BlueRoseNote/03-UnrealEngine/Rendering/RenderingPipeline/向往渲染系列文章阅读笔记/剖析虚幻渲染体系（11）- RDG.md

---
title: 剖析虚幻渲染体系（11）- RDG
date: 2024-02-04 21:42:54
excerpt: 
tags: 
rating: ⭐
---
# 前言
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