---
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;
};
```