【ue4】【架构】渲染系统

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渲染系统

UE4 的渲染器是在其自身的渲染线程中执行的

这个渲染线程通常落后于游戏线程1到2帧

所以有一些类会将游戏线程上的状态信息连接到渲染线程中来 (通常以F开头)

FScene -- 表示游戏场景, 是 UWorld 的渲染器版本

FPrimitiveSceneProxy -- UPrimitiveComponent 的渲染器版本, 为渲染线程映射 UPrimitiveComponent 状态

FPrimitiveSceneInfo -- FPrimitiveSceneProxy 的状态, 只存在于渲染器

FViewInfo -- 当前屏幕视图的渲染器 对应游戏模块中的 FSceneView

FSceneViewState -- FViewInfo 的状态, 与游戏中 UPlayer 一一对应

FLightSceneProxy 与 FLightSceneInfo -- 跟 Primitive差不多

FPrimitiveSceneProxy 存在于引擎模块中, 有两个 API 比较重要

DrawDynamicElements -- 在与之相关的任何 Pass 中绘制该拥有动态相关性的代理物体

DrawStaticElements -- 在与游戏线程相连时绘制该拥有静态相关性的代理物体

这也说明 UE4 对几何体的渲染是分了 Static 和 Dynamic 分别来处理的

  • Static Render
    • 在 FPrimitiveSceneProxy 被放入场景时, 会调用其 DrawStaticElements 来收集 FStaticMeshElements, 然后创建对应的 Draw Policy 放到 FScene 的 Draw List 里去。
  • Dynamic Render
    • 在判定 FPrimitiveSceneProxy 可见后即调用 DrawDynamicElements() 来收集 FMeshElements

【Tips】 Proxy 的意思为渲染模块在游戏模块中出现的代理, 实际存在于引擎模块中, 而非渲染模块中

材质在渲染时也需要连接游戏线程和渲染线程的接口

FMaterial -- 材质的接口, 提供属性的查询等 -- 纯虚类

FMaterialResoruce -- FMaterial 的一个具体实现, 对应 UMaterial

FMaterialRenderProxy -- 提供给渲染线程使用的代理

渲染流程

=============================================

【Pass_0】 Depth Only Pass 【Pass_1】 Base Pass 【Pass_2】 Issue Occlusion Queries 【Pass_3】 ShadowMap 【Pass_4】 Lighting 【Pass_5】 Fog 【Pass_6】 Post Processing

============================================= 【Tips】 基于 Engine\Source\Runtime\Renderer\Private\DeferredShadingRenderer.h

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class FDeferredShadingSceneRenderer : public FSceneRenderer
{
public:

	// Pass_0
	bool RenderPrePassViewDynamic(FRHICommandList& RHICmdList, const FViewInfo& View, const FDrawingPolicyRenderState& DrawRenderState);
	bool RenderPrePassView(FRHICommandList& RHICmdList, const FViewInfo& View, const FDrawingPolicyRenderState& DrawRenderState);
	
	// Pass_1
	bool RenderBasePass(FRHICommandListImmediate& RHICmdList, FExclusiveDepthStencil::Type BasePassDepthStencilAccess, IPooledRenderTarget* ForwardScreenSpaceShadowMask);
	bool RenderBasePassView(FRHICommandListImmediate& RHICmdList, FViewInfo& View, FExclusiveDepthStencil::Type BasePassDepthStencilAccess, const FDrawingPolicyRenderState& InDrawRenderState);

	// Pass_2
	void RenderOcclusion(FRHICommandListImmediate& RHICmdList);
	void FinishOcclusion(FRHICommandListImmediate& RHICmdList)

	// Pass_3
	void RenderShadowDepthMaps(FRHICommandListImmediate& RHICmdList);
	void RenderShadowDepthMapAtlases(FRHICommandListImmediate& RHICmdList);

	// Pass_4
	virtual void Render(FRHICommandListImmediate& RHICmdList) override;
	void RenderLights(FRHICommandListImmediate& RHICmdList);

	// Pass_5
	virtual void Render(FRHICommandListImmediate& RHICmdList) override;
	bool RenderFog(FRHICommandListImmediate& RHICmdList, const FLightShaftsOutput& LightShaftsOutput);
	
	// Pass_6
	virtual void Render(FRHICommandListImmediate& RHICmdList) override;


}

【Pass_0】 Depth Only Pass

深度处理阶段

绘制 Depth 到 Depth_Buffer

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/**
* Renders the scene's prepass for a particular view
* @return true if anything was rendered
*/
bool RenderPrePassView(FRHICommandList& RHICmdList, const FViewInfo& View, const FDrawingPolicyRenderState& DrawRenderState)
{
	// ...
	{ bDirty |= RenderPrePassViewDynamic(); }
	return bDirty;
}

/**
* Renders the scene's prepass for a particular view
* @return true if anything was rendered
*/
bool RenderPrePassViewDynamic(FRHICommandList& RHICmdList, const FViewInfo& View, const FDrawingPolicyRenderState& DrawRenderState) 
{
	foreach (mesh)
	{
		// 判断可见性代码
		// ...
		if (可见) { FDepthDrawingPolicyFactory::DrawDynamicMesh(); }
	}
	return true;
}

【Tips】 DrawingPolicyFactory 为绘制规则的工厂类

【Pass_1】 Base Pass

绘制 不透明的 和 Masked Material属性的 Mesh

填充 G_Buffer

UE4 中的 G_Buffer 结构 (来知乎专栏_虚幻4渲染编程)

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// Engine\Source\Runtime\Renderer\Private\BasePassRendering.cpp

/** 
* Renders the scene's base pass 
* @return true if anything was rendered
*/
bool RenderBasePass(FRHICommandListImmediate& RHICmdList, FExclusiveDepthStencil::Type BasePassDepthStencilAccess, IPooledRenderTarget* ForwardScreenSpaceShadowMask)
{
	// ...
	bDirty |= RenderBasePassView(RHICmdList, View, BasePassDepthStencilAccess, DrawRenderState);
	return bDirty;	
}

/** Renders the basepass for a given View. */
bool RenderBasePassView(FRHICommandListImmediate& RHICmdList, FViewInfo& View, FExclusiveDepthStencil::Type BasePassDepthStencilAccess, const FDrawingPolicyRenderState& InDrawRenderState)
{
	bDirty |= RenderBasePassStaticData(RHICmdList, View, DrawRenderState);
	RenderBasePassDynamicData(RHICmdList, View, DrawRenderState, bDirty);
	return bDirty;
}

此时 Shader文件可从下面代码

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// Engine\Source\Runtime\Renderer\Private\BasePassRendering.cpp 113

#define IMPLEMENT_BASEPASS_PIXELSHADER_TYPE(LightMapPolicyType,LightMapPolicyName,bEnableSkyLight,SkyLightName) \
	typedef TBasePassPS<LightMapPolicyType, bEnableSkyLight> TBasePassPS##LightMapPolicyName##SkyLightName; \
	IMPLEMENT_MATERIAL_SHADER_TYPE(template<>,TBasePassPS##LightMapPolicyName##SkyLightName, \
	TEXT(" /Engine/Private/BasePassPixelShader.usf"), \
	TEXT("MainPS"),SF_Pixel);

即 /Engine/Private/BasePassPixelShader.usf 696

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// is called in MainPS() from PixelShaderOutputCommon.usf
void FPixelShaderInOut_MainPS(
	FVertexFactoryInterpolantsVSToPS Interpolants,
	FBasePassInterpolantsVSToPS BasePassInterpolants,
	in FPixelShaderIn In, 
	inout FPixelShaderOut Out)
{
	// ...
	// Fill G_Buffer
	// ...
}

这就是 Bass Pass 的 着色器入口了

【Pass_2】 Issue Occlusion Queries

遮挡查询, 为下一帧的可见性判断提供信息

ue4 的遮挡查询是对包围盒进行深度测试来判断的

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void RenderOcclusion(FRHICommandListImmediate& RHICmdList) {}
void FinishOcclusion(FRHICommandListImmediate& RHICmdList) {}

【Pass_3】 ShadowMap

阴影计算

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void RenderShadowDepthMaps(FRHICommandListImmediate& RHICmdList) {}
void RenderShadowDepthMapAtlases(FRHICommandListImmediate& RHICmdList) {}

【Pass_4】 Lighting

光照计算

首先预处理 延迟贴花 SSAO 等, 然后再进行光照计算

UE4 的光照计算采用 TBDR (Tiled Based Deferred Rendering)

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void FDeferredShadingSceneRenderer::Render(FRHICommandListImmediate& RHICmdList)
{
	// ...
	// Pre-lighting composition lighting stage
	// e.g. deferred decals, SSAO
	// ...

	// Render lighting.
	RHICmdList.SetCurrentStat(GET_STATID(STAT_CLM_Lighting));
	RenderLights(RHICmdList);
	RHICmdList.SetCurrentStat(GET_STATID(STAT_CLM_AfterLighting));

	// Pre-lighting composition lighting stage
	// e.g. LPV indirect	
	// ...
}

然后是处理光照的着色器入口

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// Engine\Shaders\Private\TiledDeferredLightShaders.usf 55

[numthreads(THREADGROUP_SIZEX, THREADGROUP_SIZEY, 1)]
void TiledDeferredLightingMain(
	uint3 GroupId : SV_GroupID,
	uint3 DispatchThreadId : SV_DispatchThreadID,
    uint3 GroupThreadId : SV_GroupThreadID) 
{
	// ...
}

【Pass_5】 Fog

雾计算

非透明表面逐像素计算 Fog

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void FDeferredShadingSceneRenderer::Render(FRHICommandListImmediate& RHICmdList)
{
	// ...
	// Draw fog.
	if (ShouldRenderFog(ViewFamily))
	{
		SCOPE_CYCLE_COUNTER(STAT_FDeferredShadingSceneRenderer_RenderFog);
		RenderFog(RHICmdList, LightShaftOutput);
		ServiceLocalQueue();
	}
}

/** Renders the scene's fogging. */
bool RenderFog(FRHICommandListImmediate& RHICmdList, const FLightShaftsOutput& LightShaftsOutput)
{
	// ...		
}

【Pass_6】 Post Processing

后处理效果

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void FDeferredShadingSceneRenderer::Render(FRHICommandListImmediate& RHICmdList)
{
	// ...

	// Resolve the scene color for post processing.
	ResolveSceneColor(RHICmdList);

	GetRendererModule().RenderPostResolvedSceneColorExtension(RHICmdList, SceneContext);

	CopySceneCaptureComponentToTarget(RHICmdList);
}

【Tips】 UE4 移动端似乎只有 Forward Rendering -- 没有 深度渲染器 和 GBuffer