to diffuse reflection
18 ' vec3 diffuse = u_LightColor * vec3(a_Color) * nDotL;\n' +
19 ' v_Color = vec4(diffuse, a_Color.a);\n' +
20 '}\n';
Once the necessary information is available, you can carry out the calculation. First, the
vertex shader normalizes the vector at line 14. Technically, because the normal used in
this sample program is 1.0 in length, this process is not necessary. However, it is good
practice, so it is performed here:
9 In this book, the light effect with shading is calculated in the world coordinate system (see Appendix
G , “World Coordinate System Versus Local Coordinate System”) because it is simpler to program and
more intuitive with respect to the light direction. It is also safe to calculate it in the view coordinate
system but more complex.
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305
14 ' vec3 normal = normalize(vec3(a_Normal));\n' +
Although a_Normal is of type vec4 , a normal represents a direction and uses only the x, y,
and z components. So you extract these components with . xyz and then normalize. If you
pass the normal using a type vec3 , this process is not necessary. However, it is passed as
a type vec4 in this code because a vec4 will be needed when we extend the code for the
next example. We will explain the details in a later sample program. As you can see, GLSL
ES provides normalize() , a built-in function to normalize a vector specified as its argu-
ment. In the program, the normalized normal is stored in the variable normal for use later.
Next, you need to calculate the dot product 〈
〉 〈 〉 light direction surface orientation     i
from
Equation 8.4 . The light direction is stored in u_LightDirection . Because it is already
normalized, you can use it as is. The orientation of the surface is the normal that was
normalized at line 14. The dot product “ · ” can then be calculated using the built-in
function dot() , which again is provided by GLSL ES and returns the dot product of the
two vectors specified as its arguments. That is, calling dot(u_LightDirection, normal)
performs 〈 〉 〈 〉 light direction surface orientation     i . This calculation is performed at line 16.
16 ' float nDotL = max(dot(u_LightDirection, normal), 0.0);\n' +
Once the dot product is calculated, if the result is positive, it is assigned to nDotL . If it is
negative then 0.0 is assigned. The function max() used here is a GLSL ES built-in function
that returns the greater value from its two arguments.
A negative dot product means that θ in cos θ is more than 90 degrees. Because θ is the
angle between the light direction and the surface orientation, a value of θ greater than 90
degrees means that light hits the surface on its back face (see Figure 8.11 ). This is the same
as no light hitting the front face, so 0.0 is assigned to nDotL .
θ 0 9 >
orientation of
the surface
diffuse reflection
light
direction
light
source
Figure 8.11 A normal and light in case θ is greater than 90 degrees
Now that the preparation is completed, you can calculate Equation 8.4 . This is performed
at line 18, which is a direct implementation of Equation 8.4 . a_Color , which is of type
vec4 and holds the RGBA values, is converted to a vec3 ( .rgb ) because its transparency
(alpha value) is not used in lighting.
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306
In fact, transparency of an object’s surface has a significant effect on the color of the
surface. However, because the calculation of the light passing through an object is compli-
cated, we ignore transparency and don’t use the alpha value in this program:
18 ' vec3 diffuse = u_LightColor * vec3(a_Color) * nDotL;\n' +
Once calculated, the result, diffuse , is assigned to the varying variable v_Color at line 19.
Because v_Color is of type vec4 , diffuse is also converted to vec4 with 1.0 :
19 ' v_Color = vec4(diffuse, 1.0);\n' +
The result of the processing steps above is that a color, depending on the direction of
the vertex’s normal, is calculated, passed to the fragment shader, and assigned to gl_
FragColor . In this case, because you use a directional light, vertices that make up the same
surface are the same color, so each surface will be a solid color.
That completes the vertex shader code. Let’s now take a look at how the JavaScript
program passes the data needed for Equation 8.4 to the vertex shader.
Processing in the JavaScript Program
The light color ( u_LightColor ) and the light direction ( u_LightDirection ) are passed to
the vertex shader from the JavaScript program. Because the light color is white (1.0, 1.0,
1.0), it is simply written to u_LightColor using gl.uniform3f() :
69 // set the Light color (white)
70 gl.uniform3f(u_LightColor, 1.0, 1.0, 1.0);
The next step is to set up the light direction, which must be passed after normalization, as
discussed before. You can normalize it with the normalize() function for Vector3 objects
that is provided in cuon-matrix.js . Usage is simple: Create the Vector3 object that speci-
fies the vector you want to normalize as its argument (line 72), and invoke the normal-
ize() method on the object. Note that the notation in JavaScript is different from that of
GLSL ES:
71 // Set the light direction (in the world coordinate)
72 var lightDirection = new Vector3([0.5, 3.0, 4.0]);
73 lightDirection.normalize(); // Normalize
74 gl.uniform3fv(u_LightDirection, lightDirection.elements);
The result is stored in the elements property of the object in an array of type
Float32Array and then assigned to u_LightDirection using gl.uniform3fv() (line 74).
Finally, the normal data is written in initVertexBuffers() , defined at line 89. Actual
normal data is stored in the array normals at line 118 per vertex along with the color
data, as in ColoredCube.js . Data is assigned to a_Normal in the vertex shader by invoking
initArrayBuffer() at line 140:
140 if(!initArrayBuffer(gl, 'a_Normal', normals, 3, gl.FLOAT)) return -1;
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307
initArrayBuffer() , which was also used in ColoredCube , assigns the array specified by
the third argument ( normals ) to the attribute variable that has the name specified by the
second argument ( a_Normal ).
Add Shading Due to Ambient Light
Although at this stage you have successfully added lighting to the scene, as you can see
from Figure 8.9 , when you run LightedCube , the cube is a little different from the box in
the real world. In particular, the surface on the opposite side of the light source appears
almost black and not clearly visible. You can see this problem more clearly if you animate
the cube. Try the sample program LightedCube_animation (see Figure 8.12 ) to see the
problem more clearly.
Figure 8.12 The result of LightedCube_animation
Although the scene is correctly lit as the result of Equation 8.4 , our real-world experiences
tells us that something isn’t right. It is unusual to see such a sharp effect because, in the
real world, surfaces such as the back face of the cube are also lit by diffuse or reflected
light. The ambient light described in the previous section represents this indirect light
and can be used to make the scene more lifelike. Let’s add that to the scene and see if the
effect is more realistic. Because ambient light models the light that hits an object from all
directions with constant intensity, the surface color due to the reflection is determined
only by the light color and the base color of the surface. The formula that calculates this
was shown as Equation 8.2 . Let’s see it again:
〈 〉 =
〈 〉× 〈 〉
surface color by ambient reflection
light color base color of surface
      
        
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308
Let’s try to add the color due to ambient light described by this formula to the sample
program LightedCube . To do this, use Equation 8.3 shown here:
〈 〉 =
〈 〉+ 〈 〉
surface color by diffuse and ambient reflection
surface color by diffuse reflection surface color by ambient reflection
          
              
Ambient light is weak because it is the light reflected by other objects like the walls. For
example, if the ambient light color is (0.2, 0.2, 0.2) and the base color of a surface is red,
or (1.0, 0.0, 0.0), then, from Equation 8.2 , the surface color due to the ambient light is
(0.2, 0.0, 0.0). For example, if there is a white box in a blue room—that is, the base color
of the surface is (1.0, 1.0, 1.0) and the ambient light is (0.0, 0.0, 0.2)—the color becomes
slightly blue (0.0, 0.0, 0.2).
Let’s implement the effect of ambient reflection in the sample program LightedCube_
ambient , which results in the cube shown in Figure 8.13 . You can see that the surface that
the light does not directly hit is now also slightly colored and more closely resembles the
cube in the real world.
Figure 8.13 LightedCube_ambient
Sample Program (LightedCube_ambient.js)
Listing 8.2 illustrates the sample program. Because it is almost the same as LightedCube ,
only the modified parts are shown.
Listing 8.2 LightedCube_ambient.js
1 // LightedCube_ambient.js
2 // Vertex shader program
...
Lighting 3D Objects
309
8 'uniform vec3 u_LightColor;\n' + // Light color
9 'uniform vec3 u_LightDirection;\n' + // World coordinate, normalized
10 'uniform vec3 u_AmbientLight;\n' + // Color of an ambient light
11 'varying vec4 v_Color;\n' +
12 'void main() {\n' +
...
16 // The dot product of the light direction and the normal
17 ' float nDotL = max(dot(lightDirection, normal), 0.0);\n' +
18 // Calculate the color due to diffuse reflection
19 ' vec3 diffuse = u_LightColor * a_Color.rgb * nDotL;\n' +
20 // Calculate the color due to ambient reflection
21 ' vec3 ambient = u_AmbientLight * a_Color.rgb;\n' +
22 // Add surface colors due to diffuse and ambient reflection
23 ' v_Color = vec4(diffuse + ambient, a_Color.a);\n' +
24 '}\n';
...
36 function main() {
...
64 // Get the storage locations of uniform variables and so on
...
68 var u_AmbientLight = gl.getUniformLocation(gl.program, 'u_AmbientLight');
...
80 // Set the ambient light
81 gl.uniform3f(u_AmbientLight, 0.2, 0.2, 0.2);
...
95 }
u_AmbientLight at line 10 is added to the vertex shader to pass in the color of ambient
light. After Equation 8.2 is calculated using it and the base color of the surface ( a_Color ),
the result is stored in the variable ambient (line 21). Now that both diffuse and ambient
are determined, the surface color is calculated at line 23 using Equation 8.3 . The result is
passed to v_Color , just like in LightedCube , and the surface is painted with this color.
As you can see, this program simply adds ambient at line 23, causing the whole cube to
become brighter. This implements the effect of the ambient light hitting an object equally
from all directions.
The examples so far have been able to handle static objects. However, because objects are
likely to move within a scene, or the viewpoint changes, you have to be able to handle
such transformations. As you will recall from Chapter 4 , “More Transformations and Basic
Animation,” an object can be translated, scaled, or rotated using coordinate transforma-
tions. These transformations may also change the normal direction and require a recalcu-
lation of lighting as the scene changes. Let’s take a look at how to achieve that.
CHAPTER 8 Lighting Objects
310
Lighting the Translated-Rotated Object
The program LightedTranslatedRotatedCube uses a directional light source to light a cube
that is rotated 90 degrees clockwise around the z-axis and translated 0.9 units in the y-axis
direction. A part from a directional light as described in the previous section, the sample,
LightedCube_ambient , uses diffuse reflection and ambient reflection and rotates and trans-
lates the cube. The result is shown in Figure 8.14 .
Figure 8.14 LightedTranslatedRotatedCube
You saw in the previous section that the normal direction may change when coordinate
transformations are applied. Figure 8.15 shows some examples of that. The leftmost figure
in Figure 8.15 shows the cube used in this sample program looking along the negative
direction of the z-axis. The only normal (1, 0, 0), which is toward the positive direction of
the x-axis, is shown. Let’s perform some coordinate transforms on this figure, which are
the three figures on the right.
l a m r o n e h t
n o i t c e r i d
1 ( 0 ) 0
n o i t c e r i d l a m r o n e h t
1 ( 0 ) 0
n o i t c e r i d l a m r o n e h t
1 ( 1 ) 0
n o i t c e r i d l a m r o n e h t
) 0 , 5 . 0 , 1 (
x
y
n o i t a l s n a r t ) 1 (
) s i x a - y e h t g n o l a e t a l s n a r t (
n o i t a t o r ) 2 (
) s e e r g e d 5 4 e t a t o r (
g n i l a c s ) 3 (
g n o l a 2 y b e l a c s (
) s i x a - y e h t
l a m r o n l a n i g i r o e h t
Figure 8.15 The changes of the normal direction due to coordinate transformations
Lighting the Translated-Rotated Object
311
You can see the following from Figure 8.15 :
• The normal direction is not changed by a translation because the orientation of the
object does not change.
• The normal direction is changed by a rotation according to the orientation of the
object.
• Scaling has a more complicated effect on the normal. As you can see, the object in
the rightmost figure is rotated i and then scaled two times only in the y-axis. In
this case, the normal direction is changed because the orientation of the surface
changes. On the other hand, if an object is scaled equally in all axes the normal
direction is not changed. Finally, even if an object is scaled unequally, the normal
direction may not change . For example, when the leftmost figure (the original
normal) is scaled two times only in the y-axis direction, the normal direction does
not change.
Obviously, the calculation of the normal under various transformations is complex, partic-
ularly when dealing with scaling. However, a mathematical technique can help.
The Magic Matrix: Inverse Transpose Matrix
As described in Chapter 4 , the matrix that performs a coordinate transformation on an
object is called a model matrix. The normal direction can be calculated by multiplying the
normal by the inverse transpose matrix of a model matrix. The inverse transpose matrix
is the matrix that transposes the inverse of a matrix.
The inverse of the matrix M is the matrix R, where both R*M and M*R become the iden-
tity matrix. The term transpose means the operation that exchanges rows and columns of
a matrix. The details of this are explained in Appendix E , “The Inverse Transpose Matrix.”
For our purposes, it can be summarized simply using the following rule:
Rule: You can calculate the normal direction if you multiply the normal by the
inverse transpose of the model matrix.
The inverse transpose matrix is calculated as follows:
1. Invert the original matrix.
2. Transpose the resulting matrix.
This can be carried out using convenient methods supported by the Matrix4 object (see
Table 8.1 ).
CHAPTER 8 Lighting Objects
312
Table 8.1 Matrix4 Methods for an Inverse Transpose Matrix
Method Description
Matrix4.setInverseOf(m) Calculates the inverse of the matrix stored in m and stores the
result in the Matrix4 object, where m is a Matrix4 object
Matrix4.transpose() Transposes the matrix stored in the Matrix4 object and writes
the result back into the Matrix4 object
Assuming that a model matrix is stored in modelMatrix , which is a Matrix4 object, the
following code snippet will get its inverse transpose matrix. The result is stored in the vari-
able named normalMatrix , because it performs the coordinate transformation of a normal:
Matrix4 normalMatrix = new Matrix4();
// Calculate the model matrix
...
// Calculate the matrix to transform normal according to the model matrix
normalMatrix.setInverseOf(modelMatrix);
normalMatrix.transpose();
Now let’s see the program LightedTranslatedRotatedCube.js that lights the cube, which
is rotated 90 degrees clockwise around the z-axis and translated 0.9 along the y-axis, all
using directional light. You’ll use the cube that was transformed by the model matrix in
LightedCube_ambient from the previous section.
Sample Program (LightedTranslatedRotatedCube.js)
Listing 8.3 shows the sample program. The changes from LightedCube_ambient are that
u_NormalMatrix is added (line 8) to pass the matrix for coordinate transformation of the
normal to the vertex shader, and the normal is transformed at line 16 using this matrix.
u_NormalMatrix is calculated within the JavaScript.
Listing 8.3 LightedTranslatedRotatedCube.js
1 // LightedTranslatedRotatedCube.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
...
6 'attribute vec4 a_Normal;\n' +
7 'uniform mat4 u_MvpMatrix;\n' +
8 'uniform mat4 u_NormalMatrix;\n'+ // Transformation matrix of normal
9 'uniform vec3 u_LightColor;\n' + // Light color
10 'uniform vec3 u_LightDirection;\n' + // World coordinate, normalized
11 'uniform vec3 u_AmbientLight;\n' + // Ambient light color
12 'varying vec4 v_Color;\n' +
Lighting the Translated-Rotated Object
313
13 'void main() {\n' +
14 ' gl_Position = u_MvpMatrix * a_Position;\n' +
15 // Recalculate normal with normal matrix and make its length 1.0
16 ' vec3 normal = normalize(vec3(u_NormalMatrix * a_Normal));\n' +
17 // The dot product of the light direction and the normal
18 ' float nDotL = max(dot(u_LightDirection, normal), 0.0);\n' +
19 // Calculate the color due to diffuse reflection
20 ' vec3 diffuse = u_LightColor * a_Color.rgb * nDotL;\n' +
21 // Calculate the color due to ambient reflection
22 ' vec3 ambient = u_AmbientLight * a_Color.rgb;\n' +
23 // Add the surface colors due to diffuse and ambient reflection
24 ' v_Color = vec4(diffuse + ambient, a_Color.a);\n' +
25 '}\n';
...
37 function main() {
...
65 // Get the storage locations of uniform variables and so on
66 var u_MvpMatrix = gl.getUniformLocation(gl.program, 'u_MvpMatrix');
67 var u_NormalMatrix = gl.getUniformLocation(gl.program, 'u_NormalMatrix');
...
85 var modelMatrix = new Matrix4(); // Model matrix
86 var mvpMatrix = new Matrix4(); // Model view projection matrix
87 var normalMatrix = new Matrix4(); // Transformation matrix for normal
88
89 // Calculate the model matrix
90 modelMatrix.setTranslate(0, 1, 0); // Translate to y-axis direction
91 modelMatrix.rotate(90, 0, 0, 1); // Rotate around the z-axis
92 // Calculate the view projection matrix
93 mvpMatrix.setPerspective(30, canvas.width/canvas.height, 1, 100);
94 mvpMatrix.lookAt(-7, 2.5, 6, 0, 0, 0, 0, 1, 0);
95 mvpMatrix.multiply(modelMatrix);
96 // Pass the model view projection matrix to u_MvpMatrix
97 gl.uniformMatrix4fv(u_MvpMatrix, false, mvpMatrix.elements);
98
99 // Calculate matrix to transform normal based on the model matrix
100 normalMatrix.setInverseOf(modelMatrix);
101 normalMatrix.transpose();
102 // Pass the transformation matrix for normal to u_NormalMatrix
103 gl.uniformMatrix4fv(u_NormalMatrix, false, normalMatrix.elements);
...
110 }
CHAPTER 8 Lighting Objects
314
The processing in the vertex shader is almost the same as in LightedCube_ambient . The
difference, in line with the preceding rule, is that you multiply a_Normal by the inverse
transpose of the model matrix at line 16 instead of using it as-is:
15 // Recalculate normal with normal matrix and make its length 1.0
16 ' vec3 normal = normalize(vec3(u_NormalMatrix * a_Normal));\n' +
Because you passed a_Normal as type vec4 , you can multiply it by u_NormalMatrix , which
is of type mat4 . You only need the x, y, and z components of the result of the multiplica-
tion, so the result is converted into type vec3 with vec3() . It is also possible to use . xyz
as before, or write (u_NormalMatrix * a_Normal).xyz . However, vec3() is used here for
simplicity. Now that you understand how the shader calculates the normal direction
resulting from the rotation and translation of the object, let’s move on to the explanation
of the JavaScript program. The key point here is the calculation of the matrix that will be
passed to u_NormalMatrix in the vertex shader.
u_NormalMatrix is the inverse transpose of the model matrix, so the model matrix is first
calculated at lines 90 and 91. Because this program rotates an object around the z-axis
and translates it in the y-axis direction, you can use the setTranslate() and rotate()
methods of a Matrix4 object as described in Chapter 4 . It is at lines 100 and 101 that the
inverse transpose matrix is actually calculated. It is passed to u_NormalMatrix in the vertex
shader at line 103, in the same way as mvpMatrix at line 97. The second argument of gl.
uniformMatrix4f() specifies whether to transpose the matrix ( Chapter 3 ):
99 // Calculate matrix to transform normal based on the model matrix
100 normalMatrix.setInverseOf(modelMatrix);
101 normalMatrix.transpose();
102 // Pass the normal transformation matrix to u_NormalMatrix
103 gl.uniformMatrix4fv(u_NormalMatrix, false, normalMatrix.elements);
When run, the output is similar to Figure 8.14 . As you can see, the shading is the same as
LightedCube_ambient with the cube translated in the y-axis direction. That is because (1)
the translation doesn’t change the normal direction, (2) neither does the rotation by 90
degrees, because the rotation simply switches the surfaces of the cube, (3) the light direc-
tion of the directional light does not change regardless of the position of the object, and
(4) diffuse reflection reflects the light in all directions with equal intensity.
You now have a good understanding of the basics of how to implement light and shade in
3D graphics. Let’s build on this by exploring another type of light source: the point light.
Using a Point Light Object
In contrast to a directional light, the direction of the light from a point light source differs
at each position in the 3D scene (see Figure 8.16 ). So, when calculating shading, you need
to calculate the light direction at the specific position on the surface where the light hits.
Using a Point Light Object
315
α
β
a point light
Figure 8.16 The direction of a point light varies by position
In the previous sample programs, you calculated the color at each vertex by passing the
normal and the light direction for each vertex. You will use the same approach here, but
because the light direction changes, you need to pass the position of the light source and
then calculate the light direction at each vertex position.
Here, you construct the sample program PointLightedCube that displays a red cube lit
with white light from a point light source. We again use diffuse reflection and ambient
reflection. The result is shown in Figure 8.17 , which is a version of LightedCube_ambient
from the previous section but now lit with a point light.
Figure 8.17 PointLightedCube
Sample Program (PointLightedCube.js)
Listing 8.4 shows the sample program in which only the vertex shader is changed from
LightedCube_ambient . The variable u_ModelMatrix for passing the model matrix and the
variable u_LightPosition representing the light position are added. Note that because you
CHAPTER 8 Lighting Objects
316
use a point light in this program, you will use the light position instead of the light direc-
tion. Also, to make the effect easier to see, we have enlarged the cube.
Listing 8.4 PointLightedCube.js
1 // PointLightedCube.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
...
8 'uniform mat4 u_ModelMatrix;\n' + // Model matrix
9 'uniform mat4 u_NormalMatrix;\n' + // Transformation matrix of normal
10 'uniform vec3 u_LightColor;\n' + // Light color
11 'uniform vec3 u_LightPosition;\n' + // Position of the light source (in the
➥world coordinate system)
12 'uniform vec3 u_AmbientLight;\n' + // Ambient light color
13 'varying vec4 v_Color;\n' +
14 'void main() {\n' +
15 ' gl_Position = u_MvpMatrix * a_Position;\n' +
16 // Recalculate normal with normal matrix and make its length 1.0
17 ' vec3 normal = normalize(vec3(u_NormalMatrix * a_Normal));\n' +
18 // Calculate the world coordinate of the vertex
19 ' vec4 vertexPosition = u_ModelMatrix * a_Position;\n' +
20 // Calculate the light direction and make it 1.0 in length
21 ' vec3 lightDirection = normalize(u_LightPosition – vec3(vertexPosition));\n' +
22 // The dot product of the light direction and the normal
23 ' float nDotL = max(dot( lightDirection, normal), 0.0);\n' +
24 // Calculate the color due to diffuse reflection
25 ' vec3 diffuse = u_LightColor * a_Color.rgb * nDotL;\n' +
26 // Calculate the color due to ambient reflection
27 ' vec3 ambient = u_AmbientLight * a_Color.rgb;\n' +
28 // Add surface colors due to diffuse and ambient reflection
29 ' v_Color = vec4(diffuse + ambient, a_Color.a);\n' +
30 '}\n';
...
42 function main() {
...
70 // Get the storage locations of uniform variables and so on
71 var u_ModelMatrix = gl.getUniformLocation(gl.program, 'u_ModelMatrix');
...
74 var u_LightColor = gl.getUniformLocation(gl.program,'u_LightColor');
75 var u_LightPosition = gl.getUniformLocation(gl.program, 'u_LightPosition');
...
82 // Set the light color (white)
Using a Point Light Object
317
83 gl.uniform3f(u_LightColor, 1.0, 1.0, 1.0);
84 // Set the position of the light source (in the world coordinate)
85 gl.uniform3f(u_LightPosition, 0.0, 3.0, 4.0);
...
89 var modelMatrix = new Matrix4(); // Model matrix
90 var mvpMatrix = new Matrix4(); // Model view projection matrix
91 var normalMatrix = new Matrix4(); // Transformation matrix for normal
92
93 // Calculate the model matrix
94 modelMatrix.setRotate(90, 0, 1, 0); // Rotate around the y-axis
95 // Pass the model matrix to u_ModelMatrix
96 gl.uniformMatrix4fv(u_ModelMatrix, false, modelMatrix.elements);
...
The key differences in the processing within the vertex shader are at line 19 and 21. At
line 19, you transform the vertex coordinates into world coordinates in order to calculate
the light direction at the vertex coordinates. Because a point light emits light in all direc-
tions from its position, the light direction at a vertex is the result of subtracting the vertex
position from the light source position. Because the light position is passed to the variable
u_LightPosition using world coordinates at line 11, you also have to convert the vertex
coordinates into world coordinates to calculate the light direction. The light direction
is then calculated at line 21. Note that it is normalized with normalize() so that it will
be 1.0 in length. Using the resulting light direction ( lightDirection ), the dot product is
calculated at line 23 and then the surface color at each vertex is calculated based on this
light direction.
If you run this program, you will see a more realistic result, as shown in Figure 8.17 .
Although this result is more realistic, a closer look reveals an artifact: There are unnatural
lines of shade on the cube’s surface (see Figure 8.18 ). You can see this more easily if the
cube rotates as it does when you load PointLightedCube_animation .
CHAPTER 8 Lighting Objects
318
Figure 8.18 The unnatural appearance when processing the point light at each vertex
This comes about because of the interpolation process discussed in Chapter 5 , “Using
Colors and Texture Images.” As you will remember, the WebGL system interpolates the
colors between vertices based on the colors you supply at the vertices. However, because
the direction of light from a point light source varies by position to shade naturally, you
have to calculate the color at every position the light hits instead of just at each vertex.
You can see this problem more clearly using a sphere illuminated by a point light, as
shown in Figure 8.19 .
per-vertex
calculation
per-position
calculation
Figure 8.19 The spheres illuminated by a point light
As you can see, the border between the brighter parts and darker parts is unnatural in the
left figure. If the effect is hard to see on the page, the left figure is PointLightedSphere ,
and the right is PointLightedSphere_perFragment . We will describe how to draw them
correctly in the next section.
Using a Point Light Object
319
More Realistic Shading: Calculating the Color per Fragment
At first glance, it may seem daunting to have to calculate the color at every position on a
cube surface where the light hits. However, essentially it means calculating the color per
fragment, so the power of the fragment shader can now be used.
This sample program you will use is PointLightedCube_perFragment , and its result is
shown in Figure 8.20 .
Figure 8.20 PointLightedCube_perFragment
Sample Program (PointLightedCube_perFragment.js)
The sample program, which is based on PointLightedCube.js , is shown in Listing 8.5 .
Only the shader code has been modified and, as you can see, there is less processing in the
vertex shader and more processing in the fragment shader.
Listing 8.5 PointLightedCube_perFragment.js
1 // PointLightedCube_perFragment.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
...
8 'uniform mat4 u_ModelMatrix;\n' + // Model matrix
9 'uniform mat4 u_NormalMatrix;\n' + // Transformation matrix of normal
10 'varying vec4 v_Color;\n' +
11 'varying vec3 v_Normal;\n' +
12 'varying vec3 v_Position;\n' +
13 'void main() {\n' +
CHAPTER 8 Lighting Objects
320
14 ' gl_Position = u_MvpMatrix * a_Position;\n' +
15 // Calculate the vertex position in the world coordinate
16 ' v_Position = vec3(u_ModelMatrix * a_Position);\n' +
17 ' v_Normal = normalize(vec3(u_NormalMatrix * a_Normal));\n' +
18 ' v_Color = a_Color;\n' +
19 '}\n';
20
21 // Fragment shader program
22 var FSHADER_SOURCE =
...
26 'uniform vec3 u_LightColor;\n' + // Light color
27 'uniform vec3 u_LightPosition;\n' + // Position of the light source
28 'uniform vec3 u_AmbientLight;\n' + // Ambient light color
29 'varying vec3 v_Normal;\n' +
30 'varying vec3 v_Position;\n' +
31 'varying vec4 v_Color;\n' +
32 'void main() {\n' +
33 // Normalize normal because it's interpolated and not 1.0 (length)
34 ' vec3 normal = normalize(v_Normal);\n' +
35 // Calculate the light direction and make it 1.0 in length
36 ' vec3 lightDirection = normalize(u_LightPosition - v_Position);\n' +
37 // The dot product of the light direction and the normal
38 ' float nDotL = max(dot( lightDirection, normal), 0.0);\n' +
39 // Calculate the final color from diffuse and ambient reflection
40 ' vec3 diffuse = u_LightColor * v_Color.rgb * nDotL;\n' +
41 ' vec3 ambient = u_AmbientLight * v_Color.rgb;\n' +
42 ' gl_FragColor = vec4(diffuse + ambient, v_Color.a);\n' +
43 '}\n';
To calculate the color per fragment when light hits, you need (1) the position of the frag-
ment in the world coordinate system and (2) the normal direction at the fragment posi-
tion. You can utilize interpolation ( Chapter 5 ) to obtain these values per fragment by just
calculating them per vertex in the vertex shader and passing them via varying variables to
the fragment shader.
These calculations are performed at lines 16 and 17, respectively, in the vertex shader. At
line 16, the vertex position in world coordinates is calculated by multiplying each vertex
coordinate by the model matrix. After assigning the vertex position to the varying vari-
able v_Position , it will be interpolated between vertices and passed to the corresponding
variable ( v_Position ) in the fragment shader as the world coordinate of the fragment. The
normal calculation at line 17 is carried out for the same purpose.
10 By assigning the result
to v_Normal , it is also interpolated and passed to the corresponding variable ( v_Normal ) in
the fragment shader as the normal of the fragment.
Summary
321
Processing in the fragment shader is the same as that in the vertex shader of
PointLightedCube.js . First, at line 34, the interpolated normal passed from the vertex
shader is normalized. Its length may not be 1.0 anymore because of the interpolation.
Next, at line 36, the light direction is calculated and normalized. Using these results, the
dot product of the light direction and the normal is calculated at line 38. The colors due
to the diffuse reflection and ambient reflection are calculated at lines 40 and 41 and added
to get the fragment color, which is assigned to gl_FragColor at line 42.
If you have more than one light source, after calculating the color due to diffuse reflec-
tion and ambient reflection for each light source, you can obtain the final fragment color
by adding all the colors. In other words, you only have to calculate Equation 8.3 as many
times as the number of light sources.
Summary
This chapter explored how to light a 3D scene, the different types of light used, and how
light is reflected and diffused through the scene. Using this knowledge, you then imple-
mented the effects of different light sources to illuminate a 3D object and examined
various shading techniques to improve the realism of the objects. As you have seen, a
mastery of lighting is essential to adding realism to 3D scenes, which can appear flat and
uninteresting if they’re not correctly lit.
10 In this sample program, this normalization is not necessary because all normals are passed to
a_Normal with a length of 1.0. However, we normalize them here as good programming practice so
the code is more generic.
This page intentionally left blank
This chapter is the final one that describes the core features and how to program
with WebGL. Once you’ve read it, you will have mastered the basics of WebGL
and will have enough knowledge to create realistic and interactive 3D scenes.
This chapter focuses on hierarchical objects, which are important because
they allow you to progress beyond single objects like cubes or blocks to more
complex objects that you can use for game characters, robots, and even humans.
The following key points are discussed in this chapter:
• Modeling complex connected structures such as a robot arm using a hierar-
chical structure.
• Drawing and manipulating hierarchical objects made up of multiple
simpler objects.
• Combining model and rotation matrices to mimic joints such as elbow or
wrist joints
• Internally implementing initShaders() , which you’ve used but not exam-
ined so far.
By the end of this chapter, you will have all the knowledge you need to create
compelling 3D scenes populated by both simple and complex 3D objects.
Chapter 9
Hierarchical Objects
CHAPTER 9 Hierarchical Objects
324
Drawing and Manipulating Objects Composed of
Other Objects
Until now, we have described how to translate and rotate a single object, such as a two-
dimensional triangle or a three-dimensional cube. But many of the objects in 3D graphics,
game characters, robots, and so on, consist of more than one object (or segment). For a
simple example, a robot arm is shown in Figure 9.1 . As you can see, this consists of multi-
ple boxes. The program name is MultiJointModel . First, let’s load the program and experi-
ment by pressing the arrow, x, z, c, and v keys to understand what you will construct in
the following sections.
Figure 9.1 A robot arm consisting of multiple objects
One of the key issues when drawing an object consisting of multiple objects (segments)
is that you have to program to avoid conflicts when the segments move. This section will
explore this issue by describing how to draw and manipulate a robot arm that consists
of multiple segments. First, let’s consider the structure of the human body from the
shoulder to the fingertips to understand how to model our robot arm. An arm consists of
multiple segments, such as the upper arm, lower arm, palm, and fingers, each of which is
connected by a joint, as shown on the left of Figure 9.2 .
Drawing and Manipulating Objects Composed of Other Objects
325
m r a r e p p u
m l a p
t n i o j w o b l e
t n i o j r e d l u o h s
t n i o j r e d l u o h s e h t e t a t o r
t n i o j t s i r w
t n i o j t s i r w e h t e t a t o r
t n i o j r e d l u o h s
m r a r e w o l
m r a r e w o l
m r a r e p p u
m l a p
finger1
finger2 finger1
finger2
Figure 9.2 The structure and movement from the arm to the fingers
Each segment moves around a joint as follows:
• When you move the upper arm by rotating around the shoulder joint, depending
on the upper arm movement, the lower arm, palm, and fingers move (the middle of
Figure 9.2 ) accordingly.
• When you move the lower arm using an elbow joint, the palm and fingers move but
the upper arm does not.
• When you move the palm using the wrist joint, both palm and fingers move but the
upper and lower arm do not (the right of Figure 9.2 ).
• When you move fingers, the upper arm, lower arm, and palm do not move.
To summarize, when you move a segment, the segments located below it move, while the
segments located above are not affected. In addition, all movement, including twisting, is
actually rotation around a joint.
Hierarchical Structure
The typical method used to draw and manipulate the object with such features is to draw
each part object (such as a box) in the order of the object’s hierarchical structure from
upper to lower, applying each model matrix (rotation matrix) at every joint. For example,
in Figure 9.2 , shoulder, elbow, wrist, and finger joints all have respective rotation matrices.
It is important to note that, unlike humans or robots, segments in 3D graphics are not
physically joined. So if you inadvertently rotate the object corresponding to an upper arm
at the shoulder joint, the lower parts would be left behind. When you rotate the shoulder
joint, you should explicitly make the lower parts follow the movement. To do this, you
need to rotate the lower elbow and wrist joints through the same angle that you rotate the
shoulder joint.
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It is straightforward to program so that the rotation of one segment propagates to the
lower segments and simply requires that you use the same model matrix for the rotation
of the lower segments. For example, when you rotate a shoulder joint through 30 degrees
using one model matrix, you can draw the lower elbow and wrist joints rotated through
30 degrees using the same model matrix (see Figure 9.3 ). Thus, by changing only the angle
of the shoulder rotation, the lower segments are automatically rotated to follow the move-
ment of the shoulder joint.
30 degrees
30 degrees
rotate the shoulder joint
Figure 9.3 The lower segments following the rotation of the upper segment
For more complex cases, such as when you want to rotate the elbow joint 10 degrees after
rotating the shoulder joint 30 degrees, you can rotate the elbow joint by using the model
matrix and rotating 10 degrees more than the shoulder-joint model matrix. This can be
calculated by multiplying the shoulder-joint model matrix by a 10-degree rotation matrix,
which we refer to as the “elbow-joint model matrix.” The parts below the elbow will
follow the movement of the elbow when drawn using this elbow-joint model matrix.
By programming in such a way, the upper segments are not affected by rotation of the
lower segments. Thus, the upper segments will not move no matter how much the lower
segments move.
Now that you have a good understanding of the principles involved when moving multi-
segment objects, let’s look at a sample program.
Single Joint Model
Let’s begin with a simple single joint model. You will construct the program JointModel
that draws a robot arm consisting of two parts that can be manipulated with the arrow
keys. The screen shot and the hierarchy structure are shown on the left and right of Figure
9.4 , respectively. This robot arm consists of arm1 and arm2, which are joined by joint1.
You should imagine that the arm is raised above the shoulder and that arm1 is the upper
part and arm2 the lower part. When you add the hand later, it will become clearer.
Drawing and Manipulating Objects Composed of Other Objects
327
2 m r a
s i x a - y e h t d n u o r a g n i t a t o r
s i x a - z e h t d n u o r a g n i t a t o r
arm2
arm1
joint1
1 m r a
Figure 9.4 JointModel and the hierarchy structure used in the program
If you run the program, you will see that arm1 is rotated around the y-axis using the right
and left arrow keys, and joint1 is rotated around the z-axis with the up and down arrow
keys ( Figure 9.5 ). When pressing the down arrow key, joint1 is rotated and arm2 leans
forward, as shown on the left of Figure 9.5 . Then if you press the right arrow key, arm1 is
rotated, as shown on the right of Figure 9.5 .
arm2
Figure 9.5 The display change when pressing the arrow keys in JointModel
As you can see, the movement of arm2 by rotation of joint1 does not affect arm1. In
contrast, arm2 is rotated if you rotate arm1.
arm1
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Sample Program (JointModel.js)
JointModel.js is shown in Listing 9.1 . The actual vertex shader is a little complicated
because of the shading process and has been removed from the listing here to save space.
However, if you are interested in how the lessons learned in the earlier part of the chapter
are applied, please look at the full listing available by downloading the examples from the
book website. The lighting used is a directional light source and simplified diffuse reflec-
tion, which makes the robot arm look more three-dimensional. However, as you can see,
there are no special lighting calculations needed for this joint model, and all the code
required to draw and manipulate the joint model is in the JavaScript program.
Listing 9.1 JointModel.js
1 // JointModel.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'attribute vec4 a_Normal;\n' +
6 'uniform mat4 u_MvpMatrix;\n' +
...
9 'void main() {\n' +
10 ' gl_Position = u_MvpMatrix * a_Position;\n' +
11 // Shading calculation to make the arm look three-dimensional
...
17 '}\n';
...
29 function main() {
...
46 // Set the vertex coordinate.
47 var n = initVertexBuffers(gl);
...
57 // Get the storage locations of uniform variables
58 var u_MvpMatrix = gl.getUniformLocation(gl.program, 'u_MvpMatrix');
59 var u_NormalMatrix = gl.getUniformLocation(gl.program, 'u_NormalMatrix');
...
65 // Calculate the view projection matrix
66 var viewProjMatrix = new Matrix4();
67 viewProjMatrix.setPerspective(50.0, canvas.width / canvas.height, 1.0, 100.0);
68 viewProjMatrix.lookAt(20.0, 10.0, 30.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
69
70 // Register the event handler to be called when keys are pressed
71 document.onkeydown = function(ev){ keydown(ev, gl, n, viewProjMatrix,
➥u_MvpMatrix, u_NormalMatrix); };
72 // Draw robot arm
73 draw(gl, n, viewProjMatrix, u_MvpMatrix, u_NormalMatrix);
74 }
Drawing and Manipulating Objects Composed of Other Objects
329
75
76 var ANGLE_STEP = 3.0; // The increments of rotation angle (degrees)
77 var g_arm1Angle = 90.0; // The rotation angle of arm1 (degrees)
78 var g_joint1Angle = 0.0; // The rotation angle of joint1 (degrees)
79
80 function keydown(ev, gl, n, viewProjMatrix, u_MvpMatrix, u_NormalMatrix) {
81 switch (ev.keyCode) {
82 case 38: // Up arrow key -> positive rotation of joint1 (z-axis)
83 if (g_joint1Angle < 135.0) g_joint1Angle += ANGLE_STEP;
84 break;
85 case 40: // Down arrow key -> negative rotation of joint1 (z-axis)
86 if (g_joint1Angle > -135.0) g_joint1Angle -= ANGLE_STEP;
87 break;
...
91 case 37: // Left arrow key -> negative rotation of arm1 (y-axis)
92 g_arm1Angle = (g_arm1Angle - ANGLE_STEP) % 360;
93 break;
94 default: return;
95 }
96 // Draw the robot arm
97 draw(gl, n, viewProjMatrix, u_MvpMatrix, u_NormalMatrix);
98 }
99
100 function initVertexBuffers(gl) {
101 // Vertex coordinates
...
148 }
...
174 // Coordinate transformation matrix
175 var g_modelMatrix = new Matrix4(), g_mvpMatrix = new Matrix4();
176
177 function draw(gl, n, viewProjMatrix, u_MvpMatrix, u_NormalMatrix) {
...
181 // Arm1
182 var arm1Length = 10.0; // Length of arm1
183 g_modelMatrix.setTranslate(0.0, -12.0, 0.0);
184 g_modelMatrix.rotate(g_arm1Angle, 0.0, 1.0, 0.0); // Rotate y-axis
185 drawBox(gl, n, viewProjMatrix, u_MvpMatrix, u_NormalMatrix); // Draw
186
187 // Arm2
188 g_modelMatrix.translate(0.0, arm1Length, 0.0); // Move to joint1
189 g_modelMatrix.rotate(g_joint1Angle, 0.0, 0.0, 1.0);// Rotate z-axis
190 g_modelMatrix.scale(1.3, 1.0, 1.3); // Make it a little thicker
191 drawBox(gl, n, viewProjMatrix, u_MvpMatrix, u_NormalMatrix); // Draw
192 }
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193
194 var g_normalMatrix = new Matrix4(); // Transformation matrix for normal
195
196 // Draw a cube
197 function drawBox(gl, n, viewProjMatrix, u_MvpMatrix, u_NormalMatrix) {
198 //Calculate the model view project matrix and pass it to u_MvpMatrix
199 g_mvpMatrix.set(viewProjMatrix);
200 g_mvpMatrix.multiply(g_modelMatrix);
201 gl.uniformMatrix4fv(u_MvpMatrix, false, g_mvpMatrix.elements);
202 // Calculate the normal transformation matrix and pass it to u_NormalMatrix
203 g_normalMatrix.setInverseOf(g_modelMatrix);
204 g_normalMatrix.transpose();
205 gl.uniformMatrix4fv(u_NormalMatrix, false, g_normalMatrix.elements);
206 // Draw
207 gl.drawElements(gl.TRIANGLES, n, gl.UNSIGNED_BYTE, 0);
208 }
The function main() from line 29 follows the same structure as before, with the first major
difference being the initVertexBuffers() function call at line 47. In initVertexBuf-
fers() , the vertex data for arm1 and arm2 are written into the appropriate buffer objects.
Until now, you’ve been using cubes, with each side being 2.0 in length and the origin
at the center of the cube. Now, to better model the arm, you will use a cuboid like that
shown in the left side of Figure 9.6 . The cuboid has its origin at the center of the bottom
surface and is 3.0 by 3.0 and 10.0 units in height. By setting the origin at the center of the
bottom surface, its rotation around the z-axis is the same as that of joint1 in Figure 9.5 ,
making it convenient to program. Both arm1 and arm2 are drawn using this cuboid.
x
y
z
0 . 0 1
x
y
z
0 . 3
0 . 3
v6(-1, 1, -1)
v1(-1, 1, 1)
v7(-1, -1, -1)
v2(-1, -1, 1)
v3(1, -1, 1)
v4(1, -1, -1)
v0(1, 1, 1)
v5(1, 1, -1)
the previous cube
Figure 9.6 A cuboid for drawing the robot arm
Drawing and Manipulating Objects Composed of Other Objects
331
From lines 66 to 68, a view projection matrix ( viewProjMatrix ) is calculated with the
specified viewing volume, the eye position, and the view direction.
Because the robot arm in this program is moved by using the arrow keys, the event
handler keydown() is registered at line 71:
70 // Register the event handler to be called when keys are pressed
71 document.onkeydown = function(ev){ keydown(ev, gl, n, viewProjMatrix,
➥u_MvpMatrix, u_NormalMatrix); };
72 // Draw the robot arm
73 draw(gl, n, viewProjMatrix, u_MvpMatrix, u_NormalMatrix);
The keydown() function itself is defined at line 80. Before that, at lines 76, 77, and 78, the
definition of global variables used in keydown() is defined:
76 var ANGLE_STEP = 3.0; // The increments of rotation angle (degrees)
77 var g_arm1Angle = -90.0; // The rotation angle of arm1 (degrees)
78 var g_joint1Angle = 0.0; // The rotation angle of joint1 (degrees)
79
80 function keydown(ev, gl, n, u_MvpMatrix, u_NormalMatrix) {
81 switch (ev.keyCode) {
82 case 38: // Up arrow key -> the positive rotation of joint1 (z-axis)
83 if (g_joint1Angle < 135.0) g_joint1Angle += ANGLE_STEP;
84 break;
...
88 case 39: // Right arrow key -> the positive rotation of arm1 (y-axis)
89 g_arm1Angle = (g_arm1Angle + ANGLE_STEP) % 360;
90 break;
...
95 }
96 // Draw the robot arm
97 draw(gl, n, u_MvpMatrix, u_NormalMatrix);
98 }
ANGLE_STEP at line 76 is used to control how many degrees arm1 and joint1 are rotated
each time the arrow keys are pressed and is set at 3.0 degrees. g_arm1Angle (line 77) and
g_joint1Angle (line 78) are variables that store the current rotation angle of arm1 and
joint1, respectively (see Figure 9.7 ).
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g_joint1Angle
g_arm1Angle
Figure 9.7 g_joint1Angle and g_arm1Angle
The keydown() function , from line 80, increases or decreases the value of the rotation
angle of arm1 ( g_arm1Angle ) or joint1 ( g_joint1Angle ) by ANGLE_STEP , according to which
key is pressed. joint1 can only be rotated through the range from –135 degrees to 135
degrees so that arm2 does not interfere with arm1. Then the whole robot arm is drawn at
line 97 using the function draw() .
Draw the Hierarchical Structure (draw())
The draw() function draws the robotic arm according to its hierarchical structure and is
defined at line 177. Two global variables, g_modelMatrix and g_mvpMatrix , are created at
line 175 and will be used in both draw() and drawBox() :
174 // Coordinate transformation matrix
175 var g_modelMatrix = new Matrix4(), g_mvpMatrix = new Matrix4();
176
177 function draw(gl, n, viewProjMatrix, u_MvpMatrix, u_NormalMatrix) {
...
181 // Arm1
182 var arm1Length = 10.0; // Length of arm1
183 g_modelMatrix.setTranslate(0.0, -12.0, 0.0);
184 g_modelMatrix.rotate(g_arm1Angle, 0.0, 1.0, 0.0); // Rotate y-axis
185 drawBox(gl, n, viewProjMatrix, u_MvpMatrix, u_NormalMatrix); // Draw
186
187 // Arm2
188 g_modelMatrix.translate(0.0, arm1Length, 0.0); // Move to joint1
189 g_modelMatrix.rotate(g_joint1Angle, 0.0, 0.0, 1.0); // Rotate z-axis
190 g_modelMatrix.scale(1.3, 1.0, 1.3); // Make it a little thicker
191 drawBox(gl, n, viewProjMatrix, u_MvpMatrix, u_NormalMatrix); // Draw
192 }
Drawing and Manipulating Objects Composed of Other Objects
333
As you can see, draw() draws the segments by using drawBox() , starting with the upper
part (arm1) followed by the lower part (arm2).
When drawing each part, the same process is repeated: (1) translation ( setTranslate() ,
translate() ), (2) rotation ( rotate() ), and (3) drawing the part ( drawBox() ).
When drawing a hierarchical model performing a rotation, typically you will process from
upper to lower in the order of (1) translation, (2) rotation, and (3) drawing segments.
arm1 is translated to (0.0, –12.0, 0.0) with setTranslate() at line 183 to move to an easily
visible position. Because this arm is rotated around the y-axis, its model matrix ( g_model-
Matrix ) is multiplied by the rotation matrix around the y-axis at line 184. g_arm1Angle
is used here. Once arm1’s coordinate transformation has been completed, you then draw
using the drawBox() function.
Because arm2 is connected to the tip of arm1, as shown in Figure 9.7 , it has to be drawn
from the tip of arm1. This can be achieved by translating it along the y-axis in the posi-
tive direction by the length of arm1 ( arm1Length ) and applying the translation to the
model matrix, which is used when drawing arm1 ( g_modelMatrix ).
This is done as shown in line 188, where the second argument of translate() is
arm1Length . Also notice that the method uses translate() rather than setTranslate()
because arm2 is drawn at the tip of arm1:
187 // Arm2
188 g_modelMatrix.translate(0.0, arm1Length, 0.0); // Move to joint1
189 g_modelMatrix.rotate(g_joint1Angle, 0.0, 0.0, 1.0); // Rotate z-axis
190 g_modelMatrix.scale(1.3, 1.0, 1.3); // Make it a little thicker
191 drawBox(gl, n, viewProjMatrix, u_MvpMatrix, u_NormalMatrix); // Draw
Line 189 handles the rotation of arm2 which, as can be seen, uses g_joint1Angle . You
make arm2 a little thicker at line 190 by scaling it along the x and z direction. This makes
it easier to distinguish between the two arm segments but is not essential to the robotic
arm’s movement.
Now, by updating g_arm1Angle and g_joint1Angle in keydown() as described in the previ-
ous section and then invoking draw() , arm1 is rotated by g_arm1Angle and arm2 is, in
addition, rotated by g_joint1Angle .
The drawBox() function is quite simple. It calculates a model view project matrix and
passes it to the u_MvpMatrix variable at lines 199 and 200. Then it just calculates the
normal transformation matrix for shading from the model matrix, sets it to
u_NormalMatrix at lines 203 and 204, and draws the cuboid in Figure 9.6 at line 207.
This basic approach, although used here for only a single joint, can be used for any
complex hierarchical models simply by repeating the process steps used earlier.
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Obviously, our simple robot arm, although modeled on a human arm, is more like a
skeleton than a real arm. A more realistic model of a real arm would require the skin to
be modeled, which is beyond the scope of this book. Please refer to the OpenGL ES 2.0
Programming Guide for more information about skinning.
A Multijoint Model
Here, you will extend JointModel to create MultiJointModel , which draws a multijoint
robot arm consisting of two arm segments, a palm, and two fingers, all of which you can
manipulate using the keyboard. As shown in Figure 9.8 , we call the arm extending from
the base arm1, the next segment arm2, and the joint between the two arms joint1. There
is a palm at the tip of arm2. The joint between arm2 and the palm is called joint2. The
two fingers attached at the end of the palm are respectively finger1 and finger2.
s i x a - y e h t d n u o r a e t a t o r
s i x a - x e h t d n u o r a e t a t o r
s i x a - z e h t d n u o r a e t a t o r
s i x a - y e h t d n u o r a e t a t o r
1 m r a
2 m r a
m l a p
e s a b
arm1
base
palm
finger1 finger2
arm2
joint1
joint2
finger1 finger2
Figure 9.8 The hierarchical structure of MultiJointModel
Manipulation of arm1 and joint1 using the arrow keys is the same as JointModel . In addi-
tion, you can rotate joint2 (wrist) with the X and Z keys and move (rotate) the two fingers
with the C and V keys. The variables controlling the rotation angle of each part are shown
in Figure 9.9 .
Drawing and Manipulating Objects Composed of Other Objects
335
g_joint3Angle
g_joint2Angle
g_joint1Angle
g_arm1Angle
Figure 9.9 The variables controlling the rotation of segments
Sample Program (MultiJointModel.js)
This program is similar to JointModel , except for extensions to keydown() to handle the
additional control keys, and draw() , which draws the extended hierarchical structure. First
let’s look at keydown() in Listing 9.2 .
Listing 9.2 MultiJointModel.js (Code for Key Processing)
1 // MultiJointModel.js
...
76 var ANGLE_STEP = 3.0; // The increments of rotation angle (degrees)
77 var g_arm1Angle = 90.0; // The rotation angle of arm1 (degrees)
78 var g_joint1Angle = 45.0; // The rotation angle of joint1 (degrees)
79 var g_joint2Angle = 0.0; // The rotation angle of joint2 (degrees)
80 var g_joint3Angle = 0.0; // The rotation angle of joint3 (degrees)
81
82 function keydown(ev, gl, n, viewProjMatrix, u_MvpMatrix, u_NormalMatrix) {
83 switch (ev.keyCode) {
84 case 40: // Up arrow key -> positive rotation of joint1 (z-axis)
...
95 break;
96 case 90: // Z key -> the positive rotation of joint2
97 g_joint2Angle = (g_joint2Angle + ANGLE_STEP) % 360;
98 break;
99 case 88: // X key -> the negative rotation of joint2
100 g_joint2Angle = (g_joint2Angle - ANGLE_STEP) % 360;
101 break;
CHAPTER 9 Hierarchical Objects
336
102 case 86: // V key -> the positive rotation of joint3
103 if (g_joint3Angle < 60.0) g_joint3Angle = (g_joint3Angle +
➥ANGLE_STEP) % 360;
104 break;
105 case 67: // C key -> the negative rotation of joint3
106 if (g_joint3Angle > -60.0) g_joint3Angle = (g_joint3Angle –
➥ANGLE_STEP) % 360;
107 break;
108 default: return;
109 }
110 // Draw the robot arm
111 draw(gl, n, viewProjMatrix, u_MvpMatrix, u_NormalMatrix);
112 }
keydown() is basically the same as that of JointAngle , but in addition to changing
g_arm1Angle and g_joint1Angle based on key presses, it processes the Z, X, V, and C
keys at lines 96, 99, 102, and 105. These key presses change g_joint2Angle , which is the
rotation angle of joint2, and g_joint3Angle , which is the rotation angle of joint3, respec-
tively. After changing them, it calls draw() at line 111 to draw the hierarchy structure.
Let’s take a look at draw() in Listing 9.3 .
Although you are using the same cuboid for the base, arm1, arm2, palm, finger1, and
finger2, the segments are different in width, height, and depth. To make it easy to draw
these segments, let’s extend drawBox() with three more arguments than that used in the
single-joint model:
function drawBox(gl, n, width, height, depth , viewProjMatrix, u_MvpMatrix ,
➥u_NormalMatrix)
By specifying the width, height, and depth using the third to fifth argument, this function
draws a cuboid of the specified size with its origin at the center of the bottom surface.
Listing 9.3 MultiJointModel.js (Code for Drawing the Hierarchy Structure)
188 // Coordinate transformation matrix
189 var g_modelMatrix = new Matrix4(), g_mvpMatrix = new Matrix4();
190
191 function draw(gl, n, viewProjMatrix, u_MvpMatrix, u_NormalMatrix) {
192 // Clear color buffer and depth buffer
193 gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
194
195 // Draw a base
196 var baseHeight = 2.0;
197 g_modelMatrix.setTranslate(0.0, -12.0, 0.0);
198 drawBox(gl, n, 10.0, baseHeight, 10.0, viewProjMatrix, u_MvpMatrix,
➥u_NormalMatrix);
Drawing and Manipulating Objects Composed of Other Objects
337
199
200 // Arm1
201 var arm1Length = 10.0;
202 g_modelMatrix.translate(0.0, baseHeight, 0.0); // Move onto the base
203 g_modelMatrix.rotate(g_arm1Angle, 0.0, 1.0, 0.0); // Rotation
204 drawBox(gl, n, 3.0, arm1Length, 3.0, viewProjMatrix, u_MvpMatrix,
➥u_NormalMatrix); // Draw
295
206 // Arm2
...
212 // A palm
213 var palmLength = 2.0;
...
218 // Move to the center of the tip of the palm
219 g_modelMatrix.translate(0.0, palmLength, 0.0);
220
221 // Draw finger1
222 pushMatrix(g_modelMatrix);
223 g_modelMatrix.translate(0.0, 0.0, 2.0);
224 g_modelMatrix.rotate(g_joint3Angle, 1.0, 0.0, 0.0); // Rotation
225 drawBox(gl, n, 1.0, 2.0, 1.0, viewProjMatrix, u_MvpMatrix, u_NormalMatrix);
226 g_modelMatrix = popMatrix();
227
228 // Draw finger2
229 g_modelMatrix.translate(0.0, 0.0, -2.0);
230 g_modelMatrix.rotate(-g_joint3Angle, 1.0, 0.0, 0.0); // Rotation
231 drawBox(gl, n, 1.0, 2.0, 1.0, viewProjMatrix, u_MvpMatrix, u_NormalMatrix);
232 }
233
234 var g_matrixStack = []; // Array for storing a matrix
235 function pushMatrix(m) { // Store the specified matrix to the array
236 var m2 = new Matrix4(m);
237 g_matrixStack.push(m2);
238 }
239
240 function popMatrix() { // Retrieve the matrix from the array
241 return g_matrixStack.pop();
242 }
The draw() function operates in the same way as in JointModel ; that is, each part
is handled following the order of (1) translation, (2) rotation, and (3) draw (using
drawBox() ). First, because the base is not rotated, after moving to the appropriate posi-
tion at line 197, it draws a base there with drawBox() . The third to fifth arguments of
drawBox() specify a width of 10, height of 2, and depth of 10, which cause a flat stand to
be drawn.
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338
The arm1, arm2, and palm are each drawn following the same order of (1) translation,
(2) rotation, and (3) draw and by moving down the object hierarchy toward the lower
level in the same manner as JointModel .
The main difference in this sample program is the drawing of finger1 and finger2 from
line 222. Because they do not have a parent-child relationship, a little more care is needed.
In particular, you have to pay attention to the contents of the model matrix. First, let’s
look at finger1, whose position is translated 2.0 along the z-axis direction from the center
of the tip of the palm and rotated around the x-axis. finger1 can be drawn in the order of
(1) translating, (2) rotating, and (3) drawing segments as before. The program is as follows:
g_modelMatrix.translate(0.0, 0.0, 2.0);
g_modelMatrix.rotate(g_joint3Angle, 1.0, 0.0, 0.0); // Rotation
drawBox(gl, n, 1.0, 2.0, 1.0, u_MvpMatrix, u_NormalMatrix);
Next, looking at finger2, if you follow the same procedure a problem occurs. finger2’s
intended position is a translation of –2.0 units along the z-axis direction from the center
of the tip of the palm and rotated around the x-axis. However, because the model matrix
has changed, if you draw finger2, it will be drawn at the tip of finger1.
Clearly, the solution is to restore the model matrix to its state before finger1 was drawn. A
simple way to achieve this is to store the model matrix before drawing finger1 and retriev-
ing it after drawing finger1. This is actually done at lines 222 and 226 and uses the func-
tions pushMatrix() and popMatrix() to store the specified matrix and retrieve it. At line
222, you store the model matrix specified as pushMatrix() ’s argument ( g_modelMatrix ).
Then, after drawing finger1 at lines 223 to 225, you retrieve the old model matrix at line
226, with popMatrix() , and assign it to g_modelMatrix . Now, because the model matrix
has reverted back, you can draw finger2 in the same way as before.
pushMatrix() and popMatrix() are shown next. pushMatrix() stores the matrix specified
as its argument in an array named g_matrixStack at line 234. popMatrix() retrieves the
matrix stored in g_matrixStack and returns it:
234 var g_matrixStack = []; // Array for storing matrices
235 function pushMatrix(m) { // Store the specified matrix
236 var m2 = new Matrix4(m);
237 g_matrixStack.push(m2);
238 }
239
240 function popMatrix() { // Retrieve a matrix from the array
241 return g_matrixStack.pop();
242 }
This approach can be used to draw an arbitrarily long robot arm. It will scale when new
segments are added to the hierarchy. You only need to use pushMatrix() and popMatrix()
when the hierarchy structure is a sibling relation, not a parent-child relation.
Drawing and Manipulating Objects Composed of Other Objects
339
Draw Segments (drawBox())
Finally, let’s take a look at drawBox() , which draws the segments of the robot arm using
the following arguments:
247 function drawBox(gl, n, width, height, depth, viewMatrix, u_MvpMatrix,
➥u_NormalMatrix) {
The third to fifth arguments, width , height, and depth , specify the width, height, and depth
of the cuboid being drawn. As for the remaining argument, viewMatrix is a view matrix,
and u_MvpMatrix and u_NormalMatrix are the arguments for setting the coordinate trans-
formation matrices to the corresponding uniform variables in the vertex shader, just like
JointModel.js . The model view projection matrix is passed to u_MvpMatrix , and the
matrix for transforming the coordinates of the normal, described in the previous section,
is passed to u_NormalMatrix .
The three-dimensional object used here, unlike JointModel, is a cube whose side is 1.0
unit long. Its origin is located at the center of the bottom surface so that you can easily
rotate the arms, the palm, and the fingers. The function drawBox() is shown here:
244 var g_normalMatrix = new Matrix4();// Transformation matrix for normal
245
246 // Draw a cuboid
247 function drawBox(gl, n, width, height, depth, viewProjMatrix,
➥u_MvpMatrix, u_NormalMatrix) {
248 pushMatrix(g_modelMatrix); // Save the model matrix
249 // Scale a cube and draw
250 g_modelMatrix.scale(width, height, depth);
251 // Calculate model view project matrix and pass it to u_MvpMatrix
252 g_mvpMatrix.set(viewProjMatrix);
253 g_mvpMatrix.multiply(g_modelMatrix);
254 gl.uniformMatrix4fv(u_MvpMatrix, false, g_mvpMatrix.elements);
255 // Calculate transformation matrix for normals and pass it to u_NormalMatrix
...
259 // Draw
260 gl.drawElements(gl.TRIANGLES, n, gl.UNSIGNED_BYTE, 0);
261 g_modelMatrix = popMatrix(); // Retrieve the model matrix
262 }
As you can see, the model matrix is multiplied by a scaling matrix at line 250 so that
the cube will be drawn with the size specified by width , height, and depth . Note that you
store the model matrix at line 248 and retrieve it at line 261 using pushMatrix() and
popMatrix() . Otherwise, when you draw arm2 after arm1, the scaling used for arm1 is left
in the model matrix and affects the drawing of arm2. By retrieving the model matrix at
line 261, which is saved at line 248, the model matrix reverts to the state before scaling
was applied at line 250.
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As you can see, the use of pushMatrix() and popMatrix() adds an extra degree of
complexity but allows you to specify only one set of vertex coordinates and use scaling
to create different cuboids. The alternative approach, using multiple objects specified by
different sets of vertices, is also possible. Let’s take a look at how you would program that.
Draw Segments (drawSegment())
In this section, we will explain how to draw segments by switching between buffer objects
in which the vertex coordinates representing the shape of each segment are stored.
Normally, you would need to specify the vertex coordinates, the normal, and the indices
for each segment. However, in this example, because all segments are cuboids, you can
share the normals and indices and simply specify the vertices for each segment. For each
segment (the base, arm1, arm2, palm, and fingers), the vertices are stored in their respec-
tive object buffers, which are then switched when drawing the arm parts. Listing 9.4
shows the sample program.
Listing 9.4 MultiJointModel_segment.js
1 // MultiJointModel_segment.js
...
29 function main() {
...
47 var n = initVertexBuffers(gl);
...
57 // Get the storage locations of attribute and uniform variables
58 var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
...
74 draw(gl, n, viewProjMatrix, a_Position, u_MvpMatrix, u_NormalMatrix);
75 }
...
115 var g_baseBuffer = null; // Buffer object for a base
116 var g_arm1Buffer = null; // Buffer object for arm1
117 var g_arm2Buffer = null; // Buffer object for arm2
118 var g_palmBuffer = null; // Buffer object for a palm
119 var g_fingerBuffer = null; // Buffer object for fingers
120
121 function initVertexBuffers(gl){
122 // Vertex coordinate (Coordinates of cuboids for all segments)
123 var vertices_base = new Float32Array([ // Base(10x2x10)
124 5.0, 2.0, 5.0, -5.0, 2.0, 5.0, -5.0, 0.0, 5.0, 5.0, 0.0, 5.0,
125 5.0, 2.0, 5.0, 5.0, 0.0, 5.0, 5.0, 0.0,-5.0, 5.0, 2.0,-5.0,
...
129 5.0, 0.0,-5.0, -5.0, 0.0,-5.0, -5.0, 2.0,-5.0, 5.0, 2.0,-5.0
130 ]);
131
Drawing and Manipulating Objects Composed of Other Objects
341
132 var vertices_arm1 = new Float32Array([ // Arm1(3x10x3)
133 1.5, 10.0, 1.5, -1.5, 10.0, 1.5, -1.5, 0.0, 1.5, 1.5, 0.0, 1.5,
134 1.5, 10.0, 1.5, 1.5, 0.0, 1.5, 1.5, 0.0,-1.5, 1.5, 10.0,-1.5,
...
138 1.5, 0.0,-1.5, -1.5, 0.0,-1.5, -1.5, 10.0,-1.5, 1.5, 10.0,-1.5
139 ]);
...
159 var vertices_finger = new Float32Array([ // Fingers(1x2x1)
...
166 ]);
167
168 // normals
169 var normals = new Float32Array([
...
176 ]);
177
178 // Indices of vertices
179 var indices = new Uint8Array([
180 0, 1, 2, 0, 2, 3, // front
181 4, 5, 6, 4, 6, 7, // right
...
185 20,21,22, 20,22,23 // back
186 ]);
187
188 // Write coords to buffers, but don't assign to attribute variables
189 g_baseBuffer = initArrayBufferForLaterUse(gl, vertices_base, 3, gl.FLOAT);
190 g_arm1Buffer = initArrayBufferForLaterUse(gl, vertices_arm1, 3, gl.FLOAT);
...
193 g_fingerBuffer = initArrayBufferForLaterUse(gl, vertices_finger, 3, gl.FLOAT);
...
196 // Write normals to a buffer, assign it to a_Normal, and enable it
197 if (!initArrayBuffer(gl, 'a_Normal', normals, 3, gl.FLOAT)) return null;
198
199 // Write indices to a buffer
200 var indexBuffer = gl.createBuffer();
...
205 gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, indexBuffer);
206 gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, indices, gl.STATIC_DRAW);
207
208 return indices.length;
209 }
...
255 function draw(gl, n, viewProjMatrix, a_Position, u_MvpMatrix, u_NormalMatrix) {
...
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259 // Draw a base
260 var baseHeight = 2.0;
261 g_modelMatrix.setTranslate(0.0, -12.0, 0.0);
262 drawSegment(gl, n, g_baseBuffer, viewProjMatrix, a_Position,
➥u_MvpMatrix, u_NormalMatrix);
263
264 // Arm1
265 var arm1Length = 10.0;
266 g_modelMatrix.translate(0.0, baseHeight, 0.0); // Move to the tip of the base
267 g_modelMatrix.rotate(g_arm1Angle, 0.0, 1.0, 0.0); // Rotate y-axis
268 drawSegment(gl, n, g_arm1Buffer, viewProjMatrix, a_Position,
➥u_MvpMatrix, u_NormalMatrix);
269
270 // Arm2
...
292 // Finger2
...
295 drawSegment(gl, n, g_fingerBuffer, viewProjMatrix, a_Position,
➥u_MvpMatrix, u_NormalMatrix);
296 }
...
310 // Draw segments
311 function drawSegment(gl, n, buffer, viewProjMatrix, a_Position,
➥u_MvpMatrix, u_NormalMatrix) {
312 gl.bindBuffer(gl.ARRAY_BUFFER, buffer);
313 // Assign the buffer object to the attribute variable
314 gl.vertexAttribPointer(a_Position, buffer.num, buffer.type, false, 0, 0);
315 // Enable the assignment
316 gl.enableVertexAttribArray(a_Position);
317
318 // Calculate the model view project matrix and set it to u_MvpMatrix
...
322 // Calculate matrix for normal and pass it to u_NormalMatrix
...
327 gl.drawElements(gl.TRIANGLES, n, gl.UNSIGNED_BYTE, 0);
328 }
The key points in this program are (1) creating the separate buffer objects that contain the
vertex coordinates for each segment, (2) before drawing each segment, assigning the corre-
sponding buffer object to the attribute variable a_Position , and (3) enabling the buffer
and then drawing the segment.
The main() function from line 29 in the JavaScript code follows the same steps as before.
Switching between buffers for the different segments is added to initVertexBuffers() ,
Drawing and Manipulating Objects Composed of Other Objects
343
called at line 47. The stored location of a_Position is retrieved at line 58, and then draw()
is called at line 73.
Let’s examine initVertex() , defined at line 121. Lines 115 to 119 declare the buffer
objects as global variables, used to store the vertex coordinates of each segment. Within
the function, one of the main differences from MultiJointModel.js is the definition of
the vertex coordinates from line 123. Because you are not using a single cuboid trans-
formed differently for the different segments, you need to define the vertex coordinates
for all the parts separately (for example, the base ( vertices_base ) at line 123, coordinates
for arm1 ( vertices_arm1 ), at line 132. The actual creation of the buffer objects for each
part occurs in the function initArrayBufferForLaterUse() from line 189 to 193. This
function is shown here:
211 function initArrayBufferForLaterUse(gl, data, num, type){
212 var buffer = gl.createBuffer(); // Create a buffer object
...
217 // Write data to the buffer object
218 gl.bindBuffer(gl.ARRAY_BUFFER, buffer);
219 gl.bufferData(gl.ARRAY_BUFFER, data, gl.STATIC_DRAW);
220
221 // Store information to assign it to attribute variable later
222 buffer.num = num;
223 buffer.type = type;
224
225 return buffer;
226 }
initArrayBufferForLaterUse() simply creates a buffer object at line 212 and writes data
to it at lines 218 and 219. Notice that assigning it to an attribute variable ( gl.vertex
AttribPointer() ) and enabling the assignment ( gl.enableVertexAttribAray() ) are not
done within the function but later, just before drawing. To assign the buffer object to the
attribute variable a_Position later, the data needed is stored as properties of the buffer
object at lines 222 and 223.
Here you take advantage of an interesting feature of JavaScript that allows you to freely
add new properties of an object and assign data to them. You can do this by simply
appending the .property-name to the object name and assigning a value. Using this
feature, you store the number of items in the num property (line 222), and the type in the
type property (line 223). Of course, you can access the contents of the newly made prop-
erties using the same name. Note, you must be careful when referring to properties created
in this way, because JavaScript gives no error indications even if you misspell only one
character in the property name. Equally, be aware that, although convenient, appending
properties has a performance overhead. A better approach, user-defined types, is explained
in Chapter 10 , “Advanced Techniques,” but let’s stick with this approach for now.
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Finally, the draw() function, invoked at line 255, is the same as used in MultiJointModel
in terms of drawing parts according to the hierarchical structure, but it’s different in
terms of using drawSegment() to draw each segment. In particular, the third argument of
drawSegment() , shown next, is the buffer object in which the vertex coordinates of the
parts are stored.
262 drawSegment(gl, n, g_baseBuffer, viewProjMatrix, u_MvpMatrix, u_NormalMatrix);
This function is defined at line 311 and operates as follows. It assigns a buffer object to
the attribute variable a_Position and enables it at lines 312 to 316 before drawing at line
327. Here, num and type , which are just stored as buffer object properties, are used.
310 // Draw segments
311 function drawSegment(gl, n, buffer, viewProjMatrix, a_Position,
➥u_MvpMatrix, u_NormalMatrix) {
312 gl.bindBuffer(gl.ARRAY_BUFFER, buffer);
313 // Assign the buffer object to the attribute variable
314 gl.vertexAttribPointer(a_Position, buffer.num, buffer.type, false, 0, 0);
315 // Enable the assignment
316 gl.enableVertexAttribArray(a_Position);
317
318 // Calculate model view project matrix and set it to u_MvpMatrix
...
322 // Calculate transformation matrix for normal and set it to u_NormalMatrix
...
327 gl.drawElements(gl.TRIANGLES, n, gl.UNSIGNED_BYTE, 0);
328 }
This time you don’t need to scale objects with the model matrix because you have
prepared the vertex coordinates per part, so there is no need to store and retrieve the
matrix. Therefore, pushMatrix() and popMatrix() are not necessary.
Shader and Program Objects: The Role of
initShaders()
Finally, before we wrap up this chapter, let’s examine one of the convenience functions
defined for this book: initShaders() . This function has been used in all the sample
programs and has hidden quite a lot of complex detail about setting up and using shaders.
We have deliberately left this explanation to the end of this chapter to ensure you have a
good understanding of the basics of WebGL before tackling some of these complex details.
We should note that it’s not actually necessary to master these details. For some readers
it will be sufficient to simply reuse the initShaders() function we supply and skip this
section. However, for those who are interested, let’s take a look.
Shader and Program Objects: The Role of initShaders()
345
initShaders() carries out the routine work to make shaders available in WebGL. It
consists of seven steps:
1. Create shader objects ( gl.createShader() ).
2. Store the shader programs (to avoid confusion, we refer to them as “source code”) in
the shader objects ( g.shaderSource() ).
3. Compile the shader objects ( gl.compileShader() ).
4. Create a program object ( gl.createProgram() ).
5. Attach the shader objects to the program object ( gl.attachShader() ).
6. Link the program object ( gl.linkProgram() ).
7. Tell the WebGL system the program object to be used ( gl.useProgram() ).
Each step is simple but when combined can appear complex, so let’s take a look at them
one by one. First, as you know from earlier, two types of objects are necessary to use
shaders: shader objects and program objects.
Shader object A shader object manages a vertex shader or a fragment shader. One
shader object is created per shader.
Program object A program object is a container that manages the shader objects. A vertex
shader object and a fragment shader object (two shader objects in total)
must be attached to a program object in WebGL.
The relationship between a program object and shader objects is shown in Figure 9.10 .
A program object
shader object
(vertex shader)
shader object
(fragment shader)
Figure 9.10 The relationship between a program object and shader objects
Using this information, let’s discuss the preceding seven steps sequentially.
Create Shader Objects (gl.createShader())
All shader objects have to be created with a call to gl.createShader() before using them.
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gl.createShader(type)
Create a shader of the specified type .
Parameters type Specifies the type of shader object to be created: either
gl.VERTEX_SHADER (a vertex shader) or gl.FRAGMENT_
SHADER (a fragment shader).
Return value Non-null The created shader object.
null The creation of the shader object failed.
Errors INVALID_ENUM The specified type is none of the above.
gl.createShader() creates a vertex shader or a fragment shader according to the specified
type . If you do not need the shader any more, you can delete it with gl.deleteShader() .
gl.deleteShader(shader)
Delete the shader object.
Parameters shader Specifies the shader object to be deleted.
Return value None
Errors None
Note that the specified shader object is not deleted immediately if it is still in use (that is,
it is attached to a program object using gl.attachShader() , which is discussed in a few
pages). The shader object specified as an argument of gl.deleteShader() will be deleted
when a program object no longer uses it.
Store the Shader Source Code in the Shader Objects
(g.shaderSource())
A shader object has storage to store the shader source code (written as a string in the
JavaScript program or in the separate file; see Appendix F , “Loading Shader Programs from
Files”). You use gl.shaderSource() to store the source code in a shader object.
Shader and Program Objects: The Role of initShaders()
347
gl.shaderSource(shader, source)
Store the source code specified by source in the shader object specified by shader . If any
source code was previously stored in the shader object, it is replaced by new source code.
Parameters shader Specifies the shader object in which the program is stored.
source Specifies the shader source code (string)
Return value None
Errors None
Compile Shader Objects (gl.compileShader())
After storing the shader source code in the shader object, you have to compile it so that
it can be used in the WebGL system. Unlike JavaScript, and like C or C++, shaders need
to be compiled before use. In this process, the source code stored in a shader object is
compiled to executable format (binary) and kept in the WebGL system. Use gl.compile-
Shader() to compile. Note, if you replace the source code in the shader object with a call
to gl.shaderSource() after compiling, the compiled binary kept in the shader object is
not replaced. You have to recompile it explicitly.
gl.compileShader(shader)
Compile the source code stored in the shader object specified by shader .
Parameters shader Specifies the shader object in which the source code to be
compiled is stored.
Return Value None
Errors None
When executing gl.compileShader() , it is possible a compilation error occurs due to
mistakes in the source code. You can check for such errors, as well as the status of the
shader object, using gl.getShaderParameter() .
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gl.getShaderParameter(shader, pname)
Get the information specified by pname from the shader object specified by shader .
Parameters shader Specifies the shader object.
pname Specifies the information to get from the shader:
gl.SHADER_TYPE , gl.DELETE_STATUS , or
gl.COMPILE_STATUS .
Return value The following depending on pname :
gl.SHADER_TYPE The type of shader ( gl.VERTEX_SHADER or gl.FRAGMENT_
SHADER )
gl.DELETE_
STATUS
Whether the deletion has succeeded ( true or false )
gl.COMPILE_
STATUS
Whether the compilation has succeeded ( true or false )
Errors INVALID_ENUM pname is none of the above values.
To check whether the compilation succeeded, you can call gl.getShaderParameter() with
gl.COMPILE_STATUS specified in pname .
If the compilation has failed, gl.getShaderParameter() returns false , and the error infor-
mation is written in the information log for the shader in the WebGL system. This infor-
mation can be retrieved with gl.getShaderInfoLog() .
gl.getShaderInfoLog(shader)
Retrieve the information log from the shader object specified by shader .
Parameters shader Specifies the shader object from which the information log is
retrieved.
Return value non-null The string containing the logged information .
null Any errors are generated.
Errors None
Although the exact details of the logged information is implementation specific, almost
all WebGL systems return error messages containing the line numbers where the compiler
Shader and Program Objects: The Role of initShaders()
349
has detected the errors in the program. For example, assume that you compiled a fragment
shader program as follows:
var FSHADER_SOURCE =
'void main() {\n' +
' gl.FragColor = vec4(1.0, 0.0, 0.0, 1.0);\n' +
'}\n';
Because the second line is incorrect in this case ( gl. must be gl_ ), the error messages
displayed in the JavaScript console of Chrome will be similar to those shown in Figure 9.11 .
Figure 9.11 A compile error in a shader
The first message indicates that gl at line 2 is undeclared.
failed to compile shader: ERROR: 0: 2 : 'gl' : undeclared identifier
cuon-utils.js:88
The reference to cuon-utils.js:88 on the right means that the error has been detected in
gl.getShaderInfoLog() , which was invoked at line 88 of the cuon-utils.js file, where
initShaders() is defined.
Create a Program Object (gl.createProgram())
As mentioned before, a program object is a container to store the shader objects and is
created by gl.createProgram() . You are already familiar with this program object because
it is the object you pass as the first argument of gl.getAttribLocation() and
gl.getUniformLocation() .
gl.createProgram()
Create a program object.
Parameters None
Return value non-null The newly created program object.
null Failed to create a program object.
Errors None
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350
A program object can be deleted by using gl.deleteProgram() .
gl.deleteProgram(program)
Delete the program object specified by program . If the program object is not referred to
from anywhere, it is deleted immediately. Otherwise, it will be deleted when it is no
longer referred to.
Parameters program Specifies the program object to be deleted.
Return value None
Errors None
Once the program object has been created, you attach the two shader objects to it.
Attach the Shader Objects to the Program Object (gl.attachShader())
Because you always need two shaders in WebGL—a vertex shader and a fragment shader—
you must attach both of them to the program object with gl.attachShader() .
gl.attachShader(program, shader)
Attach the shader object specified by shader to the program object specified by program .
Parameters program Specifies the program object.
shader Specifies the shader object to be attached to
program .
Return value None
Errors INVALID_OPERATION Shader had already been attached to program .
It is not necessary to compile or store any source code before it is attached to the program
object. You can detach the shader object with gl.detachShader() .
Shader and Program Objects: The Role of initShaders()
351
gl.detachShader(program, shader)
Detach the shader object specified by shader from the program object specified by
program .
Parameters program Specifies the program object.
shader Specifies the shader object to be detached from
program.
Return value None
Errors INVALID_OPERATION shader is not attached to program .
Link the Program Object (gl.linkProgram())
After attaching shader objects to a program object, you need to link the shader objects.
You use gl.linkProgram() to link the shader objects in the program object.
gl.linkProgram(program)
Link the program object specified by program.
Parameters program Specifies the program object to be linked.
Return value None
Errors None
During linking, various constraints of the WebGL system are checked: (1) when varying
variables are declared in a vertex shader, whether varying variables with the same names
and types are declared in a fragment shader, (2) whether a vertex shader has written data
to varying variables used in a fragment shader, (3) when the same uniform variables
are used in both a vertex shader and a fragment shader, whether their types and names
match, (4) whether the numbers of attribute variables, uniform variables, and varying
variables does not exceed an upper limit, and so on.
After linking the program object, it is always good programming practice to check whether
it succeeded. The result of linking can be confirmed with gl.getProgramParameters() .
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352
gl.getProgramParameter(program, pname)
Return information about pname for the program object specified by program . The return
value differs depending on pname .
Parameters program Specifies the program object.
pname Specifies any one of gl.DELETE_STATUS , gl.LINK_STATUS ,
gl.VALIDATE_STATUS , gl.ATTACHED_SHADERS , gl.ACTIVE_
ATTRIBUTES , or gl.ACTIVE_UNIFORMS .
Return value Depending on pname , the following values can be returned:
gl.DELETE_STATUS Whether the program has been
deleted ( true or false )
gl.LINK_STATUS Whether the program was linked
successfully ( true or false )
gl.VALIDATE_STATUS Whether the program was validated
successfully ( true or false )
1
gl.ATTACHED_SHADERS The number of attached shader
objects
gl.ACTIVE_ATTRIBUTES The number of attribute variables in
the vertex shader
gl.ACTIVE_UNIFORMS The number of uniform variables
Errors INVALID_ENUM pname is none of the above values.
If linking succeeded, you are returned an executable program object. Otherwise, you can
get the information about the linking from the information log of the program object
with gl.getProgramInfoLog() .
gl.getProgramInfoLog(program)
Retrieve the information log from the program object specified by program .
Parameters program Specifies the program object from which the information log is
retrieved.
Return value The string containing the logged information
Errors None
1 A program object may fail to execute even if it was linked successfully, such as if no texture units are
set for the sampler. This can only be detected when drawing, not when linking. Because this check
takes time, check for these errors only when debugging and turn off otherwise.
Shader and Program Objects: The Role of initShaders()
353
Tell the WebGL System Which Program Object to Use
(gl.useProgram())
The last step is to tell the WebGL system which program object to use when drawing by
making a call to gl.useProgram() .
gl.useProgram(program)
Tell the WebGL system that the program object specified by program will be used.
Parameters program Specifies the program object to be used.
Return value None
Errors None
One powerful feature of this function is that you can use it during drawing to switch
between multiple shaders prepared in advance. This will be discussed and used in
Chapter 10 .
With this final step, the preparation for drawing with the shaders is finished. As you have
seen, initShaders() hides quite a lot of detail and can be safely used without worrying
about this detail. Essentially, once executed, the vertex and fragment shaders are set up
and can be used with calls to gl.drawArrays() or gl.drawElements() .
Now that you have an understanding of the steps and appropriate WebGL functions used
in initShaders() , let’s take a look at the program flow of initShaders() as defined in
cuon-utils.js .
The Program Flow of initShaders()
initShaders() is composed of two main functions: createProgram() , which creates a
linked program object, and loadShader() , called from createProgram() , which creates the
compiled shader objects. Both are defined in cuon-utils.js . Here, you will work through
initShader() in order from the top (see Listing 9.5 ). Note that in contrast to the normal
code samples used in the book, the comments in this code are in the JavaDoc form, which
is used in the convenience libraries.
Listing 9.5 initShaders()
1 // cuon-utils.js
2 /**
3 * Create a program object and make current
4 * @param gl GL context
5 * @param vshader a vertex shader program (string)
6 * @param fshader a fragment shader program (string)
7 * @return true, if the program object was created and successfully made current
CHAPTER 9 Hierarchical Objects
354
8 */
9 function initShaders(gl, vshader, fshader) {
10 var program = createProgram(gl, vshader, fshader);
...
16 gl.useProgram(program);
17 gl.program = program;
18
19 return true;
20 }
First, initShaders() creates a linked program object with createProgram() at line 10 and
tells the WebGL system to use the program object at line 16. Then it sets the program
object to the property named program of the gl object.
Next, look at createProgram() in Listing 9.6 .
Listing 9.6 createProgram()
22 /**
23 * Create the linked program object
24 * @param gl GL context
25 * @param vshader a vertex shader program(string)
26 * @param fshader a fragment shader program(string)
27 * @return created program object, or null if the creation has failed.
28 */
29 function createProgram(gl, vshader, fshader) {
30 // Create shader objects
31 var vertexShader = loadShader(gl, gl.VERTEX_SHADER, vshader);
32 var fragmentShader = loadShader(gl, gl.FRAGMENT_SHADER, fshader);
...
37 // Create a program object
38 var program = gl.createProgram();
...
43 // Attach the shader objects
44 gl.attachShader(program, vertexShader);
45 gl.attachShader(program, fragmentShader);
46
47 // Link the program object
48 gl.linkProgram(program);
49
50 // Check the result of linking
51 var linked = gl.getProgramParameter(program, gl.LINK_STATUS);
...
60 return program;
61 }
Shader and Program Objects: The Role of initShaders()
355
The function createProgram() creates the shader objects for the vertex and the frag-
ment shaders, which are loaded using loadShader() at lines 31 and 32. The shader
object returned from loadShader() contains the stored shader source code and compiled
versions.
The program object, to which the shader objects created here will be attached, is created
at line 38, and the vertex and fragment shader objects are attached at lines 44 and 45.
Then createProgram() links the program object at line 48 and checks the result at line 51.
If the linking has succeeded, it returns the program object at line 60.
Finally, let’s look at loadShader() ( Listing 9.7 ) which was invoked at lines 31 and 32 from
within createProgram() .
Listing 9.7 loadShader()
63 /**
64 * Create a shader object
65 * @param gl GL context
66 * @param type the type of the shader object to be created
67 * @param source a source code of a shader (string)
68 * @return created shader object, or null if the creation has failed.
69 */
70 function loadShader(gl, type, source) {
71 // Create a shader object
72 var shader = gl.createShader(type);
...
78 // Set source codes of the shader
79 gl.shaderSource(shader, source);
80
81 // Compile the shader
82 gl.compileShader(shader);
83
84 // Check the result of compilation
85 var compiled = gl.getShaderParameter(shader, gl.COMPILE_STATUS);
...
93 return shader;
94 }
First loadShader() creates a shader object at line 72. It associates the source code to the
object at line 79 and compiles it at line 82. Finally, it checks the result of compilation
at line 85 and, if no errors have occurred, returns the shader object to createShader() ,
which attaches it to the program object.
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356
Summary
This chapter is the final one to explore basic features of WebGL. It looked at how to draw
and manipulate complex 3D objects composed of multiple segments organized in a hier-
archical structure. This technique is important for understanding how to use simple 3D
objects like cubes or blocks to build up more complex objects like robots or game charac-
ters. In addition, you looked at one of the most complex convenience functions we have
provided for this book, initShaders() , which has been treated as a black box up until
now. You saw the details of how shader objects are created and managed by program
objects, so you have a better sense of the internal structure of shaders and how WebGL
manages them through program objects.
At this stage you have a full understanding of WebGL and are capable of writing your
own complex 3D scenes using the expressive power of WebGL. In the next chapter, we
will outline various advanced techniques used in 3D graphics and leverage what you have
learned so far to show how WebGL can support these techniques.
Chapter 10
Advanced Techniques
This chapter includes a “grab-bag” of interesting techniques that you should find useful
for creating your WebGL applications. The techniques are mostly stand-alone, so you
can select and read any section that interests you. Where there are dependencies, they
are clearly identified. The explanations in this chapter are terse in order to include as
many techniques as possible. However, the sample programs on the website include
comprehensive comments, so please refer to them as well.
Rotate an Object with the Mouse
When creating WebGL applications, sometimes you want users to be able to control 3D
objects with the mouse. In this section, you construct a sample program RotateObject ,
which allows users to rotate a cube by dragging it with the mouse. To make the
program simple, it uses a cube, but the basic method is applicable to any object. Figure
10.1 shows a screen shot of the cube that has a texture image mapped onto it.
Figure 10.1 A screen shot of RotateObject
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How to Implement Object Rotation
Rotating a 3D object is simply the application of a technique you’ve already studied for
2D objects—transforming the vertex coordinates by using the model view projection
matrix. The process requires you to create a rotation matrix based on the mouse move-
ment, change the model view projection matrix, and then transform the coordinates by
using the matrix.
You can obtain the amount of mouse movement by simply recording the position where
the mouse is initially clicked and then subtracting that position from the new position as
the mouse moves. Clearly, an event handler will be needed to calculate the mouse move-
ment, and then this will be converted into an angle that will rotate the object. Let’s take a
look at the sample program.
Sample Program (RotateObject.js)
Listing 10.1 shows the sample program. As you can see, the shaders do not do anything
special. Line 9 in the vertex shader transforms the vertex coordinates by using the model
view projection matrix, and line 10 maps the texture image onto the cube.
Listing 10.1 RotateObject.js
1 // RotateObject.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
...
8 'void main() {\n' +
9 ' gl_Position = u_MvpMatrix * a_Position;\n' +
10 ' v_TexCoord = a_TexCoord;\n' +
11 '}\n';
...
24 function main() {
...
42 var n = initVertexBuffers(gl);
...
61 viewProjMatrix.setPerspective(30.0, canvas.width / canvas.height,
➥1.0, 100.0);
62 viewProjMatrix.lookAt(3.0, 3.0, 7.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
63
64 // Register the event handler
65 var currentAngle = [0.0, 0.0]; // [x-axis, y-axis] degrees
66 initEventHandlers(canvas, currentAngle);
...
74 var tick = function() { // Start drawing
75 draw(gl, n, viewProjMatrix, u_MvpMatrix, currentAngle);
76 requestAnimationFrame(tick, canvas);
77 };
Rotate an Object with the Mouse
359
78 tick();
79 }
...
138 function initEventHandlers(canvas, currentAngle) {
139 var dragging = false; // Dragging or not
140 var lastX = -1, lastY = -1; // Last position of the mouse
141
142 canvas.onmousedown = function(ev) { // Mouse is pressed
143 var x = ev.clientX, y = ev.clientY;
144 // Start dragging if a mouse is in <canvas>
145 var rect = ev.target.getBoundingClientRect();
146 if (rect.left <= x && x < rect.right && rect.top <= y && y < rect.bottom) {
147 lastX = x; lastY = y;
148 dragging = true;
149 }
150 };
151 // Mouse is released
152 canvas.onmouseup = function(ev) { dragging = false; };
153
154 canvas.onmousemove = function(ev) { // Mouse is moved
155 var x = ev.clientX, y = ev.clientY;
156 if (dragging) {
157 var factor = 100/canvas.height; // The rotation ratio
158 var dx = factor * (x - lastX);
159 var dy = factor * (y - lastY);
160 // Limit x-axis rotation angle to -90 to 90 degrees
161 currentAngle[0] = Math.max(Math.min(currentAngle[0] + dy, 90.0), -90.0);
162 currentAngle[1] = currentAngle[1] + dx;
163 }
164 lastX = x, lastY = y;
165 };
166 }
167
168 var g_MvpMatrix = new Matrix4(); // The model view projection matrix
169 function draw(gl, n, viewProjMatrix, u_MvpMatrix, currentAngle) {
170 // Calculate the model view projection matrix
171 g_MvpMatrix.set(viewProjMatrix);
172 g_MvpMatrix.rotate(currentAngle[0], 1.0, 0.0, 0.0); // x-axis
173 g_MvpMatrix.rotate(currentAngle[1], 0.0, 1.0, 0.0); // y-axis
174 gl.uniformMatrix4fv(u_MvpMatrix, false, g_MvpMatrix.elements);
175
176 gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
177 gl.drawElements(gl.TRIANGLES, n, gl.UNSIGNED_BYTE, 0);
178 }
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At lines 61 and 62 of main() in JavaScript, the view projection matrix is calculated in
advance. You will have to change the model matrix on-the-fly according to the amount of
mouse movement.
The code from line 65 registers the event handlers, a key part of this sample program. The
variable currentAngle is initialized at line 65 and used to hold the current rotation angle.
Here, it is an array because it needs to handle two rotation angles around the x-axis and
y-axis. The actual registration of the event handlers is done inside initEventHandlers() ,
called at line 66. It draws the cube using the function tick() that is defined from line 74.
initEventHandlers() is defined at line 138. The code from line 142 handles mouse down,
the code from line 152 handles mouse up, and the code from line 154 handles the mouse
movement.
The processing when the mouse button is first pushed at line 142 is simple. Line 146
checks whether the mouse has been pressed inside the <canvas> element. If it is inside the
<canvas> , line 147 saves that position in lastX and lastY . Then the variable dragging ,
which indicates dragging has begun, is set to true at line 148.
The processing of the mouse button release at line 152 is simple. Because this indicates
that dragging is done, the code simply sets the variable dragging back to false .
The processing from line 154 is the critical part and tracks the movement of the mouse.
Line 156 checks whether dragging is taking place and, if it is, lines 158 and 159 calculate
how long it has moved, storing the results to dx and dy . These values are scaled, using
factor , which is a function of the canvas size. Once the distance dragged has been calcu-
lated, it can be used to determine the new angle by directly adding to the current angles
at line 161 and 162. The code limits rotation from –90 to +90 degrees simply to show the
technique; you are free to remove this. Because the mouse has moved, its position is saved
in lastX and lastY .
Once you have successfully transformed the movement of the mouse into a rotation
angle, you can let the rotation matrix handle the updates and draw the results using
tick() . These operations are done at lines 172 and 173.
This quick review of a technique to calculate the rotation angle is only one approach.
Others, such as placing virtual track balls around the object, are described in detail in the
book 3D User Interfaces .
Select an Object
When your application requires users to be able to control 3D objects interactively, you
will need a technique to allow users to select objects. There are many uses of this tech-
nique, such as selecting a 3D button created by a 3D model instead of the conventional
2D GUI button, or selecting a photo among multiple photos in a 3D scene.
Select an Object
361
Selecting a 3D object is generally more complex than selecting a 2D one because of the
mathematics required to determine if the mouse is over a nonregular shape. However,
you can use a simple trick, shown in the sample program, to avoid that complexity. In
this sample, PickObject , the user can click a rotating cube, which causes a message to be
displayed (see Figure 10.2 ). First, run the sample program and experiment with it for a
while to get the feeling of how it works.
Figure 10.2 PickObject
Figure 10.2 shows with the message displayed when clicking the cube. The message says,
“The cube was selected!” Also check what happens when you click the black part of the
background.
How to Implement Object Selection
This program goes through the following steps to check whether the cube was clicked:
1. When the mouse is pressed, draw the cube with a single color “red” (see the middle
of Figure 10.3 ).
2. Read the pixel value (color) of the selected point.
3. Redraw the cube with its original color (right in Figure 10.3 ).
4. If the color of the pixel is red, display, “The cube was selected!”
When the cube is drawn with a single color (red in this case), you can quickly see which
part of the drawing area the cube occupies. After reading the pixel value at the position
of the mouse pointer when the mouse is clicked, you can determine that the mouse was
above the cube if the pixel color is red.
CHAPTER 10 Advanced Techniques
362
Click
Figure 10.3 The object drawn at the point of mouse pressing
To ensure that the viewer doesn’t see the cube flash red, you need to draw and redraw in
the same function. Let’s take a look at the actual sample program.
Sample Program (PickObject.js)
Listing 10.2 shows the sample program. The processing in this sample mainly takes place
in the vertex shader. To implement step 1, you must inform the vertex shader that the
mouse has been clicked so that it draws the cube red. The variable u_Clicked transmits
this information and declared at line 7 in the vertex shader. When the mouse is pressed,
u_Clicked is set to true in the JavaScript and tested at line 11. If true , the color red is
assigned to v_Color ; if not, the color of the cube ( a_Color ) is directly assigned to v_Color .
This turns the cube red when the mouse is pressed.
Listing 10.2 PickObject.js
1 // PickObject.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
...
6 'uniform mat4 u_MvpMatrix;\n' +
7 'uniform bool u_Clicked;\n' + // Mouse is pressed
8 'varying vec4 v_Color;\n' +
9 'void main() {\n' +
10 ' gl_Position = u_MvpMatrix * a_Position;\n' +
11 ' if (u_Clicked) {\n' + // Draw in red if mouse is pressed <-(1)
12 ' v_Color = vec4(1.0, 0.0, 0.0, 1.0);\n' +
13 ' } else {\n' +
14 ' v_Color = a_Color;\n' +
15 ' }\n' +
16 '}\n';
17
18 // Fragment shader program
Select an Object
363
...
25 ' gl_FragColor = v_Color;\n' +
...
30 function main() {
...
60 var u_Clicked = gl.getUniformLocation(gl.program, 'u_Clicked');
...
71 gl.uniform1i(u_Clicked, 0); // Pass false to u_Clicked
72
73 var currentAngle = 0.0; // Current rotation angle
74 // Register the event handler
75 canvas.onmousedown = function(ev) { // Mouse is pressed
76 var x = ev.clientX, y = ev.clientY;
77 var rect = ev.target.getBoundingClientRect();
78 if (rect.left <= x && x < rect.right && rect.top <= y && y < rect.bottom) {
79 // Check if it is on object
80 var x_in_canvas = x - rect.left, y_in_canvas = rect.bottom - y;
81 var picked = check(gl, n, x_in_canvas, y_in_canvas, currentAngle,
➥u_Clicked, viewProjMatrix, u_MvpMatrix);
82 if (picked) alert('The cube was selected! '); <-(4)
83 }
84 }
...
92 }
...
147 function check(gl, n, x, y, currentAngle, u_Clicked, viewProjMatrix,
➥u_MvpMatrix) {
148 var picked = false;
149 gl.uniform1i(u_Clicked, 1); // Draw the cube with red
150 draw(gl, n, currentAngle, viewProjMatrix, u_MvpMatrix);
151 // Read pixel at the clicked position
152 var pixels = new Uint8Array(4); // Array for storing the pixels
153 gl.readPixels(x, y, 1, 1, gl.RGBA, gl.UNSIGNED_BYTE, pixels); <-(2)
154
155 if (pixels[0] == 255) // The mouse in on cube if pixels[0] is 255
156 picked = true;
157
158 gl.uniform1i(u_Clicked, 0); // Pass false to u_Clicked: redraw cube
159 draw(gl, n, currentAngle, viewProjMatrix, u_MvpMatrix); // <-(3)
160
161 return picked;
162 }
Let’s take a look from line 30 of main() in JavaScript. Line 60 obtains the storage location
for u_Clicked , and line 71 assigns the initial value of u_Clicked to be false .
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364
Line 75 registers the event handler to be called when the mouse has been clicked. This
event handler function does a sanity check to see if the clicked position is inside the
<canvas> element at line 78. Then it calls to check() at line 81 if it is. This function checks
whether the position, specified by the third and fourth arguments, is on the cube (see next
paragraph). If so, it returns true which causes a message to be displayed at line 82.
The function check() begins from line 147. This function processes steps (2) and (3) from
the previous section together. Line 149 informs the vertex shader that the click event has
occurred by passing 1 ( true ) to u_Clicked . Then line 150 draws the cube with the current
rotation angle. Because u_Clicked is true , the cube is drawn in red. Then the pixel value
of the clicked position is read from the color buffer at line 153. The following shows the
gl.readPixels() function used here.
gl.readPixels(x, y, width, height, format, type, pixels)
Read a block of pixels from the color buffer
1 and store it to the array pixels . x , y , width ,
and height define the block as a rectangle.
Parameters x, y Specify the position of the first pixel that is read from the
buffer.
width, height Specify the dimensions of the pixel rectangle.
format Specifies the format of the pixel data. gl.RGBA must be
specified.
type Specifies the data type of the pixel data. gl.UNSIGNED_BYTE
must be specified.
pixels Specifies the typed array ( Uint8Array ) for storing the pixel
data.
Return value None
Errors INVALID_VALUE: pixels is null . Either width or height is negative.
INVALID_OPERATION: pixels is not large enough to store the pixel data.
INVALID_ENUM: format or type is none of the above values.
The pixel value that was read is stored in the pixels array. This array is defined at line
152, and the R, G, B, and A values are stored in pixels[0] , pixels[1] , pixels[2] , and
pixels[3] , respectively. Because, in this sample program, you know that the only colors
used are red for the cube and black for the background, you can see if the mouse is on
the cube by checking the values for pixels[0] . This is done at line 155, and if it is red, it
changes picked to true .
1 If a framebuffer object is bound to gl.FRAMEBUFFER , this method reads the pixel values from the
object. We explain the object in the later section “Use What You’ve Drawn as a Texture Image.”
Select an Object
365
Then line 158 sets u_Clicked to false and redraws the cube at line 159. This turns the
cube back to its original color. Line 161 returns picked as the return value.
Note, if at this point you call any function that returns control to the browser, such as
alert() , the content of the color buffer will be displayed on the <canvas> at that point.
For example, if you execute alert('The cube was selected!') at line 156, the red cube
will be displayed when you click the cube.
This approach, although simple, can handle more than one object by assigning differ-
ent colors to each object. For example, red, blue, and green are enough if there are three
objects. For larger numbers of objects, you can use individual bits. Because there are 8 bits
for each component in RGBA, you can represent 255 objects just by using the R compo-
nent. However, if the 3D objects are complex or the drawing area is large, it will take
some time to process the selection of objects. To overcome this disadvantage, you can use
simplified models to select objects or shrink the drawing area. In such cases, you can use
the framebuffer object, which will be explained in the section “Use What You’ve Drawn as
a Texture Image” later in this chapter.
Select the Face of the Object
You can also apply the method explained in the previous section to select a particular
face of an object. Let’s customize PickObject to build PickFace , a program that turns the
selected face white. Figure 10.4 shows PickFace .
Click
Figure 10.4 PickFace
PickFace is easy once you understand how PickObject works. PickObject drew the cube
in red when the mouse was clicked, resulting in the object’s display area in the color
buffer being red. By reading the pixel value of the clicked point and seeing if the color
of the pixel at the position was red, the program could determine if the object had been
selected. PickFace goes one step further and inserts the information of which face has
been selected into the color buffer. Here, you will insert the information in the alpha
component of the RGBA value. Let’s take a look at the sample program.
CHAPTER 10 Advanced Techniques
366
Sample Program (PickFace.js)
PickFace.js is shown in Listing 10.3 . Some parts, such as the fragment shader, are
omitted for brevity.
Listing 10.3 PickFace.js
1 // PickFace.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'attribute vec4 a_Color;\n' +
6 'attribute float a_Face;\n' + // Surface number (Cannot use int)
7 'uniform mat4 u_MvpMatrix;\n' +
8 'uniform int u_PickedFace;\n' + // Surface number of selected face
9 'varying vec4 v_Color;\n' +
10 'void main() {\n' +
11 ' gl_Position = u_MvpMatrix * a_Position;\n' +
12 ' int face = int(a_Face);\n' + // Convert to int
13 ' vec3 color = (face == u_PickedFace) ? vec3(1.0):a_Color.rgb;\n'+
14 ' if(u_PickedFace == 0) {\n' + // Insert face number into alpha
15 ' v_Color = vec4(color, a_Face/255.0);\n' +
16 ' } else {\n' +
17 ' v_Color = vec4(color, a_Color.a);\n' +
18 ' }\n' +
19 '}\n';
...
33 function main() {
...
50 // Set vertex information
51 var n = initVertexBuffers(gl);
...
74 // Initialize selected surface
75 gl.uniform1i(u_PickedFace, -1);
76
77 var currentAngle = 0.0; // Current rotation angle (degrees)
78 // Register event handlers
79 canvas.onmousedown = function(ev) { // Mouse is pressed
80 var x = ev.clientX, y = ev.clientY;
81 var rect = ev.target.getBoundingClientRect();
82 if (rect.left <= x && x < rect.right && rect.top <= y && y < rect.bottom) {
83 // If clicked position is inside the <canvas> , update the face
84 var x_in_canvas = x - rect.left, y_in_canvas = rect.bottom - y;
85 var face = checkFace(gl, n, x_in_canvas, y_in_canvas,
➥currentAngle, u_PickedFace, viewProjMatrix, u_MvpMatrix);
86 gl.uniform1i(u_PickedFace, face); // Pass the surface number
87 draw(gl, n, currentAngle, viewProjMatrix, u_MvpMatrix);
Select an Object
367
88 }
89 }
...
99 function initVertexBuffers(gl) {
...
109 var vertices = new Float32Array([ // Vertex coordinates
110 1.0, 1.0, 1.0, -1.0, 1.0, 1.0, -1.0,-1.0, 1.0, 1.0,-1.0, 1.0,
111 1.0, 1.0, 1.0, 1.0,-1.0, 1.0, 1.0,-1.0,-1.0, 1.0, 1.0,-1.0,
...
115 1.0,-1.0,-1.0, -1.0,-1.0,-1.0, -1.0, 1.0,-1.0, 1.0, 1.0,-1.0
116 ]);
...
127 var faces = new Uint8Array([ // Surface number
128 1, 1, 1, 1, // v0-v1-v2-v3 Front
129 2, 2, 2, 2, // v0-v3-v4-v5 Right
...
133 6, 6, 6, 6, // v4-v7-v6-v5 Depth
134 ]);
...
154 if (!initArrayBuffer(gl, faces, gl.UNSIGNED_BYTE, 1,
➥'a_Face')) return -1; // Surface Information
...
164 }
165
166 function checkFace(gl, n, x, y, currentAngle, u_PickedFace, viewProjMatrix,
➥u_MvpMatrix) {
167 var pixels = new Uint8Array(4); // Array for storing the pixel
168 gl.uniform1i(u_PickedFace, 0); // Write surface number into alpha
169 draw(gl, n, currentAngle, viewProjMatrix, u_MvpMatrix);
170 // Read the pixels at (x, y). pixels[3] is the surface number
171 gl.readPixels(x, y, 1, 1, gl.RGBA, gl.UNSIGNED_BYTE, pixels);
172
173 return pixels[3];
174 }
Let’s take a look from the vertex shader. a_Face at line 6 is the attribute variable used to
pass the surface number, which is then “coded” into the alpha value when the mouse
is clicked. The surface numbers are set up in initVertexBuffers() defined at line 99
and simply map vertices to a surface. Lines 127 onward define these mappings. So, for
example, vertices v0-v1-v2-v3 define a surface that is numbered 1, vertices v0-v3-v4-v5 are
numbered 2, and so on. Because each vertex needs a number to pass to the vertex shader,
there are four 1s written at line 128 to represent the first face.
If a face is already selected, u_PickedFace at line 8 informs the vertex shader of the
selected face number, allowing the shader to switch the way it draws the face based on
this information.
CHAPTER 10 Advanced Techniques
368
Line 12 converts a_Face , the surface number that is a float type, into an int type because
an int type cannot be used in the attribute variables ( Chapter 6 , “The OpenGL ES Shading
Language [GLSL ES]”). If the selected surface number is the same as the surface number
currently being manipulated, white is assigned to color at line 13. Otherwise, the original
surface color is assigned. If the mouse has been clicked (that is, u_PickedFace is set to 0),
the a_Face value is inserted into the alpha value and the cube is drawn (line 15).
Now, by passing 0 into u_PickedFace when the mouse is clicked, the cube is drawn with
an alpha value set to the surface number. u_PickedFace is initialized to –1 at line 75.
There is no surface with the number –1 (refer to the faces array at line 127), so the cube is
initially drawn without surfaces selected.
Let’s take a look at the essential processing of the event handler. u_PickedFace is passed
as an argument to checkFace() at line 85, which returns the surface number of the picked
face, checkFace() , at line 166. At line 168, 0 is passed to u_PickedFace to tell the vertex
shader that the mouse has been clicked. When draw() is called in the next line, the
surface number is inserted into the alpha value and the object is redrawn. Line 171 checks
the pixel value of the clicked point, and line 173 retrieves the inserted surface number
by using pixels[3] . (It is the alpha value, so the subscript is 3.) This surface number is
returned to the main code and then used at lines 86 and 87 to draw the cube. The vertex
shader handles the rest of the processing, as described earlier.
HUD (Head Up Display)
The Head Up Display, originally developed for aircraft, is a transparent display that pres-
ents data without requiring users to look away from their usual viewpoints. A similar
effect can be achieved in 3D graphics and used to overlay textual information on the 3D
scene. Here, you will construct a sample program that will display a diagram and some
information on top of the 3D graphics (HUD), as you can see in Figure 10.5 .
Figure 10.5 HUD
HUD (Head Up Display)
369
The goal of the program is to draw a triangle and some simple information about the 3D
scene, including the current rotation angle of the cube (from PickObject ) that will change
as the cube rotates.
How to Implement a HUD
This HUD effect can be implemented using HTML and the canvas function without
WebGL. This is done as follows:
1. In the HTML file, prepare a <canvas> to draw the 3D graphics using WebGL and
another <canvas> to draw the HUD using the canvas function. In other words,
prepare two <canvas> and place the HUD on top of the WebGL canvas.
2. Draw the 3D graphics using the WebGL API on the <canvas> for WebGL.
3. Draw the HUD using the canvas functions on the <canvas> for the HUD.
As you can see, this is extremely simple and shows the power of WebGL and its ability to
mix 2D and 3D graphics within the browser. Let’s take a look at the sample program.
Sample Program (HUD.html)
Because we need to make changes to the HTML file to add the extra canvas, we show HUD.
html in Listing 10.4 , with the additions in bold.
Listing 10.4 HUD.html
1 <!DOCTYPE html>
2 <html lang="ja">
...
8 <body οnlοad="main()">
9 <canvas id="webgl" width="400" height="400" style="position:
➥absolute; z-index: 0" >
10 Please use a browser that supports "canvas"
11 </canvas>
12 <canvas id="hud" width="400" height="400" style="position:
➥absolute;z-index: 1"></canvas>
...
18 <script src="HUD.js"></script>
19 </body>
20 </html>
The style attribute, used to define how an element looks or how it is arranged, allows you
to place the HUD canvas on top of the WebGL canvas. Style information is composed of
the property name and the value separated with a : as seen at line 9: style="position:
absolute" . Multiple style elements are separated with ; .
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370
In this example, you use position , which specifies how the element is placed, and the
z-index , which specifies the hierarchical relationship.
You can specify the position of the element in the absolute coordinate if you use absolute
for the position value. Unless you specify the position, all the elements specified to this
attribute will be place at the same position. z-index specifies the order in which elements
are displayed when multiple elements are at the same position. The element with the
larger number will be displayed over the one with a smaller number. In this case, the
z-index of the <canvas> for the HUD at line 12 is 1.
The result of this code is two <canvas> elements, placed at the same location with
the <canvas> that displays the HUD on top of the <canvas> that displays the WebGL.
Conveniently, the background of the canvas element is transparent by default, so the
WebGL canvas can be seen through the HUD canvas. Anything that is drawn on the HUD
canvas will appear over the 3D objects and create the effect of a HUD.
Sample Program (HUD.js)
Next, let’s take a look at HUD.js in Listing 10.5 . There are two changes made compared to
PickObject.js :
1. Retrieve the rendering context to draw in the <canvas> for the HUD and use it to
draw.
2. Register the event handler when the mouse is clicked to the <canvas> for the HUD
and not to the <canvas> for WebGL.
Step 1 simply uses the source code used in Chapter 2 , “Your First Step with WebGL,” to
draw a triangle onto the <canvas> . Step 2 is required to ensure that mouse click informa-
tion is passed to the HUD canvas rather than the WebGL canvas. The vertex shader and
fragment shader are the same as PickObject.js .
Listing 10.5 HUD.js
1 // HUD.js
...
30 function main() {
31 // Retrieve <canvas> element
32 var canvas = document.getElementById('webgl');
33 var hud = document.getElementById('hud');
...
40 // Get the rendering context for WebGL
41 var gl = getWebGLContext(canvas);
42 // Get the rendering context for 2DCG
43 var ctx = hud.getContext('2d');
...
82 // Register the event handler
83 hud.onmousedown = function(ev) { // Mouse is pressed
HUD (Head Up Display)
371
...
89 check(gl, n, x_in_canvas, y_in_canvas, currentAngle, u_Clicked,
➥viewProjMatrix, u_MvpMatrix);
...
91 }
92
93 var tick = function() { // Start drawing
94 currentAngle = animate(currentAngle);
95 draw2D(ctx, currentAngle); // Draw 2D
96 draw(gl, n, currentAngle, viewProjMatrix, u_MvpMatrix);
97 requestAnimationFrame(tick, canvas);
98 };
99 tick();
100 }
...
184 function draw2D(ctx, currentAngle) {
185 ctx.clearRect(0, 0, 400, 400); // Clear <hud>
186 // Draw triangle with white lines
187 ctx.beginPath(); // Start drawing
188 ctx.moveTo(120, 10); ctx.lineTo(200, 150); ctx.lineTo(40, 150);
189 ctx.closePath();
190 ctx.strokeStyle = 'rgba(255, 255, 255, 1)'; // Set the line color
191 ctx.stroke(); // Draw triangle with white lines
192 // Draw white letters
193 ctx.font = '18px "Times New Roman"';
194 ctx.fillStyle = 'rgba(255, 255, 255, 1)'; // Set the letter color
195 ctx.fillText('HUD: Head Up Display', 40, 180);
196 ctx.fillText('Triangle is drawn by Hud API.', 40, 200);
197 ctx.fillText('Cube is drawn by WebGL API.', 40, 220);
198 ctx.fillText('Current Angle: '+ Math.floor(currentAngle), 40, 240);
199 }
Because the processing flow of the program is straightforward, let’s take a look from
main() at line 30. First, line 33 obtains the <canvas> element for the HUD. This is used to
get the drawing context for the 2D graphics ( Chapter 2 ) at line 43, which is used to draw
the HUD. You register the mouse-click event handler for the HUD canvas ( hud ) instead of
the WebGL canvas in PickObject.js . This is because the event goes to the HUD canvas,
which is placed on top of the WebGL canvas.
The code from line 93 handles the animation and uses draw2D() , added at line 95, to draw
the HUD information.
draw2D() is defined at line 184 and takes ctx parameters, the context to draw on the
canvas, and the current rotation angle, currentAngle . Line 185 clears the HUD canvas
using the clearRect() method, which takes the upper-left corner, the width, and the
height of the rectangle to clear. Lines 187 to 191 draw the triangle which, unlike drawing
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a rectangle as explained in Chapter 2 , requires that you define the path (outline) of a
triangle to draw it. Lines 187 to 191 define the path, set the color, and draw the triangle.
Lines 193 onward specify the text color and font and then use fillText() , which speci-
fies the letters to draw as the first parameter and the x and y coordinates to draw as the
second and third parameters, to actually write the text. Line 198 displays the current
rotation angle and uses Math.floor() to truncate the numbers below the decimal point.
Line 185 clears the canvas because the displayed value (rotation angle) changes at each
drawing.
Display a 3D Object on a Web Page (3DoverWeb)
Displaying a 3D object on a web page is simple with WebGL and the inverse of the HUD
example. In this case, the WebGL canvas is on top of the web page, and the canvas is set
to transparent. Figure 10.6 shows 3DoverWeb .
Figure 10.6 3DoverWeb
2
3DoverWeb.js is based on PickObject.js with almost no changes. The only change is that
the alpha value of the clear color is changed from 1.0 to 0.0 at line 55.
55 gl.clearColor(0.0, 0.0, 0.0, 0.0 );
By making the alpha value 0.0, the background of the WebGL canvas becomes transpar-
ent, and you can see the web page behind the WebGL <canvas> . You can also experiment
with the alpha value; any value other than 1.0 changes the transparency and makes the
web page more or less visible.
Fog (Atmospheric Effect)
In 3D graphics, the term fog is used to describe the effect that makes a distant object seem
hazy. The term describes objects in any medium, so objects underwater can also have a
2 The sentences on the web page on the background are from the book The Design of Design (by
Frederick P. Brooks Jr, Pearson).
Fog (Atmospheric Effect)
373
fog effect applied. Here, you construct a sample program Fog that realizes the fog effect.
Figure 10.7 shows a screen shot. You can adjust the density of the fog with the up/down
arrow keys. Try running the sample program and experiment with the effect.
Figure 10.7 Fog
How to Implement Fog
There are various ways to calculate fog, but here you will use a linear computation ( linear
fog ) because the calculation is easy. The linear fog method determines the density of the
fog by setting the starting point (the distance where the object starts to become hazy) and
the end point (where the object is completely obscured). The density of the fog between
these points is changed linearly. Note that the end point is not where the fog ends; rather,
it is where the fog becomes so dense that it obscures all objects. We will call how clearly
we can see the object the fog factor ; it is calculated, in the case of linear fog, as follows:
Equation 10.1
( )
( )
=
−
−
fog factor
end point distance from eye point
end point starting point
      
  
Where
≤ ≤ starting point distance from eye point end point         
When the fog factor is 1.0, you can see the object completely, and if it 0.0, you cannot see
it at all (see Figure 10.8 ). The fog factor is 1.0 when the (distance from eye point)
< (starting point) , and 0.0 when (end point) < (distance from eye point) .
CHAPTER 10 Advanced Techniques
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r o t c a f g o f
m o r f e c n a t s i d
t n i o p e y e
e c n a t s i d d n e e c n a t s i d t r a t s
0 . 1
eye point
Figure 10.8 Fog factor
You can calculate the color of a fragment based on the fog factor, as follows in Equation
10.2 .
Equation 10.2
( )
=
× + × −
fragment color
surface color fog factor fog color fog factor
  
      1   
Now, let’s take a look at the sample program.
Sample Program (Fog.js)
The sample program is shown in Listing 10.6 . Here, you (1) calculate the distance of the
object (vertex) from the eye point in the vertex shader, and based on that, you (2) calcu-
late the fog factor and the color of the object based on the fog factor in the fragment
shader. Note that this program specifies the position of the eye point with the world
coordinate system (see Appendix G , “World Coordinate System Versus Local Coordinate
System”) so the fog calculation takes place in the world coordinate system.
Listing 10.6 Fog.js
1 // Fog.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
...
7 'uniform mat4 u_ModelMatrix;\n' +
8 'uniform vec4 u_Eye;\n' + // The eye point (world coordinates)
9 'varying vec4 v_Color;\n' +
10 'varying float v_Dist;\n' +
11 'void main() {\n' +
12 ' gl_Position = u_MvpMatrix * a_Position;\n' +
13 ' v_Color = a_Color;\n' +
14 // Calculate the distance to each vertex from eye point <-(1)
15 ' v_Dist = distance(u_ModelMatrix * a_Position, u_Eye);\n' +
16 '}\n';
17
Fog (Atmospheric Effect)
375
18 // Fragment shader program
19 var FSHADER_SOURCE =
...
23 'uniform vec3 u_FogColor;\n' + // Color of Fog
24 'uniform vec2 u_FogDist;\n' + // Fog starting point, end point)
25 'varying vec4 v_Color;\n' +
26 'varying float v_Dist;\n' +
27 'void main() {\n' +
28 // Calculate the fog factor <-(2)
29 ' float fogFactor = clamp((u_FogDist.y - v_Dist) / (u_FogDist.y –
➥u_FogDist.x), 0.0, 1.0);\n' +
30 // u_FogColor * (1 - fogFactor) + v_Color * fogFactor
31 ' vec3 color = mix(u_FogColor, vec3(v_Color), fogFactor);\n' +
32 ' gl_FragColor = vec4(color, v_Color.a);\n' +
33 '}\n';
34
35 function main() {
...
53 var n = initVertexBuffers(gl);
...
59 // Color of fog
60 var fogColor = new Float32Array([0.137, 0.231, 0.423]);
61 // Distance of fog [fog starts, fog completely covers object]
62 var fogDist = new Float32Array([55, 80]);
63 // Position of eye point (world coordinates)
64 var eye = new Float32Array([25, 65, 35]);
...
76 // Pass fog color, distances, and eye point to uniform variable
77 gl.uniform3fv(u_FogColor, fogColor); // Fog color
78 gl.uniform2fv(u_FogDist, fogDist); // Starting point and end point
79 gl.uniform4fv(u_Eye, eye); // Eye point
80
81 // Set clear color and enable hidden surface removal function
82 gl.clearColor( fogColor[0], fogColor[1], fogColor[2] , 1.0);
...
93 mvpMatrix.lookAt( eye[0], eye[1], eye[2] , 0, 2, 0, 0, 1, 0);
...
97 document.onkeydown = function(ev){ keydown(ev, gl, n, u_FogDist, fogDist); };
...
The calculation of the distance from the eye point to the vertex, done by the vertex
shader, is straightforward. You simply transform the vertex coordinates to the world coor-
dinates using the model matrix and then call the built-in function distance() with the
position of the eye point (world coordinates) and the vertex coordinates. The distance()
CHAPTER 10 Advanced Techniques
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function calculates the distance between two coordinates specified by the arguments. This
calculation takes place at line 15, and the result is then written to the v_Dist variable and
passed to the fragment shader.
The fragment shader calculates the fogged color of the object using Equations 10.1 and
10.2 . The fog color, fog starting point, and fog end point, which are needed to calculate
the fogged color, are passed in the uniform variables u_FogColor and u_FogDist at lines 23
and 24. u_FogDist.x is the starting point, and u_FogDist.y is the end point.
The fog factor is calculated at line 29 using Equation 10.1 . The clamp() function is a built-
in function; if the value specified by the first parameter is outside the range specified by
the second and third parameter ([0.0 0.1] in this case), it will fix the value to one within
the range. In other words, the value is fixed to 0.0 if the value is smaller than 0.0, and 1.0
if the value is larger than 1.0. If the value is within the range, the value is unchanged.
Line 31 is the calculation of the fragment color using the fog factor. This implements
Equation 10.2 and uses a built-in function, mix() , which calculates x*(1–z)+y*z, where x is
the first parameter, y is the second, and z is the third.
The processing in JavaScript’s main() function from line 35 sets up the values necessary
for calculating the fog in the appropriate uniform variables.
You should note that there are many types of fog calculations other than linear fog, for
example exponential fog, used in OpenGL (see the book OpenGL Programming Guide ). You
can implement these fog calculations using the same approach, just changing the calcula-
tion method in the fragment shader.
Use the w Value (Fog_w.js)
Because the distance calculation within the shader can affect performance, an alternative
method allows you to easily approximate the calculation of the distance from the eye
point to the object (vertex) by using the w value of coordinates transformed by the model
view projection conversion. In this case, the coordinates are substituted in gl_Position .
The fourth component, w of gl_Position which you haven’t used explicitly before, is the
z value of each vertex in the view coordinate system multiplied by –1. The eye point is the
origin in the view coordinates, and the view direction is the negative direction of z, so z
is a negative value. The w value, which is the z value multiplied by –1, can be used as an
approximation of the distance.
If you reimplement the calculation in the vertex shader using w, as shown in Listing 10.7 ,
the fog effect will work as before.
Listing 10.7 Fog_w.js
1 // Fog_w.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
...
Make a Rounded Point
377
7 'varying vec4 v_Color;\n' +
8 'varying float v_Dist;\n' +
9 'void main() {\n' +
10 ' gl_Position = u_MvpMatrix * a_Position;\n' +
11 ' v_Color = a_Color;\n' +
12 // Use the negative z value of vertex in view coordinate system
13 ' v_Dist = gl_Position.w;\n' +
14 '}\n';
Make a Rounded Point
In Chapter 2 , you constructed a sample program that draws a point to help you under-
stand the basics of shaders. However, to allow you to focus on the operation of the
shaders, the point displayed wasn’t “round” but actually “square,” which is simpler to
draw. In this section, you construct a sample program, RoundedPoint , which draws a
round point (see Figure 10.9 ).
Figure 10.9 A screen shot of RoundedPoint
How to Implement a Rounded Point
To draw a “round” point, you just have to make the “rectangle” point round. This can be
achieved using the rasterization process that takes place between the vertex shader and
the fragment shader and was explained in Chapter 5 , “Using Colors and Texture Images.”
This rasterization process generates a rectangle consisting of multiple fragments, and each
fragment is passed to the fragment shader. If you draw these fragments as-is, a rectangle
will be displayed. So you just need to modify the fragment shader to draw only the frag-
ments inside the circle, as shown in Figure 10.10 .
CHAPTER 10 Advanced Techniques
378
d e d r a c s i d e b o t s t n e m g a r f
Figure 10.10 Discarding fragments to turn a rectangle into a circle
To achieve this, you need to know the position of each fragment created during rasteriza-
tion. In Chapter 5 , you saw a sample program that uses the built-in variable gl_FragCoord
to pass (input) the data to the fragment shader. In addition to this, there is one more
built-in variable gl_PointCoord , which is suitable for drawing a round point (see Table
10.1 ).
Table 10.1 Built-In Variables of Fragment Shader (Input)
Type and Name of Variable Description
vec4 gl_FragCoord Window coordinates of fragment
vec4 gl_PointCoord Position of fragment in the drawn point (0.0 to 1.0)
gl_PointCoord gives the position of each fragment taken from the range (0.0, 0.0) to (1.0,
1.0), as shown in Figure 10.11 . To make the rectangle round, you simply have to discard
the fragments outside the circle centered at (0.5, 0.5) with radius 0.5. You can use the
discard statement to discard these fragments.
) 5 . 0 , 5 . 0 (
5 . 0
) 0 . 0 , 0 . 0 (
) 0 . 1 , 0 . 1 (
Figure 10.11 Coordinates of gl_PointCoord
Sample Program (RoundedPoints.js)
The sample program is shown in Listing 10.8 . This is derived from MultiPoint.js , which
was used in Chapter 4 , “More Transformations and Basic Animation,” to draw multiple
Make a Rounded Point
379
points. The only difference is in the fragment shader. The vertex shader is also shown for
reference.
Listing 10.8 RoundedPoint.js
1 // RoundedPoints.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'void main() {\n' +
6 ' gl_Position = a_Position;\n' +
7 ' gl_PointSize = 10.0;\n' +
8 '}\n';
9
10 // Fragment shader program
11 var FSHADER_SOURCE =
...
15 'void main() {\n' + // Center coordinate is (0.5, 0.5)
16 ' float dist = distance(gl_PointCoord, vec2(0.5, 0.5));\n' +
17 ' if(dist < 0.5) {\n' + // Radius is 0.5
18 ' gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);\n' +
19 ' } else { discard; }\n' +
20 '}\n';
21
22 function main() {
...
53 gl.drawArrays(gl.POINTS, 0, n);
54 }
The key difference is the calculation, starting at line 16, which determines whether a frag-
ment should be discarded. gl_PointCoord holds the fragment’s position (specified in the
range 0.0 to 0.1), and the center point is (0.5, 0.5). Therefore, to make a rectangle point
round, you have to do the following:
1. Calculate the distance from the center (0.5, 0.5) to each fragment.
2. Draw those fragments for which the distance is less than 0.5.
In RoundedPoint.js , the distance calculation takes place at line 16. Here, you just have to
calculate the distance between the center point (0.5, 0.5) and gl_PointCoord . Because the
gl_PointCoord is a vec2 type, you need to pass (0.5, 0.5) to distance() as a vec2 .
Once you have calculated the distance from the center, it is used at line 17 to check
whether the distance is less than 0.5 (in other words, whether the fragment is in the
circle). If the fragment is in circle, the fragment is drawn so line 18 uses gl_FragColor to
set the draw color. Otherwise, at line 19, the discard statement causes WebGL to auto-
matically throw away the fragment.
CHAPTER 10 Advanced Techniques
380
Alpha Blending
The alpha value controls the transparency of drawn objects. If you specify 0.5 as the alpha
value, the object becomes semi-transparent, allowing anything drawn underneath it to be
partially visible. As the alpha value approaches 0, more of the background objects appear.
If you try this yourself, you’ll actually see that as you decrease the alpha value, WebGL
objects become white. This is because WebGL’s default behavior is to use the same alpha
value for both objects and the <canvas> . In the sample programs, the web page behind
the <canvas> is white, so this shows through.
Let’s construct a sample program that shows how to use alpha blending to get the desired
effect. The function that allows the use of the alpha value is called an alpha blending (or
simply blending ) function . This function is already built into WebGL, so you just need to
enable it to tell WebGL to start to use the alpha values supplied.
How to Implement Alpha Blending
You’ll need the following two steps to enable and use the alpha blending function.
1. Enable the alpha blending function:
gl.enable(gl.BLEND);
2. Specify the blending function:
gl.blendFunc(gl.SRC_ALPHA, gl.ONE_MINUS_SRC_ALPHA);
The blending function will be explained later, so let’s try using the sample program. Here,
we will reuse LookAtTrianglesWithKey_ViewVolume described in Chapter 7 , “Toward the
3D World.” As shown in Figure 10.12 , this program draws three triangles and allows the
position of the eye point to be changed using the arrow key.
Figure 10.12 A screen shot of LookAtTrianglesWithKeys_ViewVolume
Alpha Blending
381
Let’s add the code for steps 1 and 2, specify 0.4 as the alpha value of the color of the
triangles, and call the resulting program LookAtBlendedTriangles . Figure 10.13 shows the
effect when run. As you can see, all triangles became semitransparent, and you are able to
see the triangles behind. When you move the eye point with the arrow key, you can see
that the blending is continuously taking place.
Figure 10.13 A screen shot of LookAtBlendedTriangles
Let’s look at the sample program.
Sample Program (LookAtBlendedTriangles.js)
LookAtBlendedTriangles.js is shown in Listing 10.9 . The code that has changed is in
lines 51 to 54, and the alpha value (0.4) is added to the definition of color information
in initVertexBuffer() at lines 81 to 91. Accordingly, the size and stride parameters have
changed for gl.vertexAttribPointer() .
Listing 10.9 LookAtBlenderTriangles.js
1 // LookAtBlendedTriangles.js
2 // LookAtTrianglesWithKey_ViewVolume.js is the original
...
25 function main() {
...
43 var n = initVertexBuffers(gl);
...
51 // Enable alpha blending
52 gl.enable (gl.BLEND);
53 // Set blending function
54 gl.blendFunc(gl.SRC_ALPHA, gl.ONE_MINUS_SRC_ALPHA);
...
CHAPTER 10 Advanced Techniques
382
75 draw(gl, n, u_ViewMatrix, viewMatrix);
76 }
77
78 function initVertexBuffers(gl) {
79 var verticesColors = new Float32Array([
80 // Vertex coordinates and color(RGBA)
81 0.0, 0.5, -0.4, 0.4, 1.0, 0.4, 0.4 ,
82 -0.5, -0.5, -0.4, 0.4, 1.0, 0.4, 0.4 ,
...
91 0.5, -0.5, 0.0, 1.0, 0.4, 0.4, 0.4 ,
92 ]);
93 var n = 9;
...
127 return n;
128 }
Blending Function
Let’s explore the blending function gl.blendFunc() to understand how this can be used
to achieve the blending effect. You need two colors for blending: the color to blend
(source color) and the color to be blended (destination color). For example, when you
draw one triangle on top of the other, the color of the triangle already drawn is the desti-
nation color, and the color of the triangle drawn on top is the source color.
gl.blendFunc(src_factor, dst_factor)
Specify the method to blend the source color and the destination color. The blended
color is calculated as follows:
( ) = × + × color RGB source color src_factor destination color dst_factor   
Parameters src_factor Specifies the multiplier for the source color ( Table 10.2 ).
dst_factor Specifies the multiplier for the destination color ( Table
10.2 ).
Return value None
Errors INVALID_ENUM src_factor and dst_factor are none of the values in Table
10.2
Table 10.2 Constant Values that Can Be Specified as src_factor and dst_factor
3
Constant Multiplicand for R Multiplicand for G Multiplicand for B
gl.ZERO 0.0 0.0 0.0
gl.ONE 1.0 1.0 1.0
Alpha Blending
383
Constant Multiplicand for R Multiplicand for G Multiplicand for B
gl.SRC_COLOR Rs Gs Bs
gl.ONE_MINUS_SRC_COLOR (1 – Rs) (1 – Gs) (1 – Bs)
gl.DST_COLOR Rd Gd Bd
gl.ONE_MINUS_DST_COLOR (1 – Rd) (1 – Bd) (1 – Gd)
gl.SRC_ALPHA As As As
gl.ONE_MINUS_SRC_ALPHA (1 – As) (1 – As) (1 – As)
gl.DST_ALPHA Ad Ad Ad
gl.ONE_MINUS_DST_ALPHA (1 – Ad) (1 – Ad) (1 – Ad)
gl.SRC_ALPHA_SATURATE min(As, Ad) min(As, Ad) min(As, Ad)
3 gl.CONSTANT_COLOR, gl.ONE_MINUSCONSTANT_COLOR, gl.CONSTANT_ALPHA , and gl.ONE_
MINUS_CONSTANT_ALPHA are removed from OpenGL.
(Rs,Gs,Bs,As) is the source color and (Rd,Gd,Bd,Ad) is the destination color.
In the sample program, you used the following:
54 gl.blendFunc(gl.SRC_ALPHA, gl.ONE_MINUS_SRC_ALPHA);
For example, if the source color is semitransparent green (0.0, 1.0, 0.0, 0.4) and the desti-
nation color is yellow (1.0, 1.0, 0.0, 1.0), src_factor becomes the alpha value 0.4 and dst_
factor becomes (1 – 0.4)=0.6. The calculation is shown in Figure 10.14 .
= ) B G R ( r o l o c r o t c a f _ t s d * r o l o c n o i t a n i t s e d + r o t c a f _ c r s * r o l o c e c r u o s
B G R
0 . 0 0 . 1 0 . 0 r o l o c e c r u o s
4 . 0 4 . 0 4 . 0 r o t c a f _ c r s
0 . 0 4 . 0 0 . 0
B G R
0 . 0 0 . 1 0 . 1 r o l o c n o i t a n i t s e d
6 . 0 6 . 0 6 . 0 r o t c a f _ t s d
0 . 0 6 . 0 6 . 0
0 . 0 0 . 1 6 . 0 r o l o c d e d n e l B
* *
Figure 10.14 Calculation of gl.blendFunc(gl.SRC_ALPHA, gl.ONE_MINUS_SRC_ALPHA)
You can experiment with the other possible parameter values for src_factor , dst_factor , but
one that is often used is additive blending. When used, the result will become brighter
than the original value because it is a simple addition. It can be used for an indicator or
the lighting effect resulting from an explosion.
glBlendFunc(GL_SRC_ALPHA, GL_ONE);
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384
Alpha Blend 3D Objects (BlendedCube.js)
Let’s now explore the effects of alpha blending on a representative 3D object, a cube, by
making it semitransparent. You will reuse the ColoredCube sample program from Chapter
7 to create BlendedCube , which adds the two steps needed for blending (see Listing 10.10 ).
Listing 10.10 BlendedCube.js
1 // BlendedCube.js
...
47 // Set the clear color and enable the depth test
48 gl.clearColor(0.0, 0.0, 0.0, 1.0);
49 gl.enable(gl.DEPTH_TEST);
50 // Enable alpha blending
51 gl.enable (gl.BLEND);
52 // Set blending function
53 gl.blendFunc(gl.SRC_ALPHA, gl.ONE_MINUS_SRC_ALPHA);
Unfortunately, if you run this program as-is, you won’t see the expected result (right side
of Figure 10.15 ); rather, you will see something similar to the left side, which is no differ-
ent from the original ColoredCube used in Chapter 7 .
Figure 10.15 BlendedCube
This is because of the hidden surface removal function enabled at line 49. Blending only
takes place on the drawn surfaces. When the hidden surface removal function is enabled,
the hidden surfaces are not drawn, so there is no other surface to be blended with.
Therefore, you don’t see the blending effect as expected. To solve this problem, you can
simply comment out line 49 that enables the hidden surface removal function.
Alpha Blending
385
48 gl.clearColor(0.0, 0.0, 0.0, 1.0);
49 // gl.enable(gl.DEPTH_TEST);
50 // Enable alpha blending
51 gl.enable (gl.BLEND);
How to Draw When Alpha Values Coexist
This is a quick solution, but it’s not very satisfactory because, as we’ve seen in Chapter 7 ,
hidden surface removal is often needed to correctly draw a 3D scene.
You can overcome this problem by drawing objects while turning the hidden surface
removal function on and off.
1. Enable the hidden surface removal function.
gl.enable(gl.DEPTH_TEST);
2. Draw all the opaque objects (whose alpha values are 1.0).
3. Make the depth buffer ( Chapter 7 ), which is used in the hidden surface removal,
read-only.
gl.depthMask(false);
4. Draw all the transparent objects (whose alpha values are smaller than 1.0). Note,
they should be sorted by the depth order and drawn back to front.
5. Make the depth buffer readable and writable.
gl.depthMask(true);
If you completely disable the hidden surface removal function, when there are transpar-
ent objects behind opaque objects, the transparent object will not be hidden behind the
opaque objects. So you need to control that with gl.depthMask() . gl.depthMask() has the
following specification.
gl.depthMask(mask)
Enable or disable writing into the depth buffer.
Parameters mask Specifies whether the depth buffer is enabled for writing. If mask
is false , depth buffer writing is disabled.
Return value None
Errors None
CHAPTER 10 Advanced Techniques
386
The depth buffer was briefly introduced in Chapter 7 . The z values of fragments (which
are normalized to a value between 0.0 and 1.0) are written into the buffer. For example,
say there are two triangles on top of each other and you draw from the triangle on top.
First, the z value of the triangle on top is written into the depth buffer. Then, when the
triangle on bottom is drawn, the hidden surface removal function compares the z value
of its fragment that is going to be drawn, with the z value already written in the depth
buffer. Then only when the z value of the fragment that is going to be drawn is smaller
than the existing value in the buffer (that is, when it’s closer to the eye point) will the
fragment be drawn into the color buffer. This approach ensures that hidden surface
removal is achieved. Therefore, after drawing, the z value of the fragment of the surface
that can be seen from the eye point is left in the depth buffer.
Opaque objects are drawn into the color buffer in the correct order by removing the
hidden surfaces in the processing of steps 1 and 2, and the z value that represents the
order is written in the depth buffer. Transparent objects are drawn into the color buffer
using that z value in steps 3, 4, and 5, so the hidden surfaces of the transparent objects
behind the opaque objects will be removed. This results in the correct image being shown
where both objects coexist.
Switching Shaders
The sample programs in this book draw using a single vertex shader and a single fragment
shader. If all objects can be drawn with the same shaders, there is no problem. However,
if you want to change the drawing method for each object, you need to add significant
complexity to the shaders to achieve multiple effects. A solution is to prepare more than
one shader and then switch between these shaders as required. Here, you construct a
sample program, ProgramObject , which draws a cube colored with a single color and
another cube with a texture image. Figure 10.16 shows a screen shot.
Figure 10.16 A screen shot of ProgramObject
Switching Shaders
387
This program is also an example of the shading of an object with a texture image.
How to Implement Switching Shaders
The shaders can be switched easily by creating program objects, as explained in Chapter 8 ,
“Lighting Objects,” and switching them before drawing. Switching is carried out using the
function gl.useProgram() . Because you are explicitly manipulating shader objects, you
cannot use the convenience function initShaders() . However, you can use the function
createProgram() in cuon-utils.js , which is called from initShaders() .
The following is the processing flow of the sample program. It performs the same proce-
dure twice, so it looks long, but the essential code is simple:
1. Prepare the shaders to draw an object shaded with a single color.
2. Prepare the shaders to draw an object with a texture image.
3. Create a program object that has the shaders from step 1 with createProgram() .
4. Create a program object that has the shaders from step 2 with createProgram() .
5. Specify the program object created by step 3 with gl.useProgram() .
6. Enable the buffer object after assigning it to the attribute variables.
7. Draw a cube (drawn in a single color).
8. Specify the program object created in step 4 using gl.useProgram() .
9. Enable the buffer object after assigning it to the attribute variables.
10. Draw a cube (texture is pasted).
Now let’s look at the sample program.
Sample Program (ProgramObject.js)
The key program code for steps 1 to 4 is shown in Listing 10.11 . Two types of vertex
shader and fragment shader are prepared: SOLID_VSHADER_SOURCE (line 3) and SOLID_
FSHADER_SOURCE (line 19) to draw an object in a single color, and TEXTURE_VSHADER_SOURCE
(line 29) and TEXTURE_FSHADER_SOURCE (line 46) to draw an object with a texture image.
Because the focus here is on how to switch the program objects, the contents of the
shaders are omitted.
Listing 10.11 ProgramObject (Process for Steps 1 to 4)
1 // ProgramObject.js
2 // Vertex shader for single color drawing <- (1)
3 var SOLID_VSHADER_SOURCE =
...
18 // Fragment shader for single color drawing
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19 var SOLID_FSHADER_SOURCE =
...
28 // Vertex shader for texture drawing <- (2)
29 var TEXTURE_VSHADER_SOURCE =
...
45 // Fragment shader for texture drawing
46 var TEXTURE_FSHADER_SOURCE =
...
58 function main() {
...
69 // Initialize shaders
70 var solidProgram = createProgram (gl, SOLID_VSHADER_SOURCE,
➥SOLID_FSHADER_SOURCE); <- (3)
71 var texProgram = createProgram (gl, TEXTURE_VSHADER_SOURCE,
➥TEXTURE_FSHADER_SOURCE); <- (4)
...
77 // Get the variables in the program object for single color drawing
78 solidProgram.a_Position = gl.getAttribLocation(solidProgram, 'a_Position');
79 solidProgram.a_Normal = gl.getAttribLocation(solidProgram, 'a_Normal');
...
83 // Get the storage location of attribute/uniform variables
84 texProgram.a_Position = gl.getAttribLocation(texProgram, 'a_Position');
85 texProgram.a_Normal = gl.getAttribLocation(texProgram, 'a_Normal');
...
89 texProgram.u_Sampler = gl.getUniformLocation(texProgram, 'u_Sampler');
...
99 // Set vertex information
100 var cube = initVertexBuffers(gl, solidProgram);
...
106 // Set texture
107 var texture = initTextures(gl, texProgram);
...
122 // Start drawing
123 var currentAngle = 0.0; // Current rotation angle (degrees)
124 var tick = function() {
125 currentAngle = animate(currentAngle); // Update rotation angle
...
128 // Draw a cube in single color
129 drawSolidCube(gl, solidProgram, cube, -2.0, currentAngle, viewProjMatrix);
130 // Draw a cube with texture
131 drawTexCube(gl, texProgram, cube, texture, 2.0, currentAngle,
➥viewProjMatrix);
132
133 window.requestAnimationFrame(tick, canvas);
Switching Shaders
389
134 };
135 tick();
136 }
137
138 function initVertexBuffers(gl, program) {
...
148 var vertices = new Float32Array([ // Vertex coordinates
149 1.0, 1.0, 1.0, -1.0, 1.0, 1.0, -1.0,-1.0, 1.0, 1.0,-1.0, 1.0,
150 1.0, 1.0, 1.0, 1.0,-1.0, 1.0, 1.0,-1.0,-1.0, 1.0, 1.0,-1.0,
...
154 1.0,-1.0,-1.0, -1.0,-1.0,-1.0, -1.0, 1.0,-1.0, 1.0, 1.0,-1.0
155 ]);
156
157 var normals = new Float32Array([ // Normal
...
164 ]);
165
166 var texCoords = new Float32Array([ // Texture coordinates
...
173 ]);
174
175 var indices = new Uint8Array([ // Indices for vertices
...
182 ]);
183
184 var o = new Object(); // Use Object to return buffer objects
185
186 // Write vertex information to buffer object
187 o.vertexBuffer = initArrayBufferForLaterUse(gl, vertices, 3, gl.FLOAT);
188 o.normalBuffer = initArrayBufferForLaterUse(gl, normals, 3, gl.FLOAT);
189 o.texCoordBuffer = initArrayBufferForLaterUse(gl, texCoords, 2, gl.FLOAT);
190 o.indexBuffer = initElementArrayBufferForLaterUse(gl, indices,
➥gl.UNSIGNED_BYTE);
...
193 o.numIndices = indices.length;
...
199 return o;
200 }
Starting with the main() function in JavaScript, you first create a program object for
each shader with createProgram() at lines 70 and 71. The arguments of the createPro-
gram() are the same as the initShaders() , and the return value is the program object.
You save each program object in solidProgram and texProgram . Then you retrieve the
storage location of the attribute and uniform variables for each shader at lines 78 to 89.
You will store them in the corresponding properties of the program object, as you did in
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MultiJointModel_segment.js . Again, you leverage JavaScript’s ability to freely append a
new property of any type to an object.
The vertex information is then stored in the buffer object by initVertexBuffers() at line
100. You need (1) vertex coordinates, (2) the normals, and (3) indices for the shader to
draw objects in a single color. In addition, for the shader to draw objects with a texture
image, you need the texture coordinates. The processing in initVertexBuffers() handles
this and binds the correct buffer object to the corresponding attribute variables when the
program object is switched.
initVertexBuffers() prepares the vertex coordinates from line 148, normals from line
157, texture coordinates from line 166, and index arrays from line 175. Line 184 creates
object ( o ) of type Object . Then you store the buffer object to the property of the object
(lines 187 to 190). You can maintain each buffer object as a global variable, but that intro-
duces too many variables and makes it hard to understand the program. By using proper-
ties, you can more conveniently manage all four buffer objects using one object o.
4
You use initArrayBufferForLaterUse() , explained in MultiJointModel_segment.js , to
create each buffer object. This function writes vertex information into the buffer object
but does not assign it to the attribute variables. You use the buffer object name as its
property name to make it easier to understand. Line 199 returns the object o as the return
value.
Once back in main() in JavaScript, the texture image is set up in initTextures() at line
107, and then everything is ready to allow you to draw the two cube objects. First, you
draw a single color cube using drawSolidCube() at line 129, and then you draw a cube
with a texture image by using drawTexCube() at line 131. Listing 10.12 shows the latter
half of the steps, steps 5 through 10.
Listing 10.12 ProgramObject.js (Processes for Steps 5 through 10)
236 function drawSolidCube(gl, program, o, x, angle, viewProjMatrix) {
237 gl.useProgram(program); // Tell this program object is used <-(5)
238
239 // Assign the buffer objects and enable the assignment <-(6)
240 initAttributeVariable(gl, program.a_Position, o.vertexBuffer);
241 initAttributeVariable(gl, program.a_Normal, o.normalBuffer);
242 gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, o.indexBuffer);
243
244 drawCube(gl, program, o, x, angle, viewProjMatrix); // Draw <-(7)
245 }
246
247 function drawTexCube(gl, program, o, texture, x, angle, viewProjMatrix) {
4 To keep the explanation simple, the object ( o ) was used. However, it is better programming practice
to create a new user-defi ned type for managing the information about a buffer object and to use it to
manage the four buffers.
Switching Shaders
391
248 gl.useProgram(program); // Tell this program object is used <-(8)
249
250 // Assign the buffer objects and enable the assignment <-(9)
251 initAttributeVariable(gl, program.a_Position, o.vertexBuffer);
252 initAttributeVariable(gl, program.a_Normal, o.normalBuffer);
253 initAttributeVariable(gl, program.a_TexCoord, o.texCoordBuffer);
254 gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, o.indexBuffer);
255
256 // Bind texture object to texture unit 0
257 gl.activeTexture(gl.TEXTURE0);
258 gl.bindTexture(gl.TEXTURE_2D, texture);
259
260 drawCube(gl, program, o, x, angle, viewProjMatrix); // Draw <-(10)
261 }
262
263 // Assign the buffer objects and enable the assignment
264 function initAttributeVariable(gl, a_attribute, buffer) {
265 gl.bindBuffer(gl.ARRAY_BUFFER, buffer);
266 gl.vertexAttribPointer(a_attribute, buffer.num, buffer.type, false, 0, 0);
267 gl.enableVertexAttribArray(a_attribute);
268 }
...
275 function drawCube(gl, program, o, x, angle, viewProjMatrix) {
276 // Calculate a model matrix
...
281 // Calculate transformation matrix for normal
...
286 // Calculate a model view projection matrix
...
291 gl.drawElements(gl.TRIANGLES, o.numIndices, o.indexBuffer.type, 0);
292 }
drawSolidCube() is defined at line 236 and uses gl.useProgram() at line 237 to tell
the WebGL system that you will use the program (program object, solidProgram )
specified by the argument. Then you can draw using solidProgram . The buffer objects
for vertex coordinates and normals are assigned to attribute variables and enabled by
initAttributeVariable() at lines 240 and 241. This function is defined at line 264. Line
242 binds the buffer object for the indices to gl.ELEMENT_ARRAY_BUFFER . With everything
set up, you then call drawCube() at line 244, which uses gl.drawElements() at line 291 to
perform the draw operation.
drawTexCube() , defined at line 247, follows the same steps as drawSolidCube() . Line 253
is added to assign the buffer object for texture coordinates to the attribute variables, and
lines 257 and 258 are added to bind the texture object to the texture unit 0. The actual
drawing is performed in drawCube() , just like drawSolidCube() .
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Once you’ve mastered this basic technique, you can use it to switch between any number
of shader programs. This way you can use a variety of different drawing effects in a single
scene.
Use What You’ve Drawn as a Texture Image
One simple but powerful technique is to draw some 3D objects and then use the result-
ing image as a texture image for another 3D object. Essentially, if you can use the content
you’ve drawn as a texture image, you are able to generate images on-the-fly. This means
you do not need to download images from the network, and you can apply special effects
(such as motion blur and depth of field) before displaying the image. You can also use
this technique for shadowing, which will be explained in the next section. Here, you will
construct a sample program, FramebufferObject , which maps a rotating cube drawn with
WebGL to a rectangle as a texture image. Figure 10.17 shows a screen shot.
Figure 10.17 FramebufferObject
If you actually run the program, you can see a rotating cube with a texture image of a
summer sky pasted to the rectangle as its texture. Significantly, the image of the cube that
is pasted on the rectangle is not a movie prepared in advance but a rotating cube drawn
by WebGL in real time. This is quite powerful, so let’s take a look at what WebGL must do
to achieve this.
Framebuffer Object and Renderbuffer Object
By default, the WebGL system draws using a color buffer and, when using the hidden
surface removal function, a depth buffer. The final image is kept in the color buffer.
The framebuffer object is an alternative mechanism you can use instead of a color buffer
or a depth buffer ( Figure 10.18 ). Unlike a color buffer, the content drawn in a framebuffer
Use What You’ve Drawn as a Texture Image
393
object is not directly displayed on the <canvas> . Therefore, you can use it if you want to
perform different types of processing before displaying the drawn content. Or you can use
it as a texture image. Such a technique is often referred to as offscreen drawing .
m e t s y S L G b e W
r e d a h S x e t r e V t n e m g a r F
r e d a h S
t p i r c S a v a J
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
; ) … ( s r e d a h S t i n i
…
}
g n i s s e c o r p t n e m g a r f - r e p g n i s s e c o r p x e t r e v - r e p
r e f f u B r o l o C
x
y
r e f f u b e m a r F
t c e j b O
Figure 10.18 Framebuffer object
The framebuffer object has the structure shown in Figure 10.19 and supports substitutes
for the color buffer and the depth buffer. As you can see, drawing is not carried out in
the framebuffer itself, but in the drawing areas of the objects that the framebuffer points
to. These objects are attached to the framebuffer using its attachment function. A color
attachment specifies the destination for drawing to be a replacement for the color buffer.
A depth attachment and a stencil attachment specify the replacements for the depth
buffer and stencil buffer.
r e f u b e m a r F
t c e j b o
r o l o C
t n e m h c a t a
h t p e D
t n e m h c a t a
l i c n e t S
t n e m h c a t a
e r u t x e T
t c e j b o
g n i w a r D
a e r a
e r u t x e T
t c e j b o
g n i w a r D
a e r a
Render-
buffer
object
g n i w a r D
a e r a
Render-
buffer
object
g n i w a r D
a e r a
Figure 10.19 Framebuffer object, texture object, renderbuffer object
WebGL supports two types of objects that can be used to draw objects within: the texture
object that you saw in Chapter 5 , and the renderbuffer object . With the texture object,
the content drawn into the texture object can be used as a texture image. The render-
buffer object is a more general-purpose drawing area, allowing a variety of data types to
be written.
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How to Implement Using a Drawn Object as a Texture
When you want to use the content drawn into a framebuffer object as a texture object,
you actually need to use the content drawn into the color buffer for the texture object.
Because you also want to remove the hidden surfaces for drawing, you will set up the
framebuffer object as shown in Figure 10.20 .
r e f u b e m a r F
t c e j b o
r o l o C
t n e m h c a t a
h t p e D
t n e m h c a t a
l i c n e t S
t n e m h c a t a
e r u t x e T
t c e j b o
g n i w a r D
a e r a
g n i w a r D
a e r a
Render-
buffer
object
a e r a g n i w a r d h c a e f o e z i s e h T
. l a c i t n e d i e b t s u m
Figure 10.20 Configuration of framebuffer object when using drawn content as a texture
The following eight steps are needed for realizing this configuration. These processes are
similar to the process for the buffer object. Step 2 was explained in Chapter 5 , so there are
essentially seven new processes:
1. Create a framebuffer object ( gl.createFramebuffer() ).
2. Create a texture object and set its size and parameters ( gl.createTexture() ,
gl.bindTexture() , gl.texImage2D() , gl.Parameteri() ).
3. Create a renderbuffer object ( gl.createRenderbuffer() ).
4. Bind the renderbuffer object to the target and set its size ( gl.bindRenderbuffer() ,
gl.renderbufferStorage() ).
5. Attach the texture object to the color attachment of the framebuffer object
( gl.bindFramebuffer() , gl.framebufferTexture2D() ).
6. Attach the renderbuffer object to the depth attachment of the framebuffer object
( gl.framebufferRenderbuffer() ).
7. Check whether the framebuffer object is configured correctly ( gl.checkFramebuffer-
Status() ).
8. Draw using the framebuffer object ( gl.bindFramebuffer() ).
Now let’s look at the sample program. The numbers in the sample program indicate the
code used to implement the steps.
Use What You’ve Drawn as a Texture Image
395
Sample Program (FramebufferObjectj.js)
Steps 1 to 7 of FramebufferObject.js are shown in Listing 10.13 .
Listing 10.13 FramebufferObject.js (Processes for Steps 1 to 7)
1 // FramebufferObject.js
...
24 // Size of offscreen
25 var OFFSCREEN_WIDTH = 256;
26 var OFFSCREEN_HEIGHT = 256;
27
28 function main() {
...
55 // Set vertex information
56 var cube = initVertexBuffersForCube(gl);
57 var plane = initVertexBuffersForPlane(gl);
...
64 var texture = initTextures(gl);
...
70 // Initialize framebuffer object (FBO)
71 var fbo = initFramebufferObject(gl);
...
80 var viewProjMatrix = new Matrix4();/ For color buffer
81 viewProjMatrix.setPerspective(30, canvas.width/canvas.height, 1.0, 100.0);
82 viewProjMatrix.lookAt(0.0, 0.0, 7.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
83
84 var viewProjMatrixFBO = new Matrix4(); // For FBO
85 viewProjMatrixFBO.setPerspective(30.0, OFFSCREEN_WIDTH/OFFSCREEN_HEIGHT,
➥1.0, 100.0);
86 viewProjMatrixFBO.lookAt(0.0, 2.0, 7.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
...
92 draw(gl, canvas, fbo, plane, cube, currentAngle, texture, viewProjMatrix,
➥viewProjMatrixFBO);
...
96 }
...
263 function initFramebufferObject(gl) {
264 var framebuffer, texture, depthBuffer;
...
274 // Create a framebuffer object (FBO) <-(1)
275 framebuffer = gl.createFramebuffer();
...
281 // Create a texture object and set its size and parameters <-(2)
282 texture = gl.createTexture(); // Create a texture object
...
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287 gl.bindTexture(gl.TEXTURE_2D, texture);
288 gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, OFFSCREEN_WIDTH,
➥OFFSCREEN_HEIGHT, 0, gl.RGBA, gl.UNSIGNED_BYTE, null);
289 gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
290 framebuffer.texture = texture; // Store the texture object
291
292 // Create a renderbuffer object and set its size and parameters
293 depthBuffer = gl.createRenderbuffer(); // Create a renderbuffer <-(3)
...
298 gl.bindRenderbuffer(gl.RENDERBUFFER, depthBuffer); <-(4)
299 gl.renderbufferStorage(gl.RENDERBUFFER, gl.DEPTH_COMPONENT16,
➥OFFSCREEN_WIDTH, OFFSCREEN_HEIGHT);
300
301 // Attach the texture and the renderbuffer object to the FBO
302 gl.bindFramebuffer(gl.FRAMEBUFFER, framebuffer);
303 gl.framebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0,
➥gl.TEXTURE_2D, texture, 0); <-(5)
304 gl.framebufferRenderbuffer(gl.FRAMEBUFFER, gl.DEPTH_ATTACHMENT,
➥gl.RENDERBUFFER, depthBuffer); <-(6)
305
306 // Check whether FBO is configured correctly <-(7)
307 var e = gl.checkFramebufferStatus(gl.FRAMEBUFFER);
308 if (e !== gl.FRAMEBUFFER_COMPLETE) {
309 console.log('Framebuffer object is incomplete: ' + e.toString());
310 return error();
311 }
312
...
319 return framebuffer;
320 }
The vertex shader and fragment shader are omitted because this sample program uses the
same shaders as TexturedQuad.js in Chapter 5 , which pasted a texture image on a rect-
angle. The sample program in this section draws two objects: a cube and a rectangle. Just
as you did in ProgramObject.js in the previous section, you assign multiple buffer objects
needed for drawing each object as properties of an Object object. Then you store the
object to the variables cube and plane . You will use them for drawing by assigning each
buffer in the object to the attribute variable.
The key point of this program is the initialization of the framebuffer object by init-
FramebufferObject() at line 71. The initialized framebuffer object is stored in a variable
fbo and passed as the third argument of draw() at line 92. You’ll return to the function
draw() later. For now let’s examine initFramebufferObject() , at line 263, step by step.
This function performs steps 1 to 7. The view projection matrix for the framebuffer object
is prepared separately at line 84 because it is different from the one used for a color buffer.
Use What You’ve Drawn as a Texture Image
397
Create Frame Buffer Object (gl.createFramebuffer())
You must create a framebuffer object before you can use it. The sample program creates it
at line 275:
275 framebuffer = gl.createFramebuffer();
You will use gl.createFramebuffer() to create the framebuffer object.
gl.createFramebuffer()
Create a framebuffer object.
Parameters None
Return value non-null The newly created framebuffer object.
null Failed to create a framebuffer object.
Errors None
You use gl.deleteFramebuffer() to delete the created framebuffer object.
gl.deleteFramebuffer(framebuffer)
Delete a framebuffer object.
Parameters framebuffer Specifies the framebuffer object to be deleted.
Return value None
Errors None
Once you have created the framebuffer object, you need to attach a texture object to the
color attachment and a renderbuffer object to the depth attachment in the framebuffer
object. Let’s start by creating the texture object for the color attachment.
Create Texture Object and Set Its Size and Parameters
You have already seen how to create a texture object and set up its parameters
( gl.TEXTURE_MIN_FILTER ) in Chapter 5 . You should note that its width and height are
OFFSCREEN_WIDTH and OFFSCREEN_HEIGHT , respectively. The size is smaller than that of the
<canvas> to make the drawing process faster.
282 texture = gl.createTexture(); // Create a texture object
...
287 gl.bindTexture(gl.TEXTURE_2D, texture);
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288 gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, OFFSCREEN_WIDTH, OFFSCREEN_HEIGHT, 0,
➥gl.RGBA, gl.UNSIGNED_BYTE, null );
289 gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
290 framebuffer.texture = texture; // Store the texture object
The gl.texImage2D() at line 288 allocates a drawing area in a texture object. You can allo-
cate a drawing area by specifying null to the last argument, which is used to specify an
Image object. You will use this texture object later, so store it in framebuffer.texture at
line 290.
That completes the preparation for a texture object that is attached to the color attach-
ment. Next, you need to create a renderbuffer object for the depth buffer.
Create Renderbuffer Object (gl.createRenderbuffer())
Like texture buffers, you need to create a renderbuffer object before using it. The sample
program does this at line 293.
293 depthBuffer = gl.createRenderbuffer(); // Create a renderbuffer
You use gl.createRenderbuffer() to create the renderbuffer object.
gl.createRenderbuffer()
Create a renderbuffer object.
Parameters None
Return value Non-null The newly created renderbuffer object.
Null Failed to create a renderbuffer object.
Errors None
You use gl.deleteRenderbuffer() to delete the created renderbuffer object.
gl.deleteRenderbuffer(renderbuffer)
Delete a renderbuffer object.
Parameters renderbuffer Specifies the renderbuffer object to be deleted.
Return value None
Errors None
Use What You’ve Drawn as a Texture Image
399
The created renderbuffer object is used as a depth buffer here, so you store it in a variable
named depthBuffer .
Bind Renderbuffer Object to Target and Set Size
(gl.bindRenderbuffer(), gl.renderbufferStorage())
When using the created renderbuffer object, you need to bind the renderbuffer object to a
target and perform the operation on that target.
298 gl.bindRenderbuffer(gl.RENDERBUFFER, depthBuffer);
299 gl.renderbufferStorage(gl.RENDERBUFFER, gl.DEPTH_COMPONENT16,
➥OFFSCREEN_WIDTH, OFFSCREEN_HEIGHT);
The renderbuffer object is bound to a target with gl.bindRenderbuffer() .
gl.bindRenderbuffer(target, renderbuffer)
Bind the renderbuffer object specified by renderbuffer to target . If null is specified as
renderbuffer , the renderbuffer is unbound from the target .
Parameters target Must be gl.RENDERBUFFER.
renderbuffer Specifies the renderbuffer object.
Return value None
Errors INVALID_ENUM target is not gl.RENDERBUFFER
When the binding is complete, you can set the format, width, and height of the render-
buffer object by using gl.renderbufferStorage() . You must set the same width and
height as the texture object that is used as the color attachment.
gl.renderbufferStorage(target, internalformat, width, height)
Create and initialize a renderbuffer object’s data store.
Parameters target Must be gl.RENDERBUFFER.
internalformat Specifies the format of the renderbuffer.
gl.DEPTH_
COMPONENT16
The renderbuffer is used as a depth buffer.
gl.STENCIL_
INDEX8
The renderbuffer is used as a stencil buffer.
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gl.RGBA4 The renderbuffer is used as a color buffer. gl.RGBA4
(each RGBA component has 4, 4, 4, and 4 bits, respec-
tively), gl.RGB5_A1 (each RGB component has 5 bits,
and A has 1 bit), gl.RGB565 (each RGB component has
5, 6, and 5 bits, respectively)
gl.RGB5_A1
gl.RGB565
width, height Specifies the width and height of the renderbuffer in
pixels.
Return value None
Errors INVALID_ENUM Target is not gl.RENDERBUFFER or internalformat is none
of the preceding values.
INVALID_OPERATION No renderbuffer is bound to target .
The preparations of the texture object and renderbuffer object of the framebuffer object
are now complete. At this stage, you can use the object for offscreen drawing.
Set Texture Object to Framebuffer Object (gl.bindFramebuffer(),
gl.framebufferTexture2D())
You use a framebuffer object in the same way you use a renderbuffer object: You need to
bind it to a target and operate on the target, not the framebuffer object itself.
302 gl.bindFramebuffer(gl.FRAMEBUFFER, framebuffer); // Bind to target
303 gl.framebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.TEXTURE_2D,
➥texture, 0);
A framebuffer object is bound to a target with gl.bindFramebuffer() .
gl.bindFramebuffer(target, framebuffer)
Bind a framebuffer object to a target. If framebuffer is null , the binding is broken.
Parameters target Must be gl.FRAMEBUFFER.
framebuffer Specify the framebuffer object.
Return value None
Errors INVALID_ENUM target is not gl.FRAMEBUFFER
Once the framebuffer object is bound to target , you can use the target to write a texture
object to the framebuffer object. In this sample, you will use the texture object instead of
a color buffer so you attach the texture object to the color attachment of the framebuffer.
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401
You can assign the texture object to the framebuffer object with gl.
framebufferTexture2D() .
gl.framebufferTexture2D(target, attachment, textarget, texture,
level)
Attach a texture object specified by texture to the framebuffer object bound by target.
Parameters target Must be gl.FRAMEBUFFER .
attachment Specifies the attachment point of the framebuffer.
gl.COLOR_ATTACHMENT0 texture is used as a color buffer
gl.DEPTH_ATTACHMENT texture is used as a depth buffer
textarget Specifies the first argument of gl.texImage2D()
( gl.TEXTURE_2D or gl.CUBE_MAP_TEXTURE ).
texture Specifies a texture object to attach to the frame-
buffer attachment point.
level Specifies 0 (if you use a MIPMAP in texture , you
should specify its level).
Return value None
Errors INVALID_ENUM target is not gl.FRAMEBUFFER . attachment
or textarget is none of the preceding values.
INVALID_VALUE level is not valid.
INVALID_OPERATION No framebuffer object is bound to target.
The 0 in the gl.COLOR_ATTACHMENT0 used for the attachment parameter is because a frame-
buffer object in OpenGL, the basis of WebGL, can hold multiple color attachments
( gl.COLOR_ATTACHMENT0 , gl.COLOR_ATTACHMENT1 , gl.COLOR_ATTACHMENT2 ...). However,
WebGL can use just one of them.
Once the color attachment has been attached to the framebuffer object, you need to
assign a renderbuffer object as a depth attachment. This follows a similar process.
Set Renderbuffer Object to Framebuffer Object
(gl.framebufferRenderbuffer())
You will use gl.framebufferRenderbuffer() to attach a renderbuffer object to a frame-
buffer object. You need a depth buffer because this sample program will remove hidden
surfaces. So the depth attachment needs to be attached.
CHAPTER 10 Advanced Techniques
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304 gl.framebufferRenderbuffer(gl.FRAMEBUFFER, gl.DEPTH_ATTACHMENT,
gl.RENDERBUFFER, depthBuffer);
gl.framebufferRenderbuffer(target, attachment, renderbuffertarget,
renderbuffer)
Attach a renderbuffer object specified by renderbuffer to the framebuffer object bound by
target.
Parameters target Must be gl.FRAMEBUFFER .
attachment Specifies the attachment point of the framebuffer.
gl.COLOR_ATTACHMENT0 renderbuffer is used as a color buffer.
gl.DEPTH_ATTACHMENT renderbuffer is used as a depth buffer.
gl.STENCIL_ATTACHMENT renderbuffer is used as a stencil buffer.
renderbuffertarget Must be gl.RENDERBUFFER.
renderbuffer Specifies a renderbuffer object to attach to the
framebuffer attachment point
Return value None
Errors INVALID_ENUM target is not a gl.FRAMEBUFFER . attachment is
none of the above values. renderbuffertarget is
not gl.RENDERBUFFER .
Now that you’ve completed the preparation of the color attachment and depth attach-
ment to the framebuffer object, you are ready to draw. But before that, let’s check that the
configuration of the framebuffer object is correct.
Check Configuration of Framebuffer Object
(gl.checkFramebufferStatus())
Obviously, when you use a framebuffer that is not correctly configured, an error occurs. As
you have seen in the past few sections, preparing a texture object and renderbuffer object
that are needed to configure the framebuffer object is a complex process that sometimes
generates mistakes. You can check whether the created framebuffer object is configured
correctly and is available with gl.checkFramebufferStatus() .
307 var e = gl.checkFramebufferStatus(gl.FRAMEBUFFER); <- (7)
308 if (gl.FRAMEBUFFER_COMPLETE !== e) {
309 console.log('Frame buffer object is incomplete:' + e.toString());
310 return error();
311 }
Use What You’ve Drawn as a Texture Image
403
The following shows the specification of gl.checkFramebufferStatus() .
gl.checkFramebufferStatus(target)
Check the completeness status of a framebuffer bound to target.
Parameters target Must be gl.FRAMEBUFFER.
Return value 0 Target is not gl.FRAMEBUFFER.
Others
gl.FRAMEBUFFER_COMPLETE The framebuffer object is configured
correctly.
gl.FRAMEBUFFER_ INCOMPLETE_
ATTACHMENT
One of the framebuffer attachment points
is incomplete. (The attachment is not suffi-
cient. The texture object or the renderbuf-
fer object is invalid.)
gl.FRAMEBUFFER_ INCOMPLETE_
DIMENSIONS
The width or height of the texture object
or renderbuffer object of the attachment is
different.
gl.FRAMEBUFFER_ INCOMPLETE_
MISSING_ ATTACHMENT
The framebuffer does not have at least
one valid attachment.
Errors INVALID_ENUM target is not gl.FRAMEBUFFER .
That completes the preparation of the framebuffer object. Let’s now take a look at the
draw() function.
Draw Using the Framebuffer Object
Listing 10.14 shows draw() . It switches the drawing destination to fbo (the framebuffer)
and draws a cube in the texture object. Then drawTexturedPlane() uses the texture object
to draw a rectangle to the color buffer.
Listing 10.14 FramebufferObject.js (Process of (8))
321 function draw(gl, canvas, fbo, plane, cube, angle, texture, viewProjMatrix,
➥viewProjMatrixFBO) {
322 gl.bindFramebuffer(gl.FRAMEBUFFER, fbo); <-(8)
323 gl.viewport(0, 0, OFFSCREEN_WIDTH, OFFSCREEN_HEIGHT); // For FBO
324
325 gl.clearColor(0.2, 0.2, 0.4, 1.0); // Color is slightly changed
326 gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT); // Clear FBO
CHAPTER 10 Advanced Techniques
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327 // Draw the cube
328 drawTexturedCube(gl, gl.program, cube, angle, texture, viewProjMatrixFBO);
329 // Change the drawing destination to color buffer
330 gl.bindFramebuffer(gl.FRAMEBUFFER, null);
331 // Set the size of view port back to that of <canvas>
332 gl.viewport(0, 0, canvas.width, canvas.height);
333 gl.clearColor(0.0, 0.0, 0.0, 1.0);
334 gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
335 // Draw the plane
336 drawTexturedPlane(gl, gl.program, plane, angle, fbo.texture , viewProjMatrix);
337 }
Line 322 switches the drawing destination to the framebuffer object using gl.bindFrame-
buffer() . As a result, draw operations using gl.drawArrays() or gl.drawElements() are
performed for the framebuffer object. Line 332 uses gl.viewport() to specify the draw
area in the buffer (an offscreen area).
gl.viewport(x, y, width, height)
Set the viewport where gl.drawArrays() or gl.drawElements() draws. In WebGL, x and
y are specified in the <canvas> coordinate system.
Parameters x, y Specify the lower-left corner of the viewport rectangle (in
pixels).
width, height Specify the width and height of the viewport (in pixels).
Return value None
Errors None
Line 326 clears the texture image and the depth buffer bound to the framebuffer object.
When a cube is drawn at line 328, it is drawn in the texture image. To make it easier to
see the result, the clear color at line 325 is changed to a purplish blue from black. The
result of this is that the cube has been drawn into the texture buffer and is now available
for use as a texture image. The next step is to draw a rectangle ( plane ) using this texture
image. In this case, because you want to draw in the color buffer, you need to set the
drawing destination back to the color buffer. This is done at line 330 by specifying null
for the second argument of gl.bindFramebuffer() (that is, cancelling the binding). Then
line 336 draws the plane . You should note that fbo.texture is passed as the texture argu-
ment and used to map the drawn content to the rectangle. You will notice that in this
sample program, the texture image is mapped onto the back side of the rectangle. This is
because WebGL, by default, draws both sides of a polygon. You can eliminate the back
face drawing by enabling the culling function using gl.enable(gl.CULL_FACE) , which
increases the drawing speed (ideally making it twice as fast).
Display Shadows
405
Display Shadows
Chapter 8 explained shading, which is one of the phenomena when light hits an object.
We briefly mentioned shadowing, another phenomena, but didn’t explain how to imple-
ment it. Let’s take a look at that now. There are several methods to realize shadowing,
but we will explain a method that uses a shadow map (depth map). This method is quite
expressive and used in a variety of computer graphics situations and even in special effects
in movies.
How to Implement Shadows
The shadow map method is based on the idea that the sun cannot see the shadow of
objects. Essentially, it works by considering the viewer’s eye point to be at the same posi-
tion as the light source and determining what can be seen from that point. All the objects
you can see would appear to be in the light. Anything behind those objects would be in
shadow. With this method, you can use the distance to the objects (in fact, you will use
the z value, which is the depth value) from the light source to judge whether the objects
are visible. As you can see in Figure 10.21 , where there are two points on the same line,
P1 and P2, P2 is in the shadow because the distance from the light source to P2 is longer
than P1.
z
y
1 p
2 p
w o d a h s y b d e n e k r a d a e r a e h T
Figure 10.21 Theory of shadow map
You need two pairs of shaders for this process: (1) a pair of shaders that calculate the
distance from the light source to the objects, and (2) a pair of shaders that draws the
shadow using the calculated distance. Then you need a method to pass the distance data
from the light source calculated in the first pair of shaders to the second pair of shaders.
You can use a texture image for this purpose. This texture image is called the shadow
map , so this method is called shadow mapping . The shadow mapping technique consists
of the following two processes:
1. Move the eye point to the position of the light source and draw objects from there.
Because the fragments drawn from the position are hit by the light, you write the
distances from the light source to each fragment in the texture image (shadow map).
2. Move the eye point back to the position from which you want to view the objects
and draw them from there. Compare the distance from the light source to the frag-
ments drawn in this step and the distance recorded in the shadow map from step
CHAPTER 10 Advanced Techniques
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1. If the former distance is greater, you can draw the fragment as in shadow (in the
darker color).
You will use the framebuffer object in step 1 to save the distance in the texture image.
Therefore, the configurations of the framebuffer object used here is the same as that of
FramebufferObject.js in Figure 10.20 . You also need to switch pairs of shaders between
steps 1 and 2 using the technique you learned in the section “Switching Shaders,” earlier
in this chapter. Now let’s take a look at the sample program Shadow . Figure 10.22 shows
a screen shot where you can see a shadow of the red triangle cast onto the slanted white
rectangle.
Figure 10.22 Shadow
Sample Program (Shadow.js)
The key aspects of shadowing take place in the shaders, which are shown in Listing 10.15 .
Listing 10.15 Shadow.js (Shader part)
1 // Shadow.js
2 // Vertex shader program to generate a shadow map
3 var SHADOW_VSHADER_SOURCE =
...
6 'void main() {\n' +
7 ' gl_Position = u_MvpMatrix * a_Position;\n' +
8 '}\n';
9
10 // Fragment shader program for creating a shadow map
11 var SHADOW_FSHADER_SOURCE =
Display Shadows
407
...
15 'void main() {\n' +
16 ' gl_FragColor = vec4(gl_FragCoord.z, 0.0, 0.0, 0.0);\n' + <-(1)
17 '}\n';
18
19 // Vertex shader program for regular drawing
20 var VSHADER_SOURCE =
...
23 'uniform mat4 u_MvpMatrix;\n' +
24 'uniform mat4 u_MvpMatrixFromLight;\n' +
25 'varying vec4 v_PositionFromLight;\n' +
26 'varying vec4 v_Color;\n' +
27 'void main() {\n' +
28 ' gl_Position = u_MvpMatrix * a_Position;\n' +
29 ' v_PositionFromLight = u_MvpMatrixFromLight * a_Position;\n' +
30 ' v_Color = a_Color;\n' +
31 '}\n';
32
33 // Fragment shader program for regular drawing
34 var FSHADER_SOURCE =
...
38 'uniform sampler2D u_ShadowMap;\n' +
39 'varying vec4 v_PositionFromLight;\n' +
40 'varying vec4 v_Color;\n' +
41 'void main() {\n' +
42 ' vec3 shadowCoord =(v_PositionFromLight.xyz/v_PositionFromLight.w)
➥/ 2.0 + 0.5;\n' +
43 ' vec4 rgbaDepth = texture2D(u_ShadowMap, shadowCoord.xy);\n' +
44 ' float depth = rgbaDepth.r;\n' + // Retrieve the z value from R
45 ' float visibility = (shadowCoord.z > depth + 0.005) ? 0.7:1.0;\n'+ <-(2)
46 ' gl_FragColor = vec4(v_Color.rgb * visibility, v_Color.a);\n' +
47 '}\n';
Step 1 is performed in the shader responsible for the shadow map, defined from lines 3 to
17. You just switch the drawing destination to the framebuffer object, pass a model view
projection matrix in which an eye point is located at a light source to u_MvpMatrix , and
draw the objects. This results in the distance from the light source to the fragments being
written into the texture map (shadow map) attached to the framebuffer object. The vertex
shader at line 7 just multiplies the model view projection matrix by the vertex coordinates
to calculate this distance. The fragment shader is more complex and needs to calculate the
distance from the light source to the drawn fragments. For this purpose, you can utilize
the built-in variable gl_FragCoord of the fragment shader used in Chapter 5 .
gl_FragCoord is a vec4 type built-in variable that contains the coordinates of each frag-
ment. gl_FragCoord.x and gl_FragCoord.y represents the position of the fragment on the
CHAPTER 10 Advanced Techniques
408
screen, and gl_FragCoord.z contains the normalized z value in the range of [0, 1]. This is
calculated using ( gl_Position.z / gl.Position.w)/2.0+0.5. (See Section 2.12 of OpenGL ES
2.0 specification for further details.) gl_FragCoord.z is specified in the range of 0.0 to 1.0,
with 0.0 representing the fragments on the near clipping plane and 1.0 representing those
on the far clipping plane. This value is written into the R (red) component value (any
component could be used) in the shadow map at line 16.
16 ' gl_FragColor = vec4(gl_FragCoord.z, 0.0, 0.0, 0.0);\n' + <-(1)
Subsequently, the z value for each fragment drawn from the eye point placed at the light
source is written into the shadow map. This shadow map is passed to u_ShadowMap at
line 38.
For step 2, you need to draw the objects again after resetting the drawing destination
to the color buffer and moving the eye point to its original position. After drawing the
objects, you decide a fragment color by comparing the z value of the fragment with
that stored in the shadow map. This is done in the normal shaders from lines 20 to 47.
u_MvpMatrix is the model view projection matrix where the eye point is placed at the orig-
inal position and uMvpMatrixFromLight , which was used to create the shadow map, is the
model view projection matrix where the eye point is moved to the light source. The main
task of the vertex shader defined at line 20 is calculating the coordinates of each fragment
from the light source and passing them to the fragment shader (line 29) to obtain the z
value of each fragment from the light source.
The fragment shader uses the coordinates to calculate the z value (line 42). As mentioned,
the shadow map contains the value of (gl_Position.z/gl.Position.w)/2.0+0.5 . So
you could simply calculate the z value to compare with the value in the shadow map
by (v_PositionFromLight.z/v_PositionFromLight.w)/2.0+0.5 . However, because you
need to get the texel value from the shadow map, line 42 performs the following extra
calculation using the same operation. To compare to the value in the shadow map, you
need to get the texel value from the shadow map whose texture coordinates correspond
to the coordinates ( v_PositionFromLight.x, v_PositionFromLight.y ). As you know,
v_PositionFromLight.x and v_PositionFromLight.y are the x and y coordinates in the
WebGL coordinate system (see Figure 2.18 in Chapter 2 ), and they range from –1.0 to 1.0.
On the other hand, the texture coordinates s and t in the shadow map range from 0.0 to
1.0 (see Figure 5.20 in Chapter 5 ). So, you need to convert the x and y coordinates to the s
and t coordinates. You can also do this with the same expression to calculate the z value.
That is:
The texture coordinate s is (v_PositionFromLight.x/v_PositionFromLight.w)/2.0 + 0.5 .
The texture coordinate t is (v_PositionFromLight.y/v_PositionFromLight.w)/2.0 + 0.5 .
See also Section 2.12 of the OpenGL ES 2.0 specification
5 for further details about this calcu-
lation. These are carried out using the same type of calculation and can be achieved in
one line, as shown at line 42:
5 www.khronos.org/registry/gles/specs/2.0/es_full_spec_2.0.25.pdf
Display Shadows
409
42 ' vec3 shadowCoord =(v_PositionFromLight.xyz/v_PositionFromLight.w)
➥/ 2.0 + 0.5;\n' +
43 ' vec4 rgbaDepth = texture2D(u_ShadowMap, shadowCoord.xy);\n' +
44 ' float depth = rgbaDepth.r;\n' + // Retrieve the z value from R
You retrieve the value from the shadow map at lines 43 and 44. Only the R value is
retrieved using rgbaDepth.r at line 44 because you wrote it into the R component at line
16. Line 45 checks whether that fragment is in the shadow. When the position of the
fragment is determined to be greater than the depth (that is, shadowCoord.z > depth) , a
value of 0.7 is stored in visibility . The visibility is used at line 46 to draw the shadow
with a darker color:
45 ' float visibility = (shadowCoord.z > depth + 0.005) ? 0.7:1.0;\n'+
46 ' gl_FragColor = vec4(v_Color.rgb * visibility, v_Color.a);\n' +
Line 45 adds a small offset of 0.005 to the depth value. To understand why this is needed,
try running the sample program without this number. You will see a striped pattern as
shown in Figure 10.23 , referred to as the Mach band .
Figure 10.23 Striped pattern
The value of 0.005 is added to suppress the stripe pattern. The stripe pattern occurs
because of the precision of the numbers you can store in the RGBA components. It’s a
little complex, but it’s worth understanding because this problem occurs elsewhere in 3D
graphics. The z value of the shadow map is stored in the R component of RGBA in the
texture map, which is an 8-bit number. This means that the precision of R is lower than
its comparison target ( shadowCoord.z ), which is of type float . For example, let the z value
simply be 0.1234567. If you represent the value using 8 bits, in other words using 256
CHAPTER 10 Advanced Techniques
410
possibilities, you can represent the value in a precision of 1/256 (=0.0390625). So you can
represent 0.1234567 as follows:
0.1234567 / (1 / 256) = 31.6049152
Numbers below the decimal point cannot be used in 8 bits, so only 31 can be stored
in 8 bits. When you divide 31 by 256, you obtain 0.12109375 which, as you can see, is
smaller than the original value (0.1234567). This means that even if the fragment is at
the same position, its z value stored in the shadow map becomes smaller than its z value
in shadowCoord.z . As a result, the z value in shadowCoord.z becomes larger than that in
the shadow map according to the position of the fragment resulting in the stripe patterns.
Because this happens because the precision of the R value is 1/256 (=0.00390625), by
adding a small offset, such as 0.005, to the R value, you can stop the stripe pattern from
appearing. Note that any offset greater than 1/256 will work; 0.005 was chosen because it
is 1/256 plus a small margin.
Next, let’s look at the JavaScript program that passes the data to the shader (see Listing
10.16 ) with a focus on the type of transformation matrices passed. To draw a shadow
clearly, the size of a texture map for the offscreen rendering defined at line 49 is larger
than that of the <canvas> .
Listing 10.16 Shadow.js (JavaScript Part)
49 var OFFSCREEN_WIDTH = 1024, OFFSCREEN_HEIGHT = 1024;
50 var LIGHT_X = 0, LIGHT_Y = 7, LIGHT_Z = 2;
51
52 function main() {
...
63 // Initialize shaders for generating a shadow map
64 var shadowProgram = createProgram(gl, SHADOW_VSHADER_SOURCE,
➥SHADOW_FSHADER_SOURCE);
...
72 // Initialize shaders for regular drawing
73 var normalProgram = createProgram(gl, VSHADER_SOURCE, FSHADER_SOURCE);
...
85 // Set vertex information
86 var triangle = initVertexBuffersForTriangle(gl);
87 var plane = initVertexBuffersForPlane(gl);
...
93 // Initialize a framebuffer object (FBO)
94 var fbo = initFramebufferObject(gl);
...
99 gl.activeTexture(gl.TEXTURE0); // Set a texture object to the texture unit
100 gl.bindTexture(gl.TEXTURE_2D, fbo.texture);
...
106 var viewProjMatrixFromLight = new Matrix4(); // For the shadow map
Display Shadows
411
107 viewProjMatrixFromLight.setPerspective(70.0,
➥OFFSCREEN_WIDTH/OFFSCREEN_HEIGHT, 1.0, 100.0);
108 viewProjMatrixFromLight.lookAt(LIGHT_X, LIGHT_Y, LIGHT_Z, 0.0, 0.0, 0.0, 0.0,
➥1.0, 0.0);
109
110 var viewProjMatrix = new Matrix4(); // For regular drawing
111 viewProjMatrix.setPerspective(45, canvas.width/canvas.height, 1.0, 100.0);
112 viewProjMatrix.lookAt(0.0, 7.0, 9.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
113
114 var currentAngle = 0.0; // Current rotation angle [degrees]
115 var mvpMatrixFromLight_t = new Matrix4(); // For triangle
116 var mvpMatrixFromLight_p = new Matrix4(); // For plane
117 var tick = function() {
118 currentAngle = animate(currentAngle);
119 // Change the drawing destination to FBO
120 gl.bindFramebuffer(gl.FRAMEBUFFER, fbo);
...
124 gl.useProgram(shadowProgram); // For generating a shadow map
125 // Draw the triangle and the plane (for generating a shadow map)
126 drawTriangle(gl, shadowProgram, triangle, currentAngle,
➥viewProjMatrixFromLight);
127 mvpMatrixFromLight_t.set(g_mvpMatrix); // Used later
128 drawPlane(gl, shadowProgram, plane, viewProjMatrixFromLight);
129 mvpMatrixFromLight_p.set(g_mvpMatrix); // Used later
130 // Change the drawing destination to color buffer
131 gl.bindFramebuffer(gl.FRAMEBUFFER, null);
...
135 gl.useProgram(normalProgram); // For regular drawing
136 gl.uniform1i(normalProgram.u_ShadowMap, 0); // Pass gl.TEXTURE0
137 // Draw the triangle and plane (for regular drawing)
138 gl.uniformMatrix4fv(normalProgram.u_MvpMatrixFromLight, false,
➥mvpMatrixFromLight_t.elements);
139 drawTriangle(gl, normalProgram, triangle, currentAngle, viewProjMatrix);
140 gl.uniformMatrix4fv(normalProgram.u_MvpMatrixFromLight, false,
➥mvpMatrixFromLight_p.elements);
141 drawPlane(gl, normalProgram, plane, viewProjMatrix);
142
143 window.requestAnimationFrame(tick, canvas);
144 };
145 tick();
146 }
Let’s look at the main() function from line 52 in the JavaScript program. Line 64
initializes the shaders for generating the shadow map. Line 73 initializes the shaders
for normal drawing. Lines 86 and 87, which set up the vertex information and
CHAPTER 10 Advanced Techniques
412
initFramebufferObject() at line 94, are the same as the FramebufferObject.js . Line 94
prepares a framebuffer object, which contains the texture object for a shadow map. Lines
99 and 100 enable texture unit 0 and bind it to the target. This texture unit is passed to
u_ShadowMap in the shaders for normal drawing.
Lines 106 to 108 prepare a view projection matrix to generate a shadow map. The key
point is that the first three arguments (that is, the position of an eye point) at line 108 are
specified as the position of the light source. Lines 110 to 112 prepare the view projection
matrix from the eye point where you want to view the scene.
Finally, you draw the triangle and plane using all the preceding information. First you
generate the shadow map, so you switch the drawing destination to the framebuffer
object at line 120. You draw the objects by using the shaders for generating a shadow map
( shadowProgram ) at lines 126 and 128. You should note that lines 127 and 129 save the
model view projection matrices from the light source. Then the shadow map is generated,
and you use it to draw shadows with the code from line 135. Line 136 passes the map to
the fragment shader. Lines 138 and 140 pass the model view projection matrices saved at
line 127 and 129, respectively, to u_MvpMatrixFromLight .
Increasing Precision
Although you’ve successfully calculated the shadow and drawn the scene with the shadow
included, the example code is only able to handle situations in which the light source is
close to the object. To see this, let’s change the y coordinate of the light source position
to 40:
50 var LIGHT_X = 0, LIGHT_Y = 40, LIGHT_Z = 2;
If you run the modified sample program, you can see that the shadow is not displayed—as
in the left side of Figure 10.24 . Obviously, you want the shadow to be displayed correctly,
as in the figure on the right.
The reason the shadow is no longer displayed when the distance from the light source to
the object is increased is that the value of gl_FragCoord.z could not be stored in the R
component of the texture map because it has only an 8-bit precision. A simple solution to
this problem is to use not just the R component but the B, G, and A components. In other
words, you save the value separately in 4 bytes. There is a routine procedure to do this, so
let’s see the sample program. Only the fragment shader is changed.
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413
Sample Program (Shadow_highp.js)
Listing 10.17 shows the fragment shader of Shadow_highp.js . You can see that the
processing to handle the z value is more complex than that in Shadow.js .
Listing 10.17 Shadow_highp.js
1 // Shadow_highp.js
...
10 // Fragment shader program for creating a shadow map
11 var SHADOW_FSHADER_SOURCE =
...
15 'void main() {\n' +
16 ' const vec4 bitShift = vec4(1.0, 256.0, 256.0 * 256.0, 256.0 * 256.0 *
➥256.0);\n' +
17 ' const vec4 bitMask = vec4(1.0/256.0, 1.0/256.0, 1.0/256.0, 0.0);\n' +
18 ' vec4 rgbaDepth = fract(gl_FragCoord.z * bitShift);\n' +
19 ' rgbaDepth -= rgbaDepth.gbaa * bitMask;\n' +
20 ' gl_FragColor = rgbaDepth;\n' +
21 '}\n';
...
37 // Fragment shader program for regular drawing
38 var FSHADER_SOURCE =
...
45 // Recalculate the z value from the rgba
46 'float unpackDepth(const in vec4 rgbaDepth) {\n' +
47 ' const vec4 bitShift = vec4(1.0, 1.0/256.0, 1.0/(256.0 * 256.0),
➥1.0/(256.0 * 256.0 * 256.0));\n' +
Figure 10.24 The shadow is not displayed
CHAPTER 10 Advanced Techniques
414
48 ' float depth = dot(rgbaDepth, bitShift);\n' +
49 ' return depth;\n' +
50 '}\n' +
51 'void main() {\n' +
52 ' vec3 shadowCoord = (v_PositionFromLight.xyz /
➥v_PositionFromLight.w)/2.0 + 0.5;\n' +
53 ' vec4 rgbaDepth = texture2D(u_ShadowMap, shadowCoord.xy);\n' +
54 ' float depth = unpackDepth(rgbaDepth);\n' + // Recalculate the z
55 ' float visibility = (shadowCoord.z > depth + 0.0015)? 0.7:1.0;\n'+
56 ' gl_FragColor = vec4(v_Color.rgb * visibility, v_Color.a);\n' +
57 '}\n';
The code that splits gl_FragCoord.z into 4 bytes (RGBA) is from lines 16 to 19. Because 1
byte can represent up to 1/256, you can store the value greater than 1/256 in R, the value
less than 1/256 and greater than 1/(256*256) in G, the value less than 1/(256*256) and
greater than 1/(256*256*256) in B, and the rest of value in A. Line 18 calculates each value
and stores it in the RGBA components, respectively. It can be written in one line using a
vec4 data type. The function fract() is a built-in one that discards numbers below the
decimal point for the value specified as its argument. Each value in vec4 , calculated at
line 18, has more precision than 1 byte, so line 19 discards the value that does not fit in 1
byte. By substituting this result to gl_FragColor at line 20, you can save the z value using
all four components of the RGBA type and achieve higher precision.
unpackDepth() at line 54 reads out the z value from the RGBA. This function is defined at
line 46. Line 48 performs the following calculation to convert the RGBA value to the origi-
nal z value. As you can see, the calculation is the same as the inner product, so you use
dot() at line 48.
( ) ( )
= × + +
×
+
× ×
depth rgbDepth r
rgbaDepth g rgbaDepthb rgbaDepth a
. 1.0
.
256.0
.
256.0  256.0
.
256.0  256.0  256.0
Now you have retrieved the distance (z value) successfully, so you just have to draw the
shadow by comparing the distance with shadowCoord.z at line 55. In this case, 0.0015
is used as the value for adjusting the error (the stripe pattern), instead of 0.005. This is
because the precision of the z value stored in the shadow map is a float type of medium
precision (that is, its precision is 2
–10 = 0.000976563, as shown in Table 6.15 in Chapter 6 ).
So you add a little margin to it and chose 0.0015 as the value. After that, the shadow can
be drawn correctly.
Load and Display 3D Models
In the previous chapters, you drew 3D objects by specifying their vertex coordinates
and color information by hand and stored them in arrays of type Float32Array in
the JavaScript program. However, as mentioned earlier in the book, in most cases you
will actually read the vertex coordinates and color information from 3D model files
constructed by a 3D modeling tool.
Load and Display 3D Models
415
In this section, you construct a sample program that reads a 3D model constructed using
a 3D modeling tool. For this example, we use the Blender
6 modeling tool, which is a
popular tool with a free version available. Blender is able to export 3D model files using
the well-known OBJ format, which is text based and easy to read, understand, and parse.
OBJ is a geometry definition file format originally developed by Wavefront Technologies.
This file format is open and has been adopted by other 3D graphics vendors. Although
this means it is reasonably well known and used, it also means that there are a number
of variations in the format. To simplify the example code, we have made a number of
assumptions, such as not using textures. However, the example gives you a good under-
standing of how to read model data into your programs and provides a basis for you to
begin experimentation. The approach taken in the example code is designed to be reason-
ably generic and can be used for other text-based formats.
Start Blender and create a cube like that shown in Figure 10.25 . The color of one face of
this cube is orange, and the other faces are red. Then export the model to a file named
cube.obj . (You can find an example of it in the resources directory with the sample
programs.) Let’s take a look at cube.obj , which, because it is a text file, can be opened
with a simple text editor.
Figure 10.25 Blender, 3D modeling tool
6. www.blender.org/
CHAPTER 10 Advanced Techniques
416
Figure 10.26 shows the contents of cube.obj . Line numbers have been added to help with
the explanation and would not normally be in the file.
1 ' : e l i F J B O ) 0 b u s ( 0 6 . 2 v r e d n e l B #
2 g r o . r e d n e l b . w w w #
3 l t m . e b u c b i l l t m
4 e b u C o
5 0 0 0 0 0 0 . 1 - 0 0 0 0 0 0 . 1 - 0 0 0 0 0 0 . 1 v
6 0 0 0 0 0 0 . 1 0 0 0 0 0 0 . 1 - 0 0 0 0 0 0 . 1 v
7 0 0 0 0 0 0 . 1 0 0 0 0 0 0 . 1 - 0 0 0 0 0 0 . 1 - v
8 0 0 0 0 0 0 . 1 - 0 0 0 0 0 0 . 1 - 0 0 0 0 0 0 . 1 - v
9 0 0 0 0 0 0 . 1 - 0 0 0 0 0 0 . 1 0 0 0 0 0 0 . 1 v
0 1 0 0 0 0 0 . 1 0 0 0 0 0 0 . 1 0 0 0 0 0 0 . 1 v
1 0 0 0 0 0 0 . 1 0 0 0 0 0 0 . 1 0 0 0 0 0 0 . 1 - v
2 0 0 0 0 0 0 . 1 - 0 0 0 0 0 0 . 1 0 0 0 0 0 0 . 1 - v
3 l a i r e t a M l t m e s u
4 4 3 2 1 f
5 6 7 8 5 f
6 3 7 6 2 f
7 4 8 7 3 f
8 8 4 1 5 f
9 1 0 0 . l a i r e t a M l t m e s u
0 2 6 5 1 f
Figure 10.26 cube.obj
Once the model file has been created by the modeling tool, your program needs to read
the data and store it in the same type of data structures that you’ve used before. The
following steps are required:
1. Prepare the array ( vertices ) of type Float32Array and read the vertex coordinates of
the model from the file into the array.
2. Prepare the array ( colors ) of type Float32Array and read the colors of the model
from the file into the array.
3. Prepare the array ( normals ) of type Float32Array and read the normals of the model
form the file into the array.
4. Prepare the array ( indices ) of type Uint16Array (or Uint8Array ) and read the indices
of the vertices that specify the triangles that make up the model from the file into
the array.
5. Write the data read during steps 1 through 4 into the buffer object and then draw
the model using gl.drawElements() .
So in this case, you read the data described in cube.obj (shown in Figure 10.26 ) in the
appropriate arrays and then draw the model in step 5. Reading data from the file requires
understanding the format of the file cube.obj (referred to as the OBJ file).
Load and Display 3D Models
417
The OBJ File Format
An OBJ file is made up of several sections,
7 including vertex positions, face definitions,
and material definitions. There may be multiple vertices, normals, and faces within their
sections:
• Lines beginning with a hash character (#) are comments. Lines 1 and 2 in Figure
10.26 are comments generated by Blender describing its version number and origin.
The remaining lines define the 3D model.
• Line 3 references an external materials file. The OBJ format maintains the material
information of the model in an external material file called an MTL file.
mtllib <external mtl filename>
specifies that the materials file is cube.mtl .
• Line 4 specifies the named object in the following format:
<object name>
This sample program does not use this information.
• Lines 5 to 12 define vertex positions in the following format using (x,y,z[,w]) coordi-
nates, where w is optional and defaults to 1.0.
v x y z [w]
In this example, it has eight vertices because the model is a standard cube.
• Lines 13 to 20 specify a material and the faces that use the material. Line 13 specifies
the material name, as defined in the MTL file referenced at line 4, and the specific
material using the following format:
usemtl <material name>
• The following lines, 14 to 18, define faces of the model and the material to be
applied to them. Faces are defined using lists of vertex, texture, and normal indices.
f v1 v2 v3 v4 ...
v1, v2, v3, ... are the vertex indices starting from 1 and matching the correspond-
ing vertex elements of a previously defined vertex list. This sample program handles
vertex and normals. Figure 10.26 does not contain normals, but if a face has a
normal, the following format would be used:
f v1 // vn1 v2 // vn2 v3 // vn3 ...
vn1, vn2, vn3, ... are the normal indices starting from 1.
7 See http://en.wikipedia.org/wiki/Wavefront_.obj_fi le
CHAPTER 10 Advanced Techniques
418
The MTL File Format
The MTL file may define multiple materials. Figure 10.27 shows cube.mtl .
' ' : e l i F L T M r e d n e l B # 1
2 : t n u o C l a i r e t a M # 2
l a i r e t a M l t m w e n 3
0 0 0 0 0 0 . 0 0 0 0 0 0 0 . 0 0 0 0 0 0 0 . 0 a K 4
0 0 0 0 0 0 . 0 0 0 0 0 0 0 . 0 0 0 0 0 0 0 . 1 d K 5
0 0 0 0 0 0 . 0 0 0 0 0 0 0 . 0 0 0 0 0 0 0 . 0 s K 6
1 3 4 8 7 0 . 6 9 s N 7
0 0 0 0 0 0 . 1 i N 8
0 0 0 0 0 0 . 1 d 9
0 m u l l i 0 1
1 0 0 . l a i r e t a M l t m w e n 1 1
0 0 0 0 0 0 . 0 0 0 0 0 0 0 . 0 0 0 0 0 0 0 . 0 a K
0 0 0 0 0 0 . 0 0 0 0 0 5 4 . 0 0 0 0 0 0 0 . 1 d K
2 1
3 1
0 0 0 0 0 0 . 0 0 0 0 0 0 0 . 0 0 0 0 0 0 0 . 0 s K 4 1
1 3 4 8 7 0 . 6 9 s N 5 1
0 0 0 0 0 0 . 1 i N 6 1
0 0 0 0 0 0 . 1 d 7 1
0 m u l l i 8 1
Figure 10.27 cube.mtl
• Lines 1 and 2 are comments that Blender generates.
• Each new material (from line 3) starts with the newmtl command:
newmtl <material name>
This is the material name that is used in the OBJ file.
• Lines 4 to 6 define the ambient, diffuse, and specular color using Ka , Kd , and Ks ,
respectively. Color definitions are in RGB format, where each component is between
0 and 1. This sample program uses only diffuse color.
• Line 7 specifies the weight of the specular color using Ns . Line 8 specifies the optical
density for the surface using Ni . Line 9 specifies transparency using d . Line 10 speci-
fies illumination models using illum . The sample program does not use this item of
information.
Given this understanding of the structure of the OBJ and MTL files, you have to extract
the vertex coordinates, colors, normals, and indices describing a face from the file, write
them into the buffer objects, and draw with gl.drawElements() . The OBJ file may not
have the information on normals, but you can calculate them from the vertex coordinates
that make up a face by using a “cross product.”
8
Let’s look at the sample program.
8 If the vertices of a triangle are v0, v1, and v2, the vector of v0 and v1 is (x1, y1, z1), and the vector
of v1 and v2 is (x2, y2, z2), then the cross product is defi ned as (y1*z2 – z1*y2, z1*x2 – x1*z2, x1*y2
– y1*z2). The result will be the normal for the triangle. (See the book 3D Math Primer for Graphics and
Game Development. )
Load and Display 3D Models
419
Sample Program (OBJViewer.js)
The basic steps are as follows: (1) prepare an empty buffer object, (2) read an OBJ file (an
MTL file), (3) parse it, (4) write the results into the buffer object, and (5) draw. These steps
are implemented as shown in Listing 10.18 .
Listing 10.18 OBJViewer.js
1 // OBJViewer.js (
...
28 function main() {
...
40 if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
41 console.log('Failed to initialize shaders.');
42 return;
43 }
...
49 // Get the storage locations of attribute and uniform variables
50 var program = gl.program;
51 program.a_Position = gl.getAttribLocation(program, 'a_Position');
52 program.a_Normal = gl.getAttribLocation(program, 'a_Normal');
53 program.a_Color = gl.getAttribLocation(program, 'a_Color');
...
63 // Prepare empty buffer objects for vertex coordinates, colors, and normals
64 var model = initVertexBuffers(gl, program);
...
75 // Start reading the OBJ file
76 readOBJFile('../resources/cube.obj', gl, model, 60, true);
...
81 draw(gl, gl.program, currentAngle, viewProjMatrix, model);
...
85 }
86
87 // Create a buffer object and perform the initial configuration
88 function initVertexBuffers(gl, program) {
89 var o = new Object();
90 o.vertexBuffer = createEmptyArrayBuffer(gl, program.a_Position, 3, gl.FLOAT);
91 o.normalBuffer = createEmptyArrayBuffer(gl, program.a_Normal, 3, gl.FLOAT);
92 o.colorBuffer = createEmptyArrayBuffer(gl, program.a_Color, 4, gl.FLOAT);
93 o.indexBuffer = gl.createBuffer();
...
98 return o;
99 }
100
101 // Create a buffer object, assign it to attribute variables, and enable the
➥assignment
CHAPTER 10 Advanced Techniques
420
102 function createEmptyArrayBuffer(gl, a_attribute, num, type) {
103 var buffer = gl.createBuffer(); // Create a buffer object
...
108 gl.bindBuffer(gl.ARRAY_BUFFER, buffer);
109 gl.vertexAttribPointer(a_attribute, num, type, false, 0, 0);
110 gl.enableVertexAttribArray(a_attribute); // Enable the assignment
111
112 return buffer;
113 }
114
115 // Read a file
116 function readOBJFile(fileName, gl, model, scale, reverse) {
117 var request = new XMLHttpRequest();
118
119 request.onreadystatechange = function() {
120 if (request.readyState === 4 && request.status !== 404) {
121 onReadOBJFile(request.responseText, fileName, gl, model, scale, reverse);
122 }
123 }
124 request.open('GET', fileName, true); // Create a request to get file
125 request.send(); // Send the request
126 }
127
128 var g_objDoc = null; // The information of OBJ file
129 var g_drawingInfo = null; // The information for drawing 3D model
130
131 // OBJ file has been read
132 function onReadOBJFile(fileString, fileName, gl, o, scale, reverse) {
133 var objDoc = new OBJDoc(fileName); // Create a OBJDoc object
134 var result = objDoc.parse(fileString, scale, reverse);
135 if (!result) {
136 g_objDoc = null; g_drawingInfo = null;
137 console.log("OBJ file parsing error.");
138 return;
139 }
140 g_objDoc = objDoc;
141 }
Within the JavaScript, the processing in initVertexBuffers() , called at line 64, has been
changed. The function simply prepares an empty buffer object for the vertex coordinates,
colors, and normals for the 3D model to be displayed. After parsing the OBJ file, the infor-
mation corresponding to each buffer object will be written in the object.
Load and Display 3D Models
421
The initVertexBuffers() function at line 88 creates the appropriate empty buffer objects
at lines 90 to 92 using createEmptyArrayBuffer() and assigns them to an attribute vari-
able. This function is defined at line 102 and, as you can see, creates a buffer object
(line 103), assigns it to an attribute variable (line 109), and enables the assignment (line
110), but it does not write the data. After storing these buffer objects to model at line
64, the preparations of the buffer object are completed. The next step is to read the OBJ
file contents into this buffer, which takes place at line 76 using readOBJFile() . The first
argument is the location of the file (URL), the second one is gl , and the third one is the
Object object ( model ) that packages the buffer objects. The tasks carried out by this func-
tion are similar to those when loading a texture image using the Image object and are
shown here:
(2.1) Create an XMLHttpRequest object (line 117).
(2.2) Register the event handler to be called when the loading of the file is completed
(line 119).
(2.3) Create a request to acquire the file using the open() method (line 124).
(2.4) Send the request to acquire the file (line 125).
Line 117 creates the XMLHttpRequest object, which sends an HTTP request to a web server.
Line 119 is the registration of the event handler that will be called after the browser has
loaded the file. Line 124 creates the request to acquire the file using the open() method.
Because you are requesting a file, the first argument is GET , and the second one is the URL
for the file. The last one specifies whether or not the request is asynchronous. Finally, line
125 uses the send() method to send the request to the web server to get the file.
9
Once the browser has loaded the file, the event handler at line 119 is called. Line 120
checks for any errors returned by the load request. If the readyState property is 4, it indi-
cates that the loading process is completed. However, if the readyState is not 4 and the
status property is 404, it indicates that the specified file does not exist. The 404 error
is the same as “404 Not Found,” which is displayed when you try to display a web page
that does not exist. When the file has been loaded successfully, onReadOBJFile() is called,
which is defined at line 132 and takes five arguments. The first argument, responseText ,
contains the contents of the loaded file as one string. An OBJDoc object is created at line
133, which will be used, via the parse() method, to extract the results in a form that
WebGL can easily use. The details will be explained next. Line 140 assigns the objDoc ,
which contains the parsing result in g_objDoc for rendering the model later.
9 Note: When you want to run the sample programs that use external fi les in Chrome from your
local disk, you should add the option --allow-file-access-from-files to Chrome. This is for
security reasons. Chrome, by default, does not allow access to local fi les such as ../resources/
cube.obj . For Firefox, the equivalent parameter, set via account:config , is security.fileuri.
strict_origin_policy , which should be set to false . Remember to set it back as you open a
security loophole if local fi le access is enabled.
CHAPTER 10 Advanced Techniques
422
User-Defined Object
Before proceeding to the explanation of the remaining code of OBJViewer.js , you need to
understand how to create your own (user-defined) objects in JavaScript. OBJViewer.js uses
user-defined objects to parse an OBJ file. In JavaScript, you can create user-defined objects
which, once created, are treated in the same way as built-in objects like Array and Date .
The following is the StringParser object used in OBJViewer.js . The key aspects are how
to define a constructor to create a user-defined object and how to add methods to the
object. The constructor is a special method that is called when creating an object with
new . The following is the constructor for the StringParser object:
595 // Constructor
596 var StringParser = function(str) {
597 this.str; // Store the string specified by the argument
598 this.index; // Position in the string to be processed
599 this.init(str);
600 }
You can define the constructor with the anonymous function (see Chapter 2 ). Its param-
eter is the one that will be specified when creating the object with new . Lines 597 and
598 are the declaration of properties that can be used for this new object type, similar to
properties like the length property of Array. You can define the property by writing the
keyword this followed by . and the property name. Line 599 then calls init() , an initial-
ization method that has been defined for this user-defined object.
Let’s take a look at init() . You can add a method to the object by writing the method
name after the keyword prototype . The body of the method is also defined using an
anonymous function:
601 // Initialize StringParser object
602 StringParser.prototype.init = function(str) {
603 this.str = str;
604 this.index = 0;
605 }
What is convenient here is that you can access the property that is defined in the
constructor from the method. The this.str at line 603 refers to this.str defined at line
597 in the constructor. The this.index at line 604 refers to this.index at line 598 in the
constructor. Let’s try using this StringParse object:
Load and Display 3D Models
423
var sp = new StringParser('Tomorrow is another day.');
alert(sp.str); // "Tomorrow is another day." is displayed.
sp.str = 'Quo Vadis'; // The content of str is changed to "Quo Vadis".
alert(sp.str); // "Quo Vadis" is displayed
sp.init('Cinderella, tonight?');
alert(sp.str); // "Cinderella, tonight?" is displayed
Let’s look at another method, skipDelimiters() , that skips the delimiters (tab, space, (, ),
or ”) in a string:
608 StringParser.prototype.skipDelimiters = function() {
609 for(var i = this.index, len = this.str.length; i < len; i++) {
610 var c = this.str.charAt(i);
611 // Skip TAB, Space, (, ), and "
612 if (c == '\t'|| c == ' ' || c == '(' || c == ')' || c == '"') continue;
613 break;
614 }
615 this.index = i;
616 }
The charAt() method at line 610 is supported by the String object that manages a string
and retrieves the character specified by the argument from the string.
Now let’s look at the parser code in OBJViewer.js .
Sample Program (Parser Code in OBJViewer.js)
OBJViewer.js parses the content of an OBJ file line by line and converts it to the structure
shown in Figure 10.28 . Each box in Figure 10.28 is a user-defined object. Although the
parser code in OBJViewer.js looks quite complex, the core parsing process is simple. The
complexity comes because it is repeated several times. Let’s take a look at the core process-
ing, which once you understand will allow you to understand the whole process.
424
CHAPTER 10 Advanced Techniques
t c e j b O J B O t c e j b O J B O
x e t r e V
x
y
z
c o D J B O
s l t m
s t c e j b o
s l a m r o n
s e c i t r e v
t c e j b O J B O
e m a n
s e c a f
c o D L T M
e t e l p m o c
s l a i r e t a m
l a i r e t a M
e m a n
r o l o c
r o l o C
l a i r e t a M
e m a n
r o l o c
r o l o C
l a i r e t a M
e m a n
r o l o c
r o l o C
…
e m a n
s e c a f
e m a n
s e c a f
…
e c a F
e m a N l a i r e t a m
s e c i d n I v
s e c i d n I n
e c a F
e m a N l a i r e t a m
s e c i d n I v
s e c i d n I n
e c a F
e m a N l a i r e t a m
s e c i d n I v
s e c i d n I n
…
x e t r e V
x
y
z
x e t r e V
x
y
z
x e t r e V
x
y
z
x e t r e V
x
y
z
…
x
y
z
…
x
y
z
x
y
z
x
y
z
x
y
z
l a m r o N l a m r o N l a m r o N l a m r o N
l a m r o N
r g b a r g b a r g b a
Figure 10.28 The internal structure after parsing an OBJ file
Listing 10.19 shows the basic code of OBJViewer.js .
Listing 10.19 OBJViewer.js (Parser Part)
214 // OBJDoc object
215 // Constructor
216 var OBJDoc = function(fileName) {
217 this.fileName = fileName;
218 this.mtls = new Array(0); // Initialize the property for MTL
219 this.objects = new Array(0); // Initialize the property for Object
220 this.vertices = new Array(0); // Initialize the property for Vertex
221 this.normals = new Array(0); // Initialize the property for Normal
222 }
223
224 // Parsing the OBJ file
225 OBJDoc.prototype.parse = function(fileString, scale, reverseNormal) {
Load and Display 3D Models
425
226 var lines = fileString.split('\n'); // Break up into lines
227 lines.push(null); // Append null
228 var index = 0; // Initialize index of line
229
230 var currentObject = null;
231 var currentMaterialName = "";
232
233 // Parse line by line
234 var line; // A string in the line to be parsed
235 var sp = new StringParser(); // Create StringParser
236 while ((line = lines[index++]) != null) {
237 sp.init(line); // init StringParser
238 var command = sp.getWord(); // Get command
239 if(command == null) continue; // check null command
240
241 switch(command){
242 case '#':
243 continue; // Skip comments
244 case 'mtllib': // Read Material chunk
245 var path = this.parseMtllib(sp, this.fileName);
246 var mtl = new MTLDoc(); // Create MTL instance
247 this.mtls.push(mtl);
248 var request = new XMLHttpRequest();
249 request.onreadystatechange = function() {
250 if (request.readyState == 4) {
251 if (request.status != 404) {
252 onReadMTLFile(request.responseText, mtl);
253 }else{
254 mtl.complete = true;
255 }
256 }
257 }
258 request.open('GET', path, true); // Create a request to get file
259 request.send(); // Send the request
260 continue; // Go to the next line
261 case 'o':
262 case 'g': // Read Object name
263 var object = this.parseObjectName(sp);
264 this.objects.push(object);
265 currentObject = object;
266 continue; // Go to the next line
267 case 'v': // Read vertex
268 var vertex = this.parseVertex(sp, scale);
269 this.vertices.push(vertex);
270 continue; // Go to the next line
CHAPTER 10 Advanced Techniques
426
271 case 'vn': // Read normal
272 var normal = this.parseNormal(sp);
273 this.normals.push(normal);
274 continue; // Go to the next line
275 case 'usemtl': // Read Material name
276 currentMaterialName = this.parseUsemtl(sp);
277 continue; // Go to the next line
278 case 'f': // Read face
279 var face = this.parseFace(sp, currentMaterialName, this.vertices,
➥reverse);
280 currentObject.addFace(face);
281 continue; // Go to the next line
282 }
283 }
284
285 return true;
286 }
Lines 216 to 222 define the constructor for the OBJDoc object, which consists of five prop-
erties that will be parsed and set up. The actual parsing is done in the parse() method at
line 225. The content of the OBJ file is passed as one string to the argument fileString of
the parse() method and then split into manageable pieces using the split() method.
This method splits a string into pieces delimited by the characters specified as the argu-
ment. As you can see at line 226, the argument specifies “\n” (new line), so each line is
stored in this.line s as an array. null is appended at the end of the array at line 227 to
make it easy to find the end of the array. this.index indicates how many lines have been
parsed and is initialized to 0 at line 228.
You have already seen the StringParser object, which is created at line 235, in the previ-
ous section. This object is used for parsing the content of the line.
Now you are ready to start parsing the OBJ file. Each line is stored in line using this.
lines[this.index++] at line 236. Line 237 writes the line to sp ( StringParser ). Line 238
gets the first word of the line using sp.getWord() and stores it in command . You use the
methods shown in Table 10.3 , where “word” in the table indicates a string surrounded by
a delimiter (tab, space, (, ), or ”).
Table 10.3 Method that StringParser Supports
Method Description
StringParser.init(str) Initialize StringParser to be able to parse str.
StringParser.getWord() Get a word.
StringParser.skipToNext-
Word()
Skip to the beginning of the next word.
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427
Method Description
StringParser.getInt() Get a word and convert it to an integer number.
StringParser.getFloat() Get a word and convert it to a floating point number.
The switch statement at line 241 checks the command to determine how to process the
following lines in the OBJ file.
If the command is # (line 242), the line is a comment. Line 243 skips it using continue .
If the command is mtllib (line 241), the line is a reference to an MTL file. Line 245 gener-
ates the path to the file. Line 246 creates an MTLDoc object for storing the material infor-
mation in the MTL file, and line 247 stores it in this.mtls . Then lines 248 to 259 read the
file in the same way that you read an OBJ file. The MTL file is parsed by onReadMTLfile() ,
which is called when it is loaded.
If the command is o (line 261) or g (line 262), it indicates a named object or group. Line
263 parses the line and returns the results in OBJObject . This object is stored in this.
objects at line 264 and currentObject .
If the command is v , the line is a vertex position. Line 268 parses (x, y, z) and returns the
result in Vertex object. This object is stored in this.vertices at line 269.
If the command is f , it indicates that the line is a face definition. Line 279 parses it and
returns the result in the Face object. This object is stored in the currentObject . Let’s take
a look at parseVertex() , which is shown in Listing 10.20 .
Listing 10.20 OBJViewer.js (parseVertex())
302 OBJDoc.prototype.parseVertex = function(sp, scale) {
303 var x = sp.getFloat() * scale;
304 var y = sp.getFloat() * scale;
305 var z = sp.getFloat() * scale;
306 return (new Vertex(x, y, z));
307 }
Line 303 retrieves the x value from the line using sp.getFloat() . A scaling factor is
applied when the model is too small or large. After retrieving the three coordinates, line
306 creates a Vertex object using x, y, and z and returns it.
Once the OBJ file and MTL files have been fully parsed, the arrays for the vertex coordi-
nates, colors, normals, and indices are created from the structure shown in Figure 10.28 .
Then onReadComplete() is called to write them into the buffer object (see Listing 10.21 ).
CHAPTER 10 Advanced Techniques
428
Listing 10.21 OBJViewer.js (onReadComplete())
176 // OBJ File has been read completely
177 function onReadComplete(gl, model, objDoc) {
178 // Acquire the vertex coordinates and colors from OBJ file
179 var drawingInfo = objDoc.getDrawingInfo();
180
181 // Write date into the buffer object
182 gl.bindBuffer(gl.ARRAY_BUFFER, model.vertexBuffer);
183 gl.bufferData(gl.ARRAY_BUFFER, drawingInfo.vertices,gl.STATIC_DRAW);
184
185 gl.bindBuffer(gl.ARRAY_BUFFER, model.normalBuffer);
186 gl.bufferData(gl.ARRAY_BUFFER, drawingInfo.normals, gl.STATIC_DRAW);
187
188 gl.bindBuffer(gl.ARRAY_BUFFER, model.colorBuffer);
189 gl.bufferData(gl.ARRAY_BUFFER, drawingInfo.colors, gl.STATIC_DRAW);
190
191 // Write the indices to the buffer object
192 gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, model.indexBuffer);
193 gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, drawingInfo.indices, gl.STATIC_DRAW);
194
195 return drawingInfo;
196 }
This method is straightforward and starts at Line 178, which retrieves the drawing infor-
mation from objDoc that contains the results from parsing the OBJ file. Lines 183, 186,
189, and 193 write vertices, normals, colors, and indices into the respective buffer objects.
The function getDrawingInfo() at line 451 retrieves the vertices, normals, colors, and
indices from the objDoc and is shown in Listing 10.22 .
Listing 10.22 OBJViewer.js (Retrieving the Drawing Information)
450 // Retrieve the information for drawing 3D model
451 OBJDoc.prototype.getDrawingInfo = function() {
452 // Create an array for vertex coordinates, normals, colors, and indices
453 var numIndices = 0;
454 for (var i = 0; i < this.objects.length; i++){
455 numIndices += this.objects[i].numIndices;
456 }
457 var numVertices = numIndices;
458 var vertices = new Float32Array(numVertices * 3);
459 var normals = new Float32Array(numVertices * 3);
460 var colors = new Float32Array(numVertices * 4);
461 var indices = new Uint16Array(numIndices);
462
463 // Set vertex, normal, and color
Load and Display 3D Models
429
464 var index_indices = 0;
465 for (var i = 0; i < this.objects.length; i++){
466 var object = this.objects[i];
467 for (var j = 0; j < object.faces.length; j++){
468 var face = object.face[j];
469 var color = this.findColor(face.materialName);
470 var faceNormal = face.normal;
471 for (var k = 0; k < face.vIndices.length; k++){
472 // Set index
473 indices[index_indices] = index_indices;
474 // Copy vertex
475 var vIdx = face.vIndices[k];
476 var vertex = this.vertices[vIdx];
477 vertices[index_indices * 3 + 0] = vertex.x;
478 vertices[index_indices * 3 + 1] = vertex.y;
479 vertices[index_indices * 3 + 2] = vertex.z;
480 // Copy color
481 colors[index_indices * 4 + 0] = color.r;
482 colors[index_indices * 4 + 1] = color.g;
483 colors[index_indices * 4 + 2] = color.b;
484 colors[index_indices * 4 + 3] = color.a;
485 // Copy normal
486 var nIdx = face.nIndices[k];
487 if(nIdx >= 0){
488 var normal = this.normals[nIdx];
489 normals[index_indices * 3 + 0] = normal.x;
490 normals[index_indices * 3 + 1] = normal.y;
491 normals[index_indices * 3 + 2] = normal.z;
492 }else{
493 normals[index_indices * 3 + 0] = faceNormal.x;
494 normals[index_indices * 3 + 1] = faceNormal.y;
495 normals[index_indices * 3 + 2] = faceNormal.z;
4 96 }
49 7 index_indices ++;
498 }
499 }
500 }
501
502 return new DrawingInfo(vertices, normals, colors, indices);
503 };
Line 454 calculates the number of indices using a for loop. Then lines 458 to 461 create
typed arrays for storing vertices, normals, colors, and indices that are assigned to the
appropriate buffer objects. The size of each array is determined by the number of indices
at line 454.
CHAPTER 10 Advanced Techniques
430
The program traverses the OBJObject objects and its Face objects in the order shown in
Figure 10.28 and stores the information in the arrays vertices , colors , and indices .
The for statement at line 465 loops, extracting each OBJObject one by one from the result
of the earlier parsing. The for statement at line 467 does the same for each Face object
that makes up the OBJObject and performs the following steps for each Face :
1. Lines 469 finds the color of the Face using materialName and stores the color in
color . Line 468 stores the normal of the face in faceNomal for later use.
2. The for statement at line 471 loops, extracting vertex indices from the face, storing
its vertex position in vertices (lines 477 to 479), and storing the r, g, and b compo-
nents of the color in colors (lines 482 to 484). The code from line 486 handles
normals. OBJ files may or may not contain normals, so line 487 checks for that. If
normals are found in the OBJ file, lines 487 to 489 store them in normals . Lines 492
to 494 then store the normals this program generates.
Once you complete these steps for all OBJObjects , you are ready to draw. Line 502 returns
the information for drawing the model in a DrawingInfo object, which manages the
vertex information that has to be written in the buffer object, as described previously.
Although this has been, by necessity, a rapid explanation, at this stage you should under-
stand how the contents of the OBJ file can be read in, parsed, and displayed with WebGL.
If you want to read multiple model files in a single scene, you would repeat the preceding
processes. There are several other models stored as OBJ files in the resources directory of
the sample programs, which you can look at and experiment with to confirm your under-
standing (see Figure 10.29 ).
Figure 10.29 Various 3D models
Handling Lost Context
WebGL uses the underlying graphics hardware, which is a shared resource managed by the
operating system. There are several situations where this resource can be “taken away,”
resulting in information stored within the graphics hardware being lost. These include
Handling Lost Context
431
situations when another program takes over the hardware or when the machine hiber-
nates. When this happens, information that WebGL uses to draw correctly, its “context,”
can be lost. A good example is when you run a WebGL program on a notebook PC or
smart phone and it enters hibernation mode. Often, an error message is displayed before
the machine hibernates. When the machine awakes after you press the power button,
the system returns to the original state, but browser that is running the WebGL program
may display nothing on the screen, as on the right side of Figure 10.30 . Because the back-
ground color of the web page that this sample program draws is white, the web browser
shows a completely white screen.
Before Hibernation After Hibernation
Figure 10.30 WebGL program stops after returning from a hibernation mode
For example, if you are running RotatingTriangle , the following message may be
displayed on the console:
WebGL error CONTEXT_LOST_WEBGL in uniformMatrix4fv([object WebGLUniformLocation,
false, [object Float32Array]]
This indicates that the error occurred when the program performed the gl.uniformMa-
trix4fv() either before the system entered the hibernation mode or on return from hiber-
nation. The error message will differ slightly depending on what the program was trying
to do at the time of hibernation. In this section, we will explain how to deal with this
problem.
How to Implement Handling Lost Context
As previously discussed, context can be lost for any number of reasons. However, WebGL
supports two events to indicate state changes within the system: a context lost event
( webglcontextlost ) and a context restore event ( webglcontextrestored ). See Table 10.4 .
CHAPTER 10 Advanced Techniques
432
Table 10.4 The Context Events
Event Description
Webglcontextlost Occurs when the rendering context for WebGL is lost
webglcontextrestored Occurs when the browser completes a reset of the WebGL system
When the context lost event occurs, the rendering context acquired by getWebGLContext()
(that is gl in the sample programs) becomes invalid, and any operations carried out using
the gl context are invalidated. These processes include creating buffer objects and texture
objects, initializing shaders, setting the clear color, and more. After the browser resets the
WebGL system, the context restore event is generated, and your program needs to redo
these operations. The other variables in your JavaScript program are not affected and can
be used as normal.
Before taking a look at the sample program, you need to use the addEventListener()
method of the <canvas> to register the event handlers for the context lost event and the
context restore event. This is because the <canvas> does not support a specific property
that you can use to register context event handlers. Remember that in previous examples
you used the onmousedown property of <canvas> to register the event handler for the
mouse event.
canvas.addEventListener(type, handler, useCapture)
Register the event handler specified by handler to the <canvas> element.
Parameters type Specifies the name of the event to listen for (string).
handler Specifies the event handler to be called when the event
occurs. This function is called with one argument (event
object).
useCapture Specifies whether the event needs to be captured or not
(boolean). If true , the event is not dispatched to other
elements. If false , the event is dispatched to others.
Return value None
Sample Program (RotatingTriangle_contextLost.js)
In this section, you will construct a sample program, RotatingTriangle_contextLost ,
which modifies RotatingTriangle to make it possible to deal with the context lost event
(shown in Figure 10.30 ). The sample program is shown in Listing 10.23 .
Handling Lost Context
433
Listing 10.23 RotatingTriangle_contextLost.js
1 // RotatingTriangle_contextLost.js
...
16 function main() {
17 // Retrieve <canvas> element
18 var canvas = document.getElementById('webgl');
19
20 // Register event handler for context lost and restored events
21 canvas.addEventListener('webglcontextlost', contextLost, false);
22 canvas.addEventListener('webglcontextrestored', function(ev)
➥{ start(canvas); }, false);
23
24 start(canvas); // Perform WebGL-related processes
25 }
...
29 // Current rotation angle
30 var g_currentAngle = 0.0; // Changed from local variable to global
31 var g_requestID; // The return value of requestAnimationFrame()
32
33 function start(canvas) {
34 // Get the rendering context for WebGL
35 var gl = getWebGLContext(canvas);
...
41 // Initialize shaders
42 if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
...
45 }
46
47 var n = initVertexBuffers(gl); // Set vertex coordinates
...
55 // Get storage location of u_ModelMatrix
56 var u_ModelMatrix = gl.getUniformLocation(gl.program, 'u_ModelMatrix');
...
62 var modelMatrix = new Matrix4(); // Create a model matrix
63
64 var tick = function() { // Start drawing
65 g_currentAngle = animate(g_currentAngle); // Update rotation angle
66 draw(gl, n, g_currentAngle, modelMatrix, u_ModelMatrix);
67 g_requestID = requestAnimationFrame(tick, canvas);
68 };
69 tick();
70 }
71
72 function contextLost(ev) { // Event handler for context lost event
CHAPTER 10 Advanced Techniques
434
73 cancelAnimationFrame(g_requestID); // Stop animation
74 ev.preventDefault(); // Prevent the default behavior
75 }
The processing of the context lost event has no implications for the shaders, so let’s focus
on the main() function in the JavaScript program starting at line 16. Line 21 registers the
event handler for the context lost event, and line 22 registers the event handler for the
context restore event. The main() function ends by calling the function start() at
line 24.
The start() function, defined at line 33, contains the same steps as in RotatingTriangle.
js . They are the processes you have to redo when the context lost event occurs. There are
two changes from RotatingTriangle.js to handle lost context.
First, the current rotation angle, at line 65, is stored in a global variable g_currentAngle
(line 30) instead of a local variable. This allows you to draw the triangle using the angle
held in the global variable when a context restore event occurs. Line 67 stores the return
value of requestAnimationFrame() in the global variable g_requestID (line 31). This is
used to cancel the registration of the function when the context lost event occurs.
Let’s take a look at the actual event handlers. The event handler for the context lost event,
contextLost() , is defined at line 72 and has only two lines. Line 73 cancels the regis-
tration of the function used to carry out the animation, ensuring no further attempt at
drawing is made until the context is correctly restored. Then at Line 74 you prevent the
browser’s default behavior for this event. This is because, by default, the browser doesn’t
generate the context restore event. However, in our case, the event is needed, so you must
prevent this default behavior.
The event handler for the context restore event is straightforward and makes a call to
start() , which rebuilds the WebGL context. This is carried out by registering the event
handler at line 22, which calls start() by using an anonymous function.
Note that when a context lost event occurs, the following alert is always displayed on the
console:
WARNING: WebGL content on the page might have caused the graphics card to reset
By implementing these handlers for the lost context events, your WebGL applications will
be able to deal with situations where the WebGL context is lost.
Summary
This chapter explained a number of miscellaneous techniques that are useful to know
when creating WebGL applications. Due to space limitations, the explanations have been
kept brief but contain sufficient information for you to master and use the techniques
in your own WebGL applications. Although there are many more techniques you could
learn, we have chosen these because they will help you begin to apply the lessons in this
book to building your own 3D applications.
Summary
435
As you have seen, WebGL is a powerful tool for creating 3D applications and one that is
capable of creating sophisticated and visually stunning 3D graphics. Our aim in this book
has been to provide you with a step-by-step introduction to the basics of WebGL and give
you a strong enough foundation on which to begin building your own WebGL applica-
tions and exploring further. There are many other resources available to help you in that
exploration. However, our hope is that as you begin to venture out and explore WebGL
yourself, you will return to this book and find it valuable as a reference and guide as you
build your knowledge.
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Appendix A
No Need to Swap Buffers in WebGL
For those of you with some experience in developing OpenGL applications on PCs, you may have
noticed that none of the examples in this book seem to swap color buffers, which is something
that most OpenGL implementations require.
As you know, OpenGL uses two buffers: a “front” color buffer and a “back” color buffer with the
contents of the front color buffer being displayed on the screen. Usually, when you draw some-
thing using OpenGL, it is drawn into the back color buffer. When you want to actually display
something, you need to copy the contents of the back buffer to the front buffer to cause it to be
displayed. If you were to draw directly into the front buffer, you would see visual artifacts (such
as flickers) because the screen was being updated before you had finalized the data in the buffer.
To support this dual-buffer approach, OpenGL provides a mechanism to swap the back buffer
and the front buffer. In some systems this is automatic; in others, explicit calls to swap buffers,
such as glutSwapBuffers() or eglSwapBuffers() , are needed after drawing into the back buffer.
For example, a typical OpenGL application has the following user-defined “display” function:
void display(void) {
// Clear color buffer and depth buffer
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
draw(); // Draw something
glutSwapBuffers(); // Swap color buffers
}
In contrast, WebGL relies on the browser to automatically manage the display update, reliev-
ing you of the need to do it explicitly in your applications. Referring to Figure A.1 (which is the
same as Figure 2.10 ), when WebGL applications draw something in the color buffer, the browser
detects the drawing and displays the content on the screen. Therefore, WebGL supports only one
color buffer.
APPENDIX A No Need to Swap Buffers in WebGL
438
m e t s y S L G b e W
r e d a h S x e t r e V t n e m g a r F
r e d a h S
t p i r c S a v a J
{ ) ( n i a m n o i t c n u f
… L G b e W t e g = l g r a v
…
; ) … ( s r e d a h S t i n i
…
}
n o i t a r e p o t n e m g a r f - r e P n o i t a r e p o x e t r e v - r e P
r e s w o r B
x
y
: n o i t i s o P
) 0 . 1 , 0 . 0 , 0 . 0 , 0 . 0 (
0 . 0 1 : e z i S
: r o l o C
) 0 . 1 , 0 . 0 , 0 . 0 , 0 . 1 (
) d a o l n o ( d e t u c e x e s i m a r g o r p t p i r c S a v a J
r e f u b r o l o c e h t o t r e d n e R
d e t u c e x e e r a s d o h t e m d e t a l e r - L G b e W
r e f u B r o l o C
y a l p s i D
Figure A.1 The processing flow from executing a JavaScript program to displaying the result in
a browser
This approach works, because as seen in the sample programs in this book, all WebGL
programs are executed in the browser by executing the JavaScript in the form of a method
invocation from the browser.
Because the programs are not independently executed, the browser has a chance to
check whether the content of the color buffer was modified after the JavaScript program
executes and exits. If the contents have been modified, the browser is responsible for
ensuring it is displayed on the screen.
For example, in HelloPoint1 , we execute the JavaScript function ( main() ) from the HTML
file ( HelloPoint1.html ) as follows:
<body οnlοad="main()">
This causes the browser to execute the JavaScript function main() after loading the <body>
element. Within main() , the draw operation modifies the color buffer.
main(){
...
// Draw a point
gl.drawArrays(gl.POINTS, 0, 1);
}
When main() exits, the control returns to the browser that called the function. The
browser then checks the content of the color buffer, and if anything has been changed,
causes it to be displayed. One useful side effect of this approach is that the browser
No Need to Swap Buffers in WebGL
439
handles combining the color buffer with the rest of the web page, allowing you to
combine 3D graphics with your web pages. Note that HelloPoint1 shows only the
<canvas> element on the page, because HelloPoint1.html contains no other elements
than the <canvas> element.
This implies that if you call methods that return control to the browser, such as alert()
or confirm() , the browser may then display the contents of the color buffer to the screen.
This may not be what you expect, so take care when using these methods in your WebGL
programs.
The browser behaves in the same way when JavaScript draws something in an event
handler. This is because the event handler is also called from the browser, and then the
control is returned to the browser after the handler exits.
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Appendix B
Built-In Functions of GLSL ES 1.0
This appendix details all embedded functions supported by GLSL ES 1.0, including
many that are not explained in this book but which are often used in programming
shaders.
Note that, in all but texture lookup functions, the operations on vector or matrix argu-
ments are carried out component-wise. For example,
vec2 deg = vec2(60, 80);
vec2 rad = radians(deg);
In these examples, the components of the variable rad are assigned values converted
from 60 and 80 degrees, respectively.
Angle and Trigonometry Functions
Syntax Description
float radians(float degree )
vec2 radians(vec2 degree )
vec3 radians(vec3 degree )
vec4 radians(vec4 degree )
Converts degrees to radians; that is, π * degree /180.
float degrees(float radian )
vec2 degrees(vec2 radian )
vec3 degrees(vec3 radian )
vec4 degrees(vec4 radian )
Converts radians to degrees; that is, 180 * radian /π.
APPENDIX B Built-In Functions of GLSL ES 1.0
442
Syntax Description
float sin(float angle )
vec2 sin(vec2 angle )
vec3 sin(vec3 angle )
vec4 sin(vec4 angle )
The standard trigonometric sine function. angle is in
radians.
The range of the return value is [–1, 1].
float cos(float angle )
vec2 cos(vec2 angle )
vec3 cos(vec3 angle )
vec4 cos(vec4 angle )
The standard trigonometric cosine function. angle is in
radians.
The range of the return value is [–1, 1].
float tan(float angle )
vec2 tan(vec2 angle )
vec3 tan(vec3 angle )
vec4 tan(vec4 angle )
The standard trigonometric tangent function. angle is in
radians.
float asin(float x )
vec2 asin(vec2 x )
vec3 asin(vec3 x )
vec4 asin(vec4 x )
Arc sine. Returns an angle (in radians) whose sine is
x . The range of the return value is [–π/2, π/2]. Results
are undefined if x > –1 or x > +1.
float acos(float x )
vec2 acos(vec2 x )
vec3 acos(vec3 x )
vec4 acos(vec4 x )
Arc cosine. Returns an angle (in radians) whose cosine
is x . The range of the return value is [0, π]. Results are
undefined if x > –1 or x > +1.
float atan(float y , float x )
vec2 atan(vec2 y , vec2 x )
vec3 atan(vec3 y , vec3 x )
vec4 atan(vec4 y , vec4 x )
Arc tangent. Returns an angle (in radians) whose
tangent is y / x . The signs of x and y are used to deter-
mine what quadrant the angle is in. The range of the
return value is [–π, π]. Results are undefined if x and y
are both 0.
Note, for vectors, this is a component-wise operation.
float atan(float y _over_ x )
vec2 atan(vec2 y _over_ x )
vec3 atan(vec3 y _over_ x )
vec4 atan(vec4 y _over_ x )
Arc tangent. Returns an angle whose tangent is y_
over_x . The range of the return value is [–π/2, π/2].
Note, for vectors, this is a component-wise operation.
Exponential Functions
443
Exponential Functions
Syntax Description
float pow(float x , float y )
vec2 pow(vec2 x , vec2 y )
vec3 pow(vec3 x , vec3 y )
vec4 pow(vec4 x , vec4 y )
Returns x raised to the y power; that is, x
y .
Results are undefined if x < 0.
Results are undefined if x = 0 and y ≤ 0.
Note, for vectors, this is a component-wise operation.
float exp(float x )
vec2 exp(vec2 x )
vec3 exp(vec3 x )
vec4 exp(vec4 x )
Returns the natural exponentiation of x ; that is, e
x .
float log(float x )
vec2 log(vec2 x )
vec3 log(vec3 x )
vec4 log(vec4 x )
Returns the natural logarithm of x ; that is, returns the value
y , which satisfies the equation x = e
y . Results are undefined
if x ≤ 0.
float exp2(float x )
vec2 exp2(vec2 x )
vec3 exp2(vec3 x )
vec4 exp2(vec4 x )
Returns 2 raised to the x power; that is, 2
x .
float log2(float x )
vec2 log2(vec2 x )
vec3 log2(vec3 x )
vec4 log2(vec4 x )
Returns the base 2 logarithm of x ; that is, returns the value
y , which satisfies the equation x =2
y .
Results are undefined if x ≤ 0.
float sqrt(float x )
vec2 sqrt(vec2 x )
vec3 sqrt(vec3 x )
vec4 sqrt(vec4 x )
Returns x .
Results are undefined if x < 0.
float inversesqrt(float x )
vec2 inversesqrt(vec2 x )
vec3 inversesqrt(vec3 x )
vec4 inversesqrt(vec4 x )
Returns 1/ x
.
Results are undefined if x ≤ 0.
APPENDIX B Built-In Functions of GLSL ES 1.0
444
Common Functions
Syntax Description
float abs(float x )
vec2 abs(vec2 x )
vec3 abs(vec3 x )
vec4 abs(vec4 x )
Returns the non-negative value of x without
regard to its sign; that is, x if x ≥ 0, otherwise it
returns – x .
float sign(float x )
vec2 sign(vec2 x )
vec3 sign(vec3 x )
vec4 sign(vec4 x )
Returns 1.0 if x > 0, 0.0 if x = 0, or –1.0 if
x < 0.
float floor(float x )
vec2 floor(vec2 x )
vec3 floor(vec3 x )
vec4 floor(vec4 x )
Returns a value equal to the nearest integer that
is less than or equal to x.
float ceil(float x )
vec2 ceil(vec2 x )
vec3 ceil(vec3 x )
vec4 ceil(vec4 x )
Returns a value equal to the nearest integer that
is greater than or equal to x .
float fract(float x )
vec2 fract(vec2 x )
vec3 fract(vec3 x )
vec4 fract(vec4 x )
Returns the fractional part of x ; that is,
x – floor ( x ).
float mod(float x , float y )
vec2 mod(vec2 x , vec2 y )
vec3 mod(vec3 x , vec3 y )
vec4 mod(vec4 x , vec4 y )
vec2 mod(vec2 x , float y )
vec3 mod(vec3 x , float y )
vec4 mod(vec4 x , float y )
Modulus (modulo). Returns the remainder of the
division of x by y; that is, ( x – y * floor ( x / y )).
Given two positive numbers x and y, mod(x, y) is
the remainder of the division of x by y.
Note, for vectors, this is a component-wise
operation.
Common Functions
445
Syntax Description
float min(float x , float y )
vec2 min(vec2 x , vec2 y )
vec3 min(vec3 x , vec3 y )
vec4 min(vec4 x , vec4 y )
vec2 min(vec2 x , float y )
vec3 min(vec3 x , float y )
vec4 min(vec4 x , float y )
Returns the smallest value; that is, y if y < x ,
otherwise it returns x .
Note, for vectors, this is a component-wise
operation.
float max(float x , float y )
vec2 max(vec2 x , vec2 y )
vec3 max(vec3 x , vec3 y )
vec4 max(vec4 x , vec4 y )
vec2 max(vec2 x , float y )
vec3 max(vec3 x , float y )
vec4 max(vec4 x , float y )
Returns the largest value; that is, y if x < y ,
otherwise it returns x .
Note, for vectors, this is a component-wise
operation.
float clamp(float x , float minVal ,
float maxVal )
vec2 clamp(vec2 x , vec2 minVal ,
vec2 maxVal )
vec3 clamp(vec3 x , vec3 minVal ,
vec3 maxVal )
vec4 clamp(vec4 x , vec4 minVal ,
vec4 maxVal )
vec2 clamp(vec2 x , float minVal ,
float maxVal )
vec3 clamp(vec3 x , float minVal ,
float maxVal )
vec4 clamp(vec4 x , float minVal ,
float maxVal )
Constrains x to lie between minVal and maxVal;
that is, returns min (max ( x , minVal ), maxVal ).
Results are undefined if minVal > maxVal .
APPENDIX B Built-In Functions of GLSL ES 1.0
446
Syntax Description
float mix(float x , float y, float a )
vec2 mix(vec2 x , vec2 y, float a )
vec3 mix(vec3 x , vec3 y, float a )
vec4 mix(vec4 x , vec4 y, float a )
vec2 mix(vec2 x , float y, vec2 a )
vec3 mix(vec3 x , float y, vec3 a )
vec4 mix(vec4 x , float y, vec4 a )
vec2 mix(vec2 x , vec2 y, vec2 a )
vec3 mix(vec3 x , vec3 y, vec3 a )
vec4 mix(vec4 x , vec4 y, vec4 a )
Returns the linear blend of x and y ; that is, x *
(1– a ) + y * a.
float step(float edge , float x )
vec2 step(vec2 edge , vec2 x )
vec3 step(vec3 edge , vec3 x )
vec4 step(vec4 edge , vec4 x )
vec2 step(float edge , vec2 x )
vec3 step(float edge , vec3 x )
vec4 step(float edge , vec4 x )
Generates a step function by comparing two
values; that is, returns 0.0 if x < edge , otherwise
it returns 1.0.
float smoothstep(float edge0 ,
float edge1 , float x )
vec2 smoothstep(vec2 edge0 ,
vec2 edge1 , vec2 x )
vec3 smoothstep(vec3 edge0 ,
vec3 edge1 , vec3 x )
vec4 smoothstep(vec4 edge0 ,
vec4 edge1 , vec4 x )
Hermite interpolation.
Returns 0.0 if x ≤ edge0 and 1.0 if x ≥ edge1
and performs smooth Hermite interpolation
between 0 and 1 when edge0 < x < edge1 . This
is equivalent to:
// genType is float, vec2, vec3, or vec4
genType t;
t = clamp (( x – edge0 ) / ( edge1 – edge0 ), 0, 1);
return t * t * (3 – 2 * t);
Results are undefined if edge0 ≥ edge1 .
The following functions determine which components of their arguments will be used
depending on the functionality of the function.
Geometric Functions
447
Geometric Functions
Syntax Description
float length(float x )
float length(vec2 x )
float length(vec3 x )
float length(vec4 x )
Returns the length of vector x .
float distance(float p0 , float p1 )
float distance(vec2 p0 , vec2 p1 )
float distance(vec3 p0 , vec3 p1 )
float distance(vec4 p0 , vec4 p1 )
Returns the distance between p0 and p1 ; that is,
length ( p0 – p1 ).
float dot(float x , float y )
float dot(vec2 x , vec2 y )
float dot(vec3 x , vec3 y )
float dot(vec4 x , vec4 y )
Returns the dot product of x and y , in case of
vec3, x [0]* y [0]+ x [1]* y [1]+ x [2]* y [2].
vec3 cross(vec3 x , vec3 y ) Returns the cross product of x and y , in case of
vec3,
result[0] = x [1]* y [2] - y [1]* x [2]
result[1] = x [2]* y [0] - y [2]* x [0]
result[2] = x [0]* y [1] - y [0]* x [1]
float normalize(float x )
vec2 normalize(vec2 x )
vec3 normalize(vec3 x )
vec4 normalize(vec4 x )
Returns a vector in the same direction as x but
with a length of 1; that is, x /length( x ).
float faceforward(float N , float I ,
float Nref )
vec2 faceforward(vec2 N , vec2 I ,
vec2 Nref )
vec3 faceforward(vec3 N , vec3 I ,
vec3 Nref )
vec4 faceforward(vec4 N , vec4 I ,
vec4 Nref )
Reverse the normal. Adjust the vector N according to
the incident vector I and the reference vector Nref .
If dot( Nref , I ) < 0 return N , otherwise return – N .
APPENDIX B Built-In Functions of GLSL ES 1.0
448
Syntax Description
float reflect(float I , float N )
vec2 reflect(vec2 I , vec2 N )
vec3 reflect(vec3 I , vec3 N )
vec4 reflect(vec4 I , vec4 N )
Calculate reflection vector. For the incident vector
I and surface orientation N , returns the reflection
direction: I – 2 * dot( N , I ) * N
N must already be normalized to achieve the
desired result.
float refract(float I , float N ,
float eta )
vec2 refract(vec2 I , vec2 N , float
eta )
vec3 refract(vec3 I , vec3 N , float
eta )
vec4 refract(vec4 I , vec4 N , float
eta )
Calculate the change in direction of light due to its
medium by calculating the incident vector using the
ratio of indices of refraction. For the incident vector
I and surface normal N , and the ratio of indices of
refraction eta , return the refraction vector using the
following:
k = 1.0 – eta * eta * (1.0 – dot( N , I ) * dot( N , I ))
if (k < 0.0)
// genTyp is float, vec2, vec3, or vec4
return genType(0.0)
else
return eta * I - ( eta * dot( N , I ) + sqrt(k)) * N
The input parameters for the incident vector I and
the surface normal N must already be normalized.
Matrix Functions
Syntax Description
mat2 matrixCompMult(mat2 x , mat2 y )
mat3 matrixCompMult(mat3 x , mat3 y )
mat4 matrixCompMult(mat4 x , mat4 y )
Multiply matrix x by matrix y component-wise; that
is, if result = matrixCompMatrix( x , y ) then
result[i][j] = x [i][j] * y [i][j].
Vector Functions
449
Vector Functions
Syntax Description
bvec2 lessThan(vec2 x , vec2 y )
bvec3 lessThan(vec3 x , vec3 y )
bvec4 lessThan(vec4 x , vec4 y )
bvec2 lessThan(ivec2 x , ivec2 y )
bvec3 lessThan(ivec3 x , ivec3 y )
bvec4 lessThan(ivec4 x , ivec4 y )
Return the component-wise comparison of
x < y .
bvec2 lessThanEqual(vec2 x , vec2 y )
bvec3 lessThanEqual(vec3 x , vec3 y )
bvec4 lessThanEqual(vec4 x , vec4 y )
bvec2 lessThanEqual(ivec2 x , ivec2 y )
bvec3 lessThanEqual(ivec3 x , ivec3 y )
bvec4 lessThanEqual(ivec4 x , ivec4 y )
Return the component-wise comparison of
x ≤ y .
bvec2 greaterThan(vec2 x , vec2 y )
bvec3 greaterThan(vec3 x , vec3 y )
bvec4 greaterThan(vec4 x , vec4 y )
bvec2 greaterThan(ivec2 x , ivec2 y )
bvec3 greaterThan(ivec3 x , ivec3 y )
bvec4 greaterThan(ivec4 x , ivec4 y )
Return the component-wise comparison of
x > y .
bvec2 greaterThanEqual(vec2 x , vec2 y )
bvec3 greaterThanEqual(vec3 x , vec3 y )
bvec4 greaterThanEqual(vec4 x , vec4 y )
bvec2 greaterThanEqual(ivec2 x , ivec2 y )
bvec3 greaterThanEqual(ivec3 x , ivec3 y )
bvec4 greaterThanEqual(ivec4 x , ivec4 y )
Return the component-wise comparison of
x ≥ y .
APPENDIX B Built-In Functions of GLSL ES 1.0
450
Syntax Description
bvec2 equal(vec2 x , vec2 y )
bvec3 equal(vec3 x , vec3 y )
bvec4 equal(vec4 x , vec4 y )
bvec2 equal(ivec2 x , ivec2 y )
bvec3 equal(ivec3 x , ivec3 y )
bvec4 equal(ivec4 x , ivec4 y )
Return the component-wise comparison of
x == y .
bvec2 notEqual(vec2 x , vec2 y )
bvec3 notEqual(vec3 x , vec3 y )
bvec4 notEqual(vec4 x , vec4 y )
bvec2 notEqual(ivec2 x , ivec2 y )
bvec3 notEqual(ivec3 x , ivec3 y )
bvec4 notEqual(ivec4 x , ivec4 y )
Return the component-wise comparison of
x != y .
bool any(bvec2 x )
bool any(bvec3 x )
bool any(bvec4 x )
Return true if any component of x is true .
bool all(bvec2 x )
bool all(bvec3 x )
bool all(bvec4 x )
Return true only if all components of x are
true .
bvec2 not(bvec2 x )
bvec3 not(bvec3 x )
bvec4 not(bvec4 x )
Return the component-wise logical comple-
ment of x .
Texture Lookup Functions
451
Texture Lookup Functions
Syntax Description
vec4 texture2D(
sampler2D sampler , vec2 coord )
vec4 texture2D(
sampler2D sampler , vec2 coord ,
float bias )
vec4 texture2DProj(
sampler2D sampler , vec3 coord )
vec4 texture2DProj(
sampler2D sampler , vec3 coord ,
float bias )
vec4 texture2DProj(
sampler2D sampler , vec4 coord )
vec4 texture2DProj(
sampler2D sampler , vec4 coord ,
float bias )
vec4 texture2DLod(
sampler2D sampler , vec2 coord ,
float lod )
vec4 texture2DProjLod(
sampler2D sampler , vec3 coord ,
float lod )
vec4 texture2DProjLod(
sampler2D sampler , vec4 coord ,
float lod )
Use the texture coordinate coord
to read out texel values in the
2D texture currently bound to
sampler . For the projective (Proj)
versions, the texture coordinate
( coord .s, coord .t) is divided by
the last component of coord .
The third component of coord
is ignored for the vec4 coord
variant. The bias parameter
is only available in fragment
shaders. It specifies the value
to add the current lod when a
MIPMAP texture is bound to
sampler .
vec4 textureCube(
samplerCube sampler , vec3 coord )
vec4 textureCube(
samplerCube sampler , vec3 coord ,
float bias )
vec4 textureCubeLod(
samplerCube sampler , vec3 coord ,
float lod )
Use the texture coordinate
coord to read out a texel from
the cube map texture currently
bound to sampler . The direction
of coord is used to select the
face from the cube map texture.
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Appendix C
Projection Matrices
Orthogonal Projection Matrix
The following matrix is created by Matrix4.setOrtho(left , right , bottom , top , near , far) .
2
0 0
2
0 0
2
0 0
0 0 0 1
right left
t f e l t h g i r t f e l t h g i r
top bottom
top bottom top bottom
far near
far near far near
+ ⎤ ⎡
−
⎥ ⎢
− −
⎥ ⎢
+ ⎥ ⎢
−
⎥ ⎢
− −
⎥ ⎢
+ ⎥ ⎢
− −
⎥ ⎢
− −
⎥ ⎢
⎥ ⎢
⎦ ⎣
Perspective Projection Matrix
The following matrix is created by Matrix4.setPerspective(fov , aspect , near , far) .
1
0 0 0
*tan( )
2
1
0 0 0
tan( )
2
2* *
0 0
0 0 1 0
fov
aspect
fov
far near far near
far near far near
⎤ ⎡
⎥ ⎢
⎥ ⎢
⎥ ⎢
⎥ ⎢
⎥ ⎢
⎥ ⎢
⎥ ⎢
⎥ ⎢ +
− −
⎥ ⎢
− −
⎥ ⎢
⎥ ⎢
−
⎦ ⎣
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Appendix D
WebGL/OpenGL: Left or Right Handed?
In Chapter 2 , “Your First Step with WebGL,” the coordinate system of WebGL was
introduced as a right-handed system. However, you will probably come across tuto-
rials and other material on the web that contradict this. In this appendix, you’ll
learn the “real” coordinate systems used by WebGL by examining what will happen
when something is drawn using WebGL’s default settings. Because WebGL is based
on OpenGL, what you learn is equally applicable to OpenGL. You should read this
appendix after reading Chapter 7 , “Toward the 3D World,” because it refers back to
sample programs and explanations in that chapter.
Let’s start by referring to the “font of all knowledge”: the original specification.
Specifically, the authorized specification of OpenGL ES 2.0, which is the base specifi-
cation of WebGL, published by the Khronos group,
1 states in Appendix B :
7. The GL does not force left- or right-handedness on any of its coordinate systems.
If this is the case, and WebGL is agnostic about handedness, then why do many books
and tutorials, and in fact this book, describe WebGL as right handed? Essentially, it’s
a convention. When you are developing your applications, you need to decide which
coordinate system you are using and stick with it. That’s true for your applications,
but it’s also true for the many libraries that have been developed to help people use
WebGL (and OpenGL). Many of those libraries choose to adopt the right-handed
convention, so over time it becomes the accepted convention and then becomes
synonymous with the GL itself, leading people to believe that the GL is right handed.
So why the confusion? If everybody accepts the same convention, there shouldn’t be
a problem. That’s true, but the complication arises because WebGL (and OpenGL) at
certain times requires the GL to choose a handedness to carry out its operations, a
default behavior if you will, and that default isn’t always right handed!
1. www.khronos.org/registry/gles/specs/2.0/es_cm_spec_2.0.24.pdf
APPENDIX D WebGL/OpenGL: Left or Right Handed?
456
In this appendix, we explore the default behavior of WebGL to give you a clearer under-
standing of the issue and how to factor this into your own applications.
To begin the exploration of WebGL’s default behavior, let’s construct a sample program
CoordinateSystem as a test bed for experimentation. We’ll use this program to go back to
first principals, starting with the simplest method of drawing triangles and then adding
features to explore how WebGL draws multiple objects. The goal of our sample program is
to draw a blue triangle at –0.1 on the z-axis and then a red triangle at –0.5 on the z-axis.
Figure D.1 shows the triangles, their z coordinates, and colors.
d e r
e u l b
5 . 0 -
z
0 . 0
1 . 0 -
Figure D.1 The triangles used in this appendix and their colors
As this appendix will show, to achieve our relatively modest goal, we actually have to get
a number of interacting features to work together, including the basic drawing, hidden
surface removal, and viewing volume. Unless all three are set up correctly, you will get
unexpected results when drawing, which can lead to confusion about left and right
handedness.
Sample Program CoordinateSystem.js
Listing D.1 shows CoordinateSystem.js . The code for error processing and some
comments have been removed to allow all lines in the program to be shown in a limited
space, but as you can see, it is a complete program.
Listing D.1 CoordinateSystem
1 // CoordinateSystem.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'attribute vec4 a_Color;\n' +
6 'varying vec4 v_Color;\n' +
7 'void main() {\n' +
8 ' gl_Position = a_Position;\n' +
9 ' v_Color = a_Color;\n' +
Sample Program CoordinateSystem.js
457
10 '}\n';
11
12 // Fragment shader program
13 var FSHADER_SOURCE =
14 '#ifdef GL_ES\n' +
15 'precision mediump float;\n' +
16 '#endif\n' +
17 'varying vec4 v_Color;\n' +
18 'void main() {\n' +
19 ' gl_FragColor = v_Color;\n' +
20 '}\n';
21
22 function main() {
23 var canvas = document.getElementById('webgl'); // Retrieve <canvas>
24 var gl = getWebGLContext(canvas); // Get the context for WebGL
25 initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE);// Initialize shaders
26 var n = initVertexBuffers(gl); // Set vertex coordinates and colors
27
28 gl.clearColor(0.0, 0.0, 0.0, 1.0); // Specify the clear color
29 gl.clear(gl.COLOR_BUFFER_BIT); // Clear <canvas>
30 gl.drawArrays(gl.TRIANGLES, 0, n); // Draw the triangles
31 }
32
33 function initVertexBuffers(gl) {
34 var pc = new Float32Array([ // Vertex coordinates and color
35 0.0, 0.5, -0.1, 0.0, 0.0, 1.0, // The blue triangle in front
36 -0.5, -0.5, -0.1, 0.0, 0.0, 1.0,
37 0.5, -0.5, -0.1, 1.0, 1.0, 0.0,
38
39 0.5, 0.4, -0.5, 1.0, 1.0, 0.0, // The red triangle behind
40 -0.5, 0.4, -0.5, 1.0, 0.0, 0.0,
41 0.0, -0.6, -0.5, 1.0, 0.0, 0.0,
42 ]);
43 var numVertex = 3; var numColor = 3; var n = 6;
44
45 // Create a buffer object and write data to it
46 var pcbuffer = gl.createBuffer();
47 gl.bindBuffer(gl.ARRAY_BUFFER, pcbuffer);
48 gl.bufferData(gl.ARRAY_BUFFER, pc, gl.STATIC_DRAW);
49
50 var FSIZE = pc.BYTES_PER_ELEMENT; // The number of byte
51 var STRIDE = numVertex + numColor; // Calculate the stride
52
53 // Assign the vertex coordinates to attribute variable and enable it
54 var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
APPENDIX D WebGL/OpenGL: Left or Right Handed?
458
55 gl.vertexAttribPointer(a_Position, numVertex, gl.FLOAT, false, FSIZE *
➥STRIDE, 0);
56 gl.enableVertexAttribArray(a_Position);
57
58 // Assign the vertex colors to attribute variable and enable it
59 var a_Color = gl.getAttribLocation(gl.program, 'a_Color');
60 gl.vertexAttribPointer(a_Color, numColor, gl.FLOAT, false, FSIZE *
➥STRIDE, FSIZE * numVertex);
61 gl.enableVertexAttribArray(a_Color);
62
63 return n;
64 }
When the sample program is run, it produces the output shown in Figure D.2 . Although
it’s not easy to see in black and white (remember, you can run these examples in your
browser from the book’s website), the red triangle is in front of the blue triangle. This is
the opposite of what you might expect because lines 32 to 42 specify the vertex coordi-
nates of the blue triangle before the red triangle.
Figure D.2 CoordinateSystem
However, as explained in Chapter 7 , this is actually correct. What is happening is that
WebGL is first drawing the blue triangle, because its vertex coordinates are specified first,
and then it’s drawing the red triangle over the blue triangle. This is a little like oil paint-
ing; once you lay down a layer of paint, anything painted on top has to overwrite the
paint below.
For many newcomers to WebGL, this can be counterintuitive. Because WebGL is a system
for drawing 3D graphics, you’d expect it to “do the right thing” and draw the red triangle
behind the blue one. However, by default WebGL draws in the order specified in the
application code, regardless of the position on the z-axis. If you want WebGL to “do the
right thing,” you are required to enable the Hidden Surface Removal feature discussed in
Hidden Surface Removal and the Clip Coordinate System
459
Chapter 7 . As you saw in Chapter 7 , Hidden Surface Removal tells WebGL to be smart
about the 3D scene and to remove surfaces that are actually hidden. In our case, this
should deal with the red triangle problem because in the 3D scene, most of the red trian-
gle is hidden behind the blue one.
Hidden Surface Removal and the Clip Coordinate
System
Let’s turn on Hidden Surface Removal in our sample program and examine its effect. To
do that, enable the function using gl.enable(gl.DEPTH_TEST) , clear the depth buffer, and
then draw the triangles. First, you add the following at line 27.
27 gl.enable(gl.DEPTH_TEST);
Then you modify line 29 as follows:
29 gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
Now if you rerun the program after making these changes, you’d expect to see the
problem resolved and the blue triangle in front of the red one. However, what you actu-
ally see is that the red triangle is still in front. Again, although it’s difficult to see in black
and white, Figure D.3 shows the result.
Figure D.3 CoordinateSystem using the hidden surface removal function
This is unexpected and is part of the confusion surrounding WebGL’s left versus right
handedness. We have correctly programmed our example based on the belief that WebGL
is right handed, but it seems to be that WebGL is either telling us that –0.5 is located
in front of –0.1 on the z-axis or that WebGL does in fact use the left-handed coordinate
system, where the positive direction of the z-axis points into the screen ( Figure D.4 ).
APPENDIX D WebGL/OpenGL: Left or Right Handed?
460
x
z
y
Figure D.4 The left-handed coordinate system
The Clip Coordinate System and the Viewing Volume
So our application example follows the convention that WebGL is right handed, but
our program clearly shows a left-handed system is in place. What’s the explanation?
Essentially, hidden surface removal, when enabled, uses the clip coordinate system (see
Figure G.5 in Appendix G ), which itself uses the “left-handed” coordinate system, not the
right-handed one.
In WebGL (OpenGL), hidden surface removal is performed using the value of gl_
Position , the coordinates produced by the vertex shader. As you can see at line 8 in
the vertex shader in Listing D.1 , a_Position is directly assigned to gl_Position in
CoordinateSystem.js . This means that the z coordinate of the red triangle is passed as
–0.5 and that of the blue one is passed as –0.1 to the clip coordinate system (the left-
handed coordinate system). As you know, the positive direction of the z-axis in the left-
handed coordinate system points into the screen, so the smaller value of the z coordinate
(–0.5) is located in front of the bigger one (–0.1). Therefore, it is the right behavior for the
WebGL system to display the red triangle in front of the blue one in this situation.
This obviously contradicts the explanation in Chapter 3 (that WebGL uses the right-
handed coordinate system). So how do we achieve our goal of having the red triangle
displayed behind the blue triangle, and what does this tell us about WebGL’s default
behaviors? Until now, the program hasn’t considered the viewing volume that needs to be
set up correctly for Hidden Surface Removal to work with our coordinate system. When
used correctly, the viewing volume requires that the near clipping plane be located in
front of the far clipping plane (that is near < far ). However, the values of near and far are
the distance from the eye point toward the direction of line of sight and can take any
value. Therefore, it is possible to specify a value of far that is actually smaller than that of
near or even use negative values. (The negative values means the distance from the eye
point toward the opposite direction of line of sight.) Obviously, the values set for near and
far depend on whether we are assuming a right- or left-handed coordinate system.
Returning to the sample program, after setting the viewing volume correctly, let’s
carry out the hidden surface removal. Listing D.2 shows only the differences from
CoordinateSystem.js .
The Clip Coordinate System and the Viewing Volume
461
Listing D.2 CoordinateSystem_viewVolume.js
1 // CoordinateSystem_viewVolume.js
2 // Vertex shader program
3 var VSHADER_SOURCE =
4 'attribute vec4 a_Position;\n' +
5 'attribute vec4 a_Color;\n' +
6 'uniform mat4 u_MvpMatrix;\n' +
7 'varying vec4 v_Color;\n' +
8 'void main() {\n' +
9 'gl_Position = u_MvpMatrix * a_Position;\n' +
10 'v_Color = a_Color;\n' +
11 '}\n';
...
23 function main() {
...
29 gl.enable(gl.DEPTH_TEST); // Enable hidden surface removal function
30 gl.clearColor(0.0, 0.0, 0.0, 1.0); // Set the clear color
31 // Get the storage location of u_MvpMatrix
32 var u_MvpMatrix = gl.getUniformLocation(gl.program, 'u_MvpMatrix');
33
34 var mvpMatrix = new Matrix4();
35 mvpMatrix.setOrtho(-1, 1, -1, 1, 0, 1); // Set the viewing volume
36 // Pass the view matrix to u_MvpMatrix
37 gl.uniformMatrix4fv(u_MvpMatrix, false, mvpMatrix.elements);
38
39 gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
40 gl.drawArrays(gl.TRIANGLES, 0, n); // Draw the triangle
41 }
Once you run this sample program, you can see the result shown in Figure D.5 , in which
the blue triangle is displayed in front of the red one.
Figure D.5 CoordinateSystem_viewVolume
APPENDIX D WebGL/OpenGL: Left or Right Handed?
462
The critical change is that the uniform variable ( u_MvpMatrix ) for passing a view matrix
was added to the vertex shader. It was multiplied by a_Position , and then its result was
assigned to gl_Position . Although we used the setOrtho() method to specify the viewing
volume, setPerspective() has the same result.
What Is Correct?
Let’s compare the process of the vertex shader in CoordinateSystem.js with that in
CoordinateSystem_viewVolume.js .
Line 8 in CoordinateSystem.js :
8 ' gl_Position = a_Position;\n' +
became line 9 in CoordinateSystem_viewVolume.js :
9 ' gl_Position = u_MvpMatrix * a_Position;\n' +
As you can see, in CoordinateSystem_viewVolume.js , which displays the order of triangles
as was intended, the transformation matrix (in this case, a view matrix) is multiplied by
a vertex coordinate. To understand this operation, let’s examine how to rewrite line 8
in CoordinateSystem.js into the form <matrix> * <vertex coordinate> just like line 9 in
CoordinateSystem_viewVolume.js .
Line 8 assigns the vertex coordinate ( a_Position ) to gl_Position directly. To ensure that
the matrix multiplication operation has no effect the <matrix> must have the following
elements (that is, the identity matrix):
⎡
⎣
⎢
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
⎥
1 0 0 0
0 1 0 0
0 0 1 0
0 0 0 1
Therefore, line 8 in CoordinateSystem.js actually has the same effect as passing the iden-
tity matrix to u_MvpMatrix in line 9 in CoordinateSystem_viewVolume.js . In essence, this
matrix is controlling the default behavior of WebGL.
To understand this behavior better, let’s clarify what is happening if the projection matrix
is the identity matrix. You can understand this by using the matrix in Appendix C (see
Figure D.6 ) and the identify matrix to find left , right , top , bottom , near , and far.
What Is Correct?
463
2
0 0
2
0 0
2
0 0
0 0 0 1
right left
t f e l t h g i r t f e l t h g i r
top bottom
top bottom top bottom
far near
far near far near
+ ⎤ ⎡
−
⎥ ⎢
− −
⎥ ⎢
+ ⎥ ⎢
−
⎥ ⎢
− −
⎥ ⎢
+ ⎥ ⎢
− −
⎥ ⎢
− −
⎥ ⎢
⎥ ⎢
⎦ ⎣
Figure D.6 The projection matrix generated by setOrtho()
In this case, right – left = 2 and right + left = 0, which resolves to left = –1, right = 1. Equally,
far – near =–2 and far + near = 0, resolving to near = 1 and far = –1. That is:
left = -1, right = 1, bottom = -1, top = 1, near = 1, and far = -1
Using these parameters to setOrtho() as follows:
mvpMatrix.setOrtho(-1, 1, -1, 1, 1, -1);
results in near being greater than far . This means that the far clipping plane is placed in
front of the near clipping plane along the direction of the line of sight (see Figure D.7 ).
y
x z
left=-1.0
top=1.0
Eye point
near=1.0
right=1.0
far=-1.0
bottom= -1.0
Figure D.7 The viewing volume created by the identity matrix
If you specify the viewing volume by yourself, you will observe the same phenomenon
when you specify near > far to setOrtho() . That is, WebGL (OpenGL) follows the right-
handed coordinate system when you specify the viewing volume in this way.
Then look at the matrix representing the viewing volume in which the objects are
displayed correctly:
mvpMatrix.setOrtho(-1, 1, -1, 1, -1, 1);
APPENDIX D WebGL/OpenGL: Left or Right Handed?
464
This method generates the following projection matrix:
−
⎡
⎣
⎢
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
⎥
1 0 0 0
0 1 0 0
0 0 1 0
0 0 0 1
You will recognize that this matrix is a scaling matrix described in Chapter 4 , “More
Transformations and Basic Animation.” That is the matrix generated by setScale(1, 1,
-1) . You should note that the scaling factor of the z-axis is –1, meaning that the sign
of the z coordinates will be reversed. So this matrix transforms the conventional right-
handed coordinate system used in this book (and assumed by most WebGL libraries) to
the left-handed coordinate system used in the clip coordinate system by reversing the z
coordinates.
Summary
In summary, we know from the specification that WebGL doesn’t enforce either right
or left handedness. We have seen that many WebGL libraries and applications adopt
the convention that WebGL is right handed, as do we in this book. When WebGL’s
default behavior contradicts this (for example, when working in clip-space where it uses
a left-handed coordinate system), we can compensate programmatically, by reversing,
for example, the z coordinates. This allows us to continue to follow the convention that
WebGL is right handed. However, as previously stated, it’s only a convention. It’s one
that most people follow, but one that will occasionally trip you up if you aren’t aware of
WebGL’s default behaviors and how to handle them.
Appendix E
The Inverse Transpose Matrix
The inverse transpose matrix, previously introduced in Chapter 8 , “Lighting Objects,” is
a matrix that determines the inverse of a given matrix and then transposes it. As shown
in Figure E.1 , the direction of the normal vector of an object is subject to change
depending on the type of the coordinate transformation. However, if you use the
inverse transpose of the model matrix, you can safely ignore this in calculations.
l a m r o n e h t
n o i t c e r i d
1 ( 0 ) 0
n o i t c e r i d l a m r o n e h t
1 ( 0 ) 0
n o i t c e r i d l a m r o n e h t
1 ( 1 ) 0
n o i t c e r i d l a m r o n e h t
) 0 , 5 . 0 , 1 (
x
y
n o i t a l s n a r t ) 1 (
) s i x a - y e h t g n o l a e t a l s n a r t (
n o i t a t o r ) 2 (
) s e e r g e d 5 4 e t a t o r (
g n i l a c s ) 3 (
g n o l a 2 y b e l a c s (
) s i x a - y e h t
l a m r o n l a n i g i r o e h t
Figure E.1 The direction of normal vector changes along with the coordinate transformation
In Chapter 8 , you saw how to use the inverse transpose of the model matrix to trans-
form normals. However, there are actually some cases where you can also determine
the normal vector direction with the model matrix. For instance, when rotating, you
can determine the direction of the normal vector by multiplying the normal vector by
the rotation matrix. When calculating the direction of the normal vector, whether you
resort to the model matrix itself or its inverse transpose depends on which transforma-
tion (translation, rotation, and scaling) is already integrated inside the model matrix.
APPENDIX E The Inverse Transpose Matrix
466
If the model matrix already includes a translation and you multiply the normal by the
model matrix, the normal is translated, resulting in a modification of its orientation. For
example, the normal (1, 0, 0), when translated by 2.0 along the y-axis, is repositioned to
the location (1, 2, 0). You can avoid this problem by using the 3×3 submatrix extracted
from the top left area of the 4×4 model matrix. For example:
attribute vec4 a_Normal; // normal
uniform mat4 u_ModelMatrix; // model matrix
void main() {
...
vec3 normal = normalize(mat3(u_ModelMatrix) * a_Normal.xyz);
...
}
The values located in the rightmost column determine the scale of the displacement
produced by the translation matrix, as illustrated in Figure E.2 .
T
3x3 sub-matrix
Transformation Matrix (4x4 Translation Matrix)
0 0 1
x
T 0 1 0
y
T 1 0 0
z
1 0 0 0
x
y
z
1
’ x
’ y
’ z
1
= *
Figure E.2 The transformation matrix and its 3×3 submatrix
Because this submatrix also includes the components of the rotation and scaling matrices,
you need to consider rotation and scaling on a case-by-case basis:
• If you only want to perform a rotation: You can use the 3×3 submatrix of the
model matrix. If the normal is already normalized, the transformed normal does not
have to be normalized.
• If you want to perform a scaling transformation (with a uniform scale factor):
You can use the 3×3 submatrix of the model matrix. However, the transformed
normal has to be normalized.
• If you want to perform a scaling transformation (with a nonuniform scale
factor): You need to use the inverse transpose matrix of the model matrix. The
transformed normal has to be normalized.
The second case, where you want to perform a scaling transformation with a uniform
scale factor, implies that you perform a scaling with an identical scaling factor along the
x-, y-, and z-axes. For example, if you scale by a factor of 2.0 along the x-, y-, and z-axes,
you will set the same value to each of the arguments of Matrix4.scale() : Matrix4.
467
scale(2.0, 2.0, 2.0) . In this situation, even if the size of the object is modified, its
shape is left unchanged. Alternatively, those cases involving a scaling transformation with
a nonuniform scale factor require that you use a different scaling factor for each axis. For
instance, if you limit the scaling to the y-axis direction, you will use Matrix4.scale(1.0,
2.0,1.0) .
You have to resort to the inverse transpose matrix in case (3) because, if the scaling is
nonuniform, the direction of the normal vector is incorrectly modified when multiplying
it with the model matrix that incorporates the scaling transformation. Figure E.3 shows
this.
direction of normal
(1, 2, 0)
direction of normal
(1, 1, 0)
Scale by 2 along the y-axis
Figure E.3 Simply multiplying the normal vector with the model matrix results in a modification
of the normal direction
Performing a nonuniform scaling of the object (left side of the figure), with a scaling
factor of 2.0 limited to the y-axis, results in the shape on the right. Here, to determine
the normal direction after the transformation, you multiply the model matrix with the
normal (1. 1, 0) of the left side object. However, then the direction of the normal is
changed to (1, 2, 0) and is no longer at a right angle (90 degrees) to the line.
The solution to this requires a little math. We will call the model matrix M , the original
normal n , the transformation matrix M ', which transforms n without changing the direc-
tion of n , and the vector perpendicular to n , s . In addition, we define n ' and s ', as shown
in Equations E.1 and E.2:
Equation E.1 n ' = M ' × n
Equation E.2 s ' = M × s
See Figure E.4 .
APPENDIX E The Inverse Transpose Matrix
468
s
n
s’
n’
Transformation Matrix: M
Figure E.4 The relationship between n and s, and, n' and s'.
Here, you can calculate M ' so that the two vectors n ' and s ' form a right angle. If the two
vectors form a right angle, their dot product is equal to 0. Using the “·” notation for a
dot product, you can derive the following equation:
n ’. s ’ = 0
You can now rewrite this expression using the equations E.1 and E.2 ( M
T is the transpose
matrix of M ):
( M ’ × n ) · ( M × s ) = 0
( M ’ × n )
T
× ( M × s ) = 0 A . B = A
T
× B
n
T
× M ’
T
× M × s = 0 ( A × B )
T
= B
T
× A
T
Because n and s form a right angle, their dot product is also 0 ( n s = 0). Therefore, as
already stated, A . B = A
T × B , so substituting n for A and s for B will result in n . s = n T × s
= 0. Comparing this expression with the equation E.3, if the goal is for the products M '
T
× M
T between n T and s to be equal to the identity matrix ( I ), this can be reformulated as
follows:
M ’
T
× M
T
= I
Resolving this equation provides us with the following result ( M
–1 is the inverse matrix of
M ):
M ’ = ( M
–1 ) T
From this equation, you can see that M ' is the transpose matrix of the inverse matrix of
M or, in other words, the inverse transpose of M . Because M can include cases (1), (2),
and (3) enumerated earlier, if you calculate the inverse transpose matrix of M and multi-
ply it with the normal vector, you will get the correct result. Thus, this provides the solu-
tion for transforming the normal vector.
469
Obviously, the calculation of the inverse transpose matrix can be time consuming, but if
you can confirm the model matrix fits criteria (1) or (2), you can simply use the 3×3 sub-
matrix for increased efficiency.
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Appendix F
Load Shader Programs from Files
All the sample programs in this book embed the shader programs in the JavaScript
program, which increases the readability of the sample programs but makes it hard to
construct and maintain the shader programs.
As an alternative, you can load the shader programs from files by using the same
methods described in the section “Load and Display 3D Models” in Chapter 10 ,
“Advanced Techniques.” To see how that’s done, let’s modify ColoredTriangle from
Chapter 5 , “Using Colors and Texture Images,” to add support for loading shaders from
a file. The new program is called LoadShaderFromFiles , which is shown in Listing F.1 .
Listing F.1 LoadShaderFromFiles
1 // LoadShaderFromFiles.js based on ColoredTriangle.js
2 // Vertex shader program
3 var VSHADER_SOURCE = null;
4 // Fragment shader program
5 var FSHADER_SOURCE = null;
6
7 function main() {
8 // Retrieve <canvas> element
9 var canvas = document.getElementById('webgl');
10
11 // Get the rendering context for WebGL
12 var gl = getWebGLContext(canvas);
...
17 // Load the shaders from files
18 loadShaderFile(gl, 'ColoredTriangle.vert', gl.VERTEX_SHADER);
19 loadShaderFile(gl, 'ColoredTriangle.frag', gl.FRAGMENT_SHADER);
20 }
APPENDIX F Load Shader Programs from Files
472
21
22 function start(gl) {
23 // Initialize shaders
24 if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
...
43 gl.drawArrays(gl.TRIANGLES, 0, n);
44 }
...
88 function loadShaderFile(gl, fileName, shader) {
89 var request = new XMLHttpRequest();
90
91 request.onreadystatechange = function() {
92 if (request.readyState === 4 && request.status !== 404) {
93 onLoadShader(gl, request.responseText, shader);
94 }
95 }
96 request.open('GET', fileName, true);
97 request.send(); // Send the request
98 }
99
100 function onLoadShader(gl, fileString, type) {
101 if (type == gl.VERTEX_SHADER) { // The vertex shader is loaded
102 VSHADER_SOURCE = fileString;
103 } else
104 if (type == gl.FRAGMENT_SHADER) { // The fragment shader is loaded
105 FSHADER_SOURCE = fileString;
106 }
107 // Start rendering, after loading both shaders
108 if (VSHADER_SOURCE && FSHADER_SOURCE) start(gl);
109 }
Unlike ColoredTriangle.js , this sample program initializes the VSHADER_PROGRAM (line
3) and FSHADER_PROGRAM (line 5) to null to allow them to be loaded from files later. The
function main() defined at line 7 loads the shader programs at lines 18 and line 19 by
using loadShaderFile() . This function is defined at line 88, and its second argument
specifies the filename (URL) that contains the shader program. The third argument speci-
fies the type of the shader program.
The function loadShaderFile() creates a request of type XMLHttpRequest to get the file
specified by fileName and then registers an event handler ( onLoadShader() ) at line 91 to
handle the file when it is loaded. After that, it sends the request at line 97. Once the file is
acquired, onLoadShader() is called. This function is defined at line 100.
The onLoadShader() checks the third parameter type and uses it to store the fileString
containing the shader program to VSHADER_PROGRAM or FSHADER_PROGRAM . Once you load
both shader programs, call start(gl) at line 108 to draw the triangle using the shaders.
Appendix G
World Coordinate System Versus Local
Coordinate System
In Chapter 7 , “Toward the 3D World,” you used and displayed your first 3D object
(a cube), allowing the sample program to begin to feel more like a “real” 3D applica-
tion. However, to do so, you had to manually set up the vertex coordinates and the
index information of the cube, which was quite time consuming. Although you will
do the same manual setup throughout this book, this is not something you will gener-
ally do when creating your own WebGL applications. Usually you will use a dedicated
3D modeling tool to create 3D objects. This allows you to create elaborate 3D objects
through the manipulation (combination, deformation, vertex population adjustment,
vertex interval tuning, and so on) of elementary 3D shapes, such as cubes, cylinders, or
spheres. The 3D modeling tool Blender ( www.blender.org/ ) is shown in Figure G.1 .
APPENDIX G World Coordinate System Versus Local Coordinate System
474
Figure G.1 Creation of a 3D object with a 3D modeling tool
The Local Coordinate System
When creating the model of a 3D object, it is necessary to decide where the origin (that
is, (0, 0, 0)) is placed for the model. You can choose the origin of the model so that the
model can be easily built, or alternatively so that the created model can be handled easily
in a 3D scene. The cube introduced in the previous section was created with its center
set at the origin (0, 0, 0). Sphere-shaped objects, like the sun or the moon, are usually
modeled with their center at the origin.
On the other hand, in the case of game character models, such as the one shown in Figure
G.1 , most of the models are built with the origin positioned at their feet level, and the
y-axis running through the center of the body. By doing so, if you place the character at
the y coordinate = 0 height (at the ground level), the character looks like it is standing
on the ground—neither floating above the ground nor sinking down into the ground. In
this configuration, if you translate the model along the z-axis or x-axis, the character will
appear to be walking or gliding along the ground. Additionally, you can turn the character
using a simple rotation around the y-axis.
The World Coordinate System
475
In such cases, the coordinates of the vertices that constitute objects or characters config-
ured in this fashion are expressed with respect to this origin. Such a coordinate system
is called the local coordinate system . Using modeling tools (like Blender), the compo-
nents (vertex coordinates, colors, indices, and so on) of models designed this way can be
exported to a file. You can then import this information into the buffer object in WebGL,
and using gl.drawElements() , you can draw and display the model created with the 3D
modeling tool.
The World Coordinate System
Let’s consider the case of a game where multiple characters would appear in a single
space. The goal is to use the characters illustrated in Figure G.2 (right side) in the 3D scene
shown on the left side. All three characters and the world have their own origin.
(0, 0, 0)
x
y
z
x
y
z x
x
y
z
x
y
z
Figure G.2 Disposition of several characters inside a single world
When you want to display the characters as they are, you are faced with a problem.
Because all the characters are built with their origin positioned at their feet level, they
eventually are displayed on top of each other at the same location: the origin (0, 0, 0) of
the 3D scene ( Figure G.3 ).
1 That’s not something that generally happens in the real world
and certainly not what you want here.
1 To keep the fi gure understandable, the characters are placed at slightly shifted positions.
APPENDIX G World Coordinate System Versus Local Coordinate System
476
(0, 0, 0)
x
y
z
Figure G.3 All the characters are displayed at the origin.
To address this problem, you need to adjust the position of each character to avoid
them overlapping. To achieve this, you can use coordinate transformations that you
originally looked at in Chapter 3 , “Drawing and Transforming Triangles,” and 4, “More
Transformations and Basic Animation.” To prevent the characters from overlapping, you
could translate the penguin to (100, 0, 0), the monkey to (200, 10, 120), and the dog to
(10, 0, 200).
The coordinate system we use to correctly place characters created within a local coor-
dinate system is called the world coordinate system , or alternatively the global
coordinate system . The associated model transformation is referred to as the world
transformation .
Of course, to prevent the characters of the penguin, monkey, and dog from overlapping,
you can build them using the world coordinate system. For example, if you model the
penguin in a tool such as Blender with its origin set to (100, 0, 0), when you insert the
penguin into the world, it will be displayed at position (100, 0, 0), so you don’t need a
coordinate transformation to avoid overlapping. However, this approach creates its own
difficulties. For example, it becomes difficult to make the penguin spin like a ballerina
because, if you perform a rotation around the y-axis, you generate a circular motion of
radius 100. You could, of course, first translate the penguin back to the origin, rotate it,
and then translate it again to the original position, but that is quite a lot of work.
You actually already dealt with a similar case in Chapter 7 . Using the coordinates of one
set of triangles, the vertices of which were determined with respect to the origin set at the
center, PerspectiveView_mvp draws a second set of triangles. Here is the figure we referred
to in that program (see Figure G.4 ).
Transformations and the Coordinate Systems
477
z
-0.75
0.75
x
y
Figure G.4 Triangles group in PerspectiveView_mvp
Here, the local coordinate system expresses the vertex coordinates of the triangles shown
with the dotted lines, whereas the world coordinate system is used to describe their trans-
lation along the x-axis.
Transformations and the Coordinate Systems
So far, we have not considered local and world coordinate systems so that you can focus
on the core aspects of each example. However, for reference, Figure G.5 shows the rela-
tionship between the transformations and the coordinate systems and is something to
bear in mind as you deepen your knowledge of 3D graphics and experiment with model-
ing tools.
local
coordinate
system
world
coordinate
system
clipping
coordinate
system
view
coordinate
system
model matrix
translate(),
rotate(),
scale()
view matrix
lookAt()
orthographic projection matrix
setOrtho()
perspective projection matrix
setPerspective()
Figure G.5 Transformation and the coordinate system
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Appendix H
Web Browser Settings for WebGL
This appendix explains how to use advanced web browser settings to ensure that
WebGL is displayed correctly and what to do if it isn’t.
If your graphics board isn’t compatible with WebGL, you may see the message shown
in Figure H.1 .
Figure H.1 Loading a WebGL application results in an error message
If this happens, you may still be able to get WebGL to work in your browser with a
little bit of tweaking:
1. If you are using Chrome, start the browser with the option --ignore-gpu-black-
list . To specify this option, right-click the Chrome browser shortcut icon and
select Properties from the menu. You’ll see a pop-up window similar to that in
Figure H.2 . Then add the option string at the end of the command string in the
Target column on the window. After that, Chrome is always started with the
option. If this solves your problem, leave this option enabled.
APPENDIX H Web Browser Settings for WebGL
480
Figure H.2 Specifying an option in the Google Chrome Properties window
2. If you are using Firefox, enter about:config in the address bar. Firefox shows “This
might void your warranty!” Click the button labeled “I’ll be careful, I promise!” Type
webgl in the text field labeled by Search or Filter, and then Firefox will display the
WebGL-related setting names (see Figure H.3 ). Double-click webgl.force-enabled in
the list to change its value from false to true . Again, if this solves your problem,
leave this option enabled.
Figure H.3 WebGL-related settings in Firefox
If neither solution works, you will have to find another machine that has better support
for WebGL. Again, look at the Khronos wiki for more information: www.khronos.org/
webgl/wiki .
Glossary
A
alpha blending The process of using the alpha value (“A”) in RGBA to blend the
colors of two or more objects.
alpha value The value used to indicate the transparency (0.0 is transparent and 1.0
is opaque) of an object. Alpha blending uses this value.
ambient light Indirect light. Light that illuminates an object from all directions and
with the same intensity.
attach The process of establishing a connection between two existing objects.
Compare to bind .
attribute variable The variable used to pass data to a vertex shader.
B
bind The process of creating a new object and then establishing a connection (the
binding) between that object and a rendering context. Compare to attach .
buffer A block of memory allocated and dedicated to storing a specifi c kind of data,
such as color or depth values.
buffer object WebGL object used to store multiple items of vertex information.
C
canvas The HTML5 element and features to draw graphics on a web page.
clipping An operation that identifi es the area (or region) within a 3D scene that
will be drawn. Anything not in the clipping region is not drawn.
color buffer The memory area into which WebGL draws. Once drawn, the contents
are displayed on the screen.
column major A convention describing the way a matrix is stored in an array. In
column major, the columns are listed in sequence in the array.
Glossary
482
completeness Used in the context of a framebuffer, indicates the state whether a frame-
buffer object meets all the requirements for drawing.
context JavaScript object that implements the methods used to draw onto a canvas.
D
depth (value) The z value of a fragment when viewing the fragment from the eye point
along the line of sight.
depth buffer The memory area used for hidden surface removal. It stores the depth value
(z value) of all fragments.
directional light A light source that emits parallel light rays.
F
far clipping plane The farther clipping plane of the planes comprising the viewing volume
from the eye point.
fog The effect seen when fading colors to a background color based on the distance from
the observer. Fog is often used to provide depth cues to the observer.
fragment The pixel generated by the rasterization process and which has color, depth
value, texture coordinates, and more.
fragment shader The shader program to process the fragment information.
framebuffer object WebGL object used for offscreen drawing.
G
GLSL ES OpenGL ES Shading Language. ES stands for Embedded System.
H
hidden surface removal The process to determine and hide the surfaces and parts of sur-
faces that are not visible from a certain viewpoint.
I
image A rectangular array of pixels.
index (vertex) See vertex index .
L
local coordinates The vertex coordinates that are defi ned in the local coordinate system
(the coordinate system that relates specifi cally to the selected object). (Also see world coordi-
nates. )
Glossary
483
M
model matrix The matrix used to translate, rotate, or scale objects. It is also known as a
modeling matrix.
model view matrix The matrix that multiplies the view matrix by the model matrix.
model view projection matrix The matrix that multiplies the projection matrix by the
model view matrix.
N
near clipping plane The nearer clipping plane of the planes comprising the viewing vol-
ume from the eye point.
normal An imaginary line that is perpendicular to the surface of a polygon and represent-
ed by a vec3 number. It is also called the normal vector.
O
orthographic projection matrix The matrix used to defi ne a box-shaped viewing volume—
left, right, bottom, top, near, far—that defi nes the clipping planes of the box. Objects
located closer to the far clipping plane are not scaled.
P
perspective projection matrix The matrix used to defi ne a pyramid-shaped viewing
volume. Objects located closer to the far clipping plane are scaled appropriately to give
perspective.
pixel Picture element. It has an RGBA or RGB value.
point light Light source that emits light in all directions from one point.
program object WebGL object to manage shader objects.
projection matrix The generic term for the orthographic projection matrix and the per-
spective projection matrix.
R
rasterization process The process to convert shapes, defi ned in a vector format into frag-
ments (pixels or dots) for display on a video screen.
renderbuffer object WebGL object that supports a general two-dimensional drawing area.
RGBA A color format: R (red), G (green), B (blue), and A (alpha).
S
sampler A data type used to access a texture image from within a fragment shader.
shader The computer program that implements the fundamental drawing function used
in WebGL. WebGL supports vertex shaders and fragment shaders.
Glossary
484
shader object WebGL object to manage shaders.
shading The process of applying shading to each face of an object.
shadowing The process to determine and draw shadows cast by objects.
T
texel The basic element ( tex ture el ement) that makes up a texture image. It has RGB or
RGBA value.
texture coordinates Two-dimensional coordinates to be used to access a texture image.
texture image The image used in texture mapping. It is also simply called texture.
texture mapping The process of applying (mapping) a texture image to the surface of an
object.
texture object WebGL object to manage a texture image.
texture unit The mechanism to manage multiple texture objects.
transformation The process of converting the vertex coordinates of an object to new vertex
coordinates as a result of applying a transformation (translation, scaling, and so on).
U
uniform variable The variable used to pass data to the vertex shader or fragment shader.
V
varying variable The variable used to pass data from the vertex shader to the fragment
shader.
vertex index The number assigned to the vertex information elements stored in a buffer
object. It starts from 0 and is increased by 1 for each new element stored.
vertex shader The shader program that processes the vertex information.
view coordinate system The coordinate system that has the eye point at its origin, the line
of sight along the negative z-axis, and the up direction in the positive y-axis.
view matrix The matrix to transform the vertex coordinates to the coordinates that are
viewed from the eye point toward the line of sight.
view projection matrix The matrix that multiplies the projection matrix by the view ma-
trix.
viewing volume The subspace that is displayed on the screen. The objects outside the vol-
ume are not displayed.
W
world coordinates The coordinates that are obtained by multiplying the model matrix by
the local vertex coordinates of the 3D model.
References
Bowman, Doug A., Ernst Kruijff, Joseph J. LaViola Jr, and Ivan Poupyrev. 3D User
Interfaces: Theory and Practice . Addison-Wesley Professional (July 26, 2004).
Dunn, Fletcher, and Ian Parberry. 3D Math Primer for Graphics and Game Development , 2nd
Edition. A K Peters/CRC Press (November 2, 2011).
Foley, James D., Andries van Dam, Steven K. Feiner, and John F. Hughes. Computer
Graphics: Principles and Practice in C , 2nd Edition. Addison-Wesley Professional (August 4,
1995).
Munshi, Aaftab, Dan Ginsburg, and Dave Shreiner. OpenGL ES 2.0 Programming Guide .
Addison-Wesley Professional (July 24, 2008).
Shreiner, Dave. The Khronos OpenGL ARB Working Group. OpenGL Programming Guide:
The Official Guide to Learning OpenGL, Versions 3.0 and 3.1 , 7th Edition. Addison-Wesley
Professional (July 21, 2009).
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Index
Symbols
2D graphics
coloring vertices different colors,
151-160
geometric shape assembly and
rasterization, 151-155
invoking fragment shader, 155
varying variables and interpolation
process, 157-160
verifying fragment shader
invocation, 156-157
combining multiple transformations,
119-121
drawing
rectangles, 13-16, 89-91
triangles, 85-91
pasting images on, 160-183
activating texture units, 171-172
assigning texture images to texture
objects, 177-179
binding texture objects to target,
173-174
changing texture coordinates,
182-183
flipping image y-axis, 170-171
mapping texture and vertex
coordinates, 162-163, 166
multiple texture mapping, 183-190
passing texture coordinates from
vertex to fragment shader, 180-181
passing texture unit to fragment
shader, 179-180
retrieving texel color in fragment
shader, 181-182
setting texture object parameters,
174-177
setting up and loading images,
166-170
texture coordinates, explained, 162
TexturedQuad.js, 163-166
restoring clipped parts, 251-253
rotating, 96-102, 107-110, 234-235
RotatingTranslatedTriangle.js, 135-136
RotatingTriangle.js, 126-129
calling drawing function, 129-130
draw() function, 130-131
requestAnimationFrame() function,
131-133
updating rotation angle, 133-135
TranslatedTriangle.js, 92-96
translating, 92-96, 111
Index
488
3D graphics. See also WebGL
alpha blending, 384
applications
browser functionality in, 5
publishing, ease of, 4
writing in text editors, 3-4
displaying on web pages
(3DoverWeb), 372
lighting, 291-293
light sources, 293
reflected light, 294-296
loading, 414-416
modeling tools, 415, 473-475
point light objects, 314-315
shading, 292
3D models
MTL file format, 418
OBJ file format, 417
OBJViewer.js, 419-421
parser code, 423-430
user-defined objects, 422-423
3DoverWeb, 372
[ ] (array indexing) operator, 203-204
. (dot) operator, 201-202
; (semicolon), in GLSL ES, 193
A
abs() function, 444
access to members
of arrays in GLSL ES, 209
of structures in GLSL ES, 207
of vector and matrix data types,
201-204
acos() function, 442
activating texture units, 171-172
adding
color to each face (Hello Cube),
284-285
shading to ambient light, 307-308
affine transformations, 91
all() function, 450
alpha blending, 380
3D objects, 384
blending function, 382-383
drawing when alpha values coexist,
385-386
implementing, 380-381
LookAtBlendedTriangles.js, 381-382
ambient light, 294
shading, adding, 307-308
ambient reflection, 295-296
angle functions, 216, 441-442
animate() function, 129, 133-135
animation, 124-136
multiple transformations in, 135-136
RotatingTriangle.js, 126-129
calling drawing function, 129-130
draw() function, 130-131
requestAnimationFrame() function,
131-133
updating rotation angle, 133-135
anonymous functions, 52-53
any() function, 450
applications
3D graphics applications
browser functionality in, 5
publishing, ease of, 4
writing in text editors, 3-4
WebGL application structure, 6-7
Index
489
array indexing ([ ]) operator, 203-204
arrays
in GLSL ES, 208-209
interleaving, 141-145
typed arrays, 78-79
asin() function, 442
assigning
buffer objects to attribute variables,
79-81
texture images to texture objects,
177-179
values
in GLSL ES structures, 207
in GLSL ES variables, 196-197
in matrix data types, 199-201
in vector data types, 199-201
asynchronous loading of texture images,
169-170
atan() function, 442
atmospheric effects, fog, 372-373
attaching shader objects to program
objects, 350-351
attribute variables, 217-218
assiging buffer objects to, 79-81
declaring, 43
enabling assignment, 81-82
explained, 41-42
for point size (MultiAttributeSize.js),
139-140
setting value, 45-49
storage location, 44-45
B
back color buffer, 437
background objects, 267-269
z fighting, 273-275
binding
buffer objects to targets, 75-76
renderbuffer objects, 399-400
texture objects to targets, 173-174
BlendedCube.js, 384
Blender 3D modeling tool, 415, 473-475
blending function, alpha blending,
382-383
<body> element, 12
bool data type, 196
Boolean values in GLSL ES, 194
boxed-shape viewing volume
defining, 243-244
OrthoView.html, 245-246
OrthoView.js, 246-247
break statement in GLSL ES, 212-213
browsers
<canvas> element support, 12
console, viewing, 14
enabling local file access, 161
functionality in 3D graphics
applications, 5
JavaScript to WebGL processing flow,
27, 438
WebGL settings, 479-480
buffer objects
assigning to attribute variables, 79-81
binding to targets, 75-76
creating, 74-75
Index
490
creating multiple, 140-141
defined, 69
enabling attribute variable assignment,
81-82
explained, 72-74
writing vertex coordinates, colors, and
indices in, 281-284
writing data to, 76-78
buffers
color buffers, 22
drawing to, 437-439
saving content from, 56
swapping, 437
depth buffer, 22
types of, 22
built-in functions in GLSL ES, 215-216
C
calculating
color per fragment, 319
diffuse reflection, 297-299
cancelAnimationFrame() function, 133
canvas.addEventListener() function, 432
<canvas> element, 9-11, 243
browser support, 12
clearing drawing area, 16-23
coordinates for center, 23
coordinate system, 16
DrawRectangle.html, 11-13
DrawRectangle.js, 13-16
HelloCanvas.html, 17-18
HelloCanvas.js, 18-23
mapping to WebGL coordinate system,
39, 54-57
retrieving, 14, 19
canvas.getContext() function, 15
case sensitivity of GLSL ES, 193
ceil() function, 444
changing
color with varying variables, 146-151
eye point using keyboard, 238
near value, 250-251
checkFace() function, 368
Chrome
console, viewing, 14
enabling local file access, 161
WebGL browser settings, 479
clamp() function, 445
clear color, setting, 21-23
clearing
color buffer, 22
drawing area, 16-23
ClickedPoints.js, 50-52
clip coordinate system, viewing volume
and, 460-462
clipped parts, restoring, 251-253
color
adding to each face (Hello Cube),
284-285
changing with varying variables,
146-151
of points, changing, 58-66
setting, 15, 21-23
texel color, retrieving in fragment
shader, 181-182
color buffers, 22
drawing to, 437-439
Index
491
saving content from, 56
swapping, 437
ColoredCube.js, 285-289
ColoredPoints.js, 59-61
ColoredTriangle.js, 159
coloring vertices, 151-160
geometric shape assembly and
rasterization, 151-155
invoking fragment shader, 155
varying variables and interpolation
process, 157-160
verifying fragment shader invocation,
156-157
color per fragment, calculating, 319
column major order, 109-110
combining multiple transformations,
119-121
comments in GLSL ES, 193
common functions, 216, 444-446
compiling shader objects, 347-349
conditional control flow in GLSL ES,
211-213
console, viewing, 14
constant index, 203
constants of typed arrays, 79
constructors in GLSL ES, 199-201
for structures, 207
const variables in GLSL ES, 217
context, retrieving, 15, 20-21
continue statement in GLSL ES, 212-213
coordinates
center of canvas, 23
homogeneous coordinates, 35
for mouse clicks, 54-57
WebGL coordinate system, 38-39
CoordinateSystem.js, 456-459
coordinate systems
for <canvas> element, 16
clip coordinate system and viewing
volume, 460-462
CoordinateSystem.js, 456-459
handedness in default behavior,
455-464
Hidden Surface Removal tool, 459-460
local coordinate system, 474-475
projection matrices for, 462-464
texture coordinates
changing, 182-183
explained, 162
flipping image y-axis, 170-171
mapping to vertex coordinates,
162-163, 166
passing from vertex to fragment
shader, 180-181
transformations and, 477
world coordinate system, 475-477
CoordinateSystem_viewVolume.js, 461
cos() function, 442
createProgram() function, 354
cross() function, 447
ctx.fillRect() function, 16
cubes, 301
cuboids, 301
cuon-matrix.js, 116
cuon-utils.js, 20
Index
492
D
data, passing
to fragment shaders with varying
variable, 146-151
to vertex shaders, 137-151. See
also drawing; rectangles; shapes;
triangles
color changes, 146-151
creating multiple buffer objects,
140-141
interleaving, 141-145
MultiAttributeSize.js, 139-140
data types
in GLSL ES, 34, 194-196
arrays, 208-209
operators on, 197-198
precision qualifiers, 219-221
samplers, 209-210
structures, 207-208
type conversion, 196-197
type sensitivity, 195
vector and matrix types, 198-206
typed arrays, 78-79
#define preprocessor directive, 222
degrees() function, 441
deleting
shader objects, 346
texture objects, 167
depth buffer, 22
DepthBuffer.js, 272-273
diffuse reflection, 294-295
calculating, 297-299
shading, 296-297
Direct3D, 5
directional light, 293
shading, 296-297
discard statement in GLSL ES, 212-213
displaying 3D objects on web pages
(3DoverWeb), 372
distance() function, 447
document.getElementById() function,
14, 19
dot() function, 447
dot (.) operator, 201-202
draw() function, 129-131
objects composed of other objects,
332-334
processing flow of, 249
drawArrays() function, 284
drawbox() function, 339-340
drawing
to color buffers, 437-439
Hello Cube with indices and vertices
coordinates, 277-278
multiple points/vertices, 68-85
assigning buffer objects to attribute
variables, 79-81
binding buffer objects to targets,
75-76
buffer object usage, 72-74
creating buffer objects, 74-75
enabling attribute variable
assignments, 81-82
gl.drawArrays() function, 82-83
writing data to buffer objects, 76-78
objects composed of other objects,
324-325
points
assigning uniform variable values,
63-66
attribute variables, 41-42
Index
493
attribute variable storage location,
44-45
attribute variable value, 45-49
changing point color, 58-66
ClickedPoints.js, 50-52
ColoredPoints.js, 59-61
fragment shaders, 35-36
gl.drawArrays() function, 36-37
handling mouse clicks, 53-57
HelloPoint1.html, 25
HelloPoint1.js, 25-26
HelloPoint2.js, 42-43
initializing shaders, 30-33
method one, 23-41
method two, 41-50
with mouse clicks, 50-58
registering event handlers, 52-53
shaders, explained, 27-28
uniform variables, 61-62
uniform variable storage location,
62-63
vertex shaders, 33-35
WebGL coordinate system, 38-39
WebGL program structure, 28-30
rectangles, 13-16, 89-91
shapes, 85-91
animation, 124-136
multiple vertices, 68-85
rotating, 96-110
scaling, 111-113
transformation libraries, 115-124
translating, 92-96, 105-106, 111
triangles, 85-81
coloring vertices different colors,
151-160
combining multiple
transformations, 119-121
HelloTriangle.js, 85-86
restoring clipped parts, 251-253
rotating, 96-102, 107-110, 234-235
RotatingTranslatedTriangle.js,
135-136
RotatingTriangle.js, 126-135
TranslatedTriangle.js, 92-96
translating, 92-96, 111
using framebuffer objects, 403-404
when alpha values coexist, 385-386
drawing area
clearing, 16-23
defining, 12
mapping to WebGL coordinate
system, 39
drawing context. See context, retrieving
drawing function (tick()), calling
repeatedly, 129-130
DrawRectangle.html, 11-13
DrawRectangle.js, 13-16
drawSegment() function, 340
draw segments, objects composed of other
objects, 339-344
dynamic web pages, WebGL web pages
versus, 7
E
#else preprocessor directive, 222
enabling
attribute variable assignment, 81-82
local file access, 161
equal() function, 450
event handlers
for mouse clicks, 53-57
registering, 52-53
Index
494
execution order in GLSL ES, 193
exp2() function, 443
exp() function, 443
exponential functions, 216, 443
eye point, 228
changing using keyboard, 238
LookAtTrianglesWithKeys.js, 238-241
visible range, 241
F
faceforward() function, 447
face of objects, selecting, 365
PickFace.js, 366-368
files, loading shader programs from,
471-472
fill color, setting, 15
Firefox
console, viewing, 14
enabling local file access, 161
WebGL browser settings, 480
flipping image y-axis, 170-171
Float32Array object, 78
float data type, 196
floor() function, 444
flow of vertex shaders, processing,
248-249
fog, 372-373
implementing, 373-374
w value, 376-377
Fog.js, 374-376
Fog_w.js, 376-377
foreground objects, 267-269
DepthBuffer.js, 272-273
hidden surface removal, 270-271
z fighting, 273-275
for statement in GLSL ES, 211-212
fract() function, 444
fragments, 27, 35
fragment shaders, 27
drawing points, 35-36
example of, 192
geometric shape assembly and
rasterization, 151-155
invoking, 155
passing
data to, 61-62, 146-151
texture coordinates to, 180-181
texture units to, 179-180
program structure, 29-30
retrieving texel color in, 181-182
varying variables and interpolation
process, 157-160
verifying invocation, 156-157
FramebufferObject.js, 395-396, 403
framebuffer objects, 392-393
checking configurations, 402-403
creating, 397
drawing with, 403-404
renderbuffer objects set to, 401-402
setting to renderbuffer objects, 400-401
front color buffer, 437
functions
abs() function, 444
acos() function, 442
Index
495
all() function, 450
angle and trigonometry functions,
441-442
animate() function, 129, 133-135
anonymous functions, 52-53
any() function, 450
asin() function, 442
atan() function, 442
built-in functions in GLSL ES, 215-216
cancelAnimationFrame() function, 133
canvas.addEventListener()
function, 432
canvas.getContext() function, 15
ceil() function, 444
checkFace() function, 368
clamp() function, 445
common functions, 444-446
cos() function, 442
createProgram() function, 354
cross() function, 447
ctx.fillRect() function, 16
degrees() function, 441
distance() function, 447
document.getElementById() function,
14, 19
dot() function, 447
draw() function, 129-131
objects composed of other objects,
332-334
processing flow of, 249
drawArrays() function, 284
drawbox() function, 339-340
drawSegment() function, 340
equal() function, 450
exp2() function, 443
exp() function, 443
exponential functions, 216, 443
faceforward() function, 447
floor() function, 444
fract() function, 444
geometric functions, 216, 447-448
getWebGLContext() function, 20
gl.activeTexture() function, 171-172
gl.attachShader() function, 350
gl.bindBuffer() function, 75-76
gl.bindFramebuffer() function, 400
gl.bindRenderbuffer() function, 399
gl.bindTexture() function, 173-174
gl.blendFunc() function, 382-383
gl.bufferData() function, 76-78
gl.checkFramebufferStatus() function,
402-403
gl.clearColor() function, 20-21
gl.clear() function, 22, 125
gl.compileShader() function, 347-349
gl.createBuffer() function, 74-75
gl.createFramebuffer() function, 397
gl.createProgram() function, 349-350
gl.createRenderbuffer() function, 398
gl.createShader() function, 345-346
gl.createTexture() function, 167
gl.deleteBuffer() function, 75
gl.deleteFramebuffer() function, 397
gl.deleteProgram() function, 350
gl.deleteRenderbuffer() function, 398
gl.deleteShader() function, 346
gl.deleteTexture() function, 167
Index
496
gl.depthMask() function, 385
gl.detachShader() function, 351
gl.disable() function, 271
gl.disableVertexAttribArray()
function, 82
gl.drawArrays() function, 36-37, 72,
82-83, 87, 131
gl.drawElements() function, 278
gl.enable() function, 270
gl.enableVertexAttribArray() function,
81-82
gl.framebufferRenderbuffer() function,
401-402
gl.framebufferTexture2D()
function, 401
gl.getAttribLocation() function, 44-45
gl.getProgramInfoLog() function, 352
gl.getProgramParameter() function, 352
gl.getShaderInfoLog() function, 348
gl.getShaderParameter() function, 348
gl.getUniformLocation() function, 63
gl.linkProgram() function, 351-352
gl.pixelStorei() function, 171
gl.polygonOffset() function, 274
gl.readPixels() function, 364
gl.renderbufferStorage() function, 399
gl.shaderSource() function, 346-347
gl.texImage2D() function, 177-179, 398
gl.texParameteri() function, 174-177
glTranslatef() function, 116
gl.uniform1f() function, 65-66
gl.uniform1i() function, 179-180
gl.uniform2f() function, 65-66
gl.uniform3f() function, 65-66
gl.uniform4f() function, 63-66, 95
gl.uniformMatrix4fv() function, 110
gl.useProgram() function, 353, 387
gl.vertexAttrib1f() function, 47-49
gl.vertexAttrib2f() function, 47-49
gl.vertexAttrib3f() function, 45-49
gl.vertexAttrib4f() function, 47-49
gl.vertexAttribPointer() function, 79-81,
142-145
gl.viewport() function, 404
in GLSL ES, 213-215
built-in functions, 215-216
parameter qualifiers, 214-215
prototype declarations, 214
greaterThanEqual() function, 449
greaterThan() function, 449
initShaders() function, 31-32, 344-345,
353-355, 387
initTextures() function, 166-170, 187
initVertexBuffers() function, 72, 140,
152, 166, 187, 281
inversesqrt() function, 443
length() function, 447
lessThanEqual() function, 449
lessThan() function, 449
loadShader() function, 355
loadShaderFile() function, 472
loadTexture() function, 168, 170,
187-189
log() function, 443
log2() function, 443
main() function, processing flow of, 19
mathematical common functions,
444-446
Matrix4.setOrtho() function, 453
Matrix4.setPerspective() function, 453
matrix functions, 216, 448
max() function, 445
Index
497
maxtrixCompMult() function, 448
min() function, 445
mix() function, 446
mod() function, 444
normalize() function, 447
notEqual() function, 450
not() function, 450
onLoadShader() function, 472
OpenGL functions, naming
conventions, 48-49
popMatrix() function, 338
pow() function, 443
pushMatrix() function, 338
radians() function, 441
reflect() function, 448
refract() function, 448
requestAnimationFrame() function,
130-133
setInterval() function, 131
setLookAt() function, 228-229
setOrtho() function, 243
setPerspective() function, 257
setRotate() function, 117, 131
sign() function, 444
sin() function, 442
smoothstep() function, 446
sqrt() function, 443
stencil buffer, 22
step() function, 446
tan() function, 442
texture lookup functions, 451
texture2D() function, 181-182, 451
texture2DLod() function, 451
texture2DProj() function, 451
texture2DProjLod() function, 451
textureCube() function, 451
textureCubeLod() function, 451
tick() function, 129-130
trigonometry functions, 216, 441-442
type conversion, 197
vec4() function, 34-35
vector functions, 48, 216, 449
G
geometric functions, 216, 447-448
geometric shape assembly, 151-155
getWebGLContext() function, 20
gl.activeTexture() function, 171-172
gl.attachShader() function, 350
gl.bindBuffer() function, 75-76
gl.bindFramebuffer() function, 400
gl.bindRenderbuffer() function, 399
gl.bindTexture() function, 173-174
gl.blendFunc() function, 382-383
gl.bufferData() function, 76-78
gl.checkFramebufferStatus() function,
402-403
gl.clearColor() function, 20-21
gl.clear() function, 22, 125
gl.compileShader() function, 347-349
gl.createBuffer() function, 74-75
gl.createFramebuffer() function, 397
gl.createProgram() function, 349-350
gl.createRenderbuffer() function, 398
gl.createShader() function, 345-346
gl.createTexture() function, 167
gl.deleteBuffer() function, 75
gl.deleteFramebuffer() function, 397
Index
498
gl.deleteProgram() function, 350
gl.deleteRenderbuffer() function, 398
gl.deleteShader() function, 346
gl.deleteTexture() function, 167
gl.depthMask() function, 385
gl.detachShader() function, 351
gl.disable() function, 271
gl.disableVertexAttribArray() function, 82
gl.drawArrays() function, 36-37, 72, 82-83,
87, 131
gl.drawElements() function, 278
gl.enable() function, 270
gl.enableVertexAttribArray() function,
81-82
gl.framebufferRenderbuffer() function,
401-402
gl.framebufferTexture2D() function, 401
gl.getAttribLocation() function, 44-45
gl.getProgramInfoLog() function, 352
gl.getProgramParameter() function, 352
gl.getShaderInfoLog() function, 348
gl.getShaderParameter() function, 348
gl.getUniformLocation() function, 63
gl.linkProgram() function, 351-352
global coordinate system. See world
coordinate system
global variables in GLSL ES, 216
gl.pixelStorei() function, 171
gl.polygonOffset() function, 274
gl.readPixels() function, 364
gl.renderbufferStorage() function, 399
gl.shaderSource() function, 346-347
GLSL ES (OpenGL ES shading language),
6, 30
case sensitivity, 193
comments, 193
conditional control flow and iteration,
211-213
data types, 34, 194
arrays, 208-209
precision qualifiers, 219-221
samplers, 209-210
structures, 207-208
vector and matrix types, 198-206
functions, 213-215
built-in functions, 215-216
parameter qualifiers, 214-215
prototype declarations, 214
order of execution, 193
overview of, 192
preprocessor directives, 221-223
semicolon (;) usage, 193
variables
assignment of values, 196-197
data types for, 196
global and local variables, 216
keywords and reserved words,
194-195
naming conventions, 194
operator precedence, 210
operators on, 197-198
storage qualifiers, 217-219
type conversion, 196-197
type sensitivity, 195
GLSL (OpenGL shading language), 6
gl.texImage2D() function, 177-179, 398
gl.texParameteri() function, 174-177
glTranslatef() function, 116
gl.uniform1f() function, 65-66
gl.uniform1i() function, 179-180
Index
499
gl.uniform2f() function, 65-66
gl.uniform3f() function, 65-66
gl.uniform4f() function, 63-66, 95
gl.uniformMatrix4fv() function, 110
gl.useProgram() function, 353, 387
gl.vertexAttrib1f() function, 47-49
gl.vertexAttrib2f() function, 47-49
gl.vertexAttrib3f() function, 45-49
gl.vertexAttrib4f() function, 47-49
gl.vertexAttribPointer() function, 79-81,
142-145
gl.viewport() function, 404
greaterThanEqual() function, 449
greaterThan() function, 449
H
handedness of coordinate systems,
455-464
clip coordinate system and viewing
volume, 460-462
CoordinateSystem.js, 456-459
Hidden Surface Removal tool, 459-460
projection matrices for, 462-464
Head Up Display (HUD), 368
HUD.html, 369-370
HUD.js, 370-372
implementing, 369
HelloCanvas.html, 17-18
HelloCanvas.js, 18-23
Hello Cube, 275-277
adding color to each face, 284-285
ColoredCube.js, 285-289
drawing with indices and vertices
coordinates, 277-278
HelloCube.js, 278-281
writing vertex coordinates, colors, and
indices in the buffer object, 281-284
HelloCube.js, 278-281
HelloPoint1.html, 25
HelloPoint1.js, 25-26
HelloPoint2.js, 42-43
HelloQuad.js, 89-91
HelloTriangle.js, 85-86, 151-152
hidden surface removal, 270-271, 459-460
hierarchical structure, 325-326
highp precision qualifier, 220
homogeneous coordinates, 35
HTML5
<body> element, 12
<canvas> element, 9-11
browser support, 12
clearing drawing area, 16-23
coordinates for center, 23
coordinate system, 16
DrawRectangle.html, 11-13
DrawRectangle.js, 13-16
HelloCanvas.html, 17-18
HelloCanvas.js, 18-23
mapping to WebGL coordinate
system, 39, 54-57
retrieving, 14, 19
defined, 2
elements, modifying using JavaScript,
247-248
<img> element, 9
Index
500
HUD (Head Up Display), 368
HUD.html, 369-370
HUD.js, 370-372
implementing, 369
HUD.html, 369-370
HUD.js, 370-372
I
identifiers, assigning, 12
identity matrix, 119
handedness of coordinate systems,
462-463
if-else statement in GLSL ES, 211
if statement in GLSL ES, 211
images, pasting on rectangles, 160-183
activating texture units, 171-172
assigning texture images to texture
objects, 177-179
binding texture objects to target,
173-174
changing texture coordinates, 182-183
flipping image y-axis, 170-171
mapping texture and vertex
coordinates, 162-163, 166
multiple texture mapping, 183-190
passing coordinates from vertex to
fragment shader, 180-181
passing texture unit to fragment
shader, 179-180
retrieving texel color in fragment
shader, 181-182
setting texture object parameters,
174-177
setting up and loading images, 166-170
texture coordinates, explained, 162
TexturedQuad.js, 163-166
<img> element, 9
implementing
alpha blending, 380-381
fog, 373-374
HUD, 369
lost context, 431-432
object rotation, 358
object selection, 361-362
rounded points, 377-378
shadows, 405-406
switching shaders, 387
texture images, 394
indices, 282
infinity in homogenous coordinates, 35
initializing shaders, 30-33
initShaders() function, 31-32, 344-345,
353-355, 387
initTextures() function, 166-170, 187
initVertexBuffers() function, 72, 140, 152,
166, 187, 281
int data type, 196
integral constant expression, 208
interleaving, 141-145
interpolation process, varying variables
and, 157-160
inversesqrt() function, 443
inverse transpose matrix, 311-312,
465-469
iteration in GLSL ES, 211-213
Index
501
J–K
JavaScript
drawing area, mapping to WebGL
coordinate system, 39
HTML elements, modifying, 247-248
loading, 12
processing flow into WebGL, 27, 438
JointModel.js, 328-332
joints, 325
JointModel.js, 328-332
multijoint model, 334
MultiJointModel.js, 335-338
single joint model, objects composed
of other objects, 326-327
keyboard, changing eye point, 238
keywords in GLSL ES, 194-195
Khronos Group, 6
L
left-handedness of coordinate systems in
default behavior, 455-464
length() function, 447
lessThanEqual() function, 449
lessThan() function, 449
libraries, transformation, 115-124
combining multiple transformations,
119-121
cuon-matrix.js, 116
RotatedTranslatedTriangle.js, 121-124
RotatedTriangle_Matrix4.js, 117-119
light direction, calculating diffuse
reflection, 297-299
LightedCube_ambient.js, 308-309
LightedCube.js, 302-303
processing in JavaScript, 306
processing in vertex shader, 304-305
LightedTranslatedRotatedCube.js, 312-314
lighting
3D objects, 291-293
light sources, 293
reflected light, 294-296
ambient light, 294
directional light, 293
point light, 293
reflected light, 294-296
translated-rotated objects, 310-311
light sources, 293
linking program objects, 351-352
listings
array with multiple vertex information
items, 141
BlendedCube.js, 384
ClickedPoints.js, 51-52
ColoredCube.js, 286-287
ColoredPoints.js, 59-61
ColoredTriangle.js, 159
CoordinateSystem.js, 456-458
CoordinateSystem_viewVolume.js, 461
createProgram(), 354
DepthBuffer.js, 272-273
drawing multiple points, 69
DrawRectangle.html, 11
DrawRectangle.js, 13-14
Fog.js, 374-375
Fog_w.js, 376-377
fragment shader example, 192
Index
502
FramebufferObject.js
Processes for Steps 1 to 7, 395-396
Process for Step 8, 403-404
HelloCanvas.html, 18
HelloCanvas.js, 18-19
HelloCube.js, 279-280
HelloPoint1.html, 25
HelloPoint1.js, 26
HelloPoint2.js, 42-43
HelloTriangle.js, 85-86
code snippet, 151-152
HUD.html, 369
HUD.js, 370-371
initShaders(), 353-354
JointModel.js, 328-330
LightedCube_ambient.js, 308-309
LightedCube.js, 302-303
LightedTranslatedRotatedCube.js,
312-313
loadShader(), 355
LoadShaderFromFiles, 471-472
LookAtBlenderTriangles.js, 381-382
LookAtRotatedTriangles.js, 235-236
LookAtRotatedTriangles_mvMatrix.js,
237
LookAtTriangles.js, 229-231
LookAtTrianglesWithKeys.js, 239-240
LookAtTrianglesWithKeys_View
Volume.js, 252-253
MultiAttributeColor.js, 147-148
MultiAttributeSize_Interleaved.js,
142-143
MultiAttributeSize.js, 139-140
MultiJointModel.js
drawing the hierarchy structure,
336-337
key processing, 335-336
MultiJointModel_segment.js, 340-342
MultiPoint.js, 70-72
MultiTexture.js, 185-186
OBJViewer.js, 419-420
onReadComplete(), 428
parser part, 424-426
retrieving the drawing information,
428-429
OrthoView.html, 245-246
OrthoView.js, 246-247
PerspectiveView.js, 258-259
PerspectiveView_mvp.js, 263-265
PickFace.js, 366-367
PickObject.js, 362-363
PointLightedCube.js, 316-317
PointLightedCube_perFragment.js,
319-320
ProgramObject.js
Processes for Steps 1 to 4, 387-389
Processes for Steps 5 through 10,
390-391
RotatedTranslatedTriangle.js, 122
RotatedTriangle.js, 99-100
RotatedTriangle_Matrix4.html, 116
RotatedTriangle_Matrix.js, 107-108
RotateObject.js, 358-359
RotatingTriangle_contextLost.js,
433-434
RotatingTriangle.js, 126-128
RoundedPoint.js, 379
Index
503
Shadow_highp.js, 413-414
Shadow.js
JavaScript part, 410-411
Shader part, 406-407
TexturedQuad.js, 163-165
TranslatedTriangle.js, 93-94
vertex shader example, 192
Zfighting.js, 274-275
loading
3D objects, 414-416
images for texture mapping, 166-170
JavaScript, 12
shader programs from files, 471-472
loadShader() function, 355
loadShaderFile() function, 472
loadTexture() function, 168, 170, 187-189
local coordinate system, 474-475
local file access, enabling, 161
local variables in GLSL ES, 216
log() function, 443
log2() function, 443
LookAtBlendedTriangles.js, 381-382
look-at point, 228
LookAtRotatedTriangles.js, 235-238
LookAtRotatedTriangles_mvMatrix.js, 237
LookAtTriangles.js, 229-233
LookAtTrianglesWithKeys.js, 238-241
LookAtTrianglesWithKeys_ViewVolume.js,
251-253
lost context, 430-431
implementing, 431-432
RotatingTriangle_contextLost.js,
432-434
lowp precision qualifier, 220
luminance, 178
M
Mach band, 409
macros, predefined names, 222
main() function, processing flow of, 19
manipulating objects composed of other
objects, 324-325
mapping textures, 160-183
activating texture units, 171-172
assigning texture images to texture
objects, 177-179
binding texture objects to target,
173-174
changing texture coordinates, 182-183
flipping image y-axis, 170-171
mapping vertex and texture
coordinates, 162-163, 166
passing coordinates from vertex to
fragment shader, 180-181
passing texture unit to fragment
shader, 179-180
pasting multiple textures, 183-190
retrieving texel color in fragment
shader, 181-182
setting texture object parameters,
174-177
setting up and loading images, 166-170
texture coordinates, explained, 162
TexturedQuad.js, 163-166
mathematical common functions,
444-446
matrices
defined, 103
identity matrix, 119
handedness of coordinate systems,
462-463
Index
504
inverse transpose matrix, 311-312,
465-469
model matrix, 121
PerspectiveView, 262, 265
multiplication, 103, 121, 205-206
projection matrix
handedness of coordinate systems,
462-464
quadrangular pryamid, 260-261
Matrix4 object, supported methods and
properties, 118
Matrix4.setOrtho() function, 453
Matrix4.setPerspective() function, 453
matrix data types in GLSL ES, 198-206
access to components, 201-204
assignment of values, 199-201
constructors, 199-201
operators, 204-206
matrix functions, 216, 448
max() function, 445
maxtrixCompMult() function, 448
mediump precision qualifier, 220
member access in GLSL ES
arrays, 209
structures, 207
methods. See also functions
for Matrix4 object, 118
of typed arrays, 79
WebGL methods, naming conventions,
48-49
min() function, 445
MIPMAP texture format, 176
mix() function, 446
model matrix, 121
PerspectiveView, 262, 265
model transformation, 121
mod() function, 444
modifying HTML elements using
JavaScript, 247-248
mouse
drawing points, 50-58
ClickedPoints.js, 50-52
event handling, 53-57
registering event handlers, 52-53
rotating objects, 357
moving shapes, 92-96
MTL file format (3D models), 418
MultiAttributeColor.js, 147-150
MultiAttributeSize_Interleaved.js, 142-145
MultiAttributeSize.js, 139-140
multijoint model
MultiJointModel.js, 335-338
objects composed of other objects, 334
MultiJointModel.js, 335-338
MultiJointMode_segment.js, 340-342
multiple buffer objects, creating, 140-141
multiple points, drawing, 68-85
multiple textures, mapping to shapes,
183-190
multiple transformations, 115-124
in animation, 135-136
combining, 119-121
cuon-matrix.js, 116
RotatedTranslatedTriangle.js, 121-124
RotatedTriangle_Matrix4.js, 117-119
multiple vertices
basic shapes, drawing, 85-91
drawing, 68-85
assigning buffer objects to attribute
variables, 79-81
binding buffer objects to targets,
75-76
Index
505
buffer object usage, 72-74
creating buffer objects, 74-75
enabling attribute variable
assignments, 81-82
gl.drawArrays() function, 82-83
writing data to buffer objects, 76-78
multiplication
of matrices, 121
of vectors and matrices, 103, 205-206
MultiPoint.js, 70-72
MultiTexture.js, 184-190
N
naming conventions
GLSL ES variables, 194
variables, 43
WebGL methods, 48-49
near value, changing, 250-251
normal orientation of a surface, 299-301
normalize() function, 447
notEqual() function, 450
not() function, 450
numerical values in GLSL ES, 194
O
objects
composed of other objects
draw() function, 332-334
drawing, 324-325
draw segments, 339-344
hierarchical structure, 325-326
JointModel.js, 328-332
manipulating, 324-325
multijoint model, 334
single joint model, 326-327
rotation
implementing, 358
with mouse, 357
RotateObject.js, 358-360
selection, 360-362
face of objects, 365
implementing, 361-362
PickObject.js, 362-365
OBJ file format (3D models), 417
OBJViewer.js, 419-421
parser code, 423-430
onLoadShader() function, 472
OpenGL
color buffers, swapping, 437
functions, naming conventions, 48-49
in history of WebGL, 5
WebGL and, 5
OpenGL ES (Embedded Systems), 5-6, 30
OpenGL shading language (GLSL), 6
operator precedence in GLSL ES, 210
operators in GLSL ES
on arrays, 209
on structures, 208
on variables, 197-198
on vector and matrix data types,
204-206
order of execution in GLSL ES, 193
orientation of a surface
calculating diffuse reflection, 297-299
normal, 299-301
Index
506
origin
in coordinate systems, 55
in local coordinate system, 474-475
in world coordinate system, 475-477
origins of WebGL, 5-6
orthographic projection matrix, 252-253,
261, 453
OrthoView.html, 245-246
OrthoView.js, 246-247
P
parameter qualifiers in GLSL ES functions,
214-215
parameters of texture objects, setting,
174-177
parser code (OBJViewer.js), 423-430
passing data
to fragment shaders
texture units, 179-180
with varying variable, 146-151
to vertex shaders, 137-151. See
also drawing; rectangles; shapes;
triangles
color changes, 146-151
creating multiple buffer objects,
140-141
interleaving, 141-145
MultiAttributeSize.js, 139-140
pasting images on rectangles, 160-183
activating texture units, 171-172
assigning texture images to texture
objects, 177-179
binding texture objects to target,
173-174
changing texture coordinates, 182-183
flipping image y-axis, 170-171
mapping texture and vertex
coordinates, 162-163, 166
multiple texture mapping, 183-190
passing coordinates from vertex to
fragment shader, 180-181
passing texture unit to fragment
shader, 179-180
retrieving texel color in fragment
shader, 181-182
setting texture object parameters,
174-177
setting up and loading images, 166-170
texture coordinates, explained, 162
TexturedQuad.js, 163-166
perspective projection matrix, 257, 453
PerspectiveView.js, 255, 260-263
model matrix, 262, 265
PerspectiveView_mvp.js, 263-266
per-vertex operations, 93
PickFace.js, 365-368
PickObject.js, 362-365
point light, 293
point light objects, 314-315
PointLightedCube.js, 315-319
PointLightedCube_perFragment.js,
319-321
points, drawing, 23-50
attribute variables, 41-42
setting value, 45-49
storage location, 44-45
changing point color, 58-66
ClickedPoints.js, 50-52
ColoredPoints.js, 59-61
gl.drawArrays() function, 36-37
HelloPoint1.html, 25
Index
507
HelloPoint1.js, 25-26
HelloPoint2.js, 42-43
with mouse clicks, 50-58
multiple points, 68-85
registering event handlers, 52-53
shaders
explained, 27-28
fragment shaders, 35-36
initializing, 30-33
vertex shaders, 33-35
uniform variables, 61-62
assigning values, 63-66
storage location, 62-63
WebGL coordinate system 38-39
WebGL program structure, 28-30
point size, attribute variables for, 139-140
popMatrix() function, 338
positive rotation, 96
pow() function, 443
precedence of operators in GLSL ES, 210
precision qualifiers, 62, 219-221
predefined single parameters, 53
preprocessor directives in GLSL ES,
221-223
primitive assembly process. See geometric
shape assembly
primitives. See shapes
process flow
initializing shaders, 31
InitShaders() function, 353-355
JavaScript to WebGL, 27, 438
mouse click event handling, 53-57
multiple vertice drawing, 70
vertex shaders, 248-249
programmable shader functions, 6
ProgramObject.js, 387-391
program objects, 44, 353
attaching shader objects, 350-351
creating, 349-350
linking, 351-352
projection matrices, 453
handedness of coordinate systems,
462-464
quadrangular pryamid, 260-261
properties
Matrix4 object, 118
typed arrays, 79
prototype declarations in GLSL ES
functions, 214
publishing 3D graphics applications,
ease of, 4
pushMatrix() function, 338
Q
quadrangular pyramid
PerspectiveView.js, 258-260
projection matrix, 260-261
viewing volume, 256-258
visible range, 254-256
qualifiers for parameters in GLSL ES
functions, 214-215
R
radians() function, 441
rasterization, 137, 151-155
Index
508
rectangles. See also shapes; triangles
drawing, 13-16, 89-91
pasting images on, 160-183
activating texture units, 171-172
assigning texture images to texture
objects, 177-179
binding texture objects to target,
173-174
changing texture coordinates,
182-183
flipping image y-axis, 170-171
mapping texture and vertex
coordinates, 162-163, 166
multiple texture mapping, 183-190
passing texture coordinates from
vertex to fragment shader, 180-181
passing texture unit to fragment
shader, 179-180
retrieving texel color in fragment
shader, 181-182
setting texture object parameters,
174-177
setting up and loading images,
166-170
texture coordinates, explained, 162
TexturedQuad.js, 163-166
reflected light, 294-296
ambient reflection, 295-296
diffuse reflection, 294-295
reflect() function, 448
refract() function, 448
registering event handlers, 52-53
renderbuffer objects, 392-393
binding, 399-400
creating, 398
setting to framebuffer objects, 401-402
rendering context. See context, retrieving
requestAnimationFrame() function,
130-133
reserved words in GLSL ES, 194-195
resizing rotation matrix, 106-107
restoring clipped parts of triangles,
251-253
retrieving
<canvas> element, 14, 19
context, 15
for WebGL, 20-21
storage location of uniform variables,
62-63
texel color in fragment shader, 181-182
RGBA components, 409
RGBA format, 15
RGB format, 15
right-handedness of coordinate
systems, 38
in default behavior, 455-464
right-hand-rule rotation, 96
RotatedTranslatedTriangle.js, 121-124
RotatedTriangle.js, 98-102
RotatedTriangle_Matrix.js, 107-110
RotatedTriangle_Matrix4.html, 116
RotatedTriangle_Matrix4.js, 117-119
LookAtTriangles.js versus, 232-233
rotated triangles from specified positions,
234-235
RotateObject.js, 358-360
rotating
objects
implementing, 358
with mouse, 357
RotateObject.js, 358-360
Index
509
shapes, 96-102
calling drawing function, 129-130
combining multiple
transformations, 119-121
draw() function, 130-131
multiple transformations in,
135-136
requestAnimationFrame() function,
131-133
RotatingTriangle.js, 126-129
transformation matrix, 102-105
updating rotation angle, 133-135
triangles, 107-110
RotatingTranslatedTriangle.js, 135-136
RotatingTriangle_contextLost.js, 432-434
RotatingTriangle.js, 126-129
calling drawing function, 129-130
draw() function, 130-131
requestAnimationFrame() function,
131-133
updating rotation angle, 133-135
rotation angle, updating, 133-135
rotation matrix
creating, 102-105
defined, 104
inverse transpose matrix and, 465-469
resizing, 106-107
RotatedTriangle_Matrix.js, 107-110
RoundedPoint.js, 378-379
rounded points, 377
implementing, 377-378
RoundedPoint.js, 378-379
row major order, 109-110
S
sample programs
BlendedCube.js, 384
ClickedPoints.js, 50-52
ColoredCube.js, 285-289
ColoredPoints.js, 59-61
ColoredTriangle.js, 159
CoordinateSystem.js, 456-459
CoordinateSystem_viewVolume.js, 461
cuon-matrix.js, 116
cuon-utils.js, 20
DepthBuffer.js, 272-273
DrawRectangle.html, 11-13
DrawRectangle.js, 13-16
Fog.js, 374-376
Fog_w.js, 376-377
FramebufferObject.js, 395-396, 403
HelloCanvas.html, 17-18
HelloCanvas.js, 18-23
HelloCube.js, 278-281
HelloPoint1.html, 25
HelloPoint1.js, 25-26
HelloPoint2.js, 42-43
HelloQuad.js, 89-91
HelloTriangle.js, 85-86, 151-152
HUD.html, 369-370
HUD.js, 370-372
JointModel.js, 328-332
LightedCube_ambient.js, 308-309
LightedCube.js, 302-303
processing in JavaScript, 306
processing in vertex shader, 304-305
LightedTranslatedRotatedCube.js,
312-314
Index
510
LookAtBlendedTriangles.js, 381-382
LookAtRotatedTriangles.js, 235-238
LookAtRotatedTriangles_mvMatrix.js,
237
LookAtTriangles.js, 229-233
LookAtTrianglesWithKeys.js, 238-241
LookAtTrianglesWithKeys_View-
Volume.js, 251-253
MultiAttributeColor.js, 147-150
MultiAttributeSize_Interleaved.js,
142-145
MultiAttributeSize.js, 139-140
MultiJointModel.js, 335-338
MultiJointMode_segment.js, 340-342
MultiPoint.js, 70-72
MultiTexture.js, 184-190
OBJViewer.js, 419-421
parser code, 423-430
OrthoView.html, 245-246
OrthoView.js, 246-247
PerspectiveView.js, 255, 260-263
model matrix, 262, 265
PerspectiveView_mvp.js, 263-266
PickFace.js, 365-368
PickObject.js, 362-365
PointLightedCube.js, 315-319
PointLightedCube_perFragment.js,
319-321
ProgramObject.js, 387-391
RotatedTranslatedTriangle.js, 121-124
RotatedTriangle.js, 98-102
RotatedTriangle_Matrix.js, 107-110
RotatedTriangle_Matrix4.html, 116
RotatedTriangle_Matrix4.js, 117-119
LookAtTriangles.js versus, 232-233
RotateObject.js, 358-360
RotatingTranslatedTriangle.js, 135-136
RotatingTriangle_contextLost.js,
432-434
RotatingTriangle.js, 126-129
calling drawing function, 129-130
draw() function, 130-131
requestAnimationFrame() function,
131-133
updating rotation angle, 133-135
RoundedPoint.js, 378-379
Shadow_highp.js, 413-414
Shadow.js, 406-412
TexturedQuad.js, 163-166
TranslatedTriangle.js, 92-96
samplers in GLSL ES, 209-210
saving color buffer content, 56
scaling matrix
handedness of coordinate systems, 464
inverse transpose matrix and, 465-469
scaling shapes, 111-113
selecting
face of objects, 365-368
objects, 360-365
semicolon (;) in GLSL ES, 193
setInterval() function, 131
setLookAt() function, 228-229
setOrtho() function, 243
setPerspective() function, 257
setRotate() function, 117, 131
shader objects
attaching to program objects, 350-351
compiling, 347-349
creating
gl.createShader() function, 345-346
program objects, 349-350
Index
511
deleting, 346
InitShaders() function, 344-345
linking program objects, 351-352
storing shader source code, 346-347
shader programs, loading from files,
471-472
shaders, 6, 25
explained, 27-28
fragment shaders, 27
drawing points, 35-36
example of, 192
geometric shape assembly and
rasterization, 151-155
invoking, 155
passing data to, 61-62, 146-151
passing texture coordinates to,
180-181
passing texture units to, 179-180
program structure, 29-30
retrieving texel color in, 181-182
varying variables and interpolation
process, 157-160
verifying invocation, 156-157
GLSL ES. See GLSL ES
initializing, 30-33
InitShaders() function, 344-345
source code, storing, 346-347
vertex shaders, 27, 232
drawing points, 33-35
example of, 192
geometric shape assembly and
rasterization, 151-155
passing data to, 41-42, 137-151. See
also drawing; rectangles; shapes;
triangles
passing texture coordinates to
fragment shaders, 180-181
program structure, 29-30
WebGL program structure, 28-30
shading
3D objects, 292
adding ambient light, 307-308
calculating color per fragment, 319
directional light and diffuse reflection,
296-297
shading languages, 6
Shadow_highp.js, 413-414
Shadow.js, 406-412
shadow maps, 405
shadows
implementing, 405-406
increasing precision, 412
Shadow_highp.js, 413-414
Shadow.js, 406-412
shapes. See also rectangles; triangles
animation, 124-136
calling drawing function, 129-130
draw() function, 130-131
multiple transformations in,
135-136
requestAnimationFrame() function,
131-133
RotatingTriangle.js, 126-129
updating rotation angle, 133-135
drawing, 85-91
HelloTriangle.js, 85-86
list of, 87-88
multiple vertices, drawing, 68-85
Index
512
rotating, 96-102
RotatedTriangle_Matrix.js, 107-110
transformation matrix, 102-105
scaling, 111-113
transformation libraries, 115-124
combining multiple
transformations, 119-121
cuon-matrix.js, 116
RotatedTranslatedTriangle.js,
121-124
RotatedTriangle_Matrix4.js, 117-119
translating, 92-96
combining with rotation, 111
transformation matrix, 105-106
sign() function, 444
sin() function, 442
single joint model, objects composed of
other objects, 326-327
smoothstep() function, 446
sqrt() function, 443
stencil buffer, 22
step() function, 446
storage location
attribute variables, 44-45
uniform variables, 62-63
storage qualifiers, 43, 217-219
attribute variables, 218
const, 217
uniform variables, 218
varying variables, 219
storing shader source code, 346-347
StringParser object, 426
striped patterns, 409
structures in GLSL ES, 207-208
access to members, 207
assignment of values, 207
constructors, 207
operators, 208
swapping color buffers, 437
switching shaders, 386
implementing, 387
ProgramObject.js, 387-391
swizzling, 202
T
tan() function, 442
targets, binding texture objects to,
173-174
texels, 160
data formats, 178
data types, 179
retrieving color in fragment shader,
181-182
text editors, 3D graphics development
with, 3-4
texture2D() function, 181-182, 451
texture2DLod() function, 451
texture2DProj() function, 451
texture2DProjLod() function, 451
texture coordinates
changing, 182-183
explained, 162
flipping image y-axis, 170-171
mapping to vertex coordinates,
162-163, 166
passing from vertex to fragment
shader, 180-181
textureCube() function, 451
textureCubeLod() function, 451
TexturedQuad.js, 163-166
Index
513
texture images, 392
FramebufferObject.js, 395-396
framebuffer objects, 392-393
creating, 397
implementing, 394
renderbuffer objects, 392-393
creating, 398
texture lookup functions, 216, 451
texture mapping, 160-183
activating texture units, 171-172
assigning texture images to texture
objects, 177-179
binding texture objects to target,
173-174
changing texture coordinates, 182-183
flipping image y-axis, 170-171
mapping texture and vertex
coordinates, 162-163, 166
with multiple textures, 183-190
passing coordinates from vertex to
fragment shader, 180-181
passing texture unit to fragment
shader, 179-180
retrieving texel color in fragment
shader, 181-182
setting texture object parameters,
174-177
setting up and loading images, 166-170
texture coordinates, explained, 162
TexturedQuad.js, 163-166
texture objects, 170
assigning texture images to, 177-179
binding to target, 173-174
creating, 397-398
setting parameters, 174-177
setting to framebuffer objects, 400-401
texture units
activating, 171-172
passing to fragment shader, 179-180
pasting multiple, 183-190
tick() function, 129-130
transformation libraries, 115-124
combining multiple transformations,
119-121
cuon-matrix.js, 116
RotatedTranslatedTriangle.js, 121-124
RotatedTriangle_Matrix4.js, 117-119
transformation matrix
defined, 103
inverse transpose matrix and, 465-469
rotating shapes, 102-105
scaling shapes, 111-113
translating shapes, 105-106
transformations
coordinate systems and, 477
defined, 91
multiple transformations in animation,
135-136
world transformation, 476
translated-rotated objects
inverse transpose matrix, 311-312
lighting, 310-311
TranslatedTriangle.js, 92-96
translating
shapes, 92-96
combining multiple
transformations, 119-121
transformation matrix, 105-106
triangles, 111
translation matrix
combining with rotation matrix, 111
creating, 105-106
Index
514
defined, 106
inverse transpose matrix and, 465-469
triangles, 225-226. See also rectangles;
shapes
coloring vertices different colors,
151-160
geometric shape assembly and
rasterization, 151-155
invoking fragment shader, 155
varying variables and interpolation
process, 157-160
verifying fragment shader
invocation, 156-157
combining multiple transformations,
119-121
drawing, 85-91
restoring clipped parts, 251-253
rotating, 96-102, 107-110, 234-235
RotatingTranslatedTriangle.js, 135-136
RotatingTriangle.js, 126-129
calling drawing function, 129-130
draw() function, 130-131
requestAnimationFrame() function,
131-133
updating rotation angle, 133-135
TranslatedTriangle.js, 92-96
translating, 92-96, 111
trigonometry functions, 216, 441-442
type conversion in GLSL ES, 196-197
typed arrays, 78-79
typed programming languages, 34
type sensitivity in GLSL ES, 195
U
#undef preprocessor directive, 222
uniform variables, 61-62, 217-218
assigning values to, 63-66
retrieving storage location, 62-63
u_NormalMatrix, 314
updating rotation angle, 133-135
up direction, 228
user-defined objects (3D models), 422-423
V
values, assigning
to attribute variables, 45-49
to uniform variables, 63-66
variables
attribute variables, 218
declaring, 43
explained, 41-42
setting value, 45-49
storage location, 44-45
in fragment shaders, 36
in GLSL ES
arrays, 208-209
assignment of values, 196-197
data types for, 34, 196
global and local variables, 216
keywords and reserved words,
194-195
naming conventions, 194
Index
515
operator precedence, 210
operators on, 197-198
precision qualifiers, 219-221
samplers, 209-210
storage qualifiers, 217-219
structures, 207-208
type conversion, 196-197
type sensitivity, 195
vector and matrix types, 198-206
naming conventions, 43
uniform variables, 61-62, 218
assigning values to, 63-66
retrieving storage location, 62-63
in vertex shaders, 33
varying variables, 217, 219
color changes with, 146-151
interpolation process and, 157-160
vec4() function, 34-35
vector data types in GLSL ES, 198-206
access to components, 201-204
assignment of values, 199-201
constructors, 199-201
operators, 204-206
vector functions, 48, 216, 449
vector multiplication, 103, 205-206
#version preprocessor directive, 223
vertex coordinates, mapping to texture
coordinates, 162-163, 166
vertex shaders, 27, 232
drawing points, 33-35
example of, 192
geometric shape assembly and
rasterization, 151-155
passing data to, 41-42, 137-151. See
also drawing; rectangles; shapes;
triangles
color changes, 146-151
creating multiple buffer objects,
140-141
interleaving, 141-145
MultiAttributeSize.js, 139-140
passing texture coordinates to fragment
shaders, 180-181
program structure, 29-30
vertices, 27
basic shapes
drawing, 85-91
rotating, 96-102
scaling, 111-113
translating, 92-96
coloring different colors, 151-160
geometric shape assembly and
rasterization, 151-155
invoking fragment shader, 155
varying variables and interpolation
process, 157-160
verifying fragment shader
invocation, 156-157
multiple vertices, drawing, 68-85
transformation matrix
rotating shapes, 102-105
translating shapes, 105-106
view matrix, 229, 231
viewing console, 14
viewing direction, 226-227
eye point, 228
look-at point, 228
Index
516
LookAtRotatedTriangles.js, 235-236
LookAtTriangles.js, 229-232
specifying, 226-227
up direction, 228
viewing volume
clip coordinate system and, 460-462
quadrangular pyramid, 256-258
visible range, 242-243
visible range, 241-242
defining box-shaped viewing volume,
243-244
eye point, 241
quadrangular pyramid, 254-256
viewing volume, 242-243
W
web browsers
<canvas> element support, 12
console, viewing, 14
enabling local file access, 161
functionality in 3D graphics
applications, 5
JavaScript to WebGL processing flow,
27, 438
WebGL settings, 479-480
WebGL
advantages of, 3-5
application structure, 6-7
browser settings, 479-480
color, setting, 21-23
color buffer, drawing to, 437-439
coordinate system, 38-39
clip coordinate system and viewing
volume, 460-462
CoordinateSystem.js, 456-459
handedness in default behavior,
455-464
Hidden Surface Removal tool,
459-460
projection matrices for, 462-464
transforming <canvas> element
coordinates to, 54-57
defined, 1-2
JavaScript processing flow, 27, 438
methods, naming conventions, 48-49
OpenGL and, 5
origins of, 5-6
processing flow for initializing
shaders, 31
program structure for shaders, 28-30
rendering context, retrieving, 20-21
web pages (3DoverWeb), displaying 3D
objects, 372
world coordinate system, 475-477
world transformation, 476
writing
data to buffer objects, 76-78
Hello Cube vertex coordinates,
colors, and indices in the buffer
object, 281-284
w value (fog), 376-377
X-Z
y-axis, flipping, 170-171
z fighting
background objects, 273-275
foreground objects, 273-275
Zfighting.js, 274-275
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