365 lines
16 KiB
Markdown
365 lines
16 KiB
Markdown
---
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title: 剖析虚幻渲染体系（11）- RDG
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date: 2024-02-04 21:42:54
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excerpt:
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tags:
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rating: ⭐
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---
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# 前言
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原文地址:https://www.cnblogs.com/timlly/p/15217090.html
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# 概念
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- FRDGAllocator：简单的C++对象分配器, 用MemStack分配器追踪和销毁物体。
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- FComputeShaderUtils
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- Dispatch()：派发ComputeShader到RHI命令列表，携带其参数。
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- DispatchIndirect()：派发非直接的ComputeShader到RHI命令列表,，携带其参数。
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- AddPass()：派发计算着色器到**RenderGraphBuilder**， 携带其参数。
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- ClearUAV()：清理UAV。
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- AddCopyTexturePass()：增加拷贝纹理Pass。
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- AddCopyToResolveTargetPass()：增加拷贝到解析目标的Pass。
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- AddEnqueueCopyPass()：增加回读纹理的Pass。
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- AddPass()：无参数的Pass增加。
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- AddBeginUAVOverlapPass()/AddEndUAVOverlapPass()： 其它特殊Pass。
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- 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。
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- FRDGUniformBuffer、TRDGUniformBuffer
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- 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)
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- FRDGView
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- FRDGBuffer、FRDGBufferSRV、FRDGBufferUAV
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- 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
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- FRDGTexture
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- FRDGView
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- FRDGUnorderedAccessView、FRDGTextureUAV
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- FRDGShaderResourceView、FRDGTextureSRV
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- FRDGTextureDesc：创建渲染纹理的描述信息。
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- FRDGPooledTexture：贴图池里的贴图资源。
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- FRDGPooledBuffer：池化的缓冲区。
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- 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。
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- FRDGBarrierBatch：RDG屏障批。
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- FRDGBarrierBatchBegin
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- FRDGBarrierBatchEnd
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- FRDGPass
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- TRDGLambdaPass
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- FRDGSentinelPass：哨兵Pass，用于开始/结束。
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- [[#FRDGBuilder]]
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## RDG基础类型
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```c++
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enum class ERDGBuilderFlags
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{
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None = 0,
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/** Allows the builder to parallelize execution of passes. Without this flag, all passes execute on the render thread. */
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AllowParallelExecute = 1 << 0
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};
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/** Flags to annotate a pass with when calling AddPass. */
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enum class ERDGPassFlags : uint16
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{
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/** Pass doesn't have any inputs or outputs tracked by the graph. This may only be used by the parameterless AddPass function. */
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None = 0,
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/** Pass uses rasterization on the graphics pipe. */
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Raster = 1 << 0,
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/** Pass uses compute on the graphics pipe. */
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Compute = 1 << 1,
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/** Pass uses compute on the async compute pipe. */
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AsyncCompute = 1 << 2,
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/** Pass uses copy commands on the graphics pipe. */
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Copy = 1 << 3,
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/** Pass (and its producers) will never be culled. Necessary if outputs cannot be tracked by the graph. */
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NeverCull = 1 << 4,
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/** Render pass begin / end is skipped and left to the user. Only valid when combined with 'Raster'. Disables render pass merging for the pass. */
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SkipRenderPass = 1 << 5,
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/** Pass will never have its render pass merged with other passes. */
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NeverMerge = 1 << 6,
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/** Pass will never run off the render thread. */
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NeverParallel = 1 << 7,
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ParallelTranslate = 1 << 8,
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/** Pass uses copy commands but writes to a staging resource. */
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Readback = Copy | NeverCull
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};
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/** Flags to annotate a render graph buffer. */
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enum class ERDGBufferFlags : uint8
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{
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None = 0,
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/** Tag the buffer to survive through frame, that is important for multi GPU alternate frame rendering. */
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MultiFrame = 1 << 0,
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/** The buffer is ignored by RDG tracking and will never be transitioned. Use the flag when registering a buffer with no writable GPU flags.
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* 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
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* buffers that are used frequently throughout the graph. Note that it's the user's responsibility to ensure the resource is in the correct
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* readable state for use with RDG passes, as RDG does not know the exact state of the resource.
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*/
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SkipTracking = 1 << 1,
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/** When set, RDG will perform its first barrier without splitting. Practically, this means the resource is left in its initial state
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* until the first pass it's used within the graph. Without this flag, the resource is split-transitioned at the start of the graph.
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*/
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ForceImmediateFirstBarrier = 1 << 2,
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};
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/** Flags to annotate a render graph texture. */
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enum class ERDGTextureFlags : uint8
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{
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None = 0,
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/** Tag the texture to survive through frame, that is important for multi GPU alternate frame rendering. */
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MultiFrame = 1 << 0,
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/** The buffer is ignored by RDG tracking and will never be transitioned. Use the flag when registering a buffer with no writable GPU flags.
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* 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
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* buffers that are used frequently throughout the graph. Note that it's the user's responsibility to ensure the resource is in the correct
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* readable state for use with RDG passes, as RDG does not know the exact state of the resource.
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*/
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SkipTracking = 1 << 1,
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/** When set, RDG will perform its first barrier without splitting. Practically, this means the resource is left in its initial state
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* until the first pass it's used within the graph. Without this flag, the resource is split-transitioned at the start of the graph.
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*/
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ForceImmediateFirstBarrier = 1 << 2,
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/** Prevents metadata decompression on this texture. */
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MaintainCompression = 1 << 3,
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};
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ENUM_CLASS_FLAGS(ERDGTextureFlags);
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/** Flags to annotate a view with when calling CreateUAV. */
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enum class ERDGUnorderedAccessViewFlags : uint8
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{
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None = 0,
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// The view will not perform UAV barriers between consecutive usage.
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SkipBarrier = 1 << 0
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};
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ENUM_CLASS_FLAGS(ERDGUnorderedAccessViewFlags);
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/** The set of concrete parent resource types. */
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enum class ERDGViewableResourceType : uint8
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{
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Texture,
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Buffer,
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MAX
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};
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/** The set of concrete view types. */
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enum class ERDGViewType : uint8
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{
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TextureUAV,
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TextureSRV,
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BufferUAV,
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BufferSRV,
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MAX
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};
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inline ERDGViewableResourceType GetParentType(ERDGViewType ViewType)
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{
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switch (ViewType)
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{
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case ERDGViewType::TextureUAV:
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case ERDGViewType::TextureSRV:
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return ERDGViewableResourceType::Texture;
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case ERDGViewType::BufferUAV:
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case ERDGViewType::BufferSRV:
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return ERDGViewableResourceType::Buffer;
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}
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checkNoEntry();
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return ERDGViewableResourceType::MAX;
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}
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enum class ERDGResourceExtractionFlags : uint8
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{
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None = 0,
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// Allows the resource to remain transient. Only use this flag if you intend to register the resource back
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// into the graph and release the reference. This should not be used if the resource is cached for a long
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// period of time.
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AllowTransient = 1,
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};
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enum class ERDGInitialDataFlags : uint8
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{
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/** Specifies the default behavior, which is to make a copy of the initial data for replay when
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* the graph is executed. The user does not need to preserve lifetime of the data pointer.
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*/
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None = 0,
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/** Specifies that the user will maintain ownership of the data until the graph is executed. The
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* upload pass will only use a reference to store the data. Use caution with this flag since graph
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* execution is deferred! Useful to avoid the copy if the initial data lifetime is guaranteed to
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* outlive the graph.
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*/
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NoCopy = 1 << 0
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};
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enum class ERDGPooledBufferAlignment : uint8
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{
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// The buffer size is not aligned.
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None,
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// The buffer size is aligned up to the next page size.
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Page,
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// The buffer size is aligned up to the next power of two.
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PowerOfTwo
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};
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/** Returns the equivalent parent resource type for a view type. */
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inline ERDGViewableResourceType GetViewableResourceType(ERDGViewType ViewType)
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{
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switch (ViewType)
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{
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case ERDGViewType::TextureUAV:
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case ERDGViewType::TextureSRV:
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return ERDGViewableResourceType::Texture;
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case ERDGViewType::BufferUAV:
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case ERDGViewType::BufferSRV:
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return ERDGViewableResourceType::Buffer;
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default:
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checkNoEntry();
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return ERDGViewableResourceType::MAX;
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}
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}
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using ERDGTextureMetaDataAccess = ERHITextureMetaDataAccess;
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/** Returns the associated FRHITransitionInfo plane index. */
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inline int32 GetResourceTransitionPlaneForMetadataAccess(ERDGTextureMetaDataAccess Metadata)
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{
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switch (Metadata)
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{
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case ERDGTextureMetaDataAccess::CompressedSurface:
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case ERDGTextureMetaDataAccess::HTile:
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case ERDGTextureMetaDataAccess::Depth:
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return FRHITransitionInfo::kDepthPlaneSlice;
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case ERDGTextureMetaDataAccess::Stencil:
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return FRHITransitionInfo::kStencilPlaneSlice;
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default:
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return 0;
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}
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}
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```
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## FRDGBuilder
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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。
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重要函数：
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- FindExternalTexture()：查找外部纹理, 若找不到返回null.
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- 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。
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- RegisterExternalBuffer()：注册外部缓冲区到RDG, 以便RDG追踪之.
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- 资源创建接口：
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- CreateTexture()
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- CreateBuffer()
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- CreateUAV()
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- CreateSRV()
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- CreateUniformBuffer()
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- 分配内存, 内存由RDG管理生命周期。
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- Alloc()
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- AllocPOD()
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- AllocObject()
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- AllocParameters()
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- AddPass()
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- FRDGPassRef AddPass(FRDGEventName&& Name, const ParameterStructType* ParameterStruct, ERDGPassFlags Flags, ExecuteLambdaType&& ExecuteLambda); ：增加有参数的LambdaPass。
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- 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
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- FRDGPassRef AddPass(FRDGEventName&& Name, ERDGPassFlags Flags, ExecuteLambdaType&& ExecuteLambda);：增加没有参数的LambdaPass
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- 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。
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- QueueBufferExtraction()：在Builder执行末期, 提取缓冲区到指定的指针。
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- 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.
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- GetPooledTexture()/GetPooledBuffer()：立即获取底层资源, 只允许用于注册或预分配的资源。
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- SetTextureAccessFinal()/SetBufferAccessFinal()：设置执行之后的状态。
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- 变量
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- RDG对象注册表
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- FRDGPassRegistry Passes;
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- FRDGTextureRegistry Textures;
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- FRDGBufferRegistry Buffers;
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- FRDGViewRegistry Views;
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- FRDGUniformBufferRegistry UniformBuffers;
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### FRDGBuilder::Compile
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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。
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# RDG开发
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### 注册外部资源
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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：
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```c++
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// 在RDG外创建RHI资源.
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FRHIResourceCreateInfo CreateInfo;
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FTexture2DRHIRef MyRHITexture = RHICreateTexture2D(1024, 768, PF_B8G8R8A8, 1, 1, TexCreate_CPUReadback, CreateInfo);
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// 将外部创建的RHI资源注册成RDG资源.
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FRDGTextureRef MyExternalRDGTexture = GraphBuilder.RegisterExternalTexture(MyRHITexture);
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```
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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。
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如果想从RDG资源获取RHI资源的实例，以下代码可达成：
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```c++
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FRHITexture* MyRHITexture = MyRDGTexture.GetRHI();
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```
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用图例展示RHI资源和RDG资源之间的转换关系：
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- FRHIResource =>FRDGBuilder::RegisterExternalXXX =>FRDGResource
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- FRDGResource => FRDGResource::GetRHI => FRHIResource
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### 提取资源
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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：
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- FRDGBuilder::QueueTextureExtraction
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- FRDGBuilder::QueueBufferExtraction
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```c++
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// 创建RDG纹理.
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FRDGTextureRef MyRDGTexture;
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FRDGTextureDesc MyTextureDesc = FRDGTextureDesc::Create2D(OutputExtent, HistoryPixelFormat, FClearValueBinding::Black, TexCreate_ShaderResource | TexCreate_UAV);
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MyRDGTexture = GraphBuilder.CreateTexture(MyTextureDesc, "MyRDGTexture", ERDGTextureFlags::MultiFrame);
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// 创建UAV并作为Pass的shader参数.
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(......)
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PassParameters->MyRDGTextureUAV = GraphBuilder.CreateUAV(MyRDGTexture);
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(......)
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// 增加Pass, 以便渲染图像到MyRDGTextureUAV.
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FComputeShaderUtils::AddPass(GraphBuilder, RDG_EVENT_NAME("MyCustomPass", ...), ComputeShader, PassParameters, FComputeShaderUtils::GetGroupCount(8, 8));
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// 入队提取资源.
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TRefCountPtr<IPooledRenderTarget>* OutputRT;
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GraphBuilder.QueueTextureExtraction(MyRDGTexture, &OutputRT);
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// 对提取的OutputRT进行后续操作.
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(......)
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```
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# RDG调试
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RDG系统存在一些控制台命令，其名称和描述如下：
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|控制台变量|描述|
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|---|---|
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|**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。|
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|**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。|
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|**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。|
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|**r.RDG.CullPasses**|RDG是否开启裁剪无用的Pass。0-禁用，1-开启（默认）。|
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|**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。|
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|**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。|
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|**r.RDG.Debug.GraphFilter**|将某些调试事件过滤到特定的图中。|
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|**r.RDG.Debug.PassFilter**|将某些调试事件过滤到特定的Pass。|
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|**r.RDG.Debug.ResourceFilter**|将某些调试事件过滤到特定的资源。|
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|**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。|
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|**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。|
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|**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。|
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|**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）。|
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|**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。|
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|**r.RDG.TransitionLog**|输出资源转换到控制台。|
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|**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。|
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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。
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`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： |