ue4 后期处理景深_UE4 后期处理 PostProcess

在GDC2014上，UE4就提出了它移动端后期处理管线设计(参考Next_Generation_Mobile_Rendering 文章)

流程大致是

降低分辨率(Smart Reduction)(在各个Pass里处理，没有直接降低RT分辨率)->

景深(Depth of Field) (可有可无) ->

模糊(Bloom)->

光轴(LightShaft)->

混合(Vignette +Bloom + Light Shafts)->

色调映射(Tonemapping)->

抗锯齿(Anti-Aliasing)

移动端后期处理

代码在FPostProcessing::ProcessES2中

先定义了2个上下文

FRenderingCompositePassContext CompositeContext(RHICmdList, View);

FPostprocessContext Context(RHICmdList, CompositeContext.Graph, View);

CompositeContext其实其中有个后期处理Pass列表，把需要做的后期处理全部缓存在其中，然后一起运行(Processs)

FRenderingCompositePass 最基础Pass的基类

virtual const FRenderingCompositeOutputRef* GetInput(EPassInputId InPassInputId) const = 0;

virtual void SetInput(EPassInputId InPassInputId, const FRenderingCompositeOutputRef& InOutputRef) = 0;

virtual void Process(FRenderingCompositePassContext& Context) = 0;

包含Input 对象是个FRenderingCompositeOutputRef

类型，也就是其他Pass 渲染完输出的结果可以作为Input传入，在此基础上绘制

真正的Pass列表是FRenderingCompositionGraph

//注册Pass

template

T* RegisterPass(T* InPass)

{

check(InPass);

Nodes.Add(InPass);

return InPass;

}

//pass节点列表

TArray Nodes;

//迭代运行pass

void RecursivelyProcess(const FRenderingCompositeOutputRef& InOutputRef, FRenderingCompositePassContext& Context)const;

最终会执行到RecursivelyProcess方法

后面代码就是加入各种Pass加入后期自定义材质Pass

AddPostProcessMaterial(Context, BL_BeforeTranslucency, nullptr);

AddPostProcessMaterial(Context, BL_BeforeTonemapping, nullptr);

加入BloomSetUp Pass

FRenderingCompositePass* Pass= Context.Graph.RegisterPass(new(FMemStack::Get()) FRCPassPostProcessBloomSetupES2(FinalOutputViewRect, bViewRectSource));

Pass->SetInput(ePId_Input0, Context.FinalOutput);

PostProcessBloomSetup = FRenderingCompositeOutputRef(Pass);加入DOF Pass

加入7个Bloom Pass

注意每个Bloom Pass 传入的Input不一样，窗口大小(ViewportSize)也不一样

// Downsample by 2

{

FRenderingCompositePass* Pass = Context.Graph.RegisterPass(new(FMemStack::Get()) FRCPassPostProcessBloomDownES2(PrePostSourceViewportSize/4,DownScale));

Pass->SetInput(ePId_Input0, PostProcessBloomSetup);

PostProcessDownsample2 = FRenderingCompositeOutputRef(Pass);

}

// Downsample by 2

{

FRenderingCompositePass* Pass = Context.Graph.RegisterPass(new(FMemStack::Get()) FRCPassPostProcessBloomDownES2(PrePostSourceViewportSize/8, DownScale));

//使用前一个Pass

Pass->SetInput(ePId_Input0, PostProcessDownsample2);

PostProcessDownsample3 = FRenderingCompositeOutputRef(Pass);

}

加入LightShaft Pass

加入separate translucency

如果启用了SeparateTranslucency加入 SeparateTranslucency Pass

if (IsMobileSeparateTranslucencyActive(Context.View))

{

FRCSeparateTranslucensyPassES2* Pass = (FRCSeparateTranslucensyPassES2*)Context.Graph.RegisterPass(new(FMemStack::Get()) FRCSeparateTranslucensyPassES2());

Pass->SetInput(ePId_Input0, Context.FinalOutput);

Context.FinalOutput = FRenderingCompositeOutputRef(Pass);

}

加入Tonemap Pass

加入AA Pass

根据DebugViewShaderMode 类型 加入对应的Pass最后执行Pass 也就是执行RecursivelyProcess

GRenderTargetPool.CreateUntrackedElement(Desc, Temp, Item);

OverrideRenderTarget(Context.FinalOutput, Temp, Desc);

CompositeContext.Process(Context.FinalOutput.GetPass(), TEXT("PostProcessingES2"));

Pass降低分辨率方法

通过DrawRectangle设置Dst大小是Src大小 的1/4，通过降低绘制分辨率，提高性能

void FRCPassPostProcessBloomSetupES2::Process(FRenderingCompositePassContext& Context)

{

//目标是PrePostSourceViewportSize大小的一半

uint32 DstX = FMath::Max(1, PrePostSourceViewportSize.X/2);

uint32 DstY = FMath::Max(1, PrePostSourceViewportSize.Y/2);

FIntRect DstRect;

DstRect.Min.X = 0;

DstRect.Min.Y = 0;

DstRect.Max.X = DstX;

DstRect.Max.Y = DstY;

FIntPoint DstSize = PrePostSourceViewportSize / 2;

FIntPoint SrcSize;

FIntRect SrcRect;

if(bUseViewRectSource)

{

// Mobile with framebuffer fetch uses view rect as source.

const FSceneView& View = Context.View;

SrcSize = InputDesc->Extent;

// SrcRect 就是原先大小

SrcRect = PrePostSourceViewportRect;

}

…

//通过DrawRectangle缩放

DrawRectangle(

Context.RHICmdList,

0,

0,

DstX, DstY,

SrcRect.Min.X, SrcRect.Min.Y,

SrcRect.Width(), SrcRect.Height(),

DstSize,

SrcSize,

*VertexShader,

EDRF_UseTriangleOptimization);