## Ue4后处理逻辑简析
### APostProcessVolume
通常我们在后处理体积，也就是APostProcessVolume设置后处理效果。它存储了struct FPostProcessSettings Settings;
加入关卡后，会存储在UWorld的PostProcessVolumes中，之后依次调用DoPostProcessVolume=》OverridePostProcessSettings，之后修改FSceneView中的FFinalPostProcessSettings FinalPostProcessSettings。（对所有属性进行插值计算）

最后就可以通过View.FinalPostProcessSettings来读取后处理参数了。

### AddPostProcessingPasses
控制渲染的变量主要用下方式获取
- 从FViewInfo里直接获取
- 从FFinalPostProcessSettings获取（View.FinalPostProcessSettings）
- 从FEngineShowFlags获取（View.Family->EngineShowFlags）
- 从ConsoleVariable中获取

获取各种Buffer与变量之后
```c#
	const FIntRect PrimaryViewRect = View.ViewRect;

	const FSceneTextureParameters SceneTextureParameters = GetSceneTextureParameters(GraphBuilder, Inputs.SceneTextures);

	const FScreenPassRenderTarget ViewFamilyOutput = FScreenPassRenderTarget::CreateViewFamilyOutput(Inputs.ViewFamilyTexture, View);
	const FScreenPassTexture SceneDepth(SceneTextureParameters.SceneDepthTexture, PrimaryViewRect);
	const FScreenPassTexture SeparateTranslucency(Inputs.SeparateTranslucencyTextures->GetColorForRead(GraphBuilder), PrimaryViewRect);
	const FScreenPassTexture CustomDepth((*Inputs.SceneTextures)->CustomDepthTexture, PrimaryViewRect);
	const FScreenPassTexture Velocity(SceneTextureParameters.GBufferVelocityTexture, PrimaryViewRect);
	const FScreenPassTexture BlackDummy(GSystemTextures.GetBlackDummy(GraphBuilder));

	// Scene color is updated incrementally through the post process pipeline.
	FScreenPassTexture SceneColor((*Inputs.SceneTextures)->SceneColorTexture, PrimaryViewRect);

	// Assigned before and after the tonemapper.
	FScreenPassTexture SceneColorBeforeTonemap;
	FScreenPassTexture SceneColorAfterTonemap;

	// Unprocessed scene color stores the original input.
	const FScreenPassTexture OriginalSceneColor = SceneColor;

	// Default the new eye adaptation to the last one in case it's not generated this frame.
	const FEyeAdaptationParameters EyeAdaptationParameters = GetEyeAdaptationParameters(View, ERHIFeatureLevel::SM5);
	FRDGTextureRef LastEyeAdaptationTexture = GetEyeAdaptationTexture(GraphBuilder, View);
	FRDGTextureRef EyeAdaptationTexture = LastEyeAdaptationTexture;

	// Histogram defaults to black because the histogram eye adaptation pass is used for the manual metering mode.
	FRDGTextureRef HistogramTexture = BlackDummy.Texture;

	const FEngineShowFlags& EngineShowFlags = View.Family->EngineShowFlags;
	const bool bVisualizeHDR = EngineShowFlags.VisualizeHDR;
	const bool bViewFamilyOutputInHDR = GRHISupportsHDROutput && IsHDREnabled();
	const bool bVisualizeGBufferOverview = IsVisualizeGBufferOverviewEnabled(View);
	const bool bVisualizeGBufferDumpToFile = IsVisualizeGBufferDumpToFileEnabled(View);
	const bool bVisualizeGBufferDumpToPIpe = IsVisualizeGBufferDumpToPipeEnabled(View);
	const bool bOutputInHDR = IsPostProcessingOutputInHDR();
```
读取参数并设置
```c#
TOverridePassSequence<EPass> PassSequence(ViewFamilyOutput);
	PassSequence.SetNames(PassNames, UE_ARRAY_COUNT(PassNames));
	PassSequence.SetEnabled(EPass::VisualizeStationaryLightOverlap, EngineShowFlags.StationaryLightOverlap);
	PassSequence.SetEnabled(EPass::VisualizeLightCulling, EngineShowFlags.VisualizeLightCulling);
#if WITH_EDITOR
	PassSequence.SetEnabled(EPass::SelectionOutline, GIsEditor && EngineShowFlags.Selection && EngineShowFlags.SelectionOutline && !EngineShowFlags.Wireframe && !bVisualizeHDR && !IStereoRendering::IsStereoEyeView(View));
	PassSequence.SetEnabled(EPass::EditorPrimitive, FSceneRenderer::ShouldCompositeEditorPrimitives(View));
#else
	PassSequence.SetEnabled(EPass::SelectionOutline, false);
	PassSequence.SetEnabled(EPass::EditorPrimitive, false);
#endif
	PassSequence.SetEnabled(EPass::VisualizeShadingModels, EngineShowFlags.VisualizeShadingModels);
	PassSequence.SetEnabled(EPass::VisualizeGBufferHints, EngineShowFlags.GBufferHints);
	PassSequence.SetEnabled(EPass::VisualizeSubsurface, EngineShowFlags.VisualizeSSS);
	PassSequence.SetEnabled(EPass::VisualizeGBufferOverview, bVisualizeGBufferOverview || bVisualizeGBufferDumpToFile || bVisualizeGBufferDumpToPIpe);
	PassSequence.SetEnabled(EPass::VisualizeHDR, EngineShowFlags.VisualizeHDR);
#if WITH_EDITOR
	PassSequence.SetEnabled(EPass::PixelInspector, View.bUsePixelInspector);
#else
	PassSequence.SetEnabled(EPass::PixelInspector, false);
#endif
	PassSequence.SetEnabled(EPass::HMDDistortion, EngineShowFlags.StereoRendering && EngineShowFlags.HMDDistortion);
	PassSequence.SetEnabled(EPass::HighResolutionScreenshotMask, IsHighResolutionScreenshotMaskEnabled(View));
	PassSequence.SetEnabled(EPass::PrimaryUpscale, PaniniConfig.IsEnabled() || (View.PrimaryScreenPercentageMethod == EPrimaryScreenPercentageMethod::SpatialUpscale && PrimaryViewRect.Size() != View.GetSecondaryViewRectSize()));
	PassSequence.SetEnabled(EPass::SecondaryUpscale, View.RequiresSecondaryUpscale() || View.Family->GetSecondarySpatialUpscalerInterface() != nullptr);
```
这些操作一直到`PassSequence.Finalize();`。

### 后处理Pass处理
主要的Pass有这么一些：
```c#
TEXT("MotionBlur"),
TEXT("Tonemap"),
TEXT("FXAA"),
TEXT("PostProcessMaterial (AfterTonemapping)"),
TEXT("VisualizeDepthOfField"),
TEXT("VisualizeStationaryLightOverlap"),
TEXT("VisualizeLightCulling"),
TEXT("SelectionOutline"),
TEXT("EditorPrimitive"),
TEXT("VisualizeShadingModels"),
TEXT("VisualizeGBufferHints"),
TEXT("VisualizeSubsurface"),
TEXT("VisualizeGBufferOverview"),
TEXT("VisualizeHDR"),
TEXT("PixelInspector"),
TEXT("HMDDistortion"),
TEXT("HighResolutionScreenshotMask"),
TEXT("PrimaryUpscale"),
TEXT("SecondaryUpscale")
```
之前读取了参数，对这些Pass是否开启进行了设置。之后以这种格式使用Shader对传入的图形进行后处理。
```c#
if (PassSequence.IsEnabled(EPass::MotionBlur))
{
	FMotionBlurInputs PassInputs;
	PassSequence.AcceptOverrideIfLastPass(EPass::MotionBlur, PassInputs.OverrideOutput);
	PassInputs.SceneColor = SceneColor;
	PassInputs.SceneDepth = SceneDepth;
	PassInputs.SceneVelocity = Velocity;
	PassInputs.Quality = GetMotionBlurQuality();
	PassInputs.Filter = GetMotionBlurFilter();

	// Motion blur visualization replaces motion blur when enabled.
	if (bVisualizeMotionBlur)
	{
		SceneColor = AddVisualizeMotionBlurPass(GraphBuilder, View, PassInputs);
	}
	else
	{
		SceneColor = AddMotionBlurPass(GraphBuilder, View, PassInputs);
	}
}

SceneColor = AddAfterPass(EPass::MotionBlur, SceneColor);
```
这些效果的代码都在UnrealEngine\Engine\Source\Runtime\Renderer\Private\PostProcess中。

### 后处理材质调用
AddPostProcessMaterialChain
=》
AddPostProcessMaterialPass()为实际的绘制函数。最后在AddDrawScreenPass()中进行绘制。（DrawScreenPass()=>DrawPostProcessPass=>DrawPostProcessPass()）

### 推荐参考的后处理代码
PostProcessBloomSetup.h
VisualizeShadingModels.cpp