4.4 视口变换

一、投影变换和视口变换
投影变换是将viewing frustum（视景体）变换为一个cuboid（立方体）

视口变换在投影变换之后，是将这个cuboid中的物体变换到视口中，见下图

二、视口变换
glViewport(GLint x,GLint y,GLsizei width,GLsizei height)

X，Y以像素为单位，指定了视口的左下角（在第一象限内，以（0，0）为原点的）位置。
width，height表示这个视口矩形的宽度和高度，根据窗口的实时变化重绘窗口。

glViewport在默认情况下，视口被设置为占据打开窗口的整个像素矩形，窗口大小和设置视口大小相同，所以为了选择一个更小的绘图区域，就可以用glViewport函数来实现这一变换，在窗口中定义一个像素矩形，最终将图像映射到这个矩形中。

可以对窗口区域进行划分，在同一个窗口中显示分割屏幕的效果，以显示多个视图。

示例1：

    //边界
    glLineWidth(3.0f);          //设置直线宽度

    glLineStipple(1, 0x3F07);   //设置虚线的模式
    glEnable(GL_LINE_STIPPLE);  //启用虚线

    glViewport(0,0,WIDTH,HEIGHT);
    glPushMatrix(); 
        glTranslatef(0.0f,0.0f,-4.0f);
        glBegin(GL_LINES);
            glColor3f(1.0f,1.0f,1.0f);
            glVertex3f(-WIDTH/2,0.0f,0.0f);
            glVertex3f(WIDTH/2,0.0f,0.0f);
            glVertex3f(0.0f,-HEIGHT/2,0.0f);
            glVertex3f(0.0f,HEIGHT/2,0.0f);
        glEnd();
    glPopMatrix();


    //视口1
    glViewport(0,0,WIDTH/2,HEIGHT/2);
    glPushMatrix(); 
        glTranslatef(0.0f,-0.5f,-3.0f);
        glBegin(GL_TRIANGLES);
            glColor3f(1.0f,0.0f,0.0f);
            glVertex3f(0.0f,0.0f,0.0f);
            glColor3f(0.0f,0.0f,1.0f);
            glVertex3f(0.0f,1.0f,0.0f);
            glColor3f(0.0f,1.0f,0.0f);
            glVertex3f(-1.0f,0.0f,0.0f);
        glEnd();
    glPopMatrix();


    //视口2
    glViewport(WIDTH/2,0,WIDTH/2,HEIGHT/2);
    glPushMatrix(); 
        glTranslatef(0.0f,-0.5f,-3.0f);
        glBegin(GL_TRIANGLES);
            glColor3f(1.0f,0.0f,0.0f);
            glVertex3f(0.0f,0.0f,0.0f);
            glColor3f(0.0f,0.0f,1.0f);
            glVertex3f(0.0f,1.0f,0.0f);
            glColor3f(0.0f,1.0f,0.0f);
            glVertex3f(-1.0f,0.0f,0.0f);
        glEnd();
    glPopMatrix();


    //视口3
    glViewport(WIDTH/2,HEIGHT/2,WIDTH/2,HEIGHT/2);
    glPushMatrix(); 
        glTranslatef(0.0f,-0.5f,-3.0f);
        glBegin(GL_TRIANGLES);
            glColor3f(1.0f,0.0f,0.0f);
            glVertex3f(0.0f,0.0f,0.0f);
            glColor3f(0.0f,0.0f,1.0f);
            glVertex3f(0.0f,1.0f,0.0f);
            glColor3f(0.0f,1.0f,0.0f);
            glVertex3f(-1.0f,0.0f,0.0f);
        glEnd();
    glPopMatrix();


    //视口4
    glViewport(0,HEIGHT/2,WIDTH/2,HEIGHT/2);
    glPushMatrix(); 
        glTranslatef(0.0f,-0.5f,-3.0f);
        glBegin(GL_TRIANGLES);
            glColor3f(1.0f,0.0f,0.0f);
            glVertex3f(0.0f,0.0f,0.0f);
            glColor3f(0.0f,0.0f,1.0f);
            glVertex3f(0.0f,1.0f,0.0f);
            glColor3f(0.0f,1.0f,0.0f);
            glVertex3f(-1.0f,0.0f,0.0f);
        glEnd();
    glPopMatrix();
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效果图：

三、glViewport()函数和glOrtho()函数

glOrtho函数只是负责使用什么样的视景体来截取图像，并不负责使用某种规则把图像呈现在屏幕上。

glViewport负责把视景体截取的图像按照怎样的高和宽显示到屏幕上。图像会根据视口的大小拉伸变形。