73 KiB
73 KiB
title, date, excerpt, tags, rating
| title | date | excerpt | tags | rating |
|---|---|---|---|---|
| 剖析虚幻渲染体系（04）- 延迟渲染管线 | 2024-02-07 22:29:32 | ⭐ |
前言
https://www.cnblogs.com/timlly/p/14732412.html
延迟渲染管线
由于最耗时的光照计算延迟到后处理阶段，所以跟场景的物体数量解耦，只跟Render Targe尺寸相关，复杂度是O(Nlight×WRT×HRT)。所以，延迟渲染在应对复杂的场景和光源数量的场景比较得心应手，往往能得到非常好的性能提升。 但是，也存在一些缺点，如需多一个通道来绘制几何信息，需要多渲染纹理（MRT）的支持，更多的CPU和GPU显存占用，更高的带宽要求，有限的材质呈现类型，难以使用MSAA等硬件抗锯齿，存在画面较糊的情况等等。此外，应对简单场景时，可能反而得不到渲染性能方面的提升。延迟渲染可以针对不同的平台和API使用不同的优化改进技术，从而产生了诸多变种。下面是其中部分变种：
Deferred Lighting(Light Pre-Pass)
又被称为Light Pre-Pass，它和Deferred Shading的不同在于需要三个Pass：
- 第一个Pass叫Geometry Pass：只输出每个像素光照计算所需的几何属性（法线、深度）到GBuffer中。
- 第二个Pass叫Lighting Pass：存储光源属性（如Normal * LightDir、LightColor、Specular）到LBuffer（Light Buffer，光源缓冲区）。
- 第三个Pass叫Secondary Geometry Pass：获取GBuffer和LBuffer的数据，重建每个像素计算光照所需的数据，执行光照计算。
与Deferred Shading相比，Deferred lighting的优势在于G-Buffer所需的尺寸急剧减少，允许更多的材质类型呈现，较好第支持MSAA等。劣势是需要绘制场景两次，增加了Draw Call。 另外，Deferred lighting还有个优化版本，做法与上面所述略有不同，具体参见文献Light Pre-Pass。
Tiled-Based Deferred Rendering(TBDR)
Tiled-Based Deferred Rendering译名是基于瓦片的渲染，简称TBDR，它的核心思想在于将渲染纹理分成规则的一个个四边形（称为Tile），然后利用四边形的包围盒剔除该Tile内无用的光源，只保留有作用的光源列表，从而减少了实际光照计算中的无效光源的计算量。
!
- 将渲染纹理分成一个个均等面积的小块（Tile）。参见上图(b)。
Tile没有统一的固定大小，在不同的平台架构和渲染器中有所不同，不过一般是2的N次方，且长宽不一定相等，可以是16x16、32x32、64x64等等，不宜太小或太大，否则优化效果不明显。PowerVR GPU通常取32x32，而ARM Mali GPU取16x16。 !
-
根据Tile内的Depth范围计算出其Bounding Box。 !
TBDR中的每个Tile内的深度范围可能不一样，由此可得到不同大小的Bounding Box。 -
根据Tile的Bounding Box和Light的Bounding Box，执行求交。
除了无衰减的方向光，其它类型的光源都可以根据位置和衰减计算得到其Bounding Box。
- 摒弃不相交的Light，得到对Tile有作用的Light列表。参见上图(c)。
- 遍历所有Tile，获取每个Tile的有作业的光源索引列表，计算该Tile内所有像素的光照结果。
由于TBDR可以摒弃很多无作用的光源，能够避免很多无效的光照计算，目前已被广泛采用与移动端GPU架构中，形成了基于硬件加速的TBDR：!
PowerVR的TBDR架构，和立即模式的架构相比，在裁剪之后光栅化之前增加了Tiling阶段，增加了On-Chip Depth Buffer和Color Buffer，以更快地存取深度和颜色。
下图是PowerVR Rogue家族的Series7XT系列GPU和的硬件架构示意图：
!
Clustered Deferred Rendering
Clustered Deferred Rendering是分簇延迟渲染，与TBDR的不同在于对深度进行了更细粒度的划分，从而避免TBDR在深度范围跳变很大（中间无任何有效像素）时产生的光源裁剪效率降低的问题。
!
Clustered Deferred Rendering的核心思想是将深度按某种方式细分为若干份，从而更加精确地计算每个簇的包围盒，进而更精准地裁剪光源，避免深度不连续时的光源裁剪效率降低。
上图的分簇方法被称为隐式（Implicit）分簇法，实际上存在显式（Explicit）分簇法，可以进一步精确深度细分，以实际的深度范围计算每个族的包围盒：!
显式（Explicit）的深度分簇更加精确地定位到每簇的包围盒。
下图是Tiled、Implicit、Explicit深度划分法的对比图：!
VisibilityBuffer
Visibility Buffer与Deferred Texturing非常类似，是Deferred Lighting更加大胆的改进方案，核心思路是：为了减少GBuffer占用（GBuffer占用大意味着带宽大能耗大），不渲染GBuffer，改成渲染Visibility Buffer。Visibility Buffer上只存三角形和实例id，有了这些属性，在计算光照阶段（shading）分别从UAV和bindless texture里面读取真正需要的vertex attributes和贴图的属性，根据uv的差分自行计算mip-map（下图）。!
GBuffer和Visibility Buffer渲染管线对比示意图。后者在由Visiblity阶段取代前者的Gemotry Pass，此阶段只记录三角形和实例id，可将它们压缩进4bytes的Buffer中，从而极大地减少了显存的占用。
此方法虽然可以减少对Buffer的占用，但需要bindless texture的支持，对GPU Cache并不友好（相邻像素的三角形和实例id跳变大，降低Cache的空间局部性）
Deferred Adaptive Compute Shading
Deferred Adaptive Compute Shading的核心思想在于将屏幕像素按照某种方式划分为5个Level的不同粒度的像素块，以便决定是直接从相邻Level插值还是重新着色。!
此法在渲染UE4的不同场景时，在均方误差（RMSE）、峰值信噪比（PSNR）、平均结构相似性（MSSIM）都能获得良好的指标。（下图）!
渲染同一场景和画面时，对比Checkerboard（棋盘）着色方法，相同时间内，DACS的均方误差（RMSE）只是前者的21.5%，相同图像质量（MSSIM）下，DACS的时间快了4.22倍。
ForwardRendering
Forward+ Rendering
Forward+ 也被称为Tiled Forward Rendering，为了提升前向渲染光源的数量，它增加了光源剔除阶段，有3个Pass：depth prepass，light culling pass，shading pass。
light culling pass和瓦片的延迟渲染类似，将屏幕划分成若干个Tile，将每个Tile和光源求交，有效的光源写入到Tile的光源列表，以减少shading阶段的光源数量。!
+存在由于街头锥体拉长后在几何边界产生误报（False positives，可以通过separating axis theorem (SAT)改善）的情况。
-
Cluster Forward Rendering Cluster Forward Rendering和Cluster Deferred Rendering类似，将屏幕空间划分成均等Tile，深度细分成一个个簇，进而更加细粒度地裁剪光源。算法类似，这里就不累述了。
-
Volume Tiled Forward Rendering Volume Tiled Forward Rendering在Tiled和Clusterer的前向渲染基础上扩展的一种技术，旨在提升场景的光源支持数量，论文作者认为可以达到400万个光源的场景以30FPS实时渲染。它由以下步骤组成：
1.1 计算Grid（Volume Tile）的尺寸。给定Tile尺寸(tx,ty)(<28><>,<2C><>)和屏幕分辨率(w,h)(<28>,ℎ)，可以算出屏幕的细分数量(Sx,Sy)(<28><>,<2C><>)：
(Sx,Sy)=(∣∣∣wtx∣∣∣, ∣∣∣hty∣∣∣)(<28><>,<2C><>)=(|<7C><><EFBFBD>|, |ℎ<><E2848E>|)
深度方向的细分数量为：
SZ=∣∣∣log(Zfar/Znear)log(1+2tanθSy)∣∣∣<E288A3><E288A3>=|log(<28><><EFBFBD><EFBFBD>/<2F><><EFBFBD><EFBFBD><EFBFBD>)log(1+2tan<6E><E281A1><EFBFBD>)|
1.2 计算每个Volume Tile的AABB。结合下图，每个Tile的AABB边界计算如下：
knear=Znear(1+2tan(θ)Sy)kkfar=Znear(1+2tan(θ)Sy)k+1pmin=(Sx⋅i, Sy⋅j)pmax=(Sx⋅(i+1), Sy⋅(j+1))<29><><EFBFBD><EFBFBD><EFBFBD>=<3D><><EFBFBD><EFBFBD><EFBFBD>(1+2tan(<28>)<29><>)<29><><EFBFBD><EFBFBD><EFBFBD>=<3D><><EFBFBD><EFBFBD><EFBFBD>(1+2tan(<28>)<29><>)<29>+1<><31><EFBFBD><EFBFBD>=(<28><>⋅<EFBFBD>, <><C2A0>⋅<EFBFBD>)<29><><EFBFBD><EFBFBD>=(<28><>⋅(<28>+1), <><C2A0>⋅(<28>+1))
2、更新阶段：
2.1 深度Pre-pass。只记录非半透明物体的深度。
2.2 标记激活的Tile。
2.3 创建和压缩Tile列表。
2.4 将光源赋给Tile。每个线程组执行一个激活的Volume Tile，利用Tile的AABB和场景中所有的光源求交（可用BVH减少求交次数），将相交的光源索引记录到对应Tile的光源列表（每个Tile的光源数据是光源列表的起始位置和光源的数量）：
2.5 着色。此阶段与前述方法无特别差异。
基于体素分块的渲染虽然能够满足海量光源的渲染，但也存在Draw Call数量攀升和自相似体素瓦片（Self-Similar Volume Tiles，离摄像机近的体素很小而远的又相当大）的问题。
UE渲染相关
FSceneRenderer
FSceneRenderer是UE场景渲染器父类，是UE渲染体系的大脑和发动机，在整个渲染体系拥有举足轻重的位置，主要用于处理和渲染场景，生成RHI层的渲染指令。
// Engine\Source\Runtime\Renderer\Private\SceneRendering.h
// 场景渲染器
class FSceneRenderer
{
public:
FScene* Scene; // 被渲染的场景
FSceneViewFamily ViewFamily; // 被渲染的场景视图族（保存了需要渲染的所有view）。
TArray<FViewInfo> Views; // 需要被渲染的view实例。
FMeshElementCollector MeshCollector; // 网格收集器
FMeshElementCollector RayTracingCollector; // 光追网格收集器
// 可见光源信息
TArray<FVisibleLightInfo,SceneRenderingAllocator> VisibleLightInfos;
// 阴影相关的数据
TArray<FParallelMeshDrawCommandPass*, SceneRenderingAllocator> DispatchedShadowDepthPasses;
FSortedShadowMaps SortedShadowsForShadowDepthPass;
// 特殊标记
bool bHasRequestedToggleFreeze;
bool bUsedPrecomputedVisibility;
// 使用全场景阴影的点光源列表（可通过r.SupportPointLightWholeSceneShadows开关）
TArray<FName, SceneRenderingAllocator> UsedWholeScenePointLightNames;
// 平台Level信息
ERHIFeatureLevel::Type FeatureLevel;
EShaderPlatform ShaderPlatform;
(......)
public:
FSceneRenderer(const FSceneViewFamily* InViewFamily,FHitProxyConsumer* HitProxyConsumer);
virtual ~FSceneRenderer();
// FSceneRenderer接口（注意部分是空实现体和抽象接口）
// 渲染入口
virtual void Render(FRHICommandListImmediate& RHICmdList) = 0;
virtual void RenderHitProxies(FRHICommandListImmediate& RHICmdList) {}
// 场景渲染器实例
static FSceneRenderer* CreateSceneRenderer(const FSceneViewFamily* InViewFamily, FHitProxyConsumer* HitProxyConsumer);
void PrepareViewRectsForRendering();
#if WITH_MGPU // 多GPU支持
void ComputeViewGPUMasks(FRHIGPUMask RenderTargetGPUMask);
#endif
// 更新每个view所在的渲染纹理的结果
void DoCrossGPUTransfers(FRHICommandListImmediate& RHICmdList, FRHIGPUMask RenderTargetGPUMask);
// 遮挡查询接口和数据
bool DoOcclusionQueries(ERHIFeatureLevel::Type InFeatureLevel) const;
void BeginOcclusionTests(FRHICommandListImmediate& RHICmdList, bool bRenderQueries);
static FGraphEventRef OcclusionSubmittedFence[FOcclusionQueryHelpers::MaxBufferedOcclusionFrames];
void FenceOcclusionTests(FRHICommandListImmediate& RHICmdList);
void WaitOcclusionTests(FRHICommandListImmediate& RHICmdList);
bool ShouldDumpMeshDrawCommandInstancingStats() const { return bDumpMeshDrawCommandInstancingStats; }
static FGlobalBoundShaderState OcclusionTestBoundShaderState;
static bool ShouldCompositeEditorPrimitives(const FViewInfo& View);
// 等待场景渲染器执行完成和清理工作以及最终删除
static void WaitForTasksClearSnapshotsAndDeleteSceneRenderer(FRHICommandListImmediate& RHICmdList, FSceneRenderer* SceneRenderer, bool bWaitForTasks = true);
static void DelayWaitForTasksClearSnapshotsAndDeleteSceneRenderer(FRHICommandListImmediate& RHICmdList, FSceneRenderer* SceneRenderer);
// 其它接口
static FIntPoint ApplyResolutionFraction(...);
static FIntPoint QuantizeViewRectMin(const FIntPoint& ViewRectMin);
static FIntPoint GetDesiredInternalBufferSize(const FSceneViewFamily& ViewFamily);
static ISceneViewFamilyScreenPercentage* ForkScreenPercentageInterface(...);
static int32 GetRefractionQuality(const FSceneViewFamily& ViewFamily);
protected:
(......)
#if WITH_MGPU // 多GPU支持
FRHIGPUMask AllViewsGPUMask;
FRHIGPUMask GetGPUMaskForShadow(FProjectedShadowInfo* ProjectedShadowInfo) const;
#endif
// ----可在所有渲染器共享的接口----
// --渲染流程和MeshPass相关接口--
void OnStartRender(FRHICommandListImmediate& RHICmdList);
void RenderFinish(FRHICommandListImmediate& RHICmdList);
void SetupMeshPass(FViewInfo& View, FExclusiveDepthStencil::Type BasePassDepthStencilAccess, FViewCommands& ViewCommands);
void GatherDynamicMeshElements(...);
void RenderDistortion(FRHICommandListImmediate& RHICmdList);
void InitFogConstants();
bool ShouldRenderTranslucency(ETranslucencyPass::Type TranslucencyPass) const;
void RenderCustomDepthPassAtLocation(FRHICommandListImmediate& RHICmdList, int32 Location);
void RenderCustomDepthPass(FRHICommandListImmediate& RHICmdList);
void RenderPlanarReflection(class FPlanarReflectionSceneProxy* ReflectionSceneProxy);
void InitSkyAtmosphereForViews(FRHICommandListImmediate& RHICmdList);
void RenderSkyAtmosphereLookUpTables(FRHICommandListImmediate& RHICmdList);
void RenderSkyAtmosphere(FRHICommandListImmediate& RHICmdList);
void RenderSkyAtmosphereEditorNotifications(FRHICommandListImmediate& RHICmdList);
// ---阴影相关接口---
void InitDynamicShadows(FRHICommandListImmediate& RHICmdList, FGlobalDynamicIndexBuffer& DynamicIndexBuffer, FGlobalDynamicVertexBuffer& DynamicVertexBuffer, FGlobalDynamicReadBuffer& DynamicReadBuffer);
bool RenderShadowProjections(FRHICommandListImmediate& RHICmdList, const FLightSceneInfo* LightSceneInfo, IPooledRenderTarget* ScreenShadowMaskTexture, IPooledRenderTarget* ScreenShadowMaskSubPixelTexture, bool bProjectingForForwardShading, bool bMobileModulatedProjections, const struct FHairStrandsVisibilityViews* InHairVisibilityViews);
TRefCountPtr<FProjectedShadowInfo> GetCachedPreshadow(...);
void CreatePerObjectProjectedShadow(...);
void SetupInteractionShadows(...);
void AddViewDependentWholeSceneShadowsForView(...);
void AllocateShadowDepthTargets(FRHICommandListImmediate& RHICmdList);
void AllocatePerObjectShadowDepthTargets(FRHICommandListImmediate& RHICmdList, ...);
void AllocateCachedSpotlightShadowDepthTargets(FRHICommandListImmediate& RHICmdList, ...);
void AllocateCSMDepthTargets(FRHICommandListImmediate& RHICmdList, ...);
void AllocateRSMDepthTargets(FRHICommandListImmediate& RHICmdList, ...);
void AllocateOnePassPointLightDepthTargets(FRHICommandListImmediate& RHICmdList, ...);
void AllocateTranslucentShadowDepthTargets(FRHICommandListImmediate& RHICmdList, ...);
bool CheckForProjectedShadows(const FLightSceneInfo* LightSceneInfo) const;
void GatherShadowPrimitives(...);
void RenderShadowDepthMaps(FRHICommandListImmediate& RHICmdList);
void RenderShadowDepthMapAtlases(FRHICommandListImmediate& RHICmdList);
void CreateWholeSceneProjectedShadow(FLightSceneInfo* LightSceneInfo, ...);
void UpdatePreshadowCache(FSceneRenderTargets& SceneContext);
void InitProjectedShadowVisibility(FRHICommandListImmediate& RHICmdList);
void GatherShadowDynamicMeshElements(FGlobalDynamicIndexBuffer& DynamicIndexBuffer, FGlobalDynamicVertexBuffer& DynamicVertexBuffer, FGlobalDynamicReadBuffer& DynamicReadBuffer);
// --光源接口--
static void GetLightNameForDrawEvent(const FLightSceneProxy* LightProxy, FString& LightNameWithLevel);
static void GatherSimpleLights(const FSceneViewFamily& ViewFamily, ...);
static void SplitSimpleLightsByView(const FSceneViewFamily& ViewFamily, ...);
// --可见性接口--
void PreVisibilityFrameSetup(FRHICommandListImmediate& RHICmdList);
void ComputeViewVisibility(FRHICommandListImmediate& RHICmdList, ...);
void PostVisibilityFrameSetup(FILCUpdatePrimTaskData& OutILCTaskData);
// --其它接口--
void GammaCorrectToViewportRenderTarget(FRHICommandList& RHICmdList, const FViewInfo* View, float OverrideGamma);
FRHITexture* GetMultiViewSceneColor(const FSceneRenderTargets& SceneContext) const;
void UpdatePrimitiveIndirectLightingCacheBuffers();
bool ShouldRenderSkyAtmosphereEditorNotifications();
void ResolveSceneColor(FRHICommandList& RHICmdList);
(......)
};
FSceneRenderer由游戏线程的FRendererModule::BeginRenderingViewFamily负责创建和初始化，然后传递给渲染线程。渲染线程会调用FSceneRenderer::Render()，渲染完返回后，会删除FSceneRenderer的实例。也就是说，FSceneRenderer会被每帧创建和销毁。
FSceneRenderer拥有两个子类：FMobileSceneRenderer和FDeferredShadingSceneRenderer。
FMobileSceneRenderer是用于移动平台的场景渲染器，默认采用了前向渲染的流程。
FDeferredShadingSceneRenderer虽然名字叫做延迟着色场景渲染器，但其实集成了包含前向渲染和延迟渲染的两种渲染路径，是PC和主机平台的默认场景渲染器（笔者刚接触伊始也被这蜜汁取名迷惑过）。
FDeferredShadingSceneRenderer
FDeferredShadingSceneRenderer主要包含了MeshPass、光源、阴影、光线追踪、反射、可见性等几大类接口。其中最重要的接口非FDeferredShadingSceneRenderer::Render()莫属，它是FDeferredShadingSceneRenderer的渲染主入口，主流程和重要接口的调用都直接或间接发生它内部。
则可以划分成以下主要阶段：
- FScene::UpdateAllPrimitiveSceneInfos：更新所有图元的信息到GPU，若启用了GPUScene，将会用二维纹理或StructureBuffer来存储图元的信息。
- FSceneRenderTargets::Allocate：若有需要（分辨率改变、API触发），重新分配场景的渲染纹理，以保证足够大的尺寸渲染对应的view。
- InitViews：采用裁剪若干方式初始化图元的可见性，设置可见的动态阴影，有必要时会对阴影平截头体和世界求交（全场阴影和逐物体阴影）。
- PrePass / Depth only pass：提前深度Pass，用来渲染不透明物体的深度。此Pass只会写入深度而不会写入颜色，写入深度时有disabled、occlusion only、complete depths三种模式，视不同的平台和Feature Level决定。通常用来建立Hierarchical-Z，以便能够开启硬件的Early-Z技术，提升Base Pass的渲染效率。
- Base pass：也就是前面章节所说的几何通道。 用来渲染不透明物体（Opaque和Masked材质）的几何信息，包含法线、深度、颜色、AO、粗糙度、金属度等等。这些几何信息会写入若干张GBuffer中。此阶段不会计算动态光源的贡献，但会计算Lightmap和天空光的贡献。
- Issue Occlusion Queries / BeginOcclusionTests：开启遮挡渲染，此帧的渲染遮挡数据用于下一帧
InitViews阶段的遮挡剔除。此阶段主要使用物体的包围盒来渲染，也可能会打包相近物体的包围盒以减少Draw Call。 - Lighting：此阶段也就是前面章节所说的光照通道，是标准延迟着色和分块延迟着色的混合体。会计算开启阴影的光源的阴影图，也会计算每个灯光对屏幕空间像素的贡献量，并累计到Scene Color中。此外，还会计算光源也对translucency lighting volumes的贡献量。
- Fog在屏幕空间计算雾和大气对不透明物体表面像素的影响。
- Translucency：渲染半透明物体阶段。所有半透明物体由远到近（视图空间）逐个绘制到离屏渲染纹理（offscreen render target，代码中叫separate translucent render target）中，接着用单独的pass以正确计算和混合光照结果。
- Post Processing：后处理阶段。包含了不需要GBuffer的Bloom、色调映射、Gamma校正等以及需要GBuffer的SSR、SSAO、SSGI等。此阶段会将半透明的渲染纹理混合到最终的场景颜色中。
FScene::UpdateAllPrimitiveSceneInfos
FScene::UpdateAllPrimitiveSceneInfos的调用发生在FDeferredShadingSceneRenderer::Render的第一行：
// Engine\Source\Runtime\Renderer\Private\DeferredShadingRenderer.cpp
void FDeferredShadingSceneRenderer::Render(FRHICommandListImmediate& RHICmdList)
{
Scene->UpdateAllPrimitiveSceneInfos(RHICmdList, true);
(......)
}
FScene::UpdateAllPrimitiveSceneInfos的主要作用是删除、增加、更新CPU侧的图元数据，且同步到GPU端。其中GPU的图元数据存在两种方式：
- 每个图元独有一个Uniform Buffer。在shader中需要访问图元的数据时从该图元的Uniform Buffer中获取即可。这种结构简单易理解，兼容所有FeatureLevel的设备。但是会增加CPU和GPU的IO，降低GPU的Cache命中率。
- 使用Texture2D或StructuredBuffer的GPU Scene，所有图元的数据按规律放置到此。在shader中需要访问图元的数据时需要从GPU Scene中对应的位置读取数据。需要SM5支持，实现难度高，不易理解，但可减少CPU和GPU的IO，提升GPU Cache命中率，可更好地支持光线追踪和GPU Driven Pipeline。 虽然以上访问的方式不一样，但shader中已经做了封装，使用者不需要区分是哪种形式的Buffer，只需使用以下方式：
// Engine\Shaders\Private\SceneData.ush
struct FPrimitiveSceneData
{
float4x4 LocalToWorld;
float4 InvNonUniformScaleAndDeterminantSign;
float4 ObjectWorldPositionAndRadius;
float4x4 WorldToLocal;
float4x4 PreviousLocalToWorld;
float4x4 PreviousWorldToLocal;
float3 ActorWorldPosition;
float UseSingleSampleShadowFromStationaryLights;
float3 ObjectBounds;
float LpvBiasMultiplier;
float DecalReceiverMask;
float PerObjectGBufferData;
float UseVolumetricLightmapShadowFromStationaryLights;
float DrawsVelocity;
float4 ObjectOrientation;
float4 NonUniformScale;
float3 LocalObjectBoundsMin;
uint LightingChannelMask;
float3 LocalObjectBoundsMax;
uint LightmapDataIndex;
float3 PreSkinnedLocalBoundsMin;
int SingleCaptureIndex;
float3 PreSkinnedLocalBoundsMax;
uint OutputVelocity;
float4 CustomPrimitiveData[NUM_CUSTOM_PRIMITIVE_DATA];
};
由此可见，每个图元可访问的数据还是很多的，占用的显存也相当可观，每个图元大约占用576字节，如果场景存在10000个图元（游戏场景很常见），则忽略Padding情况下，这些图元数据总量达到约5.5M。
言归正传，回到C++层看看GPU Scene的定义：
// Engine\Source\Runtime\Renderer\Private\ScenePrivate.h
class FGPUScene
{
public:
// 是否更新全部图元数据，通常用于调试，运行期会导致性能下降。
bool bUpdateAllPrimitives;
// 需要更新数据的图元索引.
TArray<int32> PrimitivesToUpdate;
// 所有图元的bit,当对应索引的bit为1时表示需要更新(同时在PrimitivesToUpdate中).
TBitArray<> PrimitivesMarkedToUpdate;
// 存放图元的GPU数据结构, 可以是TextureBuffer或Texture2D, 但只有其中一种会被创建和生效, 移动端可由Mobile.UseGPUSceneTexture控制台变量设定.
FRWBufferStructured PrimitiveBuffer;
FTextureRWBuffer2D PrimitiveTexture;
// 上传的buffer
FScatterUploadBuffer PrimitiveUploadBuffer;
FScatterUploadBuffer PrimitiveUploadViewBuffer;
// 光照图
FGrowOnlySpanAllocator LightmapDataAllocator;
FRWBufferStructured LightmapDataBuffer;
FScatterUploadBuffer LightmapUploadBuffer;
};
代码略……
总结起来，FScene::UpdateAllPrimitiveSceneInfos的作用是删除、增加图元，以及更新图元的所有数据，包含变换矩阵、自定义数据、距离场数据等。
GPUScene的PrimitivesToUpdate和PrimitivesMarkedToUpdate收集好需要更新的所有图元索引后，会在FDeferredShadingSceneRenderer::Render的InitViews之后同步给GPU：
void FDeferredShadingSceneRenderer::Render(FRHICommandListImmediate& RHICmdList)
{
// 更新GPUScene的数据
Scene->UpdateAllPrimitiveSceneInfos(RHICmdList, true);
(......)
// 初始化View的数据
bDoInitViewAftersPrepass = InitViews(RHICmdList, BasePassDepthStencilAccess, ILCTaskData, UpdateViewCustomDataEvents);
(......)
// 同步CPU端的GPUScene到GPU.
UpdateGPUScene(RHICmdList, *Scene);//5.3 => Scene->GPUScene.Update(GraphBuilder, GetSceneUniforms(), *Scene, ExternalAccessQueue);
(......)
}
UpdateGPUScene()代码略……
InitViews（5.3已经变成了BeginInitViews() & EndInitViews()）
- InitViews()(BeginInitViews())
- PreVisibilityFrameSetup()：做了大量的初始化工作，如静态网格、Groom、SkinCache、特效、TAA、ViewState等等。
- 初始化特效系统（FXSystem）。
- ComputeViewVisibility()：最重要的功能就是处理图元的可见性，包含视椎体裁剪、遮挡剔除，以及收集动态网格信息、创建光源信息等。
- FPrimitiveSceneInfo::UpdateStaticMeshes：更新静态网格数据。
- ViewState::GetPrecomputedVisibilityData：获取预计算的可见性数据。
- FrustumCull：视锥体裁剪。
- ComputeAndMarkRelevanceForViewParallel：计算和标记视图并行处理的关联数据。
- GatherDynamicMeshElements：收集view的动态可见元素，上一篇中已经解析过。
- SetupMeshPass：设置网格Pass的数据，将FMeshBatch转换成FMeshDrawCommand，上一篇中已经解析过。
- CreateIndirectCapsuleShadows：创建胶囊体阴影。
- UpdateSkyIrradianceGpuBuffer：更新天空体环境光照的GPU数据。
- InitSkyAtmosphereForViews：初始化大气效果。
- PostVisibilityFrameSetup()：利用view的视锥裁剪光源，防止视线外或屏幕占比很小或没有光照强度的光源进入shader计算。此外，还会处理贴花排序、调整之前帧的RT和雾效、光束等。
- FViewInfo::InitRHIResources()：初始化每个View的UniformBuffer。
- SetupVolumetricFog：初始化和设置体积雾。
- FSceneRenderer::OnStartRender()：通知RHI已经开启了渲染，以初始化视图相关的数据和资源。
以下是旧版本代码：
// Engine\Source\Runtime\Renderer\Private\SceneVisibility.cpp
bool FDeferredShadingSceneRenderer::InitViews(FRHICommandListImmediate& RHICmdList, FExclusiveDepthStencil::Type BasePassDepthStencilAccess, struct FILCUpdatePrimTaskData& ILCTaskData, FGraphEventArray& UpdateViewCustomDataEvents)
{
// 创建可见性帧设置预备阶段.
PreVisibilityFrameSetup(RHICmdList);
RHICmdList.ImmediateFlush(EImmediateFlushType::DispatchToRHIThread);
// 特效系统初始化
{
if (Scene->FXSystem && Scene->FXSystem->RequiresEarlyViewUniformBuffer() && Views.IsValidIndex(0))
{
// 保证第一个view的RHI资源已被初始化.
Views[0].InitRHIResources();
Scene->FXSystem->PostInitViews(RHICmdList, Views[0].ViewUniformBuffer, Views[0].AllowGPUParticleUpdate() && !ViewFamily.EngineShowFlags.HitProxies);
}
}
// 创建可见性网格指令.
FViewVisibleCommandsPerView ViewCommandsPerView;
ViewCommandsPerView.SetNum(Views.Num());
// 计算可见性.
ComputeViewVisibility(RHICmdList, BasePassDepthStencilAccess, ViewCommandsPerView, DynamicIndexBufferForInitViews, DynamicVertexBufferForInitViews, DynamicReadBufferForInitViews);
RHICmdList.ImmediateFlush(EImmediateFlushType::DispatchToRHIThread);
// 胶囊阴影
CreateIndirectCapsuleShadows();
RHICmdList.ImmediateFlush(EImmediateFlushType::DispatchToRHIThread);
// 初始化大气效果.
if (ShouldRenderSkyAtmosphere(Scene, ViewFamily.EngineShowFlags))
{
InitSkyAtmosphereForViews(RHICmdList);
}
// 创建可见性帧设置后置阶段.
PostVisibilityFrameSetup(ILCTaskData);
RHICmdList.ImmediateFlush(EImmediateFlushType::DispatchToRHIThread);
(......)
// 是否可能在Prepass之后初始化view，由GDoInitViewsLightingAfterPrepass决定，而GDoInitViewsLightingAfterPrepass又可通过控制台命令r.DoInitViewsLightingAfterPrepass设定。
bool bDoInitViewAftersPrepass = !!GDoInitViewsLightingAfterPrepass;
if (!bDoInitViewAftersPrepass)
{
InitViewsPossiblyAfterPrepass(RHICmdList, ILCTaskData, UpdateViewCustomDataEvents);
}
{
// 初始化所有view的uniform buffer和RHI资源.
for (int32 ViewIndex = 0; ViewIndex < Views.Num(); ViewIndex++)
{
FViewInfo& View = Views[ViewIndex];
if (View.ViewState)
{
if (!View.ViewState->ForwardLightingResources)
{
View.ViewState->ForwardLightingResources.Reset(new FForwardLightingViewResources());
}
View.ForwardLightingResources = View.ViewState->ForwardLightingResources.Get();
}
else
{
View.ForwardLightingResourcesStorage.Reset(new FForwardLightingViewResources());
View.ForwardLightingResources = View.ForwardLightingResourcesStorage.Get();
}
#if RHI_RAYTRACING
View.IESLightProfileResource = View.ViewState ? &View.ViewState->IESLightProfileResources : nullptr;
#endif
// Set the pre-exposure before initializing the constant buffers.
if (View.ViewState)
{
View.ViewState->UpdatePreExposure(View);
}
// Initialize the view's RHI resources.
View.InitRHIResources();
}
}
// 体积雾
SetupVolumetricFog();
// 发送开始渲染事件.
OnStartRender(RHICmdList);
return bDoInitViewAftersPrepass;
}
上面的代码似乎没有做太多逻辑，然而实际上很多逻辑分散在了上面的一些重要接口中，先分析PreVisibilityFrameSetup：
// Engine\Source\Runtime\Renderer\Private\SceneVisibility.cpp
void FDeferredShadingSceneRenderer::PreVisibilityFrameSetup(FRHICommandListImmediate& RHICmdList)
{
// Possible stencil dither optimization approach
for (int32 ViewIndex = 0; ViewIndex < Views.Num(); ViewIndex++)
{
FViewInfo& View = Views[ViewIndex];
View.bAllowStencilDither = bDitheredLODTransitionsUseStencil;
}
FSceneRenderer::PreVisibilityFrameSetup(RHICmdList);
}
void FSceneRenderer::PreVisibilityFrameSetup(FRHICommandListImmediate& RHICmdList)
{
// 通知RHI已经开始渲染场景了.
RHICmdList.BeginScene();
{
static auto CVar = IConsoleManager::Get().FindConsoleVariable(TEXT("r.DoLazyStaticMeshUpdate"));
const bool DoLazyStaticMeshUpdate = (CVar->GetInt() && !GIsEditor);
// 延迟的静态网格更新.
if (DoLazyStaticMeshUpdate)
{
QUICK_SCOPE_CYCLE_COUNTER(STAT_PreVisibilityFrameSetup_EvictionForLazyStaticMeshUpdate);
static int32 RollingRemoveIndex = 0;
static int32 RollingPassShrinkIndex = 0;
if (RollingRemoveIndex >= Scene->Primitives.Num())
{
RollingRemoveIndex = 0;
RollingPassShrinkIndex++;
if (RollingPassShrinkIndex >= UE_ARRAY_COUNT(Scene->CachedDrawLists))
{
RollingPassShrinkIndex = 0;
}
// Periodically shrink the SparseArray containing cached mesh draw commands which we are causing to be regenerated with UpdateStaticMeshes
Scene->CachedDrawLists[RollingPassShrinkIndex].MeshDrawCommands.Shrink();
}
const int32 NumRemovedPerFrame = 10;
TArray<FPrimitiveSceneInfo*, TInlineAllocator<10>> SceneInfos;
for (int32 NumRemoved = 0; NumRemoved < NumRemovedPerFrame && RollingRemoveIndex < Scene->Primitives.Num(); NumRemoved++, RollingRemoveIndex++)
{
SceneInfos.Add(Scene->Primitives[RollingRemoveIndex]);
}
FPrimitiveSceneInfo::UpdateStaticMeshes(RHICmdList, Scene, SceneInfos, false);
}
}
// 转换Skin Cache
RunGPUSkinCacheTransition(RHICmdList, Scene, EGPUSkinCacheTransition::FrameSetup);
// 初始化Groom头发
if (IsHairStrandsEnable(Scene->GetShaderPlatform()) && Views.Num() > 0)
{
const EWorldType::Type WorldType = Views[0].Family->Scene->GetWorld()->WorldType;
const FShaderDrawDebugData* ShaderDrawData = &Views[0].ShaderDrawData;
auto ShaderMap = GetGlobalShaderMap(FeatureLevel);
RunHairStrandsInterpolation(RHICmdList, WorldType, Scene->GetGPUSkinCache(), ShaderDrawData, ShaderMap, EHairStrandsInterpolationType::SimulationStrands, nullptr);
}
// 特效系统
if (Scene->FXSystem && Views.IsValidIndex(0))
{
Scene->FXSystem->PreInitViews(RHICmdList, Views[0].AllowGPUParticleUpdate() && !ViewFamily.EngineShowFlags.HitProxies);
}
(......)
// 设置运动模糊参数(包含TAA的处理)
for(int32 ViewIndex = 0;ViewIndex < Views.Num();ViewIndex++)
{
FViewInfo& View = Views[ViewIndex];
FSceneViewState* ViewState = View.ViewState;
check(View.VerifyMembersChecks());
// Once per render increment the occlusion frame counter.
if (ViewState)
{
ViewState->OcclusionFrameCounter++;
}
// HighResScreenshot should get best results so we don't do the occlusion optimization based on the former frame
extern bool GIsHighResScreenshot;
const bool bIsHitTesting = ViewFamily.EngineShowFlags.HitProxies;
// Don't test occlusion queries in collision viewmode as they can be bigger then the rendering bounds.
const bool bCollisionView = ViewFamily.EngineShowFlags.CollisionVisibility || ViewFamily.EngineShowFlags.CollisionPawn;
if (GIsHighResScreenshot || !DoOcclusionQueries(FeatureLevel) || bIsHitTesting || bCollisionView)
{
View.bDisableQuerySubmissions = true;
View.bIgnoreExistingQueries = true;
}
FSceneRenderTargets& SceneContext = FSceneRenderTargets::Get(RHICmdList);
// set up the screen area for occlusion
float NumPossiblePixels = SceneContext.UseDownsizedOcclusionQueries() && IsValidRef(SceneContext.GetSmallDepthSurface()) ?
(float)View.ViewRect.Width() / SceneContext.GetSmallColorDepthDownsampleFactor() * (float)View.ViewRect.Height() / SceneContext.GetSmallColorDepthDownsampleFactor() :
View.ViewRect.Width() * View.ViewRect.Height();
View.OneOverNumPossiblePixels = NumPossiblePixels > 0.0 ? 1.0f / NumPossiblePixels : 0.0f;
// Still need no jitter to be set for temporal feedback on SSR (it is enabled even when temporal AA is off).
check(View.TemporalJitterPixels.X == 0.0f);
check(View.TemporalJitterPixels.Y == 0.0f);
// Cache the projection matrix b
// Cache the projection matrix before AA is applied
View.ViewMatrices.SaveProjectionNoAAMatrix();
if (ViewState)
{
check(View.bStatePrevViewInfoIsReadOnly);
View.bStatePrevViewInfoIsReadOnly = ViewFamily.bWorldIsPaused || ViewFamily.EngineShowFlags.HitProxies || bFreezeTemporalHistories;
ViewState->SetupDistanceFieldTemporalOffset(ViewFamily);
if (!View.bStatePrevViewInfoIsReadOnly && !bFreezeTemporalSequences)
{
ViewState->FrameIndex++;
}
if (View.OverrideFrameIndexValue.IsSet())
{
ViewState->FrameIndex = View.OverrideFrameIndexValue.GetValue();
}
}
// Subpixel jitter for temporal AA
int32 CVarTemporalAASamplesValue = CVarTemporalAASamples.GetValueOnRenderThread();
bool bTemporalUpsampling = View.PrimaryScreenPercentageMethod == EPrimaryScreenPercentageMethod::TemporalUpscale;
// Apply a sub pixel offset to the view.
if (View.AntiAliasingMethod == AAM_TemporalAA && ViewState && (CVarTemporalAASamplesValue > 0 || bTemporalUpsampling) && View.bAllowTemporalJitter)
{
float EffectivePrimaryResolutionFraction = float(View.ViewRect.Width()) / float(View.GetSecondaryViewRectSize().X);
// Compute number of TAA samples.
int32 TemporalAASamples = CVarTemporalAASamplesValue;
{
if (Scene->GetFeatureLevel() < ERHIFeatureLevel::SM5)
{
// Only support 2 samples for mobile temporal AA.
TemporalAASamples = 2;
}
else if (bTemporalUpsampling)
{
// When doing TAA upsample with screen percentage < 100%, we need extra temporal samples to have a
// constant temporal sample density for final output pixels to avoid output pixel aligned converging issues.
TemporalAASamples = float(TemporalAASamples) * FMath::Max(1.f, 1.f / (EffectivePrimaryResolutionFraction * EffectivePrimaryResolutionFraction));
}
else if (CVarTemporalAASamplesValue == 5)
{
TemporalAASamples = 4;
}
TemporalAASamples = FMath::Clamp(TemporalAASamples, 1, 255);
}
// Compute the new sample index in the temporal sequence.
int32 TemporalSampleIndex = ViewState->TemporalAASampleIndex + 1;
if(TemporalSampleIndex >= TemporalAASamples || View.bCameraCut)
{
TemporalSampleIndex = 0;
}
// Updates view state.
if (!View.bStatePrevViewInfoIsReadOnly && !bFreezeTemporalSequences)
{
ViewState->TemporalAASampleIndex = TemporalSampleIndex;
ViewState->TemporalAASampleIndexUnclamped = ViewState->TemporalAASampleIndexUnclamped+1;
}
// 根据不同的TAA采样策略和参数, 选择和计算对应的采样参数.
float SampleX, SampleY;
if (Scene->GetFeatureLevel() < ERHIFeatureLevel::SM5)
{
float SamplesX[] = { -8.0f/16.0f, 0.0/16.0f };
float SamplesY[] = { /* - */ 0.0f/16.0f, 8.0/16.0f };
check(TemporalAASamples == UE_ARRAY_COUNT(SamplesX));
SampleX = SamplesX[ TemporalSampleIndex ];
SampleY = SamplesY[ TemporalSampleIndex ];
}
else if (View.PrimaryScreenPercentageMethod == EPrimaryScreenPercentageMethod::TemporalUpscale)
{
// Uniformly distribute temporal jittering in [-.5; .5], because there is no longer any alignement of input and output pixels.
SampleX = Halton(TemporalSampleIndex + 1, 2) - 0.5f;
SampleY = Halton(TemporalSampleIndex + 1, 3) - 0.5f;
View.MaterialTextureMipBias = -(FMath::Max(-FMath::Log2(EffectivePrimaryResolutionFraction), 0.0f) ) + CVarMinAutomaticViewMipBiasOffset.GetValueOnRenderThread();
View.MaterialTextureMipBias = FMath::Max(View.MaterialTextureMipBias, CVarMinAutomaticViewMipBias.GetValueOnRenderThread());
}
else if( CVarTemporalAASamplesValue == 2 )
{
// 2xMSAA
// Pattern docs: http://msdn.microsoft.com/en-us/library/windows/desktop/ff476218(v=vs.85).aspx
// N.
// .S
float SamplesX[] = { -4.0f/16.0f, 4.0/16.0f };
float SamplesY[] = { -4.0f/16.0f, 4.0/16.0f };
check(TemporalAASamples == UE_ARRAY_COUNT(SamplesX));
SampleX = SamplesX[ TemporalSampleIndex ];
SampleY = SamplesY[ TemporalSampleIndex ];
}
else if( CVarTemporalAASamplesValue == 3 )
{
// 3xMSAA
// A..
// ..B
// .C.
// Rolling circle pattern (A,B,C).
float SamplesX[] = { -2.0f/3.0f, 2.0/3.0f, 0.0/3.0f };
float SamplesY[] = { -2.0f/3.0f, 0.0/3.0f, 2.0/3.0f };
check(TemporalAASamples == UE_ARRAY_COUNT(SamplesX));
SampleX = SamplesX[ TemporalSampleIndex ];
SampleY = SamplesY[ TemporalSampleIndex ];
}
else if( CVarTemporalAASamplesValue == 4 )
{
// 4xMSAA
// Pattern docs: http://msdn.microsoft.com/en-us/library/windows/desktop/ff476218(v=vs.85).aspx
// .N..
// ...E
// W...
// ..S.
// Rolling circle pattern (N,E,S,W).
float SamplesX[] = { -2.0f/16.0f, 6.0/16.0f, 2.0/16.0f, -6.0/16.0f };
float SamplesY[] = { -6.0f/16.0f, -2.0/16.0f, 6.0/16.0f, 2.0/16.0f };
check(TemporalAASamples == UE_ARRAY_COUNT(SamplesX));
SampleX = SamplesX[ TemporalSampleIndex ];
SampleY = SamplesY[ TemporalSampleIndex ];
}
else if( CVarTemporalAASamplesValue == 5 )
{
// Compressed 4 sample pattern on same vertical and horizontal line (less temporal flicker).
// Compressed 1/2 works better than correct 2/3 (reduced temporal flicker).
// . N .
// W . E
// . S .
// Rolling circle pattern (N,E,S,W).
float SamplesX[] = { 0.0f/2.0f, 1.0/2.0f, 0.0/2.0f, -1.0/2.0f };
float SamplesY[] = { -1.0f/2.0f, 0.0/2.0f, 1.0/2.0f, 0.0/2.0f };
check(TemporalAASamples == UE_ARRAY_COUNT(SamplesX));
SampleX = SamplesX[ TemporalSampleIndex ];
SampleY = SamplesY[ TemporalSampleIndex ];
}
else
{
float u1 = Halton( TemporalSampleIndex + 1, 2 );
float u2 = Halton( TemporalSampleIndex + 1, 3 );
// Generates samples in normal distribution
// exp( x^2 / Sigma^2 )
static auto CVar = IConsoleManager::Get().FindConsoleVariable(TEXT("r.TemporalAAFilterSize"));
float FilterSize = CVar->GetFloat();
// Scale distribution to set non-unit variance
// Variance = Sigma^2
float Sigma = 0.47f * FilterSize;
// Window to [-0.5, 0.5] output
// Without windowing we could generate samples far away on the infinite tails.
float OutWindow = 0.5f;
float InWindow = FMath::Exp( -0.5 * FMath::Square( OutWindow / Sigma ) );
// Box-Muller transform
float Theta = 2.0f * PI * u2;
float r = Sigma * FMath::Sqrt( -2.0f * FMath::Loge( (1.0f - u1) * InWindow + u1 ) );
SampleX = r * FMath::Cos( Theta );
SampleY = r * FMath::Sin( Theta );
}
View.TemporalJitterSequenceLength = TemporalAASamples;
View.TemporalJitterIndex = TemporalSampleIndex;
View.TemporalJitterPixels.X = SampleX;
View.TemporalJitterPixels.Y = SampleY;
View.ViewMatrices.HackAddTemporalAAProjectionJitter(FVector2D(SampleX * 2.0f / View.ViewRect.Width(), SampleY * -2.0f / View.ViewRect.Height()));
}
// Setup a new FPreviousViewInfo from current frame infos.
FPreviousViewInfo NewPrevViewInfo;
{
NewPrevViewInfo.ViewMatrices = View.ViewMatrices;
}
// 初始化view state
if ( ViewState )
{
// update previous frame matrices in case world origin was rebased on this frame
if (!View.OriginOffsetThisFrame.IsZero())
{
ViewState->PrevFrameViewInfo.ViewMatrices.ApplyWorldOffset(View.OriginOffsetThisFrame);
}
// determine if we are initializing or we should reset the persistent state
const float DeltaTime = View.Family->CurrentRealTime - ViewState->LastRenderTime;
const bool bFirstFrameOrTimeWasReset = DeltaTime < -0.0001f || ViewState->LastRenderTime < 0.0001f;
const bool bIsLargeCameraMovement = IsLargeCameraMovement(
View,
ViewState->PrevFrameViewInfo.ViewMatrices.GetViewMatrix(),
ViewState->PrevFrameViewInfo.ViewMatrices.GetViewOrigin(),
45.0f, 10000.0f);
const bool bResetCamera = (bFirstFrameOrTimeWasReset || View.bCameraCut || bIsLargeCameraMovement || View.bForceCameraVisibilityReset);
(......)
if (bResetCamera)
{
View.PrevViewInfo = NewPrevViewInfo;
// PT: If the motion blur shader is the last shader in the post-processing chain then it is the one that is
// adjusting for the viewport offset. So it is always required and we can't just disable the work the
// shader does. The correct fix would be to disable the effect when we don't need it and to properly mark
// the uber-postprocessing effect as the last effect in the chain.
View.bPrevTransformsReset = true;
}
else
{
View.PrevViewInfo = ViewState->PrevFrameViewInfo;
}
// Replace previous view info of the view state with this frame, clearing out references over render target.
if (!View.bStatePrevViewInfoIsReadOnly)
{
ViewState->PrevFrameViewInfo = NewPrevViewInfo;
}
// If the view has a previous view transform, then overwrite the previous view info for the _current_ frame.
if (View.PreviousViewTransform.IsSet())
{
// Note that we must ensure this transform ends up in ViewState->PrevFrameViewInfo else it will be used to calculate the next frame's motion vectors as well
View.PrevViewInfo.ViewMatrices.UpdateViewMatrix(View.PreviousViewTransform->GetTranslation(), View.PreviousViewTransform->GetRotation().Rotator());
}
// detect conditions where we should reset occlusion queries
if (bFirstFrameOrTimeWasReset ||
ViewState->LastRenderTime + GEngine->PrimitiveProbablyVisibleTime < View.Family->CurrentRealTime ||
View.bCameraCut ||
View.bForceCameraVisibilityReset ||
IsLargeCameraMovement(
View,
ViewState->PrevViewMatrixForOcclusionQuery,
ViewState->PrevViewOriginForOcclusionQuery,
GEngine->CameraRotationThreshold, GEngine->CameraTranslationThreshold))
{
View.bIgnoreExistingQueries = true;
View.bDisableDistanceBasedFadeTransitions = true;
}
// Turn on/off round-robin occlusion querying in the ViewState
static const auto CVarRROCC = IConsoleManager::Get().FindTConsoleVariableDataInt(TEXT("vr.RoundRobinOcclusion"));
const bool bEnableRoundRobin = CVarRROCC ? (CVarRROCC->GetValueOnAnyThread() != false) : false;
if (bEnableRoundRobin != ViewState->IsRoundRobinEnabled())
{
ViewState->UpdateRoundRobin(bEnableRoundRobin);
View.bIgnoreExistingQueries = true;
}
ViewState->PrevViewMatrixForOcclusionQuery = View.ViewMatrices.GetViewMatrix();
ViewState->PrevViewOriginForOcclusionQuery = View.ViewMatrices.GetViewOrigin();
(......)
// we don't use DeltaTime as it can be 0 (in editor) and is computed by subtracting floats (loses precision over time)
// Clamp DeltaWorldTime to reasonable values for the purposes of motion blur, things like TimeDilation can make it very small
// Offline renders always control the timestep for the view and always need the timescales calculated.
if (!ViewFamily.bWorldIsPaused || View.bIsOfflineRender)
{
ViewState->UpdateMotionBlurTimeScale(View);
}
ViewState->PrevFrameNumber = ViewState->PendingPrevFrameNumber;
ViewState->PendingPrevFrameNumber = View.Family->FrameNumber;
// This finishes the update of view state
ViewState->UpdateLastRenderTime(*View.Family);
ViewState->UpdateTemporalLODTransition(View);
}
else
{
// Without a viewstate, we just assume that camera has not moved.
View.PrevViewInfo = NewPrevViewInfo;
}
}
// 设置全局抖动参数和过渡uniform buffer.
for (int32 ViewIndex = 0; ViewIndex < Views.Num(); ViewIndex++)
{
FViewInfo& View = Views[ViewIndex];
FDitherUniformShaderParameters DitherUniformShaderParameters;
DitherUniformShaderParameters.LODFactor = View.GetTemporalLODTransition();
View.DitherFadeOutUniformBuffer = FDitherUniformBufferRef::CreateUniformBufferImmediate(DitherUniformShaderParameters, UniformBuffer_SingleFrame);
DitherUniformShaderParameters.LODFactor = View.GetTemporalLODTransition() - 1.0f;
View.DitherFadeInUniformBuffer = FDitherUniformBufferRef::CreateUniformBufferImmediate(DitherUniformShaderParameters, UniformBuffer_SingleFrame);
}
}
由此可知，PreVisibilityFrameSetup做了大量的初始化工作，如静态网格、Groom、SkinCache、特效、TAA、ViewState等等。接着继续分析ComputeViewVisibility：
// Engine\Source\Runtime\Renderer\Private\SceneVisibility.cpp
void FSceneRenderer::ComputeViewVisibility(FRHICommandListImmediate& RHICmdList, FExclusiveDepthStencil::Type BasePassDepthStencilAccess, FViewVisibleCommandsPerView& ViewCommandsPerView, FGlobalDynamicIndexBuffer& DynamicIndexBuffer, FGlobalDynamicVertexBuffer& DynamicVertexBuffer, FGlobalDynamicReadBuffer& DynamicReadBuffer)
{
// 分配可见光源信息列表。
if (Scene->Lights.GetMaxIndex() > 0)
{
VisibleLightInfos.AddZeroed(Scene->Lights.GetMaxIndex());
}
int32 NumPrimitives = Scene->Primitives.Num();
float CurrentRealTime = ViewFamily.CurrentRealTime;
FPrimitiveViewMasks HasDynamicMeshElementsMasks;
HasDynamicMeshElementsMasks.AddZeroed(NumPrimitives);
FPrimitiveViewMasks HasViewCustomDataMasks;
HasViewCustomDataMasks.AddZeroed(NumPrimitives);
FPrimitiveViewMasks HasDynamicEditorMeshElementsMasks;
if (GIsEditor)
{
HasDynamicEditorMeshElementsMasks.AddZeroed(NumPrimitives);
}
const bool bIsInstancedStereo = (Views.Num() > 0) ? (Views[0].IsInstancedStereoPass() || Views[0].bIsMobileMultiViewEnabled) : false;
UpdateReflectionSceneData(Scene);
// 更新不判断可见性的静态网格.
{
Scene->ConditionalMarkStaticMeshElementsForUpdate();
TArray<FPrimitiveSceneInfo*> UpdatedSceneInfos;
for (TSet<FPrimitiveSceneInfo*>::TIterator It(Scene->PrimitivesNeedingStaticMeshUpdateWithoutVisibilityCheck); It; ++It)
{
FPrimitiveSceneInfo* Primitive = *It;
if (Primitive->NeedsUpdateStaticMeshes())
{
UpdatedSceneInfos.Add(Primitive);
}
}
if (UpdatedSceneInfos.Num() > 0)
{
FPrimitiveSceneInfo::UpdateStaticMeshes(RHICmdList, Scene, UpdatedSceneInfos);
}
Scene->PrimitivesNeedingStaticMeshUpdateWithoutVisibilityCheck.Reset();
}
// 初始化所有view的数据.
uint8 ViewBit = 0x1;
for (int32 ViewIndex = 0; ViewIndex < Views.Num(); ++ViewIndex, ViewBit <<= 1)
{
STAT(NumProcessedPrimitives += NumPrimitives);
FViewInfo& View = Views[ViewIndex];
FViewCommands& ViewCommands = ViewCommandsPerView[ViewIndex];
FSceneViewState* ViewState = (FSceneViewState*)View.State;
// Allocate the view's visibility maps.
View.PrimitiveVisibilityMap.Init(false,Scene->Primitives.Num());
// we don't initialized as we overwrite the whole array (in GatherDynamicMeshElements)
View.DynamicMeshEndIndices.SetNumUninitialized(Scene->Primitives.Num());
View.PrimitiveDefinitelyUnoccludedMap.Init(false,Scene->Primitives.Num());
View.PotentiallyFadingPrimitiveMap.Init(false,Scene->Primitives.Num());
View.PrimitiveFadeUniformBuffers.AddZeroed(Scene->Primitives.Num());
View.PrimitiveFadeUniformBufferMap.Init(false, Scene->Primitives.Num());
View.StaticMeshVisibilityMap.Init(false,Scene->StaticMeshes.GetMaxIndex());
View.StaticMeshFadeOutDitheredLODMap.Init(false,Scene->StaticMeshes.GetMaxIndex());
View.StaticMeshFadeInDitheredLODMap.Init(false,Scene->StaticMeshes.GetMaxIndex());
View.StaticMeshBatchVisibility.AddZeroed(Scene->StaticMeshBatchVisibility.GetMaxIndex());
View.PrimitivesLODMask.Init(FLODMask(), Scene->Primitives.Num());
View.PrimitivesCustomData.Init(nullptr, Scene->Primitives.Num());
// We must reserve to prevent realloc otherwise it will cause memory leak if we Execute In Parallel
const bool WillExecuteInParallel = FApp::ShouldUseThreadingForPerformance() && CVarParallelInitViews.GetValueOnRenderThread() > 0;
View.PrimitiveCustomDataMemStack.Reserve(WillExecuteInParallel ? FMath::CeilToInt(((float)View.PrimitiveVisibilityMap.Num() / (float)FRelevancePrimSet<int32>::MaxInputPrims)) + 1 : 1);
View.AllocateCustomDataMemStack();
View.VisibleLightInfos.Empty(Scene->Lights.GetMaxIndex());
View.DirtyIndirectLightingCacheBufferPrimitives.Reserve(Scene->Primitives.Num());
// 创建光源信息.
for(int32 LightIndex = 0;LightIndex < Scene->Lights.GetMaxIndex();LightIndex++)
{
if( LightIndex+2 < Scene->Lights.GetMaxIndex() )
{
if (LightIndex > 2)
{
FLUSH_CACHE_LINE(&View.VisibleLightInfos(LightIndex-2));
}
}
new(View.VisibleLightInfos) FVisibleLightViewInfo();
}
View.PrimitiveViewRelevanceMap.Empty(Scene->Primitives.Num());
View.PrimitiveViewRelevanceMap.AddZeroed(Scene->Primitives.Num());
const bool bIsParent = ViewState && ViewState->IsViewParent();
if ( bIsParent )
{
ViewState->ParentPrimitives.Empty();
}
if (ViewState)
{
// 获取并解压上一帧的遮挡数据.
View.PrecomputedVisibilityData = ViewState->GetPrecomputedVisibilityData(View, Scene);
}
else
{
View.PrecomputedVisibilityData = NULL;
}
if (View.PrecomputedVisibilityData)
{
bUsedPrecomputedVisibility = true;
}
bool bNeedsFrustumCulling = true;
#if !(UE_BUILD_SHIPPING || UE_BUILD_TEST)
if( ViewState )
{
// 冻结可见性
if(ViewState->bIsFrozen)
{
bNeedsFrustumCulling = false;
for (FSceneBitArray::FIterator BitIt(View.PrimitiveVisibilityMap); BitIt; ++BitIt)
{
if (ViewState->FrozenPrimitives.Contains(Scene->PrimitiveComponentIds[BitIt.GetIndex()]))
{
BitIt.GetValue() = true;
}
}
}
}
#endif
// 平截头体裁剪.
if (bNeedsFrustumCulling)
{
// Update HLOD transition/visibility states to allow use during distance culling
FLODSceneTree& HLODTree = Scene->SceneLODHierarchy;
if (HLODTree.IsActive())
{
QUICK_SCOPE_CYCLE_COUNTER(STAT_ViewVisibilityTime_HLODUpdate);
HLODTree.UpdateVisibilityStates(View);
}
else
{
HLODTree.ClearVisibilityState(View);
}
int32 NumCulledPrimitivesForView;
const bool bUseFastIntersect = (View.ViewFrustum.PermutedPlanes.Num() == 8) && CVarUseFastIntersect.GetValueOnRenderThread();
if (View.CustomVisibilityQuery && View.CustomVisibilityQuery->Prepare())
{
if (CVarAlsoUseSphereForFrustumCull.GetValueOnRenderThread())
{
NumCulledPrimitivesForView = bUseFastIntersect ? FrustumCull<true, true, true>(Scene, View) : FrustumCull<true, true, false>(Scene, View);
}
else
{
NumCulledPrimitivesForView = bUseFastIntersect ? FrustumCull<true, false, true>(Scene, View) : FrustumCull<true, false, false>(Scene, View);
}
}
else
{
if (CVarAlsoUseSphereForFrustumCull.GetValueOnRenderThread())
{
NumCulledPrimitivesForView = bUseFastIntersect ? FrustumCull<false, true, true>(Scene, View) : FrustumCull<false, true, false>(Scene, View);
}
else
{
NumCulledPrimitivesForView = bUseFastIntersect ? FrustumCull<false, false, true>(Scene, View) : FrustumCull<false, false, false>(Scene, View);
}
}
STAT(NumCulledPrimitives += NumCulledPrimitivesForView);
UpdatePrimitiveFading(Scene, View);
}
// 处理隐藏物体.
if (View.HiddenPrimitives.Num())
{
for (FSceneSetBitIterator BitIt(View.PrimitiveVisibilityMap); BitIt; ++BitIt)
{
if (View.HiddenPrimitives.Contains(Scene->PrimitiveComponentIds[BitIt.GetIndex()]))
{
View.PrimitiveVisibilityMap.AccessCorrespondingBit(BitIt) = false;
}
}
}
(......)
// 处理静态场景.
if (View.bStaticSceneOnly)
{
for (FSceneSetBitIterator BitIt(View.PrimitiveVisibilityMap); BitIt; ++BitIt)
{
// Reflection captures should only capture objects that won't move, since reflection captures won't update at runtime
if (!Scene->Primitives[BitIt.GetIndex()]->Proxy->HasStaticLighting())
{
View.PrimitiveVisibilityMap.AccessCorrespondingBit(BitIt) = false;
}
}
}
(......)
// 非线框模式, 则只需遮挡剔除.
if (!View.Family->EngineShowFlags.Wireframe)
{
int32 NumOccludedPrimitivesInView = OcclusionCull(RHICmdList, Scene, View, DynamicVertexBuffer);
STAT(NumOccludedPrimitives += NumOccludedPrimitivesInView);
}
// 处理判断可见性的静态模型.
{
TArray<FPrimitiveSceneInfo*> AddedSceneInfos;
for (TConstDualSetBitIterator<SceneRenderingBitArrayAllocator, FDefaultBitArrayAllocator> BitIt(View.PrimitiveVisibilityMap, Scene->PrimitivesNeedingStaticMeshUpdate); BitIt; ++BitIt)
{
int32 PrimitiveIndex = BitIt.GetIndex();
AddedSceneInfos.Add(Scene->Primitives[PrimitiveIndex]);
}
if (AddedSceneInfos.Num() > 0)
{
FPrimitiveSceneInfo::UpdateStaticMeshes(RHICmdList, Scene, AddedSceneInfos);
}
}
(......)
}
(......)
// 收集所有view的动态网格元素. 上一篇已经详细解析过了.
{
GatherDynamicMeshElements(Views, Scene, ViewFamily, DynamicIndexBuffer, DynamicVertexBuffer, DynamicReadBuffer,
HasDynamicMeshElementsMasks, HasDynamicEditorMeshElementsMasks, HasViewCustomDataMasks, MeshCollector);
}
// 创建每个view的MeshPass数据.
for (int32 ViewIndex = 0; ViewIndex < Views.Num(); ViewIndex++)
{
FViewInfo& View = Views[ViewIndex];
if (!View.ShouldRenderView())
{
continue;
}
FViewCommands& ViewCommands = ViewCommandsPerView[ViewIndex];
SetupMeshPass(View, BasePassDepthStencilAccess, ViewCommands);
}
}
ComputeViewVisibility最重要的功能就是处理图元的可见性，包含平截头体裁剪、遮挡剔除，以及收集动态网格信息、创建光源信息等。下面继续粗略剖预计算可见性的过程：
// Engine\Source\Runtime\Renderer\Private\SceneOcclusion.cpp
const uint8* FSceneViewState::GetPrecomputedVisibilityData(FViewInfo& View, const FScene* Scene)
{
const uint8* PrecomputedVisibilityData = NULL;
if (Scene->PrecomputedVisibilityHandler && GAllowPrecomputedVisibility && View.Family->EngineShowFlags.PrecomputedVisibility)
{
const FPrecomputedVisibilityHandler& Handler = *Scene->PrecomputedVisibilityHandler;
FViewElementPDI VisibilityCellsPDI(&View, nullptr, nullptr);
// 绘制调试用的遮挡剔除方格.
if ((GShowPrecomputedVisibilityCells || View.Family->EngineShowFlags.PrecomputedVisibilityCells) && !GShowRelevantPrecomputedVisibilityCells)
{
for (int32 BucketIndex = 0; BucketIndex < Handler.PrecomputedVisibilityCellBuckets.Num(); BucketIndex++)
{
for (int32 CellIndex = 0; CellIndex < Handler.PrecomputedVisibilityCellBuckets[BucketIndex].Cells.Num(); CellIndex++)
{
const FPrecomputedVisibilityCell& CurrentCell = Handler.PrecomputedVisibilityCellBuckets[BucketIndex].Cells[CellIndex];
// Construct the cell's bounds
const FBox CellBounds(CurrentCell.Min, CurrentCell.Min + FVector(Handler.PrecomputedVisibilityCellSizeXY, Handler.PrecomputedVisibilityCellSizeXY, Handler.PrecomputedVisibilityCellSizeZ));
if (View.ViewFrustum.IntersectBox(CellBounds.GetCenter(), CellBounds.GetExtent()))
{
DrawWireBox(&VisibilityCellsPDI, CellBounds, FColor(50, 50, 255), SDPG_World);
}
}
}
}
// 计算哈希值和桶索引, 以减少搜索时间.
const float FloatOffsetX = (View.ViewMatrices.GetViewOrigin().X - Handler.PrecomputedVisibilityCellBucketOriginXY.X) / Handler.PrecomputedVisibilityCellSizeXY;
// FMath::TruncToInt rounds toward 0, we want to always round down
const int32 BucketIndexX = FMath::Abs((FMath::TruncToInt(FloatOffsetX) - (FloatOffsetX < 0.0f ? 1 : 0)) / Handler.PrecomputedVisibilityCellBucketSizeXY % Handler.PrecomputedVisibilityNumCellBuckets);
const float FloatOffsetY = (View.ViewMatrices.GetViewOrigin().Y -Handler.PrecomputedVisibilityCellBucketOriginXY.Y) / Handler.PrecomputedVisibilityCellSizeXY;
const int32 BucketIndexY = FMath::Abs((FMath::TruncToInt(FloatOffsetY) - (FloatOffsetY < 0.0f ? 1 : 0)) / Handler.PrecomputedVisibilityCellBucketSizeXY % Handler.PrecomputedVisibilityNumCellBuckets);
const int32 PrecomputedVisibilityBucketIndex = BucketIndexY * Handler.PrecomputedVisibilityCellBucketSizeXY + BucketIndexX;
// 绘制可见性物体对应的包围盒.
const FPrecomputedVisibilityBucket& CurrentBucket = Handler.PrecomputedVisibilityCellBuckets[PrecomputedVisibilityBucketIndex];
for (int32 CellIndex = 0; CellIndex < CurrentBucket.Cells.Num(); CellIndex++)
{
const FPrecomputedVisibilityCell& CurrentCell = CurrentBucket.Cells[CellIndex];
// 创建cell的包围盒.
const FBox CellBounds(CurrentCell.Min, CurrentCell.Min + FVector(Handler.PrecomputedVisibilityCellSizeXY, Handler.PrecomputedVisibilityCellSizeXY, Handler.PrecomputedVisibilityCellSizeZ));
// Check if ViewOrigin is inside the current cell
if (CellBounds.IsInside(View.ViewMatrices.GetViewOrigin()))
{
// 检测是否可重复使用已缓存的数据.
if (CachedVisibilityChunk
&& CachedVisibilityHandlerId == Scene->PrecomputedVisibilityHandler->GetId()
&& CachedVisibilityBucketIndex == PrecomputedVisibilityBucketIndex
&& CachedVisibilityChunkIndex == CurrentCell.ChunkIndex)
{
PrecomputedVisibilityData = &(*CachedVisibilityChunk)[CurrentCell.DataOffset];
}
else
{
const FCompressedVisibilityChunk& CompressedChunk = Handler.PrecomputedVisibilityCellBuckets[PrecomputedVisibilityBucketIndex].CellDataChunks[CurrentCell.ChunkIndex];
CachedVisibilityBucketIndex = PrecomputedVisibilityBucketIndex;
CachedVisibilityChunkIndex = CurrentCell.ChunkIndex;
CachedVisibilityHandlerId = Scene->PrecomputedVisibilityHandler->GetId();
// 解压遮挡数据.
if (CompressedChunk.bCompressed)
{
// Decompress the needed visibility data chunk
DecompressedVisibilityChunk.Reset();
DecompressedVisibilityChunk.AddUninitialized(CompressedChunk.UncompressedSize);
verify(FCompression::UncompressMemory(
NAME_Zlib,
DecompressedVisibilityChunk.GetData(),
CompressedChunk.UncompressedSize,
CompressedChunk.Data.GetData(),
CompressedChunk.Data.Num()));
CachedVisibilityChunk = &DecompressedVisibilityChunk;
}
else
{
CachedVisibilityChunk = &CompressedChunk.Data;
}
// Return a pointer to the cell containing ViewOrigin's decompressed visibility data
PrecomputedVisibilityData = &(*CachedVisibilityChunk)[CurrentCell.DataOffset];
}
(......)
}
(......)
}
return PrecomputedVisibilityData;
}
从代码中得知，可见性判定可以绘制出一些调试信息，如每个物体实际用于剔除时的包围盒大小，也可以冻结（Frozen）剔除结果，以便观察遮挡的效率。
由于可见性判定包含预计算、平截头体裁剪、遮挡剔除等，单单遮挡剔除涉及的知识点比较多（绘制、获取、数据压缩解压、存储结构、多线程传递、帧间交互、可见性判定、HLOD等等），此处只对可见性判定做了粗略的浅析，更多详细的机制和过程将会在后续的渲染优化专题中深入剖析。
接下来继续分析InitViews内的PostVisibilityFrameSetup：
// Engine\Source\Runtime\Renderer\Private\SceneVisibility.cpp
void FSceneRenderer::PostVisibilityFrameSetup(FILCUpdatePrimTaskData& OutILCTaskData)
{
// 处理贴花排序和调整历史RT.
{
QUICK_SCOPE_CYCLE_COUNTER(STAT_PostVisibilityFrameSetup_Sort);
for (int32 ViewIndex = 0; ViewIndex < Views.Num(); ViewIndex++)
{
FViewInfo& View = Views[ViewIndex];
View.MeshDecalBatches.Sort();
if (View.State)
{
((FSceneViewState*)View.State)->TrimHistoryRenderTargets(Scene);
}
}
}
(......)
// 处理光源可见性.
{
QUICK_SCOPE_CYCLE_COUNTER(STAT_PostVisibilityFrameSetup_Light_Visibility);
// 遍历所有光源, 结合view的视锥判断可见性(同一个光源可能在有的view可见但其它view不可见).
for(TSparseArray<FLightSceneInfoCompact>::TConstIterator LightIt(Scene->Lights);LightIt;++LightIt)
{
const FLightSceneInfoCompact& LightSceneInfoCompact = *LightIt;
const FLightSceneInfo* LightSceneInfo = LightSceneInfoCompact.LightSceneInfo;
// 利用每个view内的镜头视锥裁剪光源.
for(int32 ViewIndex = 0;ViewIndex < Views.Num();ViewIndex++)
{
const FLightSceneProxy* Proxy = LightSceneInfo->Proxy;
FViewInfo& View = Views[ViewIndex];
FVisibleLightViewInfo& VisibleLightViewInfo = View.VisibleLightInfos[LightIt.GetIndex()];
// 平行方向光不需要裁剪, 只有局部光源才需要裁剪
if( Proxy->GetLightType() == LightType_Point ||
Proxy->GetLightType() == LightType_Spot ||
Proxy->GetLightType() == LightType_Rect )
{
FSphere const& BoundingSphere = Proxy->GetBoundingSphere();
// 判断view的视锥是否和光源包围盒相交.
if (View.ViewFrustum.IntersectSphere(BoundingSphere.Center, BoundingSphere.W))
{
// 透视视锥需要针对最大距离做校正, 剔除距离视点太远的光源.
if (View.IsPerspectiveProjection())
{
FSphere Bounds = Proxy->GetBoundingSphere();
float DistanceSquared = (Bounds.Center - View.ViewMatrices.GetViewOrigin()).SizeSquared();
float MaxDistSquared = Proxy->GetMaxDrawDistance() * Proxy->GetMaxDrawDistance() * GLightMaxDrawDistanceScale * GLightMaxDrawDistanceScale;
// 考量了光源的半径、视图的LOD因子、最小光源屏幕半径等因素来决定最终光源是否需要绘制，以便剔除掉远距离屏幕占比很小的光源。
const bool bDrawLight = (FMath::Square(FMath::Min(0.0002f, GMinScreenRadiusForLights / Bounds.W) * View.LODDistanceFactor) * DistanceSquared < 1.0f)
&& (MaxDistSquared == 0 || DistanceSquared < MaxDistSquared);
VisibleLightViewInfo.bInViewFrustum = bDrawLight;
}
else
{
VisibleLightViewInfo.bInViewFrustum = true;
}
}
}
else
{
// 平行方向光一定可见.
VisibleLightViewInfo.bInViewFrustum = true;
(......)
}
(......)
}
在InitViews末期，会初始化每个view的RHI资源以及通知RHICommandList渲染开始事件。先看看FViewInfo::InitRHIResources：
// Engine\Source\Runtime\Renderer\Private\SceneRendering.cpp
void FViewInfo::InitRHIResources()
{
FBox VolumeBounds[TVC_MAX];
// 创建和设置缓存的视图Uniform Buffer.
CachedViewUniformShaderParameters = MakeUnique<FViewUniformShaderParameters>();
FSceneRenderTargets& SceneContext = FSceneRenderTargets::Get(FRHICommandListExecutor::GetImmediateCommandList());
SetupUniformBufferParameters(
SceneContext,
VolumeBounds,
TVC_MAX,
*CachedViewUniformShaderParameters);
// 创建和设置视图的Uniform Buffer.
ViewUniformBuffer = TUniformBufferRef<FViewUniformShaderParameters>::CreateUniformBufferImmediate(*CachedViewUniformShaderParameters, UniformBuffer_SingleFrame);
const int32 TranslucencyLightingVolumeDim = GetTranslucencyLightingVolumeDim();
// 重置缓存的Uniform Buffer.
FScene* Scene = Family->Scene ? Family->Scene->GetRenderScene() : nullptr;
if (Scene)
{
Scene->UniformBuffers.InvalidateCachedView();
}
// 初始化透明体积光照参数.
for (int32 CascadeIndex = 0; CascadeIndex < TVC_MAX; CascadeIndex++)
{
TranslucencyLightingVolumeMin[CascadeIndex] = VolumeBounds[CascadeIndex].Min;
TranslucencyVolumeVoxelSize[CascadeIndex] = (VolumeBounds[CascadeIndex].Max.X - VolumeBounds[CascadeIndex].Min.X) / TranslucencyLightingVolumeDim;
TranslucencyLightingVolumeSize[CascadeIndex] = VolumeBounds[CascadeIndex].Max - VolumeBounds[CascadeIndex].Min;
}
}
继续解析InitViews末尾的OnStartRender：
// Engine\Source\Runtime\Renderer\Private\SceneRendering.cpp
void FSceneRenderer::OnStartRender(FRHICommandListImmediate& RHICmdList)
{
// 场景MRT的初始化.
FSceneRenderTargets& SceneContext = FSceneRenderTargets::Get(RHICmdList);
FVisualizeTexturePresent::OnStartRender(Views[0]);
CompositionGraph_OnStartFrame();
SceneContext.bScreenSpaceAOIsValid = false;
SceneContext.bCustomDepthIsValid = false;
// 通知ViewState初始化.
for (FViewInfo& View : Views)
{
if (View.ViewState)
{
View.ViewState->OnStartRender(View, ViewFamily);
}
}
}
// Engine\Source\Runtime\Renderer\Private\ScenePrivate.h
void FSceneViewState::OnStartRender(FViewInfo& View, FSceneViewFamily& ViewFamily)
{
// 初始化和设置光照传输体积.
if(!(View.FinalPostProcessSettings.IndirectLightingColor * View.FinalPostProcessSettings.IndirectLightingIntensity).IsAlmostBlack())
{
SetupLightPropagationVolume(View, ViewFamily);
}
// 分配软件级场景遮挡剔除(针对不支持硬件遮挡剔除的低端机)
ConditionallyAllocateSceneSoftwareOcclusion(View.GetFeatureLevel());
}
PrePass
PrePass又被称为提前深度Pass、Depth Only Pass、Early-Z Pass，用来渲染不透明物体的深度。此Pass只会写入深度而不会写入颜色，写入深度时有disabled、occlusion only、complete depths三种模式，视不同的平台和Feature Level决定。
PrePass可以由DBuffer发起，也可由Forward Shading触发，通常用来建立Hierarchical-Z，以便能够开启硬件的Early-Z技术，还可被用于遮挡剔除，提升Base Pass的渲染效率。