## 前言
UE4 RDG(RenderDependencyGraph)渲染框架本质上是在原有渲染框架上基础上进行再次封装，它主要的设计目的就为了更好的管理每个资源的生命周期。同时Ue4的渲染管线已经都替换成了RDG框架了（但依然有很多非重要模块以及第三方插件没有替换），所以掌握以下RDG框架还是十分有必要的。

上一篇文章已经大致介绍了RDG框架的使用方法。看了前文的资料与官方的ppt，再看一下渲染管线的的代码基本就可以上手了写Shader。但作为一个工作与Ue4一点关系的业余爱好者，用的2014年的电脑通过修改渲染管线的方式来写Shader不太现实，编译一次3小时真心伤不起。同时google与Epic论坛也没有在插件中使用RDG的资料，所以我就花了些时间探索了一下用法，最后写了本文。**因为非全职开发UE4，时间精力有限，不可避免得会有些错误，还请见谅。** 代码写在我的插件里，如果感觉有用麻烦Star一下。位于在Rendering下的SimpleRDG.cpp与SimpleRenderingExample.h中。

[https://github.com/blueroseslol/BRPlugins](https://github.com/blueroseslol/BRPlugins)

首先还是从ComputeShader开始，因为比较简单。

## 参考文件
下面推荐几个参考文件，强烈推荐看GenerateMips，包含RDG Compute与GlobalShader两个案例。
- GenerateMips.cpp
- ShaderPrint.cpp
- PostProcessCombineLUTs.cpp

搜索ERDGPassFlags::Compute就可以找到RDG调用ComputeShader的代码。

## RDG的数据导入与导出
RDG需要使用RegisterExternalBuffer/Texture导入数据；GraphBuilder.QueueExternalBuffer/Texture取出渲染结果，这需要使用一个TRefCountPtr<IPooledRenderTarget>对象。直接使用RHIImmCmdList.CopyTexture尝试将FRDGTextureRef的数据拷贝出来时会触发禁止访问的断言。

## UAV
UAV用于保存ComputeShader的计算结果，它的创建步骤如下：
### 实现使用宏声明Shader变量
```
SHADER_PARAMETER_UAV(Texture2D, MyUAV)
SHADER_PARAMETER_RDG_BUFFER_UAV(RWBuffer<float4>, MyUAV)
```

### 使用对应的函数创建并且绑定到对应Shader变量上
SHADER_PARAMETER_UAV对应CreateTexture()，SHADER_PARAMETER_RDG_BUFFER_UAV对应CreateUAV()。（前者没试过）

可以使用FRDGTextureUAVDesc与Buff两种方式进行创建。

## SRV创建与使用
UAV是不能直接在Shader里读取，所以需要通过创建SRV的方式来读取。因为我并没有测试SRV，所以这里贴一下FGenerateMips中的部分代码：
```c++
TSharedPtr<FGenerateMipsStruct> FGenerateMips::SetupTexture(FRHITexture* InTexture, const FGenerateMipsParams& InParams)
{
check(InTexture->GetTexture2D());

	TSharedPtr<FGenerateMipsStruct> GenMipsStruct = MakeShareable(new FGenerateMipsStruct());

		FPooledRenderTargetDesc Desc;
		Desc.Extent.X = InTexture->GetSizeXYZ().X;
		Desc.Extent.Y = InTexture->GetSizeXYZ().Y;
		Desc.TargetableFlags = TexCreate_ShaderResource | TexCreate_RenderTargetable | TexCreate_UAV;
		Desc.Format = InTexture->GetFormat();
		Desc.NumMips = InTexture->GetNumMips();;
		Desc.DebugName = TEXT("GenerateMipPooledRTTexture");

		//Create the Pooled Render Target Resource from the input texture
		FRHIResourceCreateInfo CreateInfo(Desc.DebugName);

		//Initialise a new render target texture for creating an RDG Texture
		FSceneRenderTargetItem RenderTexture;

		//Update all the RenderTexture info
		RenderTexture.TargetableTexture = InTexture;
		RenderTexture.ShaderResourceTexture = InTexture;

		RenderTexture.SRVs.Empty(Desc.NumMips);
		RenderTexture.MipUAVs.Empty(Desc.NumMips);
		for (uint8 MipLevel = 0; MipLevel < Desc.NumMips; MipLevel++)
		{
			FRHITextureSRVCreateInfo SRVDesc;
			SRVDesc.MipLevel = MipLevel;
			RenderTexture.SRVs.Emplace(SRVDesc, RHICreateShaderResourceView((FTexture2DRHIRef&)InTexture, SRVDesc));

			RenderTexture.MipUAVs.Add(RHICreateUnorderedAccessView(InTexture, MipLevel));
		}
		RHIBindDebugLabelName(RenderTexture.TargetableTexture, Desc.DebugName);
		RenderTexture.UAV = RenderTexture.MipUAVs[0];

		//Create the RenderTarget from the PooledRenderTarget Desc and the new RenderTexture object.
	GRenderTargetPool.CreateUntrackedElement(Desc, GenMipsStruct->RenderTarget, RenderTexture);

		//Specify the Sampler details based on the input.
	GenMipsStruct->Sampler.Filter = InParams.Filter;
	GenMipsStruct->Sampler.AddressU = InParams.AddressU;
	GenMipsStruct->Sampler.AddressV = InParams.AddressV;
	GenMipsStruct->Sampler.AddressW = InParams.AddressW;

	return GenMipsStruct;
}
```
```c++

void FGenerateMips::Compute(FRHICommandListImmediate& RHIImmCmdList, FRHITexture* InTexture, TSharedPtr<FGenerateMipsStruct> GenMipsStruct)
{
    check(IsInRenderingThread());	
	//Currently only 2D textures supported
	check(InTexture->GetTexture2D());

	//Ensure the generate mips structure has been initialised correctly.
	check(GenMipsStruct);

	//Begin rendergraph for executing the compute shader
	FRDGBuilder GraphBuilder(RHIImmCmdList);
	FRDGTextureRef GraphTexture = GraphBuilder.RegisterExternalTexture(GenMipsStruct->RenderTarget, TEXT("GenerateMipsGraphTexture"));
	
	···
	
	FRDGTextureSRVDesc SRVDesc = FRDGTextureSRVDesc::CreateForMipLevel(GraphTexture, MipLevel - 1);

	FGenerateMipsCS::FParameters* PassParameters = GraphBuilder.AllocParameters<FGenerateMipsCS::FParameters>();
	PassParameters->MipInSRV = GraphBuilder.CreateSRV(SRVDesc);
}
```
可以看出是先通过CreateUntrackedElement()创建IPooledRenderTarget，之后再调用RegisterExternalTexture进行注册，最后再调用CreateSRV创建SRV。

另外IPooledRenderTarget除了有CreateUntrackedElement()，还有FindFreeElement()。这个函数就适合在多Pass RDG中使用了。

```
FRDGTextureRef GraphTexture = GraphBuilder.RegisterExternalTexture(GenMipsStruct->RenderTarget, TEXT("GenerateMipsGraphTexture"));

FRDGTextureSRVDesc SRVDesc = FRDGTextureSRVDesc::CreateForMipLevel(GraphTexture, MipLevel - 1);

FGenerateMipsCS::FParameters* PassParameters = GraphBuilder.AllocParameters<FGenerateMipsCS::FParameters>();
PassParameters->MipInSRV = GraphBuilder.CreateSRV(SRVDesc);
```
### 在Shader中读取SRV
读取SRV与读取Texture相同，首先需要创建采样器：
```
SHADER_PARAMETER_SAMPLER(SamplerState, MipSampler)

PassParameters->MipSampler = RHIImmCmdList.CreateSamplerState(GenMipsStruct->Sampler);
```
之后就可以想Texture2D那样进行取值了：
```
#pragma once
#include "Common.ush"
#include "GammaCorrectionCommon.ush"

float2 TexelSize;
Texture2D MipInSRV;
#if GENMIPS_SRGB
RWTexture2D<half4> MipOutUAV;
#else
RWTexture2D<float4> MipOutUAV;
#endif
SamplerState MipSampler;

[numthreads(8, 8, 1)]
void MainCS(uint3 DT_ID : SV_DispatchThreadID)
{
    float2 UV = TexelSize * (DT_ID.xy + 0.5f);

#if GENMIPS_SRGB
    half4 outColor = MipInSRV.SampleLevel(MipSampler, UV, 0);
    outColor = half4(LinearToSrgb(outColor.xyz), outColor.w);
#else
    float4 outColor = MipInSRV.SampleLevel(MipSampler, UV, 0);
#endif

#if GENMIPS_SWIZZLE
    MipOutUAV[DT_ID.xy] = outColor.zyxw;
#else
    MipOutUAV[DT_ID.xy] = outColor;
#endif
}
```

## RDGCompute完整代码
以下就是我写的例子，因为比较简单而且有注释，所以就不详细解释了。
```c++
void RDGCompute(FRHICommandListImmediate &RHIImmCmdList, FTexture2DRHIRef RenderTargetRHI, FSimpleShaderParameter InParameter)
{
	check(IsInRenderingThread());

	//Create PooledRenderTarget
	FPooledRenderTargetDesc RenderTargetDesc = FPooledRenderTargetDesc::Create2DDesc(RenderTargetRHI->GetSizeXY(),RenderTargetRHI->GetFormat(), FClearValueBinding::Black, TexCreate_None, TexCreate_RenderTargetable | TexCreate_ShaderResource | TexCreate_UAV, false);
	TRefCountPtr<IPooledRenderTarget> PooledRenderTarget;

	//RDG Begin
	FRDGBuilder GraphBuilder(RHIImmCmdList);
	FRDGTextureRef RDGRenderTarget = GraphBuilder.CreateTexture(RenderTargetDesc, TEXT("RDGRenderTarget"));

	//Setup Parameters
	FSimpleUniformStructParameters StructParameters;
	StructParameters.Color1 = InParameter.Color1;
	StructParameters.Color2 = InParameter.Color2;
	StructParameters.Color3 = InParameter.Color3;
	StructParameters.Color4 = InParameter.Color4;
	StructParameters.ColorIndex = InParameter.ColorIndex;

	FSimpleRDGComputeShader::FParameters *Parameters = GraphBuilder.AllocParameters<FSimpleRDGComputeShader::FParameters>();
	FRDGTextureUAVDesc UAVDesc(RDGRenderTarget);
	Parameters->SimpleUniformStruct = TUniformBufferRef<FSimpleUniformStructParameters>::CreateUniformBufferImmediate(StructParameters, UniformBuffer_SingleFrame);
	Parameters->OutTexture = GraphBuilder.CreateUAV(UAVDesc);

	//Get ComputeShader From GlobalShaderMap
	const ERHIFeatureLevel::Type FeatureLevel = GMaxRHIFeatureLevel; //ERHIFeatureLevel::SM5
	FGlobalShaderMap *GlobalShaderMap = GetGlobalShaderMap(FeatureLevel);
	TShaderMapRef<FSimpleRDGComputeShader> ComputeShader(GlobalShaderMap);

	//Compute Thread Group Count
	FIntVector ThreadGroupCount(
		RenderTargetRHI->GetSizeX() / 32,
		RenderTargetRHI->GetSizeY() / 32,
		1);

	//ValidateShaderParameters(PixelShader, Parameters);
	//ClearUnusedGraphResources(PixelShader, Parameters);

	GraphBuilder.AddPass(
		RDG_EVENT_NAME("RDGCompute"),
		Parameters,
		ERDGPassFlags::Compute,
		[Parameters, ComputeShader, ThreadGroupCount](FRHICommandList &RHICmdList) {
			FComputeShaderUtils::Dispatch(RHICmdList, ComputeShader, *Parameters, ThreadGroupCount);
		});

	GraphBuilder.QueueTextureExtraction(RDGRenderTarget, &PooledRenderTarget);
	GraphBuilder.Execute();

	//Copy Result To RenderTarget Asset
	RHIImmCmdList.CopyTexture(PooledRenderTarget->GetRenderTargetItem().ShaderResourceTexture, RenderTargetRHI->GetTexture2D(), FRHICopyTextureInfo());
	//RHIImmCmdList.CopyToResolveTarget(PooledRenderTarget->GetRenderTargetItem().ShaderResourceTexture, RenderTargetRHI->GetTexture2D(), FResolveParams());
}
```
## 调用绘制函数
与传统方法类似，调用上述渲染函数时需要使用ENQUEUE_RENDER_COMMAND(CaptureCommand)[]。下面是我写在蓝图函数库的代码。
```
void USimpleRenderingExampleBlueprintLibrary::UseRDGComput(const UObject *WorldContextObject, UTextureRenderTarget2D *OutputRenderTarget, FSimpleShaderParameter Parameter)
{
	check(IsInGameThread());

	FTexture2DRHIRef RenderTargetRHI = OutputRenderTarget->GameThread_GetRenderTargetResource()->GetRenderTargetTexture();

	ENQUEUE_RENDER_COMMAND(CaptureCommand)
	(
		[RenderTargetRHI, Parameter](FRHICommandListImmediate &RHICmdList) {
			SimpleRenderingExample::RDGCompute(RHICmdList, RenderTargetRHI, Parameter);
		});
}
```

## 如何使用
直接在蓝图中调用即可

![](https://cdn.jsdelivr.net/gh/blueroseslol/ImageBag@latest/ImageBag/Images/UseRDGCompute.png)

注意RenderTarget的格式需要与UAV的格式一一对应。

![](https://cdn.jsdelivr.net/gh/blueroseslol/ImageBag@latest/ImageBag/Images/RTFormat.png)

结果：

![](https://cdn.jsdelivr.net/gh/blueroseslol/ImageBag@latest/ImageBag/Images/RDGComputeShaderResult.png)