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## 前言
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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。
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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。
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[https://github.com/blueroseslol/BRPlugins](https://github.com/blueroseslol/BRPlugins)
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首先还是从ComputeShader开始，因为比较简单。
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## 参考文件
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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。
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- GenerateMips.cpp
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- ShaderPrint.cpp
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- PostProcessCombineLUTs.cpp
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搜索ERDGPassFlags::Compute就可以找到RDG调用ComputeShader的代码。
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## RDG的数据导入与导出
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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。
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## UAV
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UAV用于保存ComputeShader的计算结果，它的创建步骤如下：
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### 实现使用宏声明Shader变量
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```
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SHADER_PARAMETER_UAV(Texture2D, MyUAV)
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SHADER_PARAMETER_RDG_BUFFER_UAV(RWBuffer<float4>, MyUAV)
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```
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### 使用对应的函数创建并且绑定到对应Shader变量上
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SHADER_PARAMETER_UAV对应CreateTexture()，SHADER_PARAMETER_RDG_BUFFER_UAV对应CreateUAV()。（前者没试过）
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可以使用FRDGTextureUAVDesc与Buff两种方式进行创建。
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## SRV创建与使用
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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：
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```c++
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TSharedPtr<FGenerateMipsStruct> FGenerateMips::SetupTexture(FRHITexture* InTexture, const FGenerateMipsParams& InParams)
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{
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check(InTexture->GetTexture2D());
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TSharedPtr<FGenerateMipsStruct> GenMipsStruct = MakeShareable(new FGenerateMipsStruct());
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FPooledRenderTargetDesc Desc;
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Desc.Extent.X = InTexture->GetSizeXYZ().X;
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Desc.Extent.Y = InTexture->GetSizeXYZ().Y;
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Desc.TargetableFlags = TexCreate_ShaderResource | TexCreate_RenderTargetable | TexCreate_UAV;
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Desc.Format = InTexture->GetFormat();
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Desc.NumMips = InTexture->GetNumMips();;
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Desc.DebugName = TEXT("GenerateMipPooledRTTexture");
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//Create the Pooled Render Target Resource from the input texture
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FRHIResourceCreateInfo CreateInfo(Desc.DebugName);
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//Initialise a new render target texture for creating an RDG Texture
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FSceneRenderTargetItem RenderTexture;
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//Update all the RenderTexture info
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RenderTexture.TargetableTexture = InTexture;
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RenderTexture.ShaderResourceTexture = InTexture;
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RenderTexture.SRVs.Empty(Desc.NumMips);
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RenderTexture.MipUAVs.Empty(Desc.NumMips);
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for (uint8 MipLevel = 0; MipLevel < Desc.NumMips; MipLevel++)
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{
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FRHITextureSRVCreateInfo SRVDesc;
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SRVDesc.MipLevel = MipLevel;
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RenderTexture.SRVs.Emplace(SRVDesc, RHICreateShaderResourceView((FTexture2DRHIRef&)InTexture, SRVDesc));
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RenderTexture.MipUAVs.Add(RHICreateUnorderedAccessView(InTexture, MipLevel));
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}
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RHIBindDebugLabelName(RenderTexture.TargetableTexture, Desc.DebugName);
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RenderTexture.UAV = RenderTexture.MipUAVs[0];
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//Create the RenderTarget from the PooledRenderTarget Desc and the new RenderTexture object.
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GRenderTargetPool.CreateUntrackedElement(Desc, GenMipsStruct->RenderTarget, RenderTexture);
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//Specify the Sampler details based on the input.
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GenMipsStruct->Sampler.Filter = InParams.Filter;
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GenMipsStruct->Sampler.AddressU = InParams.AddressU;
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GenMipsStruct->Sampler.AddressV = InParams.AddressV;
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GenMipsStruct->Sampler.AddressW = InParams.AddressW;
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return GenMipsStruct;
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}
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```
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```c++
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void FGenerateMips::Compute(FRHICommandListImmediate& RHIImmCmdList, FRHITexture* InTexture, TSharedPtr<FGenerateMipsStruct> GenMipsStruct)
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{
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check(IsInRenderingThread());
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//Currently only 2D textures supported
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check(InTexture->GetTexture2D());
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//Ensure the generate mips structure has been initialised correctly.
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check(GenMipsStruct);
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//Begin rendergraph for executing the compute shader
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FRDGBuilder GraphBuilder(RHIImmCmdList);
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FRDGTextureRef GraphTexture = GraphBuilder.RegisterExternalTexture(GenMipsStruct->RenderTarget, TEXT("GenerateMipsGraphTexture"));
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···
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FRDGTextureSRVDesc SRVDesc = FRDGTextureSRVDesc::CreateForMipLevel(GraphTexture, MipLevel - 1);
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FGenerateMipsCS::FParameters* PassParameters = GraphBuilder.AllocParameters<FGenerateMipsCS::FParameters>();
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PassParameters->MipInSRV = GraphBuilder.CreateSRV(SRVDesc);
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}
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```
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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。
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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。
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```
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FRDGTextureRef GraphTexture = GraphBuilder.RegisterExternalTexture(GenMipsStruct->RenderTarget, TEXT("GenerateMipsGraphTexture"));
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FRDGTextureSRVDesc SRVDesc = FRDGTextureSRVDesc::CreateForMipLevel(GraphTexture, MipLevel - 1);
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FGenerateMipsCS::FParameters* PassParameters = GraphBuilder.AllocParameters<FGenerateMipsCS::FParameters>();
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PassParameters->MipInSRV = GraphBuilder.CreateSRV(SRVDesc);
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```
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### 在Shader中读取SRV
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读取SRV与读取Texture相同，首先需要创建采样器：
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```
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SHADER_PARAMETER_SAMPLER(SamplerState, MipSampler)
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PassParameters->MipSampler = RHIImmCmdList.CreateSamplerState(GenMipsStruct->Sampler);
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```
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之后就可以想Texture2D那样进行取值了：
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```
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#pragma once
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#include "Common.ush"
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#include "GammaCorrectionCommon.ush"
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float2 TexelSize;
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Texture2D MipInSRV;
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#if GENMIPS_SRGB
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RWTexture2D<half4> MipOutUAV;
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#else
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RWTexture2D<float4> MipOutUAV;
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#endif
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SamplerState MipSampler;
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[numthreads(8, 8, 1)]
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void MainCS(uint3 DT_ID : SV_DispatchThreadID)
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{
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float2 UV = TexelSize * (DT_ID.xy + 0.5f);
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#if GENMIPS_SRGB
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half4 outColor = MipInSRV.SampleLevel(MipSampler, UV, 0);
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outColor = half4(LinearToSrgb(outColor.xyz), outColor.w);
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#else
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float4 outColor = MipInSRV.SampleLevel(MipSampler, UV, 0);
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#endif
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#if GENMIPS_SWIZZLE
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MipOutUAV[DT_ID.xy] = outColor.zyxw;
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#else
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MipOutUAV[DT_ID.xy] = outColor;
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#endif
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}
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```
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## RDGCompute完整代码
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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。
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```c++
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void RDGCompute(FRHICommandListImmediate &RHIImmCmdList, FTexture2DRHIRef RenderTargetRHI, FSimpleShaderParameter InParameter)
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{
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check(IsInRenderingThread());
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//Create PooledRenderTarget
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FPooledRenderTargetDesc RenderTargetDesc = FPooledRenderTargetDesc::Create2DDesc(RenderTargetRHI->GetSizeXY(),RenderTargetRHI->GetFormat(), FClearValueBinding::Black, TexCreate_None, TexCreate_RenderTargetable | TexCreate_ShaderResource | TexCreate_UAV, false);
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TRefCountPtr<IPooledRenderTarget> PooledRenderTarget;
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//RDG Begin
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FRDGBuilder GraphBuilder(RHIImmCmdList);
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FRDGTextureRef RDGRenderTarget = GraphBuilder.CreateTexture(RenderTargetDesc, TEXT("RDGRenderTarget"));
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//Setup Parameters
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FSimpleUniformStructParameters StructParameters;
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StructParameters.Color1 = InParameter.Color1;
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StructParameters.Color2 = InParameter.Color2;
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StructParameters.Color3 = InParameter.Color3;
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StructParameters.Color4 = InParameter.Color4;
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StructParameters.ColorIndex = InParameter.ColorIndex;
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FSimpleRDGComputeShader::FParameters *Parameters = GraphBuilder.AllocParameters<FSimpleRDGComputeShader::FParameters>();
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FRDGTextureUAVDesc UAVDesc(RDGRenderTarget);
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Parameters->SimpleUniformStruct = TUniformBufferRef<FSimpleUniformStructParameters>::CreateUniformBufferImmediate(StructParameters, UniformBuffer_SingleFrame);
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Parameters->OutTexture = GraphBuilder.CreateUAV(UAVDesc);
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//Get ComputeShader From GlobalShaderMap
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const ERHIFeatureLevel::Type FeatureLevel = GMaxRHIFeatureLevel; //ERHIFeatureLevel::SM5
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FGlobalShaderMap *GlobalShaderMap = GetGlobalShaderMap(FeatureLevel);
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TShaderMapRef<FSimpleRDGComputeShader> ComputeShader(GlobalShaderMap);
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//Compute Thread Group Count
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FIntVector ThreadGroupCount(
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RenderTargetRHI->GetSizeX() / 32,
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RenderTargetRHI->GetSizeY() / 32,
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1);
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//ValidateShaderParameters(PixelShader, Parameters);
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//ClearUnusedGraphResources(PixelShader, Parameters);
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GraphBuilder.AddPass(
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RDG_EVENT_NAME("RDGCompute"),
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Parameters,
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ERDGPassFlags::Compute,
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[Parameters, ComputeShader, ThreadGroupCount](FRHICommandList &RHICmdList) {
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FComputeShaderUtils::Dispatch(RHICmdList, ComputeShader, *Parameters, ThreadGroupCount);
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});
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GraphBuilder.QueueTextureExtraction(RDGRenderTarget, &PooledRenderTarget);
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GraphBuilder.Execute();
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//Copy Result To RenderTarget Asset
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RHIImmCmdList.CopyTexture(PooledRenderTarget->GetRenderTargetItem().ShaderResourceTexture, RenderTargetRHI->GetTexture2D(), FRHICopyTextureInfo());
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//RHIImmCmdList.CopyToResolveTarget(PooledRenderTarget->GetRenderTargetItem().ShaderResourceTexture, RenderTargetRHI->GetTexture2D(), FResolveParams());
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}
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```
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## 调用绘制函数
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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。
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```
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void USimpleRenderingExampleBlueprintLibrary::UseRDGComput(const UObject *WorldContextObject, UTextureRenderTarget2D *OutputRenderTarget, FSimpleShaderParameter Parameter)
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{
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check(IsInGameThread());
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FTexture2DRHIRef RenderTargetRHI = OutputRenderTarget->GameThread_GetRenderTargetResource()->GetRenderTargetTexture();
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ENQUEUE_RENDER_COMMAND(CaptureCommand)
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(
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[RenderTargetRHI, Parameter](FRHICommandListImmediate &RHICmdList) {
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SimpleRenderingExample::RDGCompute(RHICmdList, RenderTargetRHI, Parameter);
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});
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}
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```
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## 如何使用
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直接在蓝图中调用即可
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注意RenderTarget的格式需要与UAV的格式一一对应。
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结果：
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