--- title: 剖析虚幻渲染体系（11）- RDG date: 2024-02-04 21:42:54 excerpt: tags: rating: ⭐ --- # 前言 原文地址:https://www.cnblogs.com/timlly/p/15217090.html # 概念 - FRDGAllocator：简单的C++对象分配器, 用MemStack分配器追踪和销毁物体。 - FComputeShaderUtils - Dispatch()：派发ComputeShader到RHI命令列表，携带其参数。 - DispatchIndirect()：派发非直接的ComputeShader到RHI命令列表,，携带其参数。 - AddPass()：派发计算着色器到**RenderGraphBuilder**， 携带其参数。 - ClearUAV()：清理UAV。 - AddCopyTexturePass()：增加拷贝纹理Pass。 - AddCopyToResolveTargetPass()：增加拷贝到解析目标的Pass。 - AddEnqueueCopyPass()：增加回读纹理的Pass。 - AddPass()：无参数的Pass增加。 - AddBeginUAVOverlapPass()/AddEndUAVOverlapPass()： 其它特殊Pass。 - FRDGResource：RDG资源并不是直接用RHI资源，而是包裹了RHI资源引用，然后针对不同类型的资源各自封装，且增加了额外的信息。 - FRDGUniformBuffer、TRDGUniformBuffer - FRDGParentResource：一种由图跟踪分配生命周期的渲染图资源。可能有引用它的子资源(例如视图) - FRDGView - FRDGBuffer、FRDGBufferSRV、FRDGBufferUAV - FRDGViewableResource：一种由图跟踪分配生命周期的RPGResource。可能有引用它的子资源 - FRDGTexture - FRDGView - FRDGUnorderedAccessView、FRDGTextureUAV - FRDGShaderResourceView、FRDGTextureSRV - FRDGTextureDesc：创建渲染纹理的描述信息。 - FRDGPooledTexture：贴图池里的贴图资源。 - FRDGPooledBuffer：池化的缓冲区。 - FRHITransition：用于表示RHI中挂起的资源转换的**不透明表面材质**数据结构。 - FRDGBarrierBatch：RDG屏障批。 - FRDGBarrierBatchBegin - FRDGBarrierBatchEnd - FRDGPass - TRDGLambdaPass - FRDGSentinelPass：哨兵Pass，用于开始/结束。 - [[#FRDGBuilder]] ## RDG基础类型 ```c++ enum class ERDGBuilderFlags { None = 0, /** Allows the builder to parallelize execution of passes. Without this flag, all passes execute on the render thread. */ AllowParallelExecute = 1 << 0 }; /** Flags to annotate a pass with when calling AddPass. */ enum class ERDGPassFlags : uint16 { /** Pass doesn't have any inputs or outputs tracked by the graph. This may only be used by the parameterless AddPass function. */ None = 0, /** Pass uses rasterization on the graphics pipe. */ Raster = 1 << 0, /** Pass uses compute on the graphics pipe. */ Compute = 1 << 1, /** Pass uses compute on the async compute pipe. */ AsyncCompute = 1 << 2, /** Pass uses copy commands on the graphics pipe. */ Copy = 1 << 3, /** Pass (and its producers) will never be culled. Necessary if outputs cannot be tracked by the graph. */ NeverCull = 1 << 4, /** Render pass begin / end is skipped and left to the user. Only valid when combined with 'Raster'. Disables render pass merging for the pass. */ SkipRenderPass = 1 << 5, /** Pass will never have its render pass merged with other passes. */ NeverMerge = 1 << 6, /** Pass will never run off the render thread. */ NeverParallel = 1 << 7, ParallelTranslate = 1 << 8, /** Pass uses copy commands but writes to a staging resource. */ Readback = Copy | NeverCull }; /** Flags to annotate a render graph buffer. */ enum class ERDGBufferFlags : uint8 { None = 0, /** Tag the buffer to survive through frame, that is important for multi GPU alternate frame rendering. */ MultiFrame = 1 << 0, /** The buffer is ignored by RDG tracking and will never be transitioned. Use the flag when registering a buffer with no writable GPU flags. * Write access is not allowed for the duration of the graph. This flag is intended as an optimization to cull out tracking of read-only * buffers that are used frequently throughout the graph. Note that it's the user's responsibility to ensure the resource is in the correct * readable state for use with RDG passes, as RDG does not know the exact state of the resource. */ SkipTracking = 1 << 1, /** When set, RDG will perform its first barrier without splitting. Practically, this means the resource is left in its initial state * until the first pass it's used within the graph. Without this flag, the resource is split-transitioned at the start of the graph. */ ForceImmediateFirstBarrier = 1 << 2, }; /** Flags to annotate a render graph texture. */ enum class ERDGTextureFlags : uint8 { None = 0, /** Tag the texture to survive through frame, that is important for multi GPU alternate frame rendering. */ MultiFrame = 1 << 0, /** The buffer is ignored by RDG tracking and will never be transitioned. Use the flag when registering a buffer with no writable GPU flags. * Write access is not allowed for the duration of the graph. This flag is intended as an optimization to cull out tracking of read-only * buffers that are used frequently throughout the graph. Note that it's the user's responsibility to ensure the resource is in the correct * readable state for use with RDG passes, as RDG does not know the exact state of the resource. */ SkipTracking = 1 << 1, /** When set, RDG will perform its first barrier without splitting. Practically, this means the resource is left in its initial state * until the first pass it's used within the graph. Without this flag, the resource is split-transitioned at the start of the graph. */ ForceImmediateFirstBarrier = 1 << 2, /** Prevents metadata decompression on this texture. */ MaintainCompression = 1 << 3, }; ENUM_CLASS_FLAGS(ERDGTextureFlags); /** Flags to annotate a view with when calling CreateUAV. */ enum class ERDGUnorderedAccessViewFlags : uint8 { None = 0, // The view will not perform UAV barriers between consecutive usage. SkipBarrier = 1 << 0 }; ENUM_CLASS_FLAGS(ERDGUnorderedAccessViewFlags); /** The set of concrete parent resource types. */ enum class ERDGViewableResourceType : uint8 { Texture, Buffer, MAX }; /** The set of concrete view types. */ enum class ERDGViewType : uint8 { TextureUAV, TextureSRV, BufferUAV, BufferSRV, MAX }; inline ERDGViewableResourceType GetParentType(ERDGViewType ViewType) { switch (ViewType) { case ERDGViewType::TextureUAV: case ERDGViewType::TextureSRV: return ERDGViewableResourceType::Texture; case ERDGViewType::BufferUAV: case ERDGViewType::BufferSRV: return ERDGViewableResourceType::Buffer; } checkNoEntry(); return ERDGViewableResourceType::MAX; } enum class ERDGResourceExtractionFlags : uint8 { None = 0, // Allows the resource to remain transient. Only use this flag if you intend to register the resource back // into the graph and release the reference. This should not be used if the resource is cached for a long // period of time. AllowTransient = 1, }; enum class ERDGInitialDataFlags : uint8 { /** Specifies the default behavior, which is to make a copy of the initial data for replay when * the graph is executed. The user does not need to preserve lifetime of the data pointer. */ None = 0, /** Specifies that the user will maintain ownership of the data until the graph is executed. The * upload pass will only use a reference to store the data. Use caution with this flag since graph * execution is deferred! Useful to avoid the copy if the initial data lifetime is guaranteed to * outlive the graph. */ NoCopy = 1 << 0 }; enum class ERDGPooledBufferAlignment : uint8 { // The buffer size is not aligned. None, // The buffer size is aligned up to the next page size. Page, // The buffer size is aligned up to the next power of two. PowerOfTwo }; /** Returns the equivalent parent resource type for a view type. */ inline ERDGViewableResourceType GetViewableResourceType(ERDGViewType ViewType) { switch (ViewType) { case ERDGViewType::TextureUAV: case ERDGViewType::TextureSRV: return ERDGViewableResourceType::Texture; case ERDGViewType::BufferUAV: case ERDGViewType::BufferSRV: return ERDGViewableResourceType::Buffer; default: checkNoEntry(); return ERDGViewableResourceType::MAX; } } using ERDGTextureMetaDataAccess = ERHITextureMetaDataAccess; /** Returns the associated FRHITransitionInfo plane index. */ inline int32 GetResourceTransitionPlaneForMetadataAccess(ERDGTextureMetaDataAccess Metadata) { switch (Metadata) { case ERDGTextureMetaDataAccess::CompressedSurface: case ERDGTextureMetaDataAccess::HTile: case ERDGTextureMetaDataAccess::Depth: return FRHITransitionInfo::kDepthPlaneSlice; case ERDGTextureMetaDataAccess::Stencil: return FRHITransitionInfo::kStencilPlaneSlice; default: return 0; } } ``` ## FRDGBuilder FRDGBuilder是RDG体系的心脏和发动机，也是个大管家，负责收集渲染Pass和参数，编译Pass、数据，处理资源依赖，裁剪和优化各类数据，还有提供执行接口。 重要函数： - FindExternalTexture()：查找外部纹理, 若找不到返回null. - RegisterExternalTexture()：注册外部池内RT到RDG, 以便RDG追踪之. 池内RT可能包含两种RHI纹理: MSAA和非MSAA。 - RegisterExternalBuffer()：注册外部缓冲区到RDG, 以便RDG追踪之. - 资源创建接口： - CreateTexture() - CreateBuffer() - CreateUAV() - CreateSRV() - CreateUniformBuffer() - 分配内存, 内存由RDG管理生命周期。 - Alloc() - AllocPOD() - AllocObject() - AllocParameters() - AddPass() - FRDGPassRef AddPass(FRDGEventName&& Name, const ParameterStructType* ParameterStruct, ERDGPassFlags Flags, ExecuteLambdaType&& ExecuteLambda); ：增加有参数的LambdaPass。 - FRDGPassRef AddPass(FRDGEventName&& Name, const FShaderParametersMetadata* ParametersMetadata, const void* ParameterStruct, ERDGPassFlags Flags, ExecuteLambdaType&& ExecuteLambda); ：增加带有实时生成结构体的LambdaPass - FRDGPassRef AddPass(FRDGEventName&& Name, ERDGPassFlags Flags, ExecuteLambdaType&& ExecuteLambda);：增加没有参数的LambdaPass - QueueTextureExtraction()：在Builder执行末期, 提取池内纹理到指定的指针. 对于RDG创建的资源, 这将延长GPU资源的生命周期，直到执行，指针被填充. 如果指定，纹理将转换为AccessFinal状态, 否则将转换为kDefaultAccessFinal状态。 - QueueBufferExtraction()：在Builder执行末期, 提取缓冲区到指定的指针。 - PreallocateTexture()/PreallocateBuffer()：预分配资源. 只对RDG创建的资源, 会强制立即分配底层池内资源, 有效地将其推广到外部资源. 这将增加内存压力，但允许使用GetPooled{Texture, Buffer}查询池中的资源. 主要用于增量地将代码移植到RDG. - GetPooledTexture()/GetPooledBuffer()：立即获取底层资源, 只允许用于注册或预分配的资源。 - SetTextureAccessFinal()/SetBufferAccessFinal()：设置执行之后的状态。 - 变量 - RDG对象注册表 - FRDGPassRegistry Passes; - FRDGTextureRegistry Textures; - FRDGBufferRegistry Buffers; - FRDGViewRegistry Views; - FRDGUniformBufferRegistry UniformBuffers; ### FRDGBuilder::Compile RDG编译期间的逻辑非常复杂，步骤繁多，先后经历构建生产者和消费者的依赖关系，确定Pass的裁剪等各类标记，调整资源的生命周期，裁剪Pass，处理Pass的资源转换和屏障，处理异步计算Pass的依赖和引用关系，查找并建立分叉和合并Pass节点，合并所有具体相同渲染目标的光栅化Pass等步骤。 # RDG开发 ### 注册外部资源 如果我们已有非RDG创建的资源，可以在RDG使用么？答案是可以，通过FRDGBuilder::RegisterExternalXXX接口可以完成将外部资源注册到RDG系统中。下面以注册纹理为例： ```c++ // 在RDG外创建RHI资源. FRHIResourceCreateInfo CreateInfo; FTexture2DRHIRef MyRHITexture = RHICreateTexture2D(1024, 768, PF_B8G8R8A8, 1, 1, TexCreate_CPUReadback, CreateInfo); // 将外部创建的RHI资源注册成RDG资源. FRDGTextureRef MyExternalRDGTexture = GraphBuilder.RegisterExternalTexture(MyRHITexture); ``` 需要注意的是，外部注册的资源，RDG无法控制和管理其生命周期，需要保证RDG使用期间外部资源的生命周期处于正常状态，否则将引发异常甚至程序崩溃。 如果想从RDG资源获取RHI资源的实例，以下代码可达成： ```c++ FRHITexture* MyRHITexture = MyRDGTexture.GetRHI(); ``` 用图例展示RHI资源和RDG资源之间的转换关系： - FRHIResource =>FRDGBuilder::RegisterExternalXXX =>FRDGResource - FRDGResource => FRDGResource::GetRHI => FRHIResource ### 提取资源 RDG收集Pass之后并非立即执行，而是延迟执行（包括资源被延迟创建或分配），这就导致了一个问题：如果想将渲染后的资源赋值给某个变量，无法使用立即模式，需要适配延迟执行模式。这种适配延迟执行的资源提取是通过以下接口来实现的： - FRDGBuilder::QueueTextureExtraction - FRDGBuilder::QueueBufferExtraction ```c++ // 创建RDG纹理. FRDGTextureRef MyRDGTexture; FRDGTextureDesc MyTextureDesc = FRDGTextureDesc::Create2D(OutputExtent, HistoryPixelFormat, FClearValueBinding::Black, TexCreate_ShaderResource | TexCreate_UAV); MyRDGTexture = GraphBuilder.CreateTexture(MyTextureDesc, "MyRDGTexture", ERDGTextureFlags::MultiFrame); // 创建UAV并作为Pass的shader参数. (......) PassParameters->MyRDGTextureUAV = GraphBuilder.CreateUAV(MyRDGTexture); (......) // 增加Pass, 以便渲染图像到MyRDGTextureUAV. FComputeShaderUtils::AddPass(GraphBuilder, RDG_EVENT_NAME("MyCustomPass", ...), ComputeShader, PassParameters, FComputeShaderUtils::GetGroupCount(8, 8)); // 入队提取资源. TRefCountPtr* OutputRT; GraphBuilder.QueueTextureExtraction(MyRDGTexture, &OutputRT); // 对提取的OutputRT进行后续操作. (......) ``` # RDG调试 RDG系统存在一些控制台命令，其名称和描述如下： |控制台变量|描述| |---|---| |**r.RDG.AsyncCompute**|控制异步计算策略：0-禁用；1-为异步计算Pass启用标记（默认）；2-开启所有使用compute命令列表的计算通道。| |**r.RDG.Breakpoint**|当满足某些条件时，断点到调试器的断点位置。0-禁用，1~4-不同的特殊调试模式。| |**r.RDG.ClobberResources**|在分配时间用指定的清理颜色清除所有渲染目标和纹理/缓冲UAV。用于调试。| |**r.RDG.CullPasses**|RDG是否开启裁剪无用的Pass。0-禁用，1-开启（默认）。| |**r.RDG.Debug**|允许输出在连接和执行过程中发现的效率低下的警告。| |**r.RDG.Debug.FlushGPU**|开启每次Pass执行后刷新指令到GPU。当设置(r.RDG.AsyncCompute=0)时禁用异步计算。| |**r.RDG.Debug.GraphFilter**|将某些调试事件过滤到特定的图中。| |**r.RDG.Debug.PassFilter**|将某些调试事件过滤到特定的Pass。| |**r.RDG.Debug.ResourceFilter**|将某些调试事件过滤到特定的资源。| |**r.RDG.DumpGraph**|将多个可视化日志转储到磁盘。0-禁用，1-显示生产者、消费者Pass依赖，2-显示资源状态和转换，3-显示图形、异步计算的重叠。| |**r.RDG.ExtendResourceLifetimes**|RDG将把资源生命周期扩展到图的全部长度。会增加内存的占用。| |**r.RDG.ImmediateMode**|在创建Pass时执行Pass。当在Pass的Lambda中崩溃时，连接代码的调用堆栈非常有用。| |**r.RDG.MergeRenderPasses**|图形将合并相同的、连续的渲染通道到一个单一的渲染通道。0-禁用，1-开启（默认）。| |**r.RDG.OverlapUAVs**|RDG将在需要时重叠UAV的工作。如果禁用，UAV屏障总是插入。| |**r.RDG.TransitionLog**|输出资源转换到控制台。| |**r.RDG.VerboseCSVStats**|控制RDG的CSV分析统计的详细程度。0-为图形执行生成一个CSV配置文件，1-为图形执行的每个阶段生成一个CSV文件。| 除了以上列出的RDG控制台，还有一些命令可以显示RDG系统运行过程中的有用信息。 `vis`列出所有有效的纹理，输入之后可能显示如下所示的信息：