---
title: 剖析虚幻渲染体系（08）- Shader体系
date: 2024-02-04 21:44:10
excerpt: 
tags: 
rating: ⭐
---
# 前言
原文地址:https://www.cnblogs.com/timlly/p/15092257.html

# FShader
```c++
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）。
```c++
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`。

![[UE_Uniform.png]]

### 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。
![[UE_VertexFactory.png]]

**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。**

```c++
// 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：
![[UE_VertexFactory_FStaticMeshDataType.jpg]]
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：
```c++

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);
```
下面进入CableComponent相关类型关于FLocalVertexFactory的使用：
```c++
// Engine\Plugins\Runtime\CableComponent\Source\CableComponent\Private\CableComponent.cpp

class FCableSceneProxy final : public FPrimitiveSceneProxy
{
public:
    FCableSceneProxy(UCableComponent* Component)
        : FPrimitiveSceneProxy(Component)
        , Material(NULL)
        // 构造顶点工厂.
        , VertexFactory(GetScene().GetFeatureLevel(), "FCableSceneProxy")
        (......)
    {
        // 利用顶点工厂初始化缓冲区.
        VertexBuffers.InitWithDummyData(&VertexFactory, GetRequiredVertexCount());
        (......)
    }

    virtual ~FCableSceneProxy()
    {
        // 释放顶点工厂.
        VertexFactory.ReleaseResource();
        (......)
    }

    // 构建Cable网格.
    void BuildCableMesh(const TArray<FVector>& InPoints, TArray<FDynamicMeshVertex>& OutVertices, TArray<int32>& OutIndices)
    {
        (......)
    }

    // 设置动态数据(渲染线程调用)
    void SetDynamicData_RenderThread(FCableDynamicData* NewDynamicData)
    {
        // 释放旧数据.
        if(DynamicData)
        {
            delete DynamicData;
            DynamicData = NULL;
        }
        DynamicData = NewDynamicData;

        // 从Cable点构建顶点.
        TArray<FDynamicMeshVertex> Vertices;
        TArray<int32> Indices;
        BuildCableMesh(NewDynamicData->CablePoints, Vertices, Indices);

        // 填充顶点缓冲区数据.
        for (int i = 0; i < Vertices.Num(); i++)
        {
            const FDynamicMeshVertex& Vertex = Vertices[i];

            VertexBuffers.PositionVertexBuffer.VertexPosition(i) = Vertex.Position;
            VertexBuffers.StaticMeshVertexBuffer.SetVertexTangents(i, Vertex.TangentX.ToFVector(), Vertex.GetTangentY(), Vertex.TangentZ.ToFVector());
            VertexBuffers.StaticMeshVertexBuffer.SetVertexUV(i, 0, Vertex.TextureCoordinate[0]);
            VertexBuffers.ColorVertexBuffer.VertexColor(i) = Vertex.Color;
        }

        // 更新顶点缓冲区数据到RHI.
        {
            auto& VertexBuffer = VertexBuffers.PositionVertexBuffer;
            void* VertexBufferData = RHILockVertexBuffer(VertexBuffer.VertexBufferRHI, 0, VertexBuffer.GetNumVertices() * VertexBuffer.GetStride(), RLM_WriteOnly);
            FMemory::Memcpy(VertexBufferData, VertexBuffer.GetVertexData(), VertexBuffer.GetNumVertices() * VertexBuffer.GetStride());
            RHIUnlockVertexBuffer(VertexBuffer.VertexBufferRHI);
        }

        (......)
    }

    virtual void GetDynamicMeshElements(const TArray<const FSceneView*>& Views, const FSceneViewFamily& ViewFamily, uint32 VisibilityMap, FMeshElementCollector& Collector) const override
    {
        (......)

        for (int32 ViewIndex = 0; ViewIndex < Views.Num(); ViewIndex++)
        {
            if (VisibilityMap & (1 << ViewIndex))
            {
                const FSceneView* View = Views[ViewIndex];
                
                // 构造FMeshBatch实例.
                FMeshBatch& Mesh = Collector.AllocateMesh();
                // 将顶点工厂实例传给FMeshBatch实例.
                Mesh.VertexFactory = &VertexFactory;
                
                (......)
                
                Collector.AddMesh(ViewIndex, Mesh);
            }
        }
    }

    (......)

private:
    // 材质
    UMaterialInterface* Material;
    // 顶点和索引缓冲.
    FStaticMeshVertexBuffers VertexBuffers;
    FCableIndexBuffer IndexBuffer;
    // 顶点工厂.
    FLocalVertexFactory VertexFactory;
    // 动态数据.
    FCableDynamicData* DynamicData;

    (......)
};
```

主要步骤：
1. 在构造函数中初始化LocalVertexFactory成员变量。
2. SetDynamicData_RenderThread()
	1. 通过控制点构建CableMesh顶点数据。
	2. 填充顶点缓冲区数据。FStaticMeshVertexBuffers VertexBuffers
		1. PositionVertexBuffer.VertexPosition
		2. StaticMeshVertexBuffer.SetVertexTangents
		3. StaticMeshVertexBuffer.SetVertexUV
		4. ColorVertexBuffer.VertexColor
	3. 更新IndexBufferData到RHI。
3. GetDynamicMeshElements()：将顶点工厂实例传给FMeshBatch实例
	1. FMeshBatch& Mesh = Collector.AllocateMesh();
	2. Mesh.VertexFactory = &VertexFactory;
	3. Collector.AddMesh(ViewIndex, Mesh);

不过，无论是使用已有的还是自定义的顶点工厂，顶点工厂的顶点声明的顺序、类型、组件数量和插槽需要和HLSL层的FVertexFactoryInput保持一致。比如说FLocalVertexFactory::InitRHI的顶点声明顺序是位置、切线、颜色、纹理坐标、光照图，那么我们进入FLocalVertexFactory对应的HLSL文件（由IMPLEMENT_VERTEX_FACTORY_TYPE等宏指定）看看：
```c++
// Engine\Shaders\Private\LocalVertexFactory.ush
// 局部顶点工厂对应的输入结构体.
struct FVertexFactoryInput
{
    // 位置
    float4    Position    : ATTRIBUTE0;

    // 切线和颜色
#if !MANUAL_VERTEX_FETCH
    #if METAL_PROFILE
        float3    TangentX    : ATTRIBUTE1;
        // TangentZ.w contains sign of tangent basis determinant
        float4    TangentZ    : ATTRIBUTE2;

        float4    Color        : ATTRIBUTE3;
    #else
        half3    TangentX    : ATTRIBUTE1;
        // TangentZ.w contains sign of tangent basis determinant
        half4    TangentZ    : ATTRIBUTE2;

        half4    Color        : ATTRIBUTE3;
    #endif
#endif

    // 纹理坐标
#if NUM_MATERIAL_TEXCOORDS_VERTEX
    #if !MANUAL_VERTEX_FETCH
        #if GPUSKIN_PASS_THROUGH
            // These must match GPUSkinVertexFactory.usf
            float2    TexCoords[NUM_MATERIAL_TEXCOORDS_VERTEX] : ATTRIBUTE4;
            #if NUM_MATERIAL_TEXCOORDS_VERTEX > 4
                #error Too many texture coordinate sets defined on GPUSkin vertex input. Max: 4.
            #endif
        #else
            #if NUM_MATERIAL_TEXCOORDS_VERTEX > 1
                float4    PackedTexCoords4[NUM_MATERIAL_TEXCOORDS_VERTEX/2] : ATTRIBUTE4;
            #endif
            #if NUM_MATERIAL_TEXCOORDS_VERTEX == 1
                float2    PackedTexCoords2 : ATTRIBUTE4;
            #elif NUM_MATERIAL_TEXCOORDS_VERTEX == 3
                float2    PackedTexCoords2 : ATTRIBUTE5;
            #elif NUM_MATERIAL_TEXCOORDS_VERTEX == 5
                float2    PackedTexCoords2 : ATTRIBUTE6;
            #elif NUM_MATERIAL_TEXCOORDS_VERTEX == 7
                float2    PackedTexCoords2 : ATTRIBUTE7;
            #endif
        #endif
    #endif
#elif USE_PARTICLE_SUBUVS && !MANUAL_VERTEX_FETCH
    float2    TexCoords[1] : ATTRIBUTE4;
#endif
    (......)
};
```
因此可知，FVertexFactoryInput结构体的数据顺序和FLocalVertexFactory的顶点声明是一一对应的。

# ShaderMap
ShaderMap的作用是**存储编译后的shader代码**，分为FGlobalShaderMap、FMaterialShaderMap、FMeshMaterialShaderMap三种类型。

- FGlobalShaderMap：FGlobalShaderMap保存并管理着所有编译好的FGlobalShader代码。（**没有材质和顶点工程的**）
- FMaterialShaderMap：存储和管理着一组FMaterialShader实例的对象。（**额外关联一个材质和一个顶点工厂**）
	- 因此可以找到，每个FMaterial都有一个FMaterialShaderMap（游戏线程一个，渲染线程一个），如果要获取FMaterial的指定类型的Shader，就需要从该FMaterial的FMaterialShaderMap实例中获取，从而完成了它们之间的链接。
- FMeshMaterialShaderMap：存储和管理FMeshMaterialShader。

### 编译相关代码
如果需要了解编译过程可以查看`RecompileShaders`命令。
```c++
// Engine\Source\Runtime\Launch\Private\LaunchEngineLoop.cpp
// 引擎预初始化.
int32 FEngineLoop::PreInitPreStartupScreen(const TCHAR* CmdLine)
{
    (......)
    
    // 是否开启shader编译, 一般情况下都会开启.
    bool bEnableShaderCompile = !FParse::Param(FCommandLine::Get(), TEXT("NoShaderCompile"));
    
    (......)
    
    if (bEnableShaderCompile && !IsRunningDedicatedServer() && !bIsCook)
    {
        (......)
        
        // 编译GlobalShaderMap
        CompileGlobalShaderMap(false);
        
        (......)
    }
    
    (......)
}

// Engine\Source\Runtime\Engine\Private\ShaderCompiler\ShaderCompiler.cpp
void CompileGlobalShaderMap(EShaderPlatform Platform, const ITargetPlatform* TargetPlatform, bool bRefreshShaderMap)
{
    (......)

    // 如果对应平台的GlobalShaderMap未创建, 则创建之.
    if (!GGlobalShaderMap[Platform])
    {
        (......)

        // 创建对应平台的FGlobalShaderMap.
        GGlobalShaderMap[Platform] = new FGlobalShaderMap(Platform);

        // Cooked模式.
        if (FPlatformProperties::RequiresCookedData())
        {
            (......)
        }
        // Uncooked模式
        else
        {
            // FGlobalShaderMap的id.
            FGlobalShaderMapId ShaderMapId(Platform);

            const int32 ShaderFilenameNum = ShaderMapId.GetShaderFilenameToDependeciesMap().Num();
            const float ProgressStep = 25.0f / ShaderFilenameNum;

            TArray<uint32> AsyncDDCRequestHandles;
            AsyncDDCRequestHandles.SetNum(ShaderFilenameNum);

            int32 HandleIndex = 0;

            // 提交DDC请求.
            for (const auto& ShaderFilenameDependencies : ShaderMapId.GetShaderFilenameToDependeciesMap())
            {
                SlowTask.EnterProgressFrame(ProgressStep);

                const FString DataKey = GetGlobalShaderMapKeyString(ShaderMapId, Platform, TargetPlatform, ShaderFilenameDependencies.Value);

                AsyncDDCRequestHandles[HandleIndex] = GetDerivedDataCacheRef().GetAsynchronous(*DataKey, TEXT("GlobalShaderMap"_SV));

                ++HandleIndex;
            }

            // 处理已经结束的DDC请求.
            TArray<uint8> CachedData;
            HandleIndex = 0;
            for (const auto& ShaderFilenameDependencies : ShaderMapId.GetShaderFilenameToDependeciesMap())
            {
                SlowTask.EnterProgressFrame(ProgressStep);
                CachedData.Reset();
                
                GetDerivedDataCacheRef().WaitAsynchronousCompletion(AsyncDDCRequestHandles[HandleIndex]);
                if (GetDerivedDataCacheRef().GetAsynchronousResults(AsyncDDCRequestHandles[HandleIndex], CachedData))
                {
                    FMemoryReader MemoryReader(CachedData);
                    GGlobalShaderMap[Platform]->AddSection(FGlobalShaderMapSection::CreateFromArchive(MemoryReader));
                }
                else
                {
                    // 没有在DDC中找到, 忽略之.
                }

                ++HandleIndex;
            }
        }

        // 如果有shader没有被加载, 编译之.
        VerifyGlobalShaders(Platform, bLoadedFromCacheFile);

        // 创建所有着色器.
        if (GCreateShadersOnLoad && Platform == GMaxRHIShaderPlatform)
        {
            GGlobalShaderMap[Platform]->BeginCreateAllShaders();
        }
    }
}
```

# Shader调试
修改`Engine\Config\ConsoleVariables.ini`配置
- r.ShaderDevelopmentMode=1 获得关于着色器编译的详细日志和错误重试的机会。
- r.DumpShaderDebugInfo=1 将编译的所有着色器的文件保存到磁盘ProjectName/Saved/ShaderDebugInfo的目录。包含源文件、预处理后的版本、一个批处理文件（用于使用编译器等效的命令行选项来编译预处理版本）。
- r.DumpShaderDebugShortNames=1 保存的Shader路径将被精简。
- r.Shaders.Optimize=0 禁用着色器优化，使得shader的调试信息被保留。
- r.Shaders.KeepDebugInfo=1 保留调试信息，配合RenderDoc等截帧工具时特别有用。
- r.Shaders.SkipCompression=1 忽略shader压缩，可以节省调试shader的时间。

另外，如果修改了Shader的某些文件（如BasePassPixelShader.ush），不需要重启UE编辑器，可以在控制台输入`RecompileShaders`命令重新编译指定的shader文件。其中`RecompileShaders`的具体含义如下：
- RecompileShaders all 编译源码有修改的所有shader，包含global、material、meshmaterial。
- RecompileShaders changed 编译源码有修改的shader。
- RecompileShaders global 编译源码有修改的global shader。
- RecompileShaders material 编译源码有修改的material shader。
- RecompileShaders material 编译指定名称的材质。
- RecompileShaders 编译指定路径的shader源文件。

# 案例
新增加FVertexFactory子类的过程如下：
```c++
// FMyVertexFactory.h

// 声明顶点工厂着色器参数.
BEGIN_GLOBAL_SHADER_PARAMETER_STRUCT(FMyVertexFactoryParameters, )
    SHADER_PARAMETER(FVector4, Color)
END_GLOBAL_SHADER_PARAMETER_STRUCT()

// 声明类型.
typedef TUniformBufferRef<FMyVertexFactoryParameters> FMyVertexFactoryBufferRef;

// 索引缓冲.
class FMyMeshIndexBuffer : public FIndexBuffer
{
public:
    FMyMeshIndexBuffer(int32 InNumQuadsPerSide) : NumQuadsPerSide(InNumQuadsPerSide) {}

    void InitRHI() override
    {
        if (NumQuadsPerSide < 256)
        {
            IndexBufferRHI = CreateIndexBuffer<uint16>();
        }
        else
        {
            IndexBufferRHI = CreateIndexBuffer<uint32>();
        }
    }

    int32 GetIndexCount() const { return NumIndices; };

private:
    template <typename IndexType>
    FIndexBufferRHIRef CreateIndexBuffer()
    {
        TResourceArray<IndexType, INDEXBUFFER_ALIGNMENT> Indices;

        // 分配顶点索引内存.
        Indices.Reserve(NumQuadsPerSide * NumQuadsPerSide * 6);

        // 用Morton顺序构建索引缓冲, 以更好地重用顶点.
        for (int32 Morton = 0; Morton < NumQuadsPerSide * NumQuadsPerSide; Morton++)
        {
            int32 SquareX = FMath::ReverseMortonCode2(Morton);
            int32 SquareY = FMath::ReverseMortonCode2(Morton >> 1);

            bool ForwardDiagonal = false;

            if (SquareX % 2)
            {
                ForwardDiagonal = !ForwardDiagonal;
            }
            if (SquareY % 2)
            {
                ForwardDiagonal = !ForwardDiagonal;
            }

            int32 Index0 = SquareX + SquareY * (NumQuadsPerSide + 1);
            int32 Index1 = Index0 + 1;
            int32 Index2 = Index0 + (NumQuadsPerSide + 1);
            int32 Index3 = Index2 + 1;

            Indices.Add(Index3);
            Indices.Add(Index1);
            Indices.Add(ForwardDiagonal ? Index2 : Index0);
            Indices.Add(Index0);
            Indices.Add(Index2);
            Indices.Add(ForwardDiagonal ? Index1 : Index3);
        }

        NumIndices = Indices.Num();
        const uint32 Size = Indices.GetResourceDataSize();
        const uint32 Stride = sizeof(IndexType);

        // Create index buffer. Fill buffer with initial data upon creation
        FRHIResourceCreateInfo CreateInfo(&Indices);
        return RHICreateIndexBuffer(Stride, Size, BUF_Static, CreateInfo);
    }

    int32 NumIndices = 0;
    const int32 NumQuadsPerSide = 0;
};

// 顶点索引.
class FMyMeshVertexBuffer : public FVertexBuffer
{
public:
    FMyMeshVertexBuffer(int32 InNumQuadsPerSide) : NumQuadsPerSide(InNumQuadsPerSide) {}

    virtual void InitRHI() override
    {
        const uint32 NumVertsPerSide = NumQuadsPerSide + 1;
        
        NumVerts = NumVertsPerSide * NumVertsPerSide;

        FRHIResourceCreateInfo CreateInfo;
        void* BufferData = nullptr;
        VertexBufferRHI = RHICreateAndLockVertexBuffer(sizeof(FVector4) * NumVerts, BUF_Static, CreateInfo, BufferData);
        FVector4* DummyContents = (FVector4*)BufferData;

        for (uint32 VertY = 0; VertY < NumVertsPerSide; VertY++)
        {
            FVector4 VertPos;
            VertPos.Y = (float)VertY / NumQuadsPerSide - 0.5f;

            for (uint32 VertX = 0; VertX < NumVertsPerSide; VertX++)
            {
                VertPos.X = (float)VertX / NumQuadsPerSide - 0.5f;

                DummyContents[NumVertsPerSide * VertY + VertX] = VertPos;
            }
        }

        RHIUnlockVertexBuffer(VertexBufferRHI);
    }

    int32 GetVertexCount() const { return NumVerts; }

private:
    int32 NumVerts = 0;
    const int32 NumQuadsPerSide = 0;
};

// 顶点工厂.
class FMyVertexFactory : public FVertexFactory
{
    DECLARE_VERTEX_FACTORY_TYPE(FMyVertexFactory);

public:
    using Super = FVertexFactory;

    FMyVertexFactory(ERHIFeatureLevel::Type InFeatureLevel);
    ~FMyVertexFactory();

    virtual void InitRHI() override;
    virtual void ReleaseRHI() override;

    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);

    inline const FUniformBufferRHIRef GetMyVertexFactoryUniformBuffer() const { return UniformBuffer; }

private:
    void SetupUniformData();

    FMyMeshVertexBuffer* VertexBuffer = nullptr;
    FMyMeshIndexBuffer* IndexBuffer = nullptr;

    FMyVertexFactoryBufferRef UniformBuffer;
};


// FMyVertexFactory.cpp
#include "ShaderParameterUtils.h"

// 实现FMyVertexFactoryParameters, 注意在shader的名字是MyVF.
IMPLEMENT_GLOBAL_SHADER_PARAMETER_STRUCT(FMyVertexFactoryParameters, "MyVF");

// 顶点工厂着色器参数.
class FMyVertexFactoryShaderParameters : public FVertexFactoryShaderParameters
{
    DECLARE_TYPE_LAYOUT(FMyVertexFactoryShaderParameters, NonVirtual);

public:
    
    void Bind(const FShaderParameterMap& ParameterMap)
    {
    }

    void GetElementShaderBindings(
        const class FSceneInterface* Scene,
        const class FSceneView* View,
        const class FMeshMaterialShader* Shader,
        const EVertexInputStreamType InputStreamType,
        ERHIFeatureLevel::Type FeatureLevel,
        const class FVertexFactory* InVertexFactory,
        const struct FMeshBatchElement& BatchElement,
        class FMeshDrawSingleShaderBindings& ShaderBindings,
        FVertexInputStreamArray& VertexStreams) const
    {
        // 强制转换成FMyVertexFactory.
        FMyVertexFactory* VertexFactory = (FMyVertexFactory*)InVertexFactory;

        // 增加shader帮定到表格.
        ShaderBindings.Add(Shader->GetUniformBufferParameter<FMyVertexFactoryShaderParameters>(), VertexFactory->GetMyVertexFactoryUniformBuffer());

        // 填充顶点流.
        if (VertexStreams.Num() > 0)
        {
            // 处理顶点流索引.
            for (int32 i = 0; i < 2; ++i)
            {
                FVertexInputStream* InstanceInputStream = VertexStreams.FindByPredicate([i](const FVertexInputStream& InStream) { return InStream.StreamIndex == i+1; });
                // 绑定顶点流索引.
                InstanceInputStream->VertexBuffer = InstanceDataBuffers->GetBuffer(i);
            }

            // 处理偏移.
            if (InstanceOffsetValue > 0)
            {
                VertexFactory->OffsetInstanceStreams(InstanceOffsetValue, InputStreamType, VertexStreams);
            }
        }
    }
};

// ----------- 实现顶点工厂 -----------
FMyVertexFactory::FMyVertexFactory(ERHIFeatureLevel::Type InFeatureLevel)
{
    VertexBuffer = new FMyMeshVertexBuffer(16);
    IndexBuffer = new FMyMeshIndexBuffer(16);
}

FMyVertexFactory::~FMyVertexFactory()
{
    delete VertexBuffer;
    delete IndexBuffer;
}

void FMyVertexFactory::InitRHI()
{
    Super::InitRHI();

    // 设置Uniform数据.
    SetupUniformData();

    VertexBuffer->InitResource();
    IndexBuffer->InitResource();

    // 顶点流: 位置
    FVertexStream PositionVertexStream;
    PositionVertexStream.VertexBuffer = VertexBuffer;
    PositionVertexStream.Stride = sizeof(FVector4);
    PositionVertexStream.Offset = 0;
    PositionVertexStream.VertexStreamUsage = EVertexStreamUsage::Default;

    // 简单的实例化顶点流数据 其中VertexBuffer在绑定时设置.
    FVertexStream InstanceDataVertexStream;
    InstanceDataVertexStream.VertexBuffer = nullptr;
    InstanceDataVertexStream.Stride = sizeof(FVector4);
    InstanceDataVertexStream.Offset = 0;
    InstanceDataVertexStream.VertexStreamUsage = EVertexStreamUsage::Instancing;

    FVertexElement VertexPositionElement(Streams.Add(PositionVertexStream), 0, VET_Float4, 0, PositionVertexStream.Stride, false);

    // 顶点声明.
    FVertexDeclarationElementList Elements;
    Elements.Add(VertexPositionElement);

    // 添加索引顶点流.
    for (int32 StreamIdx = 0; StreamIdx < NumAdditionalVertexStreams; ++StreamIdx)
    {
        FVertexElement InstanceElement(Streams.Add(InstanceDataVertexStream), 0, VET_Float4, 8 + StreamIdx, InstanceDataVertexStream.Stride, true);
        Elements.Add(InstanceElement);
    }

    // 初始化声明.
    InitDeclaration(Elements);
}

void FMyVertexFactory::ReleaseRHI()
{
    UniformBuffer.SafeRelease();
    
    if (VertexBuffer)
    {
        VertexBuffer->ReleaseResource();
    }

    if (IndexBuffer)
    {
        IndexBuffer->ReleaseResource();
    }

    Super::ReleaseRHI();
}

void FMyVertexFactory::SetupUniformData()
{
    FMyVertexFactoryParameters UniformParams;
    UniformParams.Color = FVector4(1,0,0,1);

    UniformBuffer = FMyVertexFactoryBufferRef::CreateUniformBufferImmediate(UniformParams, UniformBuffer_MultiFrame);
}

void FMyVertexFactory::ShouldCompilePermutation(const FVertexFactoryShaderPermutationParameters& Parameters)
{
    return true;
}

void FMyVertexFactory::ModifyCompilationEnvironment(const FVertexFactoryShaderPermutationParameters& Parameters, FShaderCompilerEnvironment& OutEnvironment)
{
    OutEnvironment.SetDefine(TEXT("MY_MESH_FACTORY"), 1);
}

void FMyVertexFactory::ValidateCompiledResult(const FVertexFactoryType* Type, EShaderPlatform Platform, const FShaderParameterMap& ParameterMap, TArray<FString>& OutErrors)
{
}
```
C++层的逻辑已经完成，但HLSL层也需要编写对应的代码：
```c++
#include "/Engine/Private/VertexFactoryCommon.ush"

// VS插值到PS的结构体。
struct FVertexFactoryInterpolantsVSToPS
{
#if NUM_TEX_COORD_INTERPOLATORS
    float4    TexCoords[(NUM_TEX_COORD_INTERPOLATORS+1)/2] : TEXCOORD0;
#endif

#if VF_USE_PRIMITIVE_SCENE_DATA
    nointerpolation uint PrimitiveId : PRIMITIVE_ID;
#endif

#if INSTANCED_STEREO
    nointerpolation uint EyeIndex : PACKED_EYE_INDEX;
#endif
};

struct FVertexFactoryInput
{
    float4    Position    : ATTRIBUTE0;

    float4 InstanceData0 : ATTRIBUTE8;
    float4 InstanceData1 : ATTRIBUTE9; 

#if VF_USE_PRIMITIVE_SCENE_DATA
    uint PrimitiveId : ATTRIBUTE13;
#endif
};

struct FPositionOnlyVertexFactoryInput
{
    float4    Position    : ATTRIBUTE0;

    float4 InstanceData0 : ATTRIBUTE8;
    float4 InstanceData1 : ATTRIBUTE9; 

#if VF_USE_PRIMITIVE_SCENE_DATA
    uint PrimitiveId : ATTRIBUTE1;
#endif
};

struct FPositionAndNormalOnlyVertexFactoryInput
{
    float4    Position    : ATTRIBUTE0;
    float4    Normal        : ATTRIBUTE2;

    float4 InstanceData0 : ATTRIBUTE8;
    float4 InstanceData1 : ATTRIBUTE9; 

#if VF_USE_PRIMITIVE_SCENE_DATA
    uint PrimitiveId : ATTRIBUTE1;
#endif
};

struct FVertexFactoryIntermediates
{
    float3 OriginalWorldPos;
    
    uint PrimitiveId;
};

uint GetPrimitiveId(FVertexFactoryInterpolantsVSToPS Interpolants)
{
#if VF_USE_PRIMITIVE_SCENE_DATA
    return Interpolants.PrimitiveId;
#else
    return 0;
#endif
}

void SetPrimitiveId(inout FVertexFactoryInterpolantsVSToPS Interpolants, uint PrimitiveId)
{
#if VF_USE_PRIMITIVE_SCENE_DATA
    Interpolants.PrimitiveId = PrimitiveId;
#endif
}

#if NUM_TEX_COORD_INTERPOLATORS
float2 GetUV(FVertexFactoryInterpolantsVSToPS Interpolants, int UVIndex)
{
    float4 UVVector = Interpolants.TexCoords[UVIndex / 2];
    return UVIndex % 2 ? UVVector.zw : UVVector.xy;
}

void SetUV(inout FVertexFactoryInterpolantsVSToPS Interpolants, int UVIndex, float2 InValue)
{
    FLATTEN
    if (UVIndex % 2)
    {
        Interpolants.TexCoords[UVIndex / 2].zw = InValue;
    }
    else
    {
        Interpolants.TexCoords[UVIndex / 2].xy = InValue;
    }
}
#endif

FMaterialPixelParameters GetMaterialPixelParameters(FVertexFactoryInterpolantsVSToPS Interpolants, float4 SvPosition)
{
    // GetMaterialPixelParameters is responsible for fully initializing the result
    FMaterialPixelParameters Result = MakeInitializedMaterialPixelParameters();

#if NUM_TEX_COORD_INTERPOLATORS
    UNROLL
    for (int CoordinateIndex = 0; CoordinateIndex < NUM_TEX_COORD_INTERPOLATORS; CoordinateIndex++)
    {
        Result.TexCoords[CoordinateIndex] = GetUV(Interpolants, CoordinateIndex);
    }
#endif    //NUM_MATERIAL_TEXCOORDS

    Result.TwoSidedSign = 1;
    Result.PrimitiveId = GetPrimitiveId(Interpolants);

    return Result;
}

FMaterialVertexParameters GetMaterialVertexParameters(FVertexFactoryInput Input, FVertexFactoryIntermediates Intermediates, float3 WorldPosition, half3x3 TangentToLocal)
{
    FMaterialVertexParameters Result = (FMaterialVertexParameters)0;
    
    Result.WorldPosition = WorldPosition;
    Result.TangentToWorld = float3x3(1,0,0,0,1,0,0,0,1);
    Result.PreSkinnedPosition = Input.Position.xyz;
    Result.PreSkinnedNormal = float3(0,0,1);

#if NUM_MATERIAL_TEXCOORDS_VERTEX
    UNROLL
    for(int CoordinateIndex = 0; CoordinateIndex < NUM_MATERIAL_TEXCOORDS_VERTEX; CoordinateIndex++)
    {
        Result.TexCoords[CoordinateIndex] = Intermediates.MorphedWorldPosRaw.xy;
    }
#endif  //NUM_MATERIAL_TEXCOORDS_VERTEX

    return Result;
}

FVertexFactoryIntermediates GetVertexFactoryIntermediates(FVertexFactoryInput Input)
{
    FVertexFactoryIntermediates Intermediates;

    // Get the packed instance data
    float4 Data0 = Input.InstanceData0;
    float4 Data1 = Input.InstanceData1;

    const float3 Translation = Data0.xyz;
    const float3 Scale = float3(Data1.zw, 1.0f);
    const uint PackedDataChannel = asuint(Data1.x);

    // Lod level is in first 8 bits and ShouldMorph bit is in the 9th bit
    const float LODLevel = (float)(PackedDataChannel & 0xFF);
    const uint ShouldMorph = ((PackedDataChannel >> 8) & 0x1); 

    // Calculate the world pos
    Intermediates.OriginalWorldPos = float3(Input.Position.xy, 0.0f) * Scale + Translation;

#if VF_USE_PRIMITIVE_SCENE_DATA
    Intermediates.PrimitiveId = Input.PrimitiveId;
#else
    Intermediates.PrimitiveId = 0;
#endif

    return Intermediates;
}

half3x3 VertexFactoryGetTangentToLocal(FVertexFactoryInput Input, FVertexFactoryIntermediates Intermediates)
{
    return half3x3(1,0,0,0,1,0,0,0,1);
}

float4 VertexFactoryGetRasterizedWorldPosition(FVertexFactoryInput Input, FVertexFactoryIntermediates Intermediates, float4 InWorldPosition)
{
    return InWorldPosition;
}

float3 VertexFactoryGetPositionForVertexLighting(FVertexFactoryInput Input, FVertexFactoryIntermediates Intermediates, float3 TranslatedWorldPosition)
{
    return TranslatedWorldPosition;
}

FVertexFactoryInterpolantsVSToPS VertexFactoryGetInterpolantsVSToPS(FVertexFactoryInput Input, FVertexFactoryIntermediates Intermediates, FMaterialVertexParameters VertexParameters)
{
    FVertexFactoryInterpolantsVSToPS Interpolants;

    Interpolants = (FVertexFactoryInterpolantsVSToPS)0;

#if NUM_TEX_COORD_INTERPOLATORS
    float2 CustomizedUVs[NUM_TEX_COORD_INTERPOLATORS];
    GetMaterialCustomizedUVs(VertexParameters, CustomizedUVs);
    GetCustomInterpolators(VertexParameters, CustomizedUVs);
    
    UNROLL
    for (int CoordinateIndex = 0; CoordinateIndex < NUM_TEX_COORD_INTERPOLATORS; CoordinateIndex++)
    {
        SetUV(Interpolants, CoordinateIndex, CustomizedUVs[CoordinateIndex]);
    }
#endif

#if INSTANCED_STEREO
    Interpolants.EyeIndex = 0;
#endif

    SetPrimitiveId(Interpolants, Intermediates.PrimitiveId);

    return Interpolants;
}

float4 VertexFactoryGetWorldPosition(FPositionOnlyVertexFactoryInput Input)
{
    return Input.Position;
}

float4 VertexFactoryGetPreviousWorldPosition(FVertexFactoryInput Input, FVertexFactoryIntermediates Intermediates)
{
    float4x4 PreviousLocalToWorldTranslated = GetPrimitiveData(Intermediates.PrimitiveId).PreviousLocalToWorld;
    PreviousLocalToWorldTranslated[3][0] += ResolvedView.PrevPreViewTranslation.x;
    PreviousLocalToWorldTranslated[3][1] += ResolvedView.PrevPreViewTranslation.y;
    PreviousLocalToWorldTranslated[3][2] += ResolvedView.PrevPreViewTranslation.z;

    return mul(Input.Position, PreviousLocalToWorldTranslated);
}

float4 VertexFactoryGetTranslatedPrimitiveVolumeBounds(FVertexFactoryInterpolantsVSToPS Interpolants)
{
    float4 ObjectWorldPositionAndRadius = GetPrimitiveData(GetPrimitiveId(Interpolants)).ObjectWorldPositionAndRadius;
    return float4(ObjectWorldPositionAndRadius.xyz + ResolvedView.PreViewTranslation.xyz, ObjectWorldPositionAndRadius.w);
}

uint VertexFactoryGetPrimitiveId(FVertexFactoryInterpolantsVSToPS Interpolants)
{
    return GetPrimitiveId(Interpolants);
}

float3 VertexFactoryGetWorldNormal(FPositionAndNormalOnlyVertexFactoryInput Input)
{
    return Input.Normal.xyz;
}

float3 VertexFactoryGetWorldNormal(FVertexFactoryInput Input, FVertexFactoryIntermediates Intermediates)
{
    return float3(0.0f, 0.0f, 1.0f);
}
```

由此可见，如果新增加了FVertexFactory的自定义类型，需要在HLSL实现以下接口：
- FVertexFactoryInput 定义输入到VS的数据布局，需要匹配c++侧的FVertexFactory的类型。
- FVertexFactoryIntermediates 用于存储将在多个顶点工厂函数中使用的缓存中间数据，比如TangentToLocal。
- FVertexFactoryInterpolantsVSToPS 从VS传递到PS的顶点工厂数据。
- VertexFactoryGetWorldPosition 从顶点着色器调用来获得世界空间的顶点位置。
- VertexFactoryGetInterpolantsVSToPS 转换FVertexFactoryInput到FVertexFactoryInterpolants，在硬件光栅化插值之前计算需要插值或传递到PS的数据。
- GetMaterialPixelParameters 由PS调用，根据FVertexFactoryInterpolants计算并填充FMaterialPixelParameters结构体。