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剖析虚幻渲染体系（08）- Shader体系	2024-02-04 21:44:10			⭐

前言

原文地址:https://www.cnblogs.com/timlly/p/15092257.html

FShader

class FShader
{
public:
	// 在编译触发之前修改编译环境参数, 可由子类覆盖.
	static void ModifyCompilationEnvironment(const FShaderPermutationParameters&, FShaderCompilerEnvironment&) {}

	// 是否需要编译指定的排列, 可由子类覆盖.
	static bool ShouldCompilePermutation(const FShaderPermutationParameters&) { return true; }

	// 检测编译结果是否有效, 可由子类覆盖.
	static bool ValidateCompiledResult(EShaderPlatform InPlatform, const FShaderParameterMap& InParameterMap, TArray<FString>& OutError) { return true; }

	// 取得RayTracingPayloadType
	static ERayTracingPayloadType GetRayTracingPayloadType(const int32 PermutationId) { return static_cast<ERayTracingPayloadType>(0); }

	// 获取各类数据的Hash的接口.
	RENDERCORE_API const FSHAHash& GetHash() const;
	RENDERCORE_API const FSHAHash& GetVertexFactoryHash() const;	
	RENDERCORE_API const FSHAHash& GetOutputHash() const;

	/** Returns an identifier suitable for deterministic sorting of shaders between sessions. */
	uint32 GetSortKey() const { return SortKey; }
	// 保存并检测shader代码的编译结果.
	RENDERCORE_API void Finalize(const FShaderMapResourceCode* Code);

	// 数据获取接口.
	inline FShaderType* GetType(const FShaderMapPointerTable& InPointerTable) const { return Type.Get(InPointerTable.ShaderTypes); }
	inline FShaderType* GetType(const FPointerTableBase* InPointerTable) const { return Type.Get(InPointerTable); }
	inline FVertexFactoryType* GetVertexFactoryType(const FShaderMapPointerTable& InPointerTable) const { return VFType.Get(InPointerTable.VFTypes); }
	inline FVertexFactoryType* GetVertexFactoryType(const FPointerTableBase* InPointerTable) const { return VFType.Get(InPointerTable); }
	inline FShaderType* GetTypeUnfrozen() const { return Type.GetUnfrozen(); }
	inline int32 GetResourceIndex() const { checkSlow(ResourceIndex != INDEX_NONE); return ResourceIndex; }
	inline EShaderPlatform GetShaderPlatform() const { return Target.GetPlatform(); }
	inline EShaderFrequency GetFrequency() const { return Target.GetFrequency(); }
	inline const FShaderTarget GetTarget() const { return Target; }
	inline bool IsFrozen() const { return Type.IsFrozen(); }
	inline uint32 GetNumInstructions() const { return NumInstructions; }

#if WITH_EDITORONLY_DATA
	inline uint32 GetNumTextureSamplers() const { return NumTextureSamplers; }
	inline uint32 GetCodeSize() const { return CodeSize; }
	inline void SetNumInstructions(uint32 Value) { NumInstructions = Value; }
#else
	inline uint32 GetNumTextureSamplers() const { return 0u; }
	inline uint32 GetCodeSize() const { return 0u; }
#endif

	// 尝试返回匹配指定类型的自动绑定的Uniform Buffer, 如果不存在则返回未绑定的.
	template<typename UniformBufferStructType>
	FORCEINLINE_DEBUGGABLE const TShaderUniformBufferParameter<UniformBufferStructType>& GetUniformBufferParameter() const
	{
		const FShaderUniformBufferParameter& FoundParameter = GetUniformBufferParameter(UniformBufferStructType::FTypeInfo::GetStructMetadata());
		return static_cast<const TShaderUniformBufferParameter<UniformBufferStructType>&>(FoundParameter);
	}

	FORCEINLINE_DEBUGGABLE const FShaderUniformBufferParameter& GetUniformBufferParameter(const FShaderParametersMetadata* SearchStruct) const
	{
		const FHashedName SearchName = SearchStruct->GetShaderVariableHashedName();
		
		return GetUniformBufferParameter(SearchName);
	}

	FORCEINLINE_DEBUGGABLE const FShaderUniformBufferParameter& GetUniformBufferParameter(const FHashedName SearchName) const
	{
		int32 FoundIndex = INDEX_NONE;
		TArrayView<const FHashedName> UniformBufferParameterStructsView(UniformBufferParameterStructs);
		for (int32 StructIndex = 0, Count = UniformBufferParameterStructsView.Num(); StructIndex < Count; StructIndex++)
		{
			if (UniformBufferParameterStructsView[StructIndex] == SearchName)
			{
				FoundIndex = StructIndex;
				break;
			}
		}

		if (FoundIndex != INDEX_NONE)
		{
			const FShaderUniformBufferParameter& FoundParameter = UniformBufferParameters[FoundIndex];
			return FoundParameter;
		}
		else
		{
			// This can happen if the uniform buffer was not bound
			// There's no good way to distinguish not being bound due to temporary debugging / compiler optimizations or an actual code bug,
			// Hence failing silently instead of an error message
			static FShaderUniformBufferParameter UnboundParameter;
			return UnboundParameter;
		}
	}

	RENDERCORE_API const FShaderParametersMetadata* FindAutomaticallyBoundUniformBufferStruct(int32 BaseIndex) const;

	RENDERCORE_API void DumpDebugInfo(const FShaderMapPointerTable& InPtrTable);

#if WITH_EDITOR
	RENDERCORE_API void SaveShaderStableKeys(const FShaderMapPointerTable& InPtrTable, EShaderPlatform TargetShaderPlatform, int32 PermutationId, const struct FStableShaderKeyAndValue& SaveKeyVal);
#endif // WITH_EDITOR

	/** Returns the meta data for the root shader parameter struct. */
	static inline const FShaderParametersMetadata* GetRootParametersMetadata()
	{
		return nullptr;
	}

private:
	RENDERCORE_API void BuildParameterMapInfo(const TMap<FString, FParameterAllocation>& ParameterMap);

public:
	// 着色器参数绑定.
	LAYOUT_FIELD(FShaderParameterBindings, Bindings);
	// 着色器参数绑定的映射信息.
	LAYOUT_FIELD(FShaderParameterMapInfo, ParameterMapInfo);

protected:
	LAYOUT_FIELD(TMemoryImageArray<FHashedName>, UniformBufferParameterStructs);
	LAYOUT_FIELD(TMemoryImageArray<FShaderUniformBufferParameter>, UniformBufferParameters);

	// 下面3个是编辑器参数. 
	// 着色器的编译输出和结果参数映射的哈希值, 用于查找匹配的资源.
	LAYOUT_FIELD_EDITORONLY(FSHAHash, OutputHash);
	// 顶点工厂资源哈希值
	LAYOUT_FIELD_EDITORONLY(FSHAHash, VFSourceHash);
	// Shader资源哈希值.
	LAYOUT_FIELD_EDITORONLY(FSHAHash, SourceHash);

private:
	// 着色器类型.
	LAYOUT_FIELD(TIndexedPtr<FShaderType>, Type);
	// 顶点工厂类型.
	LAYOUT_FIELD(TIndexedPtr<FVertexFactoryType>, VFType);
	// 目标平台和着色频率(frequency).
	LAYOUT_FIELD(FShaderTarget, Target);
	// 在FShaderMapResource的shader索引.
	LAYOUT_FIELD(int32, ResourceIndex);
	// shader指令数.
	LAYOUT_FIELD(uint32, NumInstructions);
	/** Truncated version of OutputHash, intended for sorting. Not suitable for unique shader identification. */
	LAYOUT_FIELD(uint32, SortKey);
	// 纹理采样器数量.
	LAYOUT_FIELD_EDITORONLY(uint32, NumTextureSamplers);
	// shader代码尺寸.
	LAYOUT_FIELD_EDITORONLY(uint32, CodeSize);
};

以上可知，FShader存储着Shader关联的绑定参数、顶点工厂、编译后的各类资源等数据，并提供了编译器修改和检测接口，还有各类数据获取接口。

FShader实际上是个基础父类，它的子类有：

FGlobalShader：全局着色器，它的子类在内存中只有唯一的实例，常用于屏幕方块绘制、后处理等。相比父类FShader，增加了SetParameters设置视图统一缓冲的接口。FGlobalShader包含了后处理、光照、工具类、可视化、地形、虚拟纹理等方面的Shader代码，可以是VS、PS、CS，但CS必然是FGlobalShader的子类
FMaterialShader：材质着色器，由FMaterialShaderType指定的材质引用的着色器，是材质蓝图在实例化后的一个shader子集。FMaterialShader主要包含了模型、专用Pass、体素化等方面的Shader代码，可以是VS、PS、GS等，但不会有CS。

Shader Parameter

位于Engine\Source\Runtime\RenderCore\Public\ShaderParameters.h。

FShaderParameter：着色器的寄存器绑定参数, 它的类型可以是float1/2/3/4，数组等。
FShaderResourceParameter：着色器资源绑定(纹理或采样器)。
FRWShaderParameter：绑定了UAV或SRV资源的类型。
FShaderUniformBufferParameter：着色器统一缓冲参数。

Uniform Buffer

位于Engine\Source\Runtime\RHI\Public\RHIResources.h。 UE的Uniform Buffer涉及了几个核心的概念，最底层的是RHI层的FRHIUniformBuffer，封装了各种图形API的统一缓冲区（也叫Constant Buffer）。

class FRHIUniformBuffer : public FRHIResource
{
public:
    // 构造函数.
    FRHIUniformBuffer(const FRHIUniformBufferLayout& InLayout);

    // 引用计数操作.
    uint32 AddRef() const;
    uint32 Release() const;
    
    // 数据获取接口.
    uint32 GetSize() const;
    const FRHIUniformBufferLayout& GetLayout() const;
    bool IsGlobal() const;

private:
    // RHI Uniform Buffer的布局.
    const FRHIUniformBufferLayout* Layout;
    // 缓冲区尺寸.
    uint32 LayoutConstantBufferSize;
};

// 定义FRHIUniformBuffer的引用类型.
typedef TRefCountPtr<FRHIUniformBuffer> FUniformBufferRHIRef;

// Engine\Source\Runtime\RenderCore\Public\ShaderParameterMacros.h
// 引用了指定类型的FRHIUniformBuffer的实例资源. 注意是继承了FUniformBufferRHIRef.
template<typename TBufferStruct>
class TUniformBufferRef : public FUniformBufferRHIRef
{
public:
    TUniformBufferRef();

    // 根据给定的值创建Uniform Buffer, 并返回结构体引用. (模板)
    static TUniformBufferRef<TBufferStruct> CreateUniformBufferImmediate(const TBufferStruct& Value, EUniformBufferUsage Usage, EUniformBufferValidation Validation = EUniformBufferValidation::ValidateResources);
    // 根据给定的值创建[局部]的Uniform Buffer, 并返回结构体引用.
    static FLocalUniformBuffer CreateLocalUniformBuffer(FRHICommandList& RHICmdList, const TBufferStruct& Value, EUniformBufferUsage Usage);

    // 立即刷新缓冲区数据到RHI.
    void UpdateUniformBufferImmediate(const TBufferStruct& Value);

private:
    // 私有构造体, 只能给TUniformBuffer和TRDGUniformBuffer创建.
    TUniformBufferRef(FRHIUniformBuffer* InRHIRef);

    template<typename TBufferStruct2>
    friend class TUniformBuffer;

    friend class TRDGUniformBuffer<TBufferStruct>;
};

最后TUniformBuffer和TRDGUniformBuffer引用了FUniformBufferRHIRef。TUniformBuffer为TUniformBufferRef<TBufferStruct> UniformBufferRHI成员变量；TRDGUniformBuffer为TRefCountPtr<FRHIUniformBuffer> UniformBufferRHI。

定义宏

SHADER_PARAMETER_STRUCT_REF：引用着色器参数结构体(全局的才行)
SHADER_PARAMETER_STRUCT_INCLUDE：包含着色器参数结构体(局部或全局都行)

Vertex Factory

我们知道，在引擎中存在着静态网格、蒙皮骨骼、程序化网格以及地形等等类型的网格类型，而材质就是通过顶点工厂FVertexFactory来支持这些网格类型。实际上，顶点工厂要涉及各方面的数据和类型，包含但不限于：

顶点着色器。顶点着色器的输入输出需要顶点工厂来表明数据的布局。
顶点工厂的参数和RHI资源。这些数据将从C++层传入到顶点着色器中进行处理。
顶点缓冲和顶点布局。通过顶点布局，我们可以自定义和扩展顶点缓冲的输入，从而实现定制化的Shader代码。
几何预处理。顶点缓冲、网格资源、材质参数等等都可以在真正渲染前预处理它们。 !

顶点工厂在渲染层级中的关系。由图可知，顶点工厂是渲染线程的对象，横跨于CPU和GPU两端。

// Engine\Source\Runtime\RHI\Public\RHI.h
// 顶点元素.
struct FVertexElement
{
    uint8 StreamIndex;      // 流索引
    uint8 Offset;          // 偏移
    TEnumAsByte<EVertexElementType> Type; // 类型
    uint8 AttributeIndex;// 属性索引
    uint16 Stride;          // 步长
    // 实例索引或顶点索引是否实例化的, 若是0, 则元素会对每个实例进行重复.
    uint16 bUseInstanceIndex;

    FVertexElement();
    FVertexElement(uint8 InStreamIndex, ...);
    
    void operator=(const FVertexElement& Other);
    friend FArchive& operator<<(FArchive& Ar,FVertexElement& Element);
    
    FString ToString() const;
    void FromString(const FString& Src);
    void FromString(const FStringView& Src);
};

// 顶点声明元素列表的类型.
typedef TArray<FVertexElement,TFixedAllocator<MaxVertexElementCount> > FVertexDeclarationElementList;

// Engine\Source\Runtime\RHI\Public\RHIResources.h
// 顶点声明的RHI资源
class FRHIVertexDeclaration : public FRHIResource
{
public:
    virtual bool GetInitializer(FVertexDeclarationElementList& Init) { return false; }
};

// 顶点缓冲区
class FRHIVertexBuffer : public FRHIResource
{
public:
    FRHIVertexBuffer(uint32 InSize,uint32 InUsage);

    uint32 GetSize() const;
    uint32 GetUsage() const;

protected:
    FRHIVertexBuffer();

    void Swap(FRHIVertexBuffer& Other);
    void ReleaseUnderlyingResource();

private:
    // 尺寸.
    uint32 Size;
    // 缓冲区标记, 如BUF_UnorderedAccess
    uint32 Usage;
};

// Engine\Source\Runtime\RenderCore\Public\VertexFactory.h
// 顶点输入流.
struct FVertexInputStream
{
    // 顶点流索引
    uint32 StreamIndex : 4;
    // 在VertexBuffer的偏移.
    uint32 Offset : 28;
    // 顶点缓存区
    FRHIVertexBuffer* VertexBuffer;

    FVertexInputStream();
    FVertexInputStream(uint32 InStreamIndex, uint32 InOffset, FRHIVertexBuffer* InVertexBuffer);

    inline bool operator==(const FVertexInputStream& rhs) const;
    inline bool operator!=(const FVertexInputStream& rhs) const;
};

// 顶点输入流数组.
typedef TArray<FVertexInputStream, TInlineAllocator<4>> FVertexInputStreamArray;

// 顶点流标记
enum class EVertexStreamUsage : uint8
{
    Default            = 0 << 0, // 默认
    Instancing        = 1 << 0, // 实例化
    Overridden        = 1 << 1, // 覆盖
    ManualFetch        = 1 << 2  // 手动获取
};

// 顶点输入流类型.
enum class EVertexInputStreamType : uint8
{
    Default = 0,  // 默认
    PositionOnly, // 只有位置
    PositionAndNormalOnly // 只有位置和法线
};

// 顶点流组件.
struct FVertexStreamComponent
{
    // 流数据的顶点缓冲区, 如果为null, 则不会有数据从此顶点流被读取.
    const FVertexBuffer* VertexBuffer = nullptr;

    // vertex buffer的偏移.
    uint32 StreamOffset = 0;
    // 数据的偏移, 相对于顶点缓冲区中每个元素的开头.
    uint8 Offset = 0;
    // 数据的步长.
    uint8 Stride = 0;
    // 从流读取的数据类型.
    TEnumAsByte<EVertexElementType> Type = VET_None;
    // 顶点流标记.
    EVertexStreamUsage VertexStreamUsage = EVertexStreamUsage::Default;

    (......)
};

// 着色器使用的顶点工厂的参数绑定接口.
class FVertexFactoryShaderParameters
{
public:
    // 绑定参数到ParameterMap. 具体逻辑由子类完成.
    void Bind(const class FShaderParameterMap& ParameterMap) {}

    // 获取顶点工厂的着色器绑定和顶点流. 具体逻辑由子类完成.
    void GetElementShaderBindings(
        const class FSceneInterface* Scene,
        const class FSceneView* View,
        const class FMeshMaterialShader* Shader,
        const EVertexInputStreamType InputStreamType,
        ERHIFeatureLevel::Type FeatureLevel,
        const class FVertexFactory* VertexFactory,
        const struct FMeshBatchElement& BatchElement,
        class FMeshDrawSingleShaderBindings& ShaderBindings,
        FVertexInputStreamArray& VertexStreams) const {}

    (......)
};

// 用来表示顶点工厂类型的类.
class FVertexFactoryType
{
public:
    // 类型定义
    typedef FVertexFactoryShaderParameters* (*ConstructParametersType)(EShaderFrequency ShaderFrequency, const class FShaderParameterMap& ParameterMap);
    typedef const FTypeLayoutDesc* (*GetParameterTypeLayoutType)(EShaderFrequency ShaderFrequency);
    (......)

    // 获取顶点工厂类型数量.
    static int32 GetNumVertexFactoryTypes();

    // 获取全局的着色器工厂列表.
    static RENDERCORE_API TLinkedList<FVertexFactoryType*>*& GetTypeList();
    // 获取已存的材质类型列表.
    static RENDERCORE_API const TArray<FVertexFactoryType*>& GetSortedMaterialTypes();
    // 通过名字查找FVertexFactoryType
    static RENDERCORE_API FVertexFactoryType* GetVFByName(const FHashedName& VFName);

    // 初始化FVertexFactoryType静态成员, 必须在VF类型创建之前调用.
    static void Initialize(const TMap<FString, TArray<const TCHAR*> >& ShaderFileToUniformBufferVariables);
    static void Uninitialize();

    // 构造/析构函数.
    RENDERCORE_API FVertexFactoryType(...);
    virtual ~FVertexFactoryType();

    // 数据获取接口.
    const TCHAR* GetName() const;
    FName GetFName() const;
    const FHashedName& GetHashedName() const;
    const TCHAR* GetShaderFilename() const;

    // 着色器参数接口.
    FVertexFactoryShaderParameters* CreateShaderParameters(...) const;
    const FTypeLayoutDesc* GetShaderParameterLayout(...) const;
    void GetShaderParameterElementShaderBindings(...) const;

    // 标记访问.
    bool IsUsedWithMaterials() const;
    bool SupportsStaticLighting() const;
    bool SupportsDynamicLighting() const;
    bool SupportsPrecisePrevWorldPos() const;
    bool SupportsPositionOnly() const;
    bool SupportsCachingMeshDrawCommands() const;
    bool SupportsPrimitiveIdStream() const;

    // 获取哈希.
    friend uint32 GetTypeHash(const FVertexFactoryType* Type);
    // 基于顶点工厂类型的源码和包含计算出来的哈希.
    const FSHAHash& GetSourceHash(EShaderPlatform ShaderPlatform) const;
    // 是否需要缓存材质的着色器类型.
    bool ShouldCache(const FVertexFactoryShaderPermutationParameters& Parameters) const;

    void ModifyCompilationEnvironment(...);
    void ValidateCompiledResult(EShaderPlatform Platform, ...);

    bool SupportsTessellationShaders() const;

    // 增加引用的Uniform Buffer包含.
    void AddReferencedUniformBufferIncludes(...);
    void FlushShaderFileCache(...);
    const TMap<const TCHAR*, FCachedUniformBufferDeclaration>& GetReferencedUniformBufferStructsCache() const;

private:
    static uint32 NumVertexFactories;
    static bool bInitializedSerializationHistory;

    // 顶点工厂类型的各类数据和标记.
    const TCHAR* Name;
    const TCHAR* ShaderFilename;
    FName TypeName;
    FHashedName HashedName;
    uint32 bUsedWithMaterials : 1;
    uint32 bSupportsStaticLighting : 1;
    uint32 bSupportsDynamicLighting : 1;
    uint32 bSupportsPrecisePrevWorldPos : 1;
    uint32 bSupportsPositionOnly : 1;
    uint32 bSupportsCachingMeshDrawCommands : 1;
    uint32 bSupportsPrimitiveIdStream : 1;
    ConstructParametersType ConstructParameters;
    GetParameterTypeLayoutType GetParameterTypeLayout;
    GetParameterTypeElementShaderBindingsType GetParameterTypeElementShaderBindings;
    ShouldCacheType ShouldCacheRef;
    ModifyCompilationEnvironmentType ModifyCompilationEnvironmentRef;
    ValidateCompiledResultType ValidateCompiledResultRef;
    SupportsTessellationShadersType SupportsTessellationShadersRef;

    // 全局顶点工厂类型列表.
    TLinkedList<FVertexFactoryType*> GlobalListLink;
    // 缓存引用的Uniform Buffer的包含.
    TMap<const TCHAR*, FCachedUniformBufferDeclaration> ReferencedUniformBufferStructsCache;
    // 跟踪ReferencedUniformBufferStructsCache缓存了哪些平台的声明.
    bool bCachedUniformBufferStructDeclarations;
};

// ------顶点工厂的工具宏------
// 实现顶点工厂参数类型
#define IMPLEMENT_VERTEX_FACTORY_PARAMETER_TYPE(FactoryClass, ShaderFrequency, ParameterClass)
// 顶点工厂类型的声明
#define DECLARE_VERTEX_FACTORY_TYPE(FactoryClass)
// 顶点工厂类型的实现
#define IMPLEMENT_VERTEX_FACTORY_TYPE(FactoryClass,ShaderFilename,bUsedWithMaterials,bSupportsStaticLighting,bSupportsDynamicLighting,bPrecisePrevWorldPos,bSupportsPositionOnly)
// 顶点工厂的虚函数表实现
#define IMPLEMENT_VERTEX_FACTORY_VTABLE(FactoryClass

// 顶点工厂
class FVertexFactory : public FRenderResource
{
public:
    FVertexFactory(ERHIFeatureLevel::Type InFeatureLevel);

    virtual FVertexFactoryType* GetType() const;

    // 获取顶点数据流.
    void GetStreams(ERHIFeatureLevel::Type InFeatureLevel, EVertexInputStreamType VertexStreamType, FVertexInputStreamArray& OutVertexStreams) const
    {
        // Default顶点流类型
        if (VertexStreamType == EVertexInputStreamType::Default)
        {
            bool bSupportsVertexFetch = SupportsManualVertexFetch(InFeatureLevel);

            // 将顶点工厂的数据构造到FVertexInputStream中并添加到输出列表
            for (int32 StreamIndex = 0;StreamIndex < Streams.Num();StreamIndex++)
            {
                const FVertexStream& Stream = Streams[StreamIndex];

                if (!(EnumHasAnyFlags(EVertexStreamUsage::ManualFetch, Stream.VertexStreamUsage) && bSupportsVertexFetch))
                {
                    if (!Stream.VertexBuffer)
                    {
                        OutVertexStreams.Add(FVertexInputStream(StreamIndex, 0, nullptr));
                    }
                    else
                    {
                        if (EnumHasAnyFlags(EVertexStreamUsage::Overridden, Stream.VertexStreamUsage) && !Stream.VertexBuffer->IsInitialized())
                        {
                            OutVertexStreams.Add(FVertexInputStream(StreamIndex, 0, nullptr));
                        }
                        else
                        {
                            OutVertexStreams.Add(FVertexInputStream(StreamIndex, Stream.Offset, Stream.VertexBuffer->VertexBufferRHI));
                        }
                    }
                }
            }
        }
        // 只有位置和的顶点流类型
        else if (VertexStreamType == EVertexInputStreamType::PositionOnly)
        {
            // Set the predefined vertex streams.
            for (int32 StreamIndex = 0; StreamIndex < PositionStream.Num(); StreamIndex++)
            {
                const FVertexStream& Stream = PositionStream[StreamIndex];
                OutVertexStreams.Add(FVertexInputStream(StreamIndex, Stream.Offset, Stream.VertexBuffer->VertexBufferRHI));
            }
        }
        // 只有位置和法线的顶点流类型
        else if (VertexStreamType == EVertexInputStreamType::PositionAndNormalOnly)
        {
            // Set the predefined vertex streams.
            for (int32 StreamIndex = 0; StreamIndex < PositionAndNormalStream.Num(); StreamIndex++)
            {
                const FVertexStream& Stream = PositionAndNormalStream[StreamIndex];
                OutVertexStreams.Add(FVertexInputStream(StreamIndex, Stream.Offset, Stream.VertexBuffer->VertexBufferRHI));
            }
        }
        else
        {
            // NOT_IMPLEMENTED
        }
    }
    
    // 偏移实例的数据流.
    void OffsetInstanceStreams(uint32 InstanceOffset, EVertexInputStreamType VertexStreamType, FVertexInputStreamArray& VertexStreams) const;
    
    static void ModifyCompilationEnvironment(...);
    static void ValidateCompiledResult(...);

    static bool SupportsTessellationShaders();

    // FRenderResource接口, 释放RHI资源.
    virtual void ReleaseRHI();

    // 设置/获取顶点声明的RHI引用.
    FVertexDeclarationRHIRef& GetDeclaration();
    void SetDeclaration(FVertexDeclarationRHIRef& NewDeclaration);

    // 根据类型获取顶点声明的RHI引用.
    const FVertexDeclarationRHIRef& GetDeclaration(EVertexInputStreamType InputStreamType) const 
    {
        switch (InputStreamType)
        {
        case EVertexInputStreamType::Default:                return Declaration;
        case EVertexInputStreamType::PositionOnly:            return PositionDeclaration;
        case EVertexInputStreamType::PositionAndNormalOnly:    return PositionAndNormalDeclaration;
        }
        return Declaration;
    }

    // 各类标记.
    virtual bool IsGPUSkinned() const;
    virtual bool SupportsPositionOnlyStream() const;
    virtual bool SupportsPositionAndNormalOnlyStream() const;
    virtual bool SupportsNullPixelShader() const;

    // 用面向摄像机精灵的方式渲染图元.
    virtual bool RendersPrimitivesAsCameraFacingSprites() const;

    // 是否需要顶点声明.
    bool NeedsDeclaration() const;
    // 是否支持手动的顶点获取.
    inline bool SupportsManualVertexFetch(const FStaticFeatureLevel InFeatureLevel) const;
    // 根据流类型获取索引.
    inline int32 GetPrimitiveIdStreamIndex(EVertexInputStreamType InputStreamType) const;

protected:
    inline void SetPrimitiveIdStreamIndex(EVertexInputStreamType InputStreamType, int32 StreamIndex)
    {
        PrimitiveIdStreamIndex[static_cast<uint8>(InputStreamType)] = StreamIndex;
    }

    // 为顶点流组件创建顶点元素.
    FVertexElement AccessStreamComponent(const FVertexStreamComponent& Component,uint8 AttributeIndex);
    FVertexElement AccessStreamComponent(const FVertexStreamComponent& Component, uint8 AttributeIndex, EVertexInputStreamType InputStreamType);
    // 初始化顶点声明.
    void InitDeclaration(const FVertexDeclarationElementList& Elements, EVertexInputStreamType StreamType = EVertexInputStreamType::Default)
    {
        if (StreamType == EVertexInputStreamType::PositionOnly)
        {
            PositionDeclaration = PipelineStateCache::GetOrCreateVertexDeclaration(Elements);
        }
        else if (StreamType == EVertexInputStreamType::PositionAndNormalOnly)
        {
            PositionAndNormalDeclaration = PipelineStateCache::GetOrCreateVertexDeclaration(Elements);
        }
        else // (StreamType == EVertexInputStreamType::Default)
        {
            // Create the vertex declaration for rendering the factory normally.
            Declaration = PipelineStateCache::GetOrCreateVertexDeclaration(Elements);
        }
    }

    // 顶点流, 需要设置到顶点流的信息体.
    struct FVertexStream
    {
        const FVertexBuffer* VertexBuffer = nullptr;
        uint32 Offset = 0;
        uint16 Stride = 0;
        EVertexStreamUsage VertexStreamUsage = EVertexStreamUsage::Default;
        uint8 Padding = 0;

        friend bool operator==(const FVertexStream& A,const FVertexStream& B);
        FVertexStream();
    };

    // 用于渲染顶点工厂的顶点流.
    TArray<FVertexStream,TInlineAllocator<8> > Streams;

    // VF(顶点工厂)可以显式地将此设置为false，以避免在没有声明的情况下出现错误. 主要用于需要直接从缓冲区获取数据的VF(如Niagara).
    bool bNeedsDeclaration = true;
    bool bSupportsManualVertexFetch = false;
    int8 PrimitiveIdStreamIndex[3] = { -1, -1, -1 };

private:
    // 只有位置的顶点流, 用于渲染深度Pass的顶点工厂.
    TArray<FVertexStream,TInlineAllocator<2> > PositionStream;
    // 只有位置和法线的顶点流.
    TArray<FVertexStream, TInlineAllocator<3> > PositionAndNormalStream;

    // 用于常规渲染顶点工厂的RHI顶点声明.
    FVertexDeclarationRHIRef Declaration;

    // PositionStream和PositionAndNormalStream对应的RHI资源.
    FVertexDeclarationRHIRef PositionDeclaration;
    FVertexDeclarationRHIRef PositionAndNormalDeclaration;
};

上面展示了Vertex Factory的很多类型，有好几个是核心类，比如FVertexFactory、FVertexElement、FRHIVertexDeclaration、FRHIVertexBuffer、FVertexFactoryType、FVertexStreamComponent、FVertexInputStream、FVertexFactoryShaderParameters等。那么它们之间的关系是什么呢？

为了更好地说明它们之间的关系，以静态模型的FStaticMeshDataType为例： ! FStaticMeshDataType会包含若干个FVertexStreamComponent实例，每个FVertexStreamComponent包含了一个在FVertexDeclarationElementList的FVertexElement实例索引和一个在FVertexInputStreamArray列表的FVertexStream实例索引。此外，FVertexFactory是个基类，内置的子类主要有：

FGeometryCacheVertexVertexFactory：几何缓存顶点的顶点工厂，常用于预生成的布料、动作等网格类型。
FGPUBaseSkinVertexFactory：GPU蒙皮骨骼网格的父类，它的子类有：
- TGPUSkinVertexFactory：可指定骨骼权重方式的GPU蒙皮的顶点工厂。
FLocalVertexFactory：局部顶点工厂，常用于静态网格，它拥有数量较多的子类：
- FInstancedStaticMeshVertexFactory：实例化的静态网格顶点工厂。
- FSplineMeshVertexFactory：样条曲线网格顶点工厂。
- FGeometryCollectionVertexFactory：几何收集顶点工厂。
- FGPUSkinPassthroughVertexFactory：启用了Skin Cache模式的蒙皮骨骼顶点工厂。
- FSingleTriangleMeshVertexFactory：单个三角形网格的顶点工厂，用于体积云渲染。
- ......
FParticleVertexFactoryBase：用于粒子渲染的顶点工厂基类。
FLandscapeVertexFactory：用于渲染地形的顶点工厂。

除了以上继承自FVertexFactory，还有一些不是继承自FVertexFactory的类型，如：

FGPUBaseSkinAPEXClothVertexFactory：布料顶点工厂。
- TGPUSkinAPEXClothVertexFactory：可带骨骼权重模式的布料顶点工厂。

除了FVertexFactory，相应的其它核心类也有继承体系。比如FVertexFactoryShaderParameters的子类有：

FGeometryCacheVertexFactoryShaderParameters
FGPUSkinVertexFactoryShaderParameters
FMeshParticleVertexFactoryShaderParameters
FParticleSpriteVertexFactoryShaderParameters
FGPUSpriteVertexFactoryShaderParametersVS
FGPUSpriteVertexFactoryShaderParametersPS
FSplineMeshVertexFactoryShaderParameters
FLocalVertexFactoryShaderParametersBase
FLandscapeVertexFactoryVertexShaderParameters
FLandscapeVertexFactoryPixelShaderParameters
......

另外，有部分顶点工厂还会在内部派生FStaticMeshDataType的类型，以复用静态网格相关的数据成员。为了更好地说明顶点工厂的使用方式，下面就以最常见的FLocalVertexFactory和使用了FLocalVertexFactory的CableComponent为例：


class ENGINE_API FLocalVertexFactory : public FVertexFactory
{
public:
    FLocalVertexFactory(ERHIFeatureLevel::Type InFeatureLevel, const char* InDebugName)；

    // 派生自FStaticMeshDataType的数据类型.
    struct FDataType : public FStaticMeshDataType
    {
        FRHIShaderResourceView* PreSkinPositionComponentSRV = nullptr;
    };

    // 环境变量更改和校验.
    static bool ShouldCompilePermutation(const FVertexFactoryShaderPermutationParameters& Parameters);
    static void ModifyCompilationEnvironment(const FVertexFactoryShaderPermutationParameters& Parameters, FShaderCompilerEnvironment& OutEnvironment);
    static void ValidateCompiledResult(const FVertexFactoryType* Type, EShaderPlatform Platform, const FShaderParameterMap& ParameterMap, TArray<FString>& OutErrors);

    // 由TSynchronizedResource从游戏线程更新而来的数据.
    void SetData(const FDataType& InData);
    // 从其它顶点工厂复制数据.
    void Copy(const FLocalVertexFactory& Other);

    // FRenderResource接口.
    virtual void InitRHI() override;
    virtual void ReleaseRHI() override
    {
        UniformBuffer.SafeRelease();
        FVertexFactory::ReleaseRHI();
    }

    // 顶点颜色接口.
    void SetColorOverrideStream(FRHICommandList& RHICmdList, const FVertexBuffer* ColorVertexBuffer) const;
    void GetColorOverrideStream(const FVertexBuffer* ColorVertexBuffer, FVertexInputStreamArray& VertexStreams) const;
    
    // 着色器参数和其它数据接口.
    inline FRHIShaderResourceView* GetPositionsSRV() const;
    inline FRHIShaderResourceView* GetPreSkinPositionSRV() const;
    inline FRHIShaderResourceView* GetTangentsSRV() const;
    inline FRHIShaderResourceView* GetTextureCoordinatesSRV() const;
    inline FRHIShaderResourceView* GetColorComponentsSRV() const;
    inline const uint32 GetColorIndexMask() const;
    inline const int GetLightMapCoordinateIndex() const;
    inline const int GetNumTexcoords() const;
    FRHIUniformBuffer* GetUniformBuffer() const;
    
    (......)

protected:
    // 从游戏线程传入的数据. FDataType是FStaticMeshDataType的子类.
    FDataType Data;
    // 局部顶点工厂的着色器参数.
    TUniformBufferRef<FLocalVertexFactoryUniformShaderParameters> UniformBuffer;
    // 顶点颜色流索引.
    int32 ColorStreamIndex;

    (......)
};

// Engine\Source\Runtime\Engine\Public\LocalVertexFactory.cpp
void FLocalVertexFactory::InitRHI()
{
    // 是否使用gpu场景.
    const bool bCanUseGPUScene = UseGPUScene(GMaxRHIShaderPlatform, GMaxRHIFeatureLevel);

    // 初始化位置流和位置声明.
    if (Data.PositionComponent.VertexBuffer != Data.TangentBasisComponents[0].VertexBuffer)
    {
        // 增加顶点声明.
        auto AddDeclaration = [this, bCanUseGPUScene](EVertexInputStreamType InputStreamType, bool bAddNormal)
        {
            // 顶点流元素.
            FVertexDeclarationElementList StreamElements;
            StreamElements.Add(AccessStreamComponent(Data.PositionComponent, 0, InputStreamType));

            bAddNormal = bAddNormal && Data.TangentBasisComponents[1].VertexBuffer != NULL;
            if (bAddNormal)
            {
                StreamElements.Add(AccessStreamComponent(Data.TangentBasisComponents[1], 2, InputStreamType));
            }

            const uint8 TypeIndex = static_cast<uint8>(InputStreamType);
            PrimitiveIdStreamIndex[TypeIndex] = -1;
            if (GetType()->SupportsPrimitiveIdStream() && bCanUseGPUScene)
            {
                // When the VF is used for rendering in normal mesh passes, this vertex buffer and offset will be overridden
                StreamElements.Add(AccessStreamComponent(FVertexStreamComponent(&GPrimitiveIdDummy, 0, 0, sizeof(uint32), VET_UInt, EVertexStreamUsage::Instancing), 1, InputStreamType));
                PrimitiveIdStreamIndex[TypeIndex] = StreamElements.Last().StreamIndex;
            }

            // 初始化声明.
            InitDeclaration(StreamElements, InputStreamType);
        };

        // 增加PositionOnly和PositionAndNormalOnly两种顶点声明, 其中前者不需要法线.
        AddDeclaration(EVertexInputStreamType::PositionOnly, false);
        AddDeclaration(EVertexInputStreamType::PositionAndNormalOnly, true);
    }

    // 顶点声明元素列表.
    FVertexDeclarationElementList Elements;
    
    // 顶点位置
    if(Data.PositionComponent.VertexBuffer != NULL)
    {
        Elements.Add(AccessStreamComponent(Data.PositionComponent,0));
    }

    // 图元id
    {
        const uint8 Index = static_cast<uint8>(EVertexInputStreamType::Default);
        PrimitiveIdStreamIndex[Index] = -1;
        if (GetType()->SupportsPrimitiveIdStream() && bCanUseGPUScene)
        {
            // When the VF is used for rendering in normal mesh passes, this vertex buffer and offset will be overridden
            Elements.Add(AccessStreamComponent(FVertexStreamComponent(&GPrimitiveIdDummy, 0, 0, sizeof(uint32), VET_UInt, EVertexStreamUsage::Instancing), 13));
            PrimitiveIdStreamIndex[Index] = Elements.Last().StreamIndex;
        }
    }

    // 切线和法线, 切线法线才需要被顶点流使用, 副法线由shader生成.
    uint8 TangentBasisAttributes[2] = { 1, 2 };
    for(int32 AxisIndex = 0;AxisIndex < 2;AxisIndex++)
    {
        if(Data.TangentBasisComponents[AxisIndex].VertexBuffer != NULL)
        {
            Elements.Add(AccessStreamComponent(Data.TangentBasisComponents[AxisIndex],TangentBasisAttributes[AxisIndex]));
        }
    }

    if (Data.ColorComponentsSRV == nullptr)
    {
        Data.ColorComponentsSRV = GNullColorVertexBuffer.VertexBufferSRV;
        Data.ColorIndexMask = 0;
    }

    // 顶点颜色
    ColorStreamIndex = -1;
    if(Data.ColorComponent.VertexBuffer)
    {
        Elements.Add(AccessStreamComponent(Data.ColorComponent,3));
        ColorStreamIndex = Elements.Last().StreamIndex;
    }
    else
    {
        FVertexStreamComponent NullColorComponent(&GNullColorVertexBuffer, 0, 0, VET_Color, EVertexStreamUsage::ManualFetch);
        Elements.Add(AccessStreamComponent(NullColorComponent, 3));
        ColorStreamIndex = Elements.Last().StreamIndex;
    }

    // 纹理坐标
    if(Data.TextureCoordinates.Num())
    {
        const int32 BaseTexCoordAttribute = 4;
        for(int32 CoordinateIndex = 0;CoordinateIndex < Data.TextureCoordinates.Num();CoordinateIndex++)
        {
            Elements.Add(AccessStreamComponent(
                Data.TextureCoordinates[CoordinateIndex],
                BaseTexCoordAttribute + CoordinateIndex
                ));
        }

        for (int32 CoordinateIndex = Data.TextureCoordinates.Num(); CoordinateIndex < MAX_STATIC_TEXCOORDS / 2; CoordinateIndex++)
        {
            Elements.Add(AccessStreamComponent(
                Data.TextureCoordinates[Data.TextureCoordinates.Num() - 1],
                BaseTexCoordAttribute + CoordinateIndex
                ));
        }
    }

    // 光照图
    if(Data.LightMapCoordinateComponent.VertexBuffer)
    {
        Elements.Add(AccessStreamComponent(Data.LightMapCoordinateComponent,15));
    }
    else if(Data.TextureCoordinates.Num())
    {
        Elements.Add(AccessStreamComponent(Data.TextureCoordinates[0],15));
    }

    // 初始化顶点声明
    InitDeclaration(Elements);

    const int32 DefaultBaseVertexIndex = 0;
    const int32 DefaultPreSkinBaseVertexIndex = 0;
    if (RHISupportsManualVertexFetch(GMaxRHIShaderPlatform) || bCanUseGPUScene)
    {
        SCOPED_LOADTIMER(FLocalVertexFactory_InitRHI_CreateLocalVFUniformBuffer);
        UniformBuffer = CreateLocalVFUniformBuffer(this, Data.LODLightmapDataIndex, nullptr, DefaultBaseVertexIndex, DefaultPreSkinBaseVertexIndex);
    }
}

// 实现FLocalVertexFactory的参数类型.
IMPLEMENT_VERTEX_FACTORY_PARAMETER_TYPE(FLocalVertexFactory, SF_Vertex, FLocalVertexFactoryShaderParameters);

// 实现FLocalVertexFactory.
IMPLEMENT_VERTEX_FACTORY_TYPE_EX(FLocalVertexFactory,"/Engine/Private/LocalVertexFactory.ush",true,true,true,true,true,true,true);

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前言

FShader

Shader Parameter

Uniform Buffer

定义宏

Vertex Factory

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