剖析虚幻渲染体系（12）- 移动端专题Part 1（UE移动端渲染分析）
12.1 本篇概述
前面的所有篇章都是基于PC端的延迟渲染管线阐述UE的渲染体系的，特别是剖析虚幻渲染体系（04）- 延迟渲染管线详尽地阐述了在PC端的延迟渲染管线的流程和步骤。
此篇只要针对UE的移动端的渲染管线进行阐述，最终还会对比移动和和PC端的渲染差异，以及特殊的优化措施。本篇主要阐述UE渲染体系的以下内容：
- FMobileSceneRenderer的主要流程和步骤。
- 移动端的前向和延迟渲染管线。
- 移动端的光影和阴影。
- 移动端和PC端的异同，以及涉及的特殊优化技巧。
特别要指出的是，本篇分析的UE源码升级到了4.27.1，需要同步看源码的同学注意更新了。
如果要在PC的UE编辑器开启移动端渲染管线，可以选择如下所示的菜单：
等待Shader编译完成，UE编辑器的视口内便是移动端的预览效果。
12.1.1 移动设备的特点
相比PC桌面平台，移动端在尺寸、电量、硬件性能等诸多方面都存在显著的差异，具体表现在：
-
更小的尺寸。移动端的便携性就要求整机设备必须轻巧，可置于掌中或口袋内，所以整机只能限制在非常小的体积之内。
-
有限的能量和功率。受限于电池存储技术，目前的主流锂电池普通在1万毫安，但移动设备的分辨率、画质却越来越高，为了满足足够长的续航和散热限制，必须严格控制移动设备的整机功率，通常在5w以内。
-
散热方式受限。PC设备通常可以安装散热风扇、甚至水冷系统，而移动设备不具备这些主动散热方式，只能靠热传导散热。如果散热不当，CPU和GPU都会主动降频，以非常有限的性能运行，以免设备元器件因过热而损毁。
-
有限的硬件性能。移动设备的各类元件（CPU、带宽、内存、GPU等）的性能都只是PC设备的数十分之一。
2018年，主流PC设备（NV GV100-400-A1 Titan V）和主流移动设备（Samsung Exynos 9 8895）的性能对比图。移动设备的很多硬件性能只是PC设备的几十分之一，但分辨率却接近PC的一半，更加突显了移动设备的挑战和窘境。
到了2020年，主流的移动设备性能如下所示：
-
特殊的硬件架构。如CPU和GPU共享内存存储设备，被称为耦合式架构，还有GPU的TB(D)R架构，目的都是为了在低功耗内完成尽可能多的操作。
PC设备的解耦硬件架构和移动设备的耦合式硬件架构对比图。
除此之外，不同于PC端的CPU和GPU纯粹地追求计算性能，衡量移动端的性能有三个指标：性能（Performance）、能量（Power）、面积（Area），俗称PPA。（下图）
衡量移动设备的三个基本参数：Performance、Area、Power，其中Compute Density（计算密度）涉及性能和面积，能耗比涉及性能和能力消耗，越大越好。
与移动设备一起崛起的还有XR设备，它是移动设备的一个重要发展分支。目前存在着各种不同大小、功能、应用场景的XR设备：
各种形式的XR设备。
随着近来元宇宙（Metaverse）的爆火，以及FaceBook改名为Meta，加之Apple、MicroSoft、NVidia、Google等科技巨头都在加紧布局面向未来的沉浸式体验，XR设备作为能够最接近元宇宙畅想的载体和入口，自然成为一条未来非常有潜力能够出现巨无霸的全新赛道。
12.2 UE移动端渲染特性
本章阐述一下UE4.27在移动端的渲染特性。
12.2.1 Feature Level
UE在移动端支持以下图形API：
| Feature Level | 说明 |
|---|---|
| OpenGL ES 3.1 | 安卓系统的默认特性等级，可以在工程设置（Project Settings > Platforms > Android Material Quality - ES31）配置具体的材质参数。 |
| Android Vulkan | 可用于某些特定Android设备的高端渲染器，支持Vulkan 1.2 API，轻量级设计理念的Vulkan，多数情况下会比OpenGL更高效。 |
| Metal 2.0 | 专用于iOS设备的特性等级。可以在Project Settings > Platforms > iOS Material Quality配置材质参数。 |
在目前的主流安卓设备，使用Vulkan能获得更好的性能，原因在于Vulkan轻量级的设计理念，使得UE等应用程序能够更加精准地执行优化。下面是Vulkan和OpenGL的对照表：
| Vulkan | OpenGL |
|---|---|
| 基于对象的状态，没有全局状态。 | 单一的全局状态机。 |
| 所有的状态概念都放置到命令缓冲区中。 | 状态被绑定到单个上下文。 |
| 可以多线程编码。 | 渲染操作只能被顺序执行。 |
| 可以精确、显式地操控GPU的内存和同步。 | GPU的内存和同步细节通常被驱动程序隐藏起来。 |
| 驱动程序没有运行时错误检测，但存在针对开发人员的验证层。 | 广泛的运行时错误检测。 |
如果在Windows平台，UE编辑器也可以启动OpenGL、Vulkan、Metal的模拟器，以便在编辑器时预览效果，但可能跟实际的运行设备的画面有差异，不可完全依赖此功能。
开启Vulkan前需要在工程中配置一些参数，具体参看官方文档Android Vulkan Mobile Renderer。
另外，UE早前几个版本就移除了windows下的OpenGL支持，虽然目前UE编辑器还存在OpenGL的模拟选项，但实际上底层是用D3D渲染的。
12.2.2 Deferred Shading
UE的Deferred Shading（延迟着色）是在4.26才加入的功能，使得开发者能够在移动端实现较复杂的光影效果，诸如高质量反射、多动态光照、贴花、高级光照特性。
上：前向渲染；下：延迟渲染。
如果要在移动端开启延迟渲染，需要在工程配置目录下的DefaultEngine.ini添加r.Mobile.ShadingPath=1字段，然后重启编辑器。
12.2.3 Ground Truth Ambient Occlusion
Ground Truth Ambient Occlusion (GTAO)是接近现实世界的环境遮挡技术，是阴影的一种补偿，能够遮蔽一部分非直接光照，从而获得良好的软阴影效果。
开启了GTAO的效果，注意机器人靠近墙面时，会在墙面留下具有渐变的软阴影效果。
为了开启GTAO，需要勾选以下所示的选项：
此外，GTAO依赖Mobile HDR的选项，为了在对应目标设备开启，还需要在[Platform]Scalability.ini的配置中添加r.Mobile.AmbientOcclusionQuality字段，并且值需要大于0，否则GTAO将被禁用。
值得注意的是，GTAO在Mali设备上存在性能问题，因为它们的最大Compute Shader线程数量少于1024个。
12.2.4 Dynamic Lighting and Shadow
UE在移动端实现的光源特性有：
- 线性空间的HDR光照。
- 带方向的光照图（考虑了法线）。
- 太阳（平行光）支持距离场阴影 + 解析的镜面高光。
- IBL光照：每个物体采样了最近的一个反射捕捉器，而没有视差校正。
- 动态物体能正确地接受光照，也可以投射阴影。
UE移动端支持的动态光源的类型、数量、阴影等信息如下：
| 光源类型 | 最大数量 | 阴影 | 描述 |
|---|---|---|---|
| 平行光 | 1 | CSM | CSM默认是2级，最多支持4级。 |
| 点光源 | 4 | 不支持 | 点光源阴影需要立方体阴影图，而单Pass渲染立方体阴影（OnePassPointLightShadow）的技术需要GS（SM5才有）才支持。 |
| 聚光灯 | 4 | 支持 | 默认禁用，需要在工程中开启。 |
| 区域光 | 0 | 不支持 | 目前不支持动态区域光照效果。 |
动态的聚光灯需要在工程配置中显式开启：
在移动BasePass的像素着色器中，聚光灯阴影图与CSM共享相同的纹理采样器，并且聚光灯阴影和CSM使用相同的阴影图图集。CSM能够保证有足够的空间，而聚光灯将按阴影分辨率排序。
默认情况下，可见阴影的最大数量被限制为8个，但可以通过改变r.Mobile.MaxVisibleMovableSpotLightsShadow的值来改变上限值。聚光灯阴影的分辨率是基于屏幕大小和r.Shadow.TexelsPerPixelSpotlight。
在前向渲染路径中，局部光源（点光源和聚光灯）的总数不能超过4个。
移动端还支持一种特殊的阴影模式，那就是调制阴影（Modulated Shadows），只能用于固定（Stationary）的平行光。开启了调制阴影的效果图如下：
调制阴影还支持改变阴影颜色和混合比例：
左：动态阴影；右：调制阴影。
移动端的阴影同样支持自阴影、阴影质量等级（r.shadowquality）、深度偏移等参数的设置。
此外，移动端默认使用了GGX的高光反射，如果想切换到传统的高光着色模型，可以在以下配置里修改：
12.2.5 Pixel Projected Reflection
UE针对移动端做了一个优化版的SSR，被称为Pixel Projected Reflection（PPR），也是复用屏幕空间像素的核心思想。
PPR效果图。
为了开启PPR效果，需要满足以下条件：
-
开启MobileHDR选项。
-
r.Mobile.PixelProjectedReflectionQuality的值大于0。 -
设置Project Settings > Mobile and set the Planar Reflection Mode成正确的模式：
Planar Reflection Mode有3个选项：
- Usual：平面反射Actor在所有平台上的作用都是相同的。
- MobilePPR：平面反射Actor在PC/主机平台上正常工作，但在移动平台上使用PPR渲染。
- MobilePPRExclusive：平面反射Actor将只用于移动平台上的PPR，为PC和Console项目留下了使用传统SSR的空间。
默认只有高端移动设备才会在[Project]Scalability.ini开启r.Mobile.PixelProjectedReflectionQuality。
12.2.6 Mesh Auto-Instancing
PC端的网格绘制管线已经支持了网格的自动化实例和合并绘制，这个特性可以极大提升渲染性能。4.27已经在移动端支持了这一特性。
若想开启，则需要打开工程配置目录下的DefaultEngine.ini，添加以下字段：
r.Mobile.SupportGPUScene=1
r.Mobile.UseGPUSceneTexture=1
重启编辑器，等待Shader编译完即可预览效果。
由于需要GPUSceneTexture支持，而Mali设备的Uniform Buffer最大只有64kb，以致无法支持足够大的空间，所以，Mali设备会使用纹理而非缓冲区来存储GPUScene数据。
但也存在一些限制：
-
移动设备上的自动实例化主要有利于CPU密集型项目，而不是GPU密集型项目。虽然启用自动实例化不太可能会对GPU密集型的项目造成损害，但不太可能看到使用它带来的显著性能改进。
-
如果一款游戏或应用需要大量内存，那么关闭
r.Mobile.UseGPUSceneTexture并使用缓冲区可能会更有好处，因为它无法在Mali设备上正常运行。也可以针对Mali设备关闭
r.Mobile.UseGPUSceneTexture，而其它GPU厂商的设备正常使用。
自动实例化的有效性很大程度上取决于项目的确切规范和定位，建议创建一个启用了自动实例化的构建，并对其进行概要分析，以确定是否会看到实质性的性能提升。
12.2.7 Post Processing
由于移动设备存在更慢的依赖纹理读取（dependent texture read）、有限的硬件特性、特殊的硬件架构、额外的渲染目标解析、有限的带宽等限制性因素，故而后处理在移动设备上执行起来会比较耗性能，有些极端情况会卡住渲染管线。
尽管如此，在某些画质要求高的游戏或应用，依然非常依赖后处理的强劲表现力，为高品质迈上几个台阶。UE不会限制开发者使用后处理。
为了开启后处理，必须先开启MobileHDR选项：
开启后处理之后，就可以在后处理体积（Post Process Volume）设置各种后处理效果。
在移动端可以支持的后处理有Mobile Tonemapper、Color Grading、Lens、Bloom、Dirt Mask、Auto Exposure、Lens Flares、Depth of Field等等。
为了获得更好的性能，官方给出的建议是在移动端只开启Bloom和TAA。
12.2.8 其它特性和限制
- Reflection Capture Compression
移动端支持反射捕捉器组件（Reflection Capture Component）的压缩，可以减少Reflection Capture运行时的内存和带宽，提升渲染效率。需要在工程配置中开启：
开启之后，默认使用ETC2进行压缩。另外，也可以针对每个Reflection Capture Component进行调整：
- 材质特性
移动平台上的材质(特性级别Open ES 3.1)使用与其他平台相同的基于节点的创建过程，并且绝大多数节点在移动端都支持。
移动平台支持的材质属性有：BaseColor、Roughness、Metallic、Specular、Normal、Emissive、Refraction，但不支持Scene Color表达式、Tessellation输入、次表面散射着色模型。
移动平台支持的材质存在一些限制：
- 由于硬件限制，只能使用16个纹理采样器。
- 只有DefaultLit和Unlit着色模型可用。
- 自定义UV应该用来避免依赖纹理读取（没有纹理uv的数学计算）。
- 半透明和Masked材质是及其耗性能， 建议尽量使用不透明材质。
- 深度淡出（Depth fade）可以在iOS平台的半透明材质中使用，但在硬件不支持从深度缓冲区获取数据的平台上，是不受支持的，将导致不可接受的性能成本。
材质属性面板存在一些针对移动端的特殊选项：
这些属性的说明如下：
-
Mobile Separate Translucency：是否在移动端开启单独的半透明渲染纹理。
-
Use Full Precision：是否使用全精度，如果否，可以减少带宽占用和能耗，提升性能，但可能会出现远处物体的瑕疵：
左：全精度材质；右：半精度材质，远处的太阳出现了瑕疵。
-
Use Lightmap Directionality：是否开启光照图的方向性，若勾选，会考虑光照图的方向和像素法线，但会提升性能消耗。
-
Use Alpha to Coverage：是否为Masked材质开启MSAA抗锯齿，若勾选，会开启MSAA。
-
Fully Rough：是否完全粗糙，如果勾选，将极大提升此材质的渲染效率。
此外，移动端支持的网格类型有：
- Skeletal Mesh
- Static Mesh
- Landscape
- CPU particle sprites, particle meshes
除上述类型以外的其它都不被支持。其它限制还有：
- 单个网格最多只能到65k，因为顶点索引只有16位。
- 单个Skeletal Mesh的骨骼数量必须在75个以内，因为受硬件性能的限制。
12.3 FMobileSceneRenderer
FMobileSceneRenderer继承自FSceneRenderer，它负责移动端的场景渲染流程，而PC端是同样继承自FSceneRenderer的FDeferredShadingSceneRenderer。它们的继承关系图如下：
classDiagram-v2
FSceneRenderer <|-- FMobileSceneRenderer
FSceneRenderer <|-- FDeferredShadingSceneRenderer
前述多篇文章已经提及了FDeferredShadingSceneRenderer，它的渲染流程尤为复杂，包含了复杂的光影和渲染步骤。相比之下，FMobileSceneRenderer的逻辑和步骤会简单许多，下面是RenderDoc的截帧：
以上主要包含了InitViews、ShadowDepths、PrePass、BasePass、OcclusionTest、ShadowProjectionOnOpaque、Translucency、PostProcessing等步骤。其中这些步骤在PC端都是存在的，但实现过程可能会有所不同。见后续章节剖析。
12.3.1 渲染器主流程
移动端的场景渲染器的主流程也发生在FMobileSceneRenderer::Render中，代码和解析如下：
// Engine\Source\Runtime\Renderer\Private\MobileShadingRenderer.cpp
void FMobileSceneRenderer::Render(FRHICommandListImmediate& RHICmdList)
{
// 更新图元场景信息。
Scene->UpdateAllPrimitiveSceneInfos(RHICmdList);
// 准备视图的渲染区域.
PrepareViewRectsForRendering(RHICmdList);
// 准备天空大气的数据
if (ShouldRenderSkyAtmosphere(Scene, ViewFamily.EngineShowFlags))
{
for (int32 LightIndex = 0; LightIndex < NUM_ATMOSPHERE_LIGHTS; ++LightIndex)
{
if (Scene->AtmosphereLights[LightIndex])
{
PrepareSunLightProxy(*Scene->GetSkyAtmosphereSceneInfo(), LightIndex, *Scene->AtmosphereLights[LightIndex]);
}
}
}
else
{
Scene->ResetAtmosphereLightsProperties();
}
if(!ViewFamily.EngineShowFlags.Rendering)
{
return;
}
// 等待遮挡剔除测试.
WaitOcclusionTests(RHICmdList);
FRHICommandListExecutor::GetImmediateCommandList().PollOcclusionQueries();
RHICmdList.ImmediateFlush(EImmediateFlushType::DispatchToRHIThread);
// 初始化视图, 查找可见图元, 准备渲染所需的RT和缓冲区等数据.
InitViews(RHICmdList);
if (GRHINeedsExtraDeletionLatency || !GRHICommandList.Bypass())
{
QUICK_SCOPE_CYCLE_COUNTER(STAT_FMobileSceneRenderer_PostInitViewsFlushDel);
// 可能会暂停遮挡查询，所以最好在等待时让RHI线程和GPU工作. 此外，当执行RHI线程时，这是唯一将处理挂起删除的位置.
FRHICommandListExecutor::GetImmediateCommandList().PollOcclusionQueries();
FRHICommandListExecutor::GetImmediateCommandList().ImmediateFlush(EImmediateFlushType::FlushRHIThreadFlushResources);
}
GEngine->GetPreRenderDelegate().Broadcast();
// 在渲染开始前提交全局动态缓冲.
DynamicIndexBuffer.Commit();
DynamicVertexBuffer.Commit();
DynamicReadBuffer.Commit();
RHICmdList.ImmediateFlush(EImmediateFlushType::DispatchToRHIThread);
RHICmdList.SetCurrentStat(GET_STATID(STAT_CLMM_SceneSim));
if (ViewFamily.bLateLatchingEnabled)
{
BeginLateLatching(RHICmdList);
}
FSceneRenderTargets& SceneContext = FSceneRenderTargets::Get(RHICmdList);
// 处理虚拟纹理
if (bUseVirtualTexturing)
{
SCOPED_GPU_STAT(RHICmdList, VirtualTextureUpdate);
FVirtualTextureSystem::Get().Update(RHICmdList, FeatureLevel, Scene);
// Clear virtual texture feedback to default value
FUnorderedAccessViewRHIRef FeedbackUAV = SceneContext.GetVirtualTextureFeedbackUAV();
RHICmdList.Transition(FRHITransitionInfo(FeedbackUAV, ERHIAccess::SRVMask, ERHIAccess::UAVMask));
RHICmdList.ClearUAVUint(FeedbackUAV, FUintVector4(~0u, ~0u, ~0u, ~0u));
RHICmdList.Transition(FRHITransitionInfo(FeedbackUAV, ERHIAccess::UAVMask, ERHIAccess::UAVMask));
RHICmdList.BeginUAVOverlap(FeedbackUAV);
}
// 已排序的光源信息.
FSortedLightSetSceneInfo SortedLightSet;
// 延迟渲染.
if (bDeferredShading)
{
// 收集和排序光源.
GatherAndSortLights(SortedLightSet);
int32 NumReflectionCaptures = Views[0].NumBoxReflectionCaptures + Views[0].NumSphereReflectionCaptures;
bool bCullLightsToGrid = (NumReflectionCaptures > 0 || GMobileUseClusteredDeferredShading != 0);
FRDGBuilder GraphBuilder(RHICmdList);
// 计算光源格子.
ComputeLightGrid(GraphBuilder, bCullLightsToGrid, SortedLightSet);
GraphBuilder.Execute();
}
// 生成天空/大气LUT.
const bool bShouldRenderSkyAtmosphere = ShouldRenderSkyAtmosphere(Scene, ViewFamily.EngineShowFlags);
if (bShouldRenderSkyAtmosphere)
{
FRDGBuilder GraphBuilder(RHICmdList);
RenderSkyAtmosphereLookUpTables(GraphBuilder);
GraphBuilder.Execute();
}
// 通知特效系统场景准备渲染.
if (FXSystem && ViewFamily.EngineShowFlags.Particles)
{
FXSystem->PreRender(RHICmdList, NULL, !Views[0].bIsPlanarReflection);
if (FGPUSortManager* GPUSortManager = FXSystem->GetGPUSortManager())
{
GPUSortManager->OnPreRender(RHICmdList);
}
}
// 轮询遮挡剔除请求.
FRHICommandListExecutor::GetImmediateCommandList().PollOcclusionQueries();
RHICmdList.ImmediateFlush(EImmediateFlushType::DispatchToRHIThread);
RHICmdList.SetCurrentStat(GET_STATID(STAT_CLMM_Shadows));
// 渲染阴影.
RenderShadowDepthMaps(RHICmdList);
FRHICommandListExecutor::GetImmediateCommandList().PollOcclusionQueries();
RHICmdList.ImmediateFlush(EImmediateFlushType::DispatchToRHIThread);
// 收集视图列表.
TArray<const FViewInfo*> ViewList;
for (int32 ViewIndex = 0; ViewIndex < Views.Num(); ViewIndex++)
{
ViewList.Add(&Views[ViewIndex]);
}
// 渲染自定义深度.
if (bShouldRenderCustomDepth)
{
FRDGBuilder GraphBuilder(RHICmdList);
FSceneTextureShaderParameters SceneTextures = CreateSceneTextureShaderParameters(GraphBuilder, Views[0].GetFeatureLevel(), ESceneTextureSetupMode::None);
RenderCustomDepthPass(GraphBuilder, SceneTextures);
GraphBuilder.Execute();
}
// 渲染深度PrePass.
if (bIsFullPrepassEnabled)
{
// SDF和AO需要完整的PrePass深度.
FRHIRenderPassInfo DepthPrePassRenderPassInfo(
SceneContext.GetSceneDepthSurface(),
EDepthStencilTargetActions::ClearDepthStencil_StoreDepthStencil);
DepthPrePassRenderPassInfo.NumOcclusionQueries = ComputeNumOcclusionQueriesToBatch();
DepthPrePassRenderPassInfo.bOcclusionQueries = DepthPrePassRenderPassInfo.NumOcclusionQueries != 0;
RHICmdList.BeginRenderPass(DepthPrePassRenderPassInfo, TEXT("DepthPrepass"));
RHICmdList.SetCurrentStat(GET_STATID(STAT_CLM_MobilePrePass));
// 渲染完整的深度PrePass.
RenderPrePass(RHICmdList);
// 提交遮挡剔除.
RHICmdList.SetCurrentStat(GET_STATID(STAT_CLMM_Occlusion));
RenderOcclusion(RHICmdList);
RHICmdList.EndRenderPass();
// SDF阴影
if (bRequiresDistanceFieldShadowingPass)
{
CSV_SCOPED_TIMING_STAT_EXCLUSIVE(RenderSDFShadowing);
RenderSDFShadowing(RHICmdList);
}
// HZB.
if (bShouldRenderHZB)
{
RenderHZB(RHICmdList, SceneContext.SceneDepthZ);
}
// AO.
if (bRequiresAmbientOcclusionPass)
{
RenderAmbientOcclusion(RHICmdList, SceneContext.SceneDepthZ);
}
}
FRHITexture* SceneColor = nullptr;
// 延迟渲染.
if (bDeferredShading)
{
SceneColor = RenderDeferred(RHICmdList, ViewList, SortedLightSet);
}
// 前向渲染.
else
{
SceneColor = RenderForward(RHICmdList, ViewList);
}
// 渲染速度缓冲.
if (bShouldRenderVelocities)
{
FRDGBuilder GraphBuilder(RHICmdList);
FRDGTextureMSAA SceneDepthTexture = RegisterExternalTextureMSAA(GraphBuilder, SceneContext.SceneDepthZ);
FRDGTextureRef VelocityTexture = TryRegisterExternalTexture(GraphBuilder, SceneContext.SceneVelocity);
if (VelocityTexture != nullptr)
{
AddClearRenderTargetPass(GraphBuilder, VelocityTexture);
}
// 渲染可移动物体的速度缓冲.
AddSetCurrentStatPass(GraphBuilder, GET_STATID(STAT_CLMM_Velocity));
RenderVelocities(GraphBuilder, SceneDepthTexture.Resolve, VelocityTexture, FSceneTextureShaderParameters(), EVelocityPass::Opaque, false);
AddSetCurrentStatPass(GraphBuilder, GET_STATID(STAT_CLMM_AfterVelocity));
// 渲染透明物体的速度缓冲.
AddSetCurrentStatPass(GraphBuilder, GET_STATID(STAT_CLMM_TranslucentVelocity));
RenderVelocities(GraphBuilder, SceneDepthTexture.Resolve, VelocityTexture, GetSceneTextureShaderParameters(CreateMobileSceneTextureUniformBuffer(GraphBuilder, EMobileSceneTextureSetupMode::SceneColor)), EVelocityPass::Translucent, false);
GraphBuilder.Execute();
}
// 处理场景渲染后的逻辑.
{
FRendererModule& RendererModule = static_cast<FRendererModule&>(GetRendererModule());
FRDGBuilder GraphBuilder(RHICmdList);
RendererModule.RenderPostOpaqueExtensions(GraphBuilder, Views, SceneContext);
if (FXSystem && Views.IsValidIndex(0))
{
AddUntrackedAccessPass(GraphBuilder, [this](FRHICommandListImmediate& RHICmdList)
{
check(RHICmdList.IsOutsideRenderPass());
FXSystem->PostRenderOpaque(
RHICmdList,
Views[0].ViewUniformBuffer,
nullptr,
nullptr,
Views[0].AllowGPUParticleUpdate()
);
if (FGPUSortManager* GPUSortManager = FXSystem->GetGPUSortManager())
{
GPUSortManager->OnPostRenderOpaque(RHICmdList);
}
});
}
GraphBuilder.Execute();
}
// 刷新/提交命令缓冲.
if (bSubmitOffscreenRendering)
{
RHICmdList.SubmitCommandsHint();
RHICmdList.ImmediateFlush(EImmediateFlushType::DispatchToRHIThread);
}
// 转换场景颜色成SRV, 以供后续步骤读取.
if (!bGammaSpace || bRenderToSceneColor)
{
RHICmdList.Transition(FRHITransitionInfo(SceneColor, ERHIAccess::Unknown, ERHIAccess::SRVMask));
}
if (bDeferredShading)
{
// 释放场景渲染目标上的原始引用.
SceneContext.AdjustGBufferRefCount(RHICmdList, -1);
}
RHICmdList.SetCurrentStat(GET_STATID(STAT_CLMM_Post));
// 处理虚拟纹理.
if (bUseVirtualTexturing)
{
SCOPED_GPU_STAT(RHICmdList, VirtualTextureUpdate);
// No pass after this should make VT page requests
RHICmdList.EndUAVOverlap(SceneContext.VirtualTextureFeedbackUAV);
RHICmdList.Transition(FRHITransitionInfo(SceneContext.VirtualTextureFeedbackUAV, ERHIAccess::UAVMask, ERHIAccess::SRVMask));
TArray<FIntRect, TInlineAllocator<4>> ViewRects;
ViewRects.AddUninitialized(Views.Num());
for (int32 ViewIndex = 0; ViewIndex < Views.Num(); ++ViewIndex)
{
ViewRects[ViewIndex] = Views[ViewIndex].ViewRect;
}
FVirtualTextureFeedbackBufferDesc Desc;
Desc.Init2D(SceneContext.GetBufferSizeXY(), ViewRects, SceneContext.GetVirtualTextureFeedbackScale());
SubmitVirtualTextureFeedbackBuffer(RHICmdList, SceneContext.VirtualTextureFeedback, Desc);
}
FMemMark Mark(FMemStack::Get());
FRDGBuilder GraphBuilder(RHICmdList);
FRDGTextureRef ViewFamilyTexture = TryCreateViewFamilyTexture(GraphBuilder, ViewFamily);
// 解析场景
if (ViewFamily.bResolveScene)
{
if (!bGammaSpace || bRenderToSceneColor)
{
// 完成每个视图的渲染或完整的立体声缓冲区(如果启用)
{
RDG_EVENT_SCOPE(GraphBuilder, "PostProcessing");
SCOPE_CYCLE_COUNTER(STAT_FinishRenderViewTargetTime);
TArray<TRDGUniformBufferRef<FMobileSceneTextureUniformParameters>, TInlineAllocator<1, SceneRenderingAllocator>> MobileSceneTexturesPerView;
MobileSceneTexturesPerView.SetNumZeroed(Views.Num());
const auto SetupMobileSceneTexturesPerView = [&]()
{
for (int32 ViewIndex = 0; ViewIndex < Views.Num(); ++ViewIndex)
{
EMobileSceneTextureSetupMode SetupMode = EMobileSceneTextureSetupMode::SceneColor;
if (Views[ViewIndex].bCustomDepthStencilValid)
{
SetupMode |= EMobileSceneTextureSetupMode::CustomDepth;
}
if (bShouldRenderVelocities)
{
SetupMode |= EMobileSceneTextureSetupMode::SceneVelocity;
}
MobileSceneTexturesPerView[ViewIndex] = CreateMobileSceneTextureUniformBuffer(GraphBuilder, SetupMode);
}
};
SetupMobileSceneTexturesPerView();
FMobilePostProcessingInputs PostProcessingInputs;
PostProcessingInputs.ViewFamilyTexture = ViewFamilyTexture;
// 渲染后处理效果.
for (int32 ViewIndex = 0; ViewIndex < Views.Num(); ViewIndex++)
{
RDG_EVENT_SCOPE_CONDITIONAL(GraphBuilder, Views.Num() > 1, "View%d", ViewIndex);
PostProcessingInputs.SceneTextures = MobileSceneTexturesPerView[ViewIndex];
AddMobilePostProcessingPasses(GraphBuilder, Views[ViewIndex], PostProcessingInputs, NumMSAASamples > 1);
}
}
}
}
GEngine->GetPostRenderDelegate().Broadcast();
RHICmdList.SetCurrentStat(GET_STATID(STAT_CLMM_SceneEnd));
if (bShouldRenderVelocities)
{
SceneContext.SceneVelocity.SafeRelease();
}
if (ViewFamily.bLateLatchingEnabled)
{
EndLateLatching(RHICmdList, Views[0]);
}
RenderFinish(GraphBuilder, ViewFamilyTexture);
GraphBuilder.Execute();
// 轮询遮挡剔除请求.
FRHICommandListExecutor::GetImmediateCommandList().PollOcclusionQueries();
FRHICommandListExecutor::GetImmediateCommandList().ImmediateFlush(EImmediateFlushType::DispatchToRHIThread);
}
看过剖析虚幻渲染体系（04）- 延迟渲染管线篇章的同学应该都知道，移动端的场景渲染过程精简了很多很多步骤，相当于是PC端场景渲染器的一个子集。当然，为了适应移动端特有的GPU硬件架构，移动端的场景渲染也有区别于PC端的地方。后面会详细剖析。移动端场景的主要步骤和流程如下所示：
stateDiagram-v2
state bDeferredShading <<choice>>
state bIsFullPrepassEnabled <<choice>>
[*] --> UpdateAllPrimitiveSceneInfos
UpdateAllPrimitiveSceneInfos --> PrepareViewRectsForRendering
PrepareViewRectsForRendering --> InitViews
InitViews --> bDeferredShading
bDeferredShading --> RenderSkyAtmosphereLookUpTables* : No
bDeferredShading --> GatherAndSortLights: Yes
GatherAndSortLights --> ComputeLightGrid
ComputeLightGrid --> RenderSkyAtmosphereLookUpTables*
RenderSkyAtmosphereLookUpTables* --> RenderShadowDepthMaps
RenderShadowDepthMaps --> RenderCustomDepthPass*
RenderCustomDepthPass* --> bIsFullPrepassEnabled
bIsFullPrepassEnabled --> bDeferredShading2: No
bIsFullPrepassEnabled --> RenderPrePass:Yes
RenderPrePass --> RenderOcclusion
RenderOcclusion --> RenderSDFShadowing*
RenderSDFShadowing* --> RenderHZB*
RenderHZB* --> RenderAmbientOcclusion*
RenderAmbientOcclusion* --> bDeferredShading2
bDeferredShading2-->RenderForward:No
bDeferredShading2-->RenderDeferred:Yes
RenderDeferred --> RenderVelocities*
RenderForward --> RenderVelocities*
RenderVelocities* --> AddMobilePostProcessingPasses*
AddMobilePostProcessingPasses*-->RenderFinish
RenderFinish --> [*]
关于上面的流程图，有以下几点需要加以说明：
- 流程图节点
bDeferredShading和bDeferredShading2是同一个变量，这里区分开主要是为了防止mermaid语法绘图错误。 - 带*的节点是有条件的，非必然执行的步骤。
UE4.26便加入了移动端的延迟渲染管线，所以上述代码中有前向渲染分支RenderForward和延迟渲染分支RenderDeferred，它们返回的都是渲染结果SceneColor。
移动端也支持了图元GPU场景、SDF阴影、AO、天空大气、虚拟纹理、遮挡剔除等渲染特性。
自UE4.26开始，渲染体系广泛地使用了RDG系统，移动端的场景渲染器也不例外。上述代码中总共声明了数个FRDGBuilder实例，用于计算光源格子，以及渲染天空大气LUT、自定义深度、速度缓冲、渲染后置事件、后处理等，它们都是相对独立的功能模块或渲染阶段。
12.3.2 RenderForward
RenderForward在移动端场景渲染器中负责前向渲染的分支，它的代码和解析如下：
FRHITexture* FMobileSceneRenderer::RenderForward(FRHICommandListImmediate& RHICmdList, const TArrayView<const FViewInfo*> ViewList)
{
const FViewInfo& View = *ViewList[0];
FSceneRenderTargets& SceneContext = FSceneRenderTargets::Get(RHICmdList);
FRHITexture* SceneColor = nullptr;
FRHITexture* SceneColorResolve = nullptr;
FRHITexture* SceneDepth = nullptr;
ERenderTargetActions ColorTargetAction = ERenderTargetActions::Clear_Store;
EDepthStencilTargetActions DepthTargetAction = EDepthStencilTargetActions::ClearDepthStencil_DontStoreDepthStencil;
// 是否启用移动端MSAA.
bool bMobileMSAA = NumMSAASamples > 1 && SceneContext.GetSceneColorSurface()->GetNumSamples() > 1;
// 是否启用移动端多试图模式.
static const auto CVarMobileMultiView = IConsoleManager::Get().FindTConsoleVariableDataInt(TEXT("vr.MobileMultiView"));
const bool bIsMultiViewApplication = (CVarMobileMultiView && CVarMobileMultiView->GetValueOnAnyThread() != 0);
// gamma空间的渲染分支.
if (bGammaSpace && !bRenderToSceneColor)
{
// 如果开启MSAA, 则从SceneContext获取渲染纹理(包含场景颜色和解析纹理)
if (bMobileMSAA)
{
SceneColor = SceneContext.GetSceneColorSurface();
SceneColorResolve = ViewFamily.RenderTarget->GetRenderTargetTexture();
ColorTargetAction = ERenderTargetActions::Clear_Resolve;
RHICmdList.Transition(FRHITransitionInfo(SceneColorResolve, ERHIAccess::Unknown, ERHIAccess::RTV | ERHIAccess::ResolveDst));
}
// 非MSAA，从视图家族获取渲染纹理.
else
{
SceneColor = ViewFamily.RenderTarget->GetRenderTargetTexture();
RHICmdList.Transition(FRHITransitionInfo(SceneColor, ERHIAccess::Unknown, ERHIAccess::RTV));
}
SceneDepth = SceneContext.GetSceneDepthSurface();
}
// 线性空间或渲染到场景纹理.
else
{
SceneColor = SceneContext.GetSceneColorSurface();
if (bMobileMSAA)
{
SceneColorResolve = SceneContext.GetSceneColorTexture();
ColorTargetAction = ERenderTargetActions::Clear_Resolve;
RHICmdList.Transition(FRHITransitionInfo(SceneColorResolve, ERHIAccess::Unknown, ERHIAccess::RTV | ERHIAccess::ResolveDst));
}
else
{
SceneColorResolve = nullptr;
ColorTargetAction = ERenderTargetActions::Clear_Store;
}
SceneDepth = SceneContext.GetSceneDepthSurface();
if (bRequiresMultiPass)
{
// store targets after opaque so translucency render pass can be restarted
ColorTargetAction = ERenderTargetActions::Clear_Store;
DepthTargetAction = EDepthStencilTargetActions::ClearDepthStencil_StoreDepthStencil;
}
if (bKeepDepthContent)
{
// store depth if post-processing/capture needs it
DepthTargetAction = EDepthStencilTargetActions::ClearDepthStencil_StoreDepthStencil;
}
}
// prepass的深度纹理状态.
if (bIsFullPrepassEnabled)
{
ERenderTargetActions DepthTarget = MakeRenderTargetActions(ERenderTargetLoadAction::ELoad, GetStoreAction(GetDepthActions(DepthTargetAction)));
ERenderTargetActions StencilTarget = MakeRenderTargetActions(ERenderTargetLoadAction::ELoad, GetStoreAction(GetStencilActions(DepthTargetAction)));
DepthTargetAction = MakeDepthStencilTargetActions(DepthTarget, StencilTarget);
}
FRHITexture* ShadingRateTexture = nullptr;
if (!View.bIsSceneCapture && !View.bIsReflectionCapture)
{
TRefCountPtr<IPooledRenderTarget> ShadingRateTarget = GVRSImageManager.GetMobileVariableRateShadingImage(ViewFamily);
if (ShadingRateTarget.IsValid())
{
ShadingRateTexture = ShadingRateTarget->GetRenderTargetItem().ShaderResourceTexture;
}
}
// 场景颜色渲染Pass信息.
FRHIRenderPassInfo SceneColorRenderPassInfo(
SceneColor,
ColorTargetAction,
SceneColorResolve,
SceneDepth,
DepthTargetAction,
nullptr, // we never resolve scene depth on mobile
ShadingRateTexture,
VRSRB_Sum,
FExclusiveDepthStencil::DepthWrite_StencilWrite
);
SceneColorRenderPassInfo.SubpassHint = ESubpassHint::DepthReadSubpass;
if (!bIsFullPrepassEnabled)
{
SceneColorRenderPassInfo.NumOcclusionQueries = ComputeNumOcclusionQueriesToBatch();
SceneColorRenderPassInfo.bOcclusionQueries = SceneColorRenderPassInfo.NumOcclusionQueries != 0;
}
// 如果场景颜色不是多视图，但应用程序是，需要渲染为单视图的多视图给着色器.
SceneColorRenderPassInfo.MultiViewCount = View.bIsMobileMultiViewEnabled ? 2 : (bIsMultiViewApplication ? 1 : 0);
// 开始渲染场景颜色.
RHICmdList.BeginRenderPass(SceneColorRenderPassInfo, TEXT("SceneColorRendering"));
if (GIsEditor && !View.bIsSceneCapture)
{
DrawClearQuad(RHICmdList, Views[0].BackgroundColor);
}
if (!bIsFullPrepassEnabled)
{
RHICmdList.SetCurrentStat(GET_STATID(STAT_CLM_MobilePrePass));
// 渲染深度pre-pass
RenderPrePass(RHICmdList);
}
RHICmdList.SetCurrentStat(GET_STATID(STAT_CLMM_Opaque));
// 渲染BasePass: 不透明和masked物体.
RenderMobileBasePass(RHICmdList, ViewList);
RHICmdList.ImmediateFlush(EImmediateFlushType::DispatchToRHIThread);
//渲染调试模式.
#if !(UE_BUILD_SHIPPING || UE_BUILD_TEST)
if (ViewFamily.UseDebugViewPS())
{
// Here we use the base pass depth result to get z culling for opaque and masque.
// The color needs to be cleared at this point since shader complexity renders in additive.
DrawClearQuad(RHICmdList, FLinearColor::Black);
RenderMobileDebugView(RHICmdList, ViewList);
RHICmdList.ImmediateFlush(EImmediateFlushType::DispatchToRHIThread);
}
#endif // !(UE_BUILD_SHIPPING || UE_BUILD_TEST)
const bool bAdrenoOcclusionMode = CVarMobileAdrenoOcclusionMode.GetValueOnRenderThread() != 0;
if (!bIsFullPrepassEnabled)
{
// 遮挡剔除
if (!bAdrenoOcclusionMode)
{
// 提交遮挡剔除
RHICmdList.SetCurrentStat(GET_STATID(STAT_CLMM_Occlusion));
RenderOcclusion(RHICmdList);
RHICmdList.ImmediateFlush(EImmediateFlushType::DispatchToRHIThread);
}
}
// 后置事件, 处理插件渲染.
{
CSV_SCOPED_TIMING_STAT_EXCLUSIVE(ViewExtensionPostRenderBasePass);
QUICK_SCOPE_CYCLE_COUNTER(STAT_FMobileSceneRenderer_ViewExtensionPostRenderBasePass);
for (int32 ViewExt = 0; ViewExt < ViewFamily.ViewExtensions.Num(); ++ViewExt)
{
for (int32 ViewIndex = 0; ViewIndex < ViewFamily.Views.Num(); ++ViewIndex)
{
ViewFamily.ViewExtensions[ViewExt]->PostRenderBasePass_RenderThread(RHICmdList, Views[ViewIndex]);
}
}
}
// 如果需要渲染透明物体或像素投影的反射, 则需要拆分pass.
if (bRequiresMultiPass || bRequiresPixelProjectedPlanarRelfectionPass)
{
RHICmdList.EndRenderPass();
}
RHICmdList.SetCurrentStat(GET_STATID(STAT_CLMM_Translucency));
// 如果需要, 则重新开启透明渲染通道.
if (bRequiresMultiPass || bRequiresPixelProjectedPlanarRelfectionPass)
{
check(RHICmdList.IsOutsideRenderPass());
// 如果当前硬件不支持读写相同的深度缓冲区，则复制场景深度.
ConditionalResolveSceneDepth(RHICmdList, View);
if (bRequiresPixelProjectedPlanarRelfectionPass)
{
const FPlanarReflectionSceneProxy* PlanarReflectionSceneProxy = Scene ? Scene->GetForwardPassGlobalPlanarReflection() : nullptr;
RenderPixelProjectedReflection(RHICmdList, SceneContext, PlanarReflectionSceneProxy);
FRHITransitionInfo TranslucentRenderPassTransitions[] = {
FRHITransitionInfo(SceneColor, ERHIAccess::SRVMask, ERHIAccess::RTV),
FRHITransitionInfo(SceneDepth, ERHIAccess::SRVMask, ERHIAccess::DSVWrite)
};
RHICmdList.Transition(MakeArrayView(TranslucentRenderPassTransitions, UE_ARRAY_COUNT(TranslucentRenderPassTransitions)));
}
DepthTargetAction = EDepthStencilTargetActions::LoadDepthStencil_DontStoreDepthStencil;
FExclusiveDepthStencil::Type ExclusiveDepthStencil = FExclusiveDepthStencil::DepthRead_StencilRead;
if (bModulatedShadowsInUse)
{
ExclusiveDepthStencil = FExclusiveDepthStencil::DepthRead_StencilWrite;
}
// 用于移动端像素投影反射的不透明网格必须将深度写入深度RT, 因为只渲染一次网格(如果质量水平低于或等于BestPerformance).
if (IsMobilePixelProjectedReflectionEnabled(View.GetShaderPlatform())
&& GetMobilePixelProjectedReflectionQuality() == EMobilePixelProjectedReflectionQuality::BestPerformance)
{
ExclusiveDepthStencil = FExclusiveDepthStencil::DepthWrite_StencilWrite;
}
if (bKeepDepthContent && !bMobileMSAA)
{
DepthTargetAction = EDepthStencilTargetActions::LoadDepthStencil_StoreDepthStencil;
}
#if PLATFORM_HOLOLENS
if (bShouldRenderDepthToTranslucency)
{
ExclusiveDepthStencil = FExclusiveDepthStencil::DepthWrite_StencilWrite;
}
#endif
// 透明物体渲染Pass.
FRHIRenderPassInfo TranslucentRenderPassInfo(
SceneColor,
SceneColorResolve ? ERenderTargetActions::Load_Resolve : ERenderTargetActions::Load_Store,
SceneColorResolve,
SceneDepth,
DepthTargetAction,
nullptr,
ShadingRateTexture,
VRSRB_Sum,
ExclusiveDepthStencil
);
TranslucentRenderPassInfo.NumOcclusionQueries = 0;
TranslucentRenderPassInfo.bOcclusionQueries = false;
TranslucentRenderPassInfo.SubpassHint = ESubpassHint::DepthReadSubpass;
// 开始渲染半透明物体.
RHICmdList.BeginRenderPass(TranslucentRenderPassInfo, TEXT("SceneColorTranslucencyRendering"));
}
// 场景深度是只读的，可以获取.
RHICmdList.NextSubpass();
if (!View.bIsPlanarReflection)
{
// 渲染贴花.
if (ViewFamily.EngineShowFlags.Decals)
{
CSV_SCOPED_TIMING_STAT_EXCLUSIVE(RenderDecals);
RenderDecals(RHICmdList);
}
// 渲染调制阴影投射.
if (ViewFamily.EngineShowFlags.DynamicShadows)
{
CSV_SCOPED_TIMING_STAT_EXCLUSIVE(RenderShadowProjections);
RenderModulatedShadowProjections(RHICmdList);
}
}
// 绘制半透明.
if (ViewFamily.EngineShowFlags.Translucency)
{
CSV_SCOPED_TIMING_STAT_EXCLUSIVE(RenderTranslucency);
SCOPE_CYCLE_COUNTER(STAT_TranslucencyDrawTime);
RenderTranslucency(RHICmdList, ViewList);
FRHICommandListExecutor::GetImmediateCommandList().PollOcclusionQueries();
RHICmdList.ImmediateFlush(EImmediateFlushType::DispatchToRHIThread);
}
if (!bIsFullPrepassEnabled)
{
// Adreno遮挡剔除模式.
if (bAdrenoOcclusionMode)
{
RHICmdList.SetCurrentStat(GET_STATID(STAT_CLMM_Occlusion));
// flush
RHICmdList.SubmitCommandsHint();
bSubmitOffscreenRendering = false; // submit once
// Issue occlusion queries
RenderOcclusion(RHICmdList);
RHICmdList.ImmediateFlush(EImmediateFlushType::DispatchToRHIThread);
}
}
// 在MSAA被解析前预计算色调映射(只在iOS有效)
if (!bGammaSpace)
{
PreTonemapMSAA(RHICmdList);
}
// 结束场景颜色渲染.
RHICmdList.EndRenderPass();
// 优化返回场景颜色的解析纹理(开启了MSAA才有).
return SceneColorResolve ? SceneColorResolve : SceneColor;
}
移动端前向渲染主要步骤跟PC端类似，依次渲染PrePass、BasePass、特殊渲染（贴花、AO、遮挡剔除等）、半透明物体。它们的流程图如下：
stateDiagram-v2
[*] --> DrawClearQuad*
DrawClearQuad* --> RenderPrePass*
RenderPrePass* --> RenderMobileBasePass
RenderMobileBasePass --> RenderOcclusion*
RenderOcclusion* --> RenderDecals*
RenderDecals* --> RenderModulatedShadowProjections*
RenderModulatedShadowProjections* --> RenderTranslucency*
RenderTranslucency* --> PreTonemapMSAA*
PreTonemapMSAA* --> [*]
其中遮挡剔除和GPU厂商相关，比如高通Adreno系列GPU芯片要求在Flush渲染指令和Switch FBO之间：
Render Opaque -> Render Translucent -> Flush -> Render Queries -> Switch FBO
那么UE也遵循了Adreno系列芯片的特殊要求，对其的遮挡剔除做了特殊的处理。
Adreno系列芯片支持TBDR架构的Bin和普通的Direct两种混合模式的渲染，会在遮挡查询时自动切换到Direct模式，以降低遮挡查询的开销。如果不在Flush渲染指令和Switch FBO之间提交查询，会卡住整个渲染管线，引发渲染性能下降。
MSAA由于天然硬件支持且效果和效率达到很好的平衡，是UE在移动端前向渲染的首选抗锯齿。因此，上述代码中出现了不少处理MSAA的逻辑，包含颜色和深度纹理及其资源状态。如果开启了MSAA，默认情况下是在RHICmdList.EndRenderPass()解析场景颜色（同时将芯片分块上的数据写回到系统显存中），由此获得抗锯齿的纹理。移动端的MSAA默认不开启，但可在以下界面中设置：
前向渲染支持Gamma空间和HDR（线性空间）两种颜色空间模式。如果是线性空间，则渲染后期需要色调映射等步骤。默认是HDR，可在项目配置中更改：
上述代码的bRequiresMultiPass标明了是否需要专用的渲染Pass绘制半透明物体，决定它的值由以下代码完成：
// Engine\Source\Runtime\Renderer\Private\MobileShadingRenderer.cpp
bool FMobileSceneRenderer::RequiresMultiPass(FRHICommandListImmediate& RHICmdList, const FViewInfo& View) const
{
// Vulkan uses subpasses
if (IsVulkanPlatform(ShaderPlatform))
{
return false;
}
// All iOS support frame_buffer_fetch
if (IsMetalMobilePlatform(ShaderPlatform))
{
return false;
}
if (IsMobileDeferredShadingEnabled(ShaderPlatform))
{
// TODO: add GL support
return true;
}
// Some Androids support frame_buffer_fetch
if (IsAndroidOpenGLESPlatform(ShaderPlatform) && (GSupportsShaderFramebufferFetch || GSupportsShaderDepthStencilFetch))
{
return false;
}
// Always render reflection capture in single pass
if (View.bIsPlanarReflection || View.bIsSceneCapture)
{
return false;
}
// Always render LDR in single pass
if (!IsMobileHDR())
{
return false;
}
// MSAA depth can't be sampled or resolved, unless we are on PC (no vulkan)
if (NumMSAASamples > 1 && !IsSimulatedPlatform(ShaderPlatform))
{
return false;
}
return true;
}
与之类似但意义不同的是bIsMultiViewApplication和bIsMobileMultiViewEnabled标记，标明是否开启多视图渲染以及多视图的个数。只用于VR，由控制台变量vr.MobileMultiView及图形API等因素决定。MultiView用于XR，用于优化渲染两次的情形，它存在Basic和Advanced两种模式：
用于优化VR等渲染的MultiView对比图。上：未采用MultiView模式的渲染，两个眼睛各自提交绘制指令；中：基础MultiView模式，复用提交指令，在GPU层复制多一份Command List；下：高级MultiView模式，可以复用DC、Command List、几何信息。
bKeepDepthContent标明是否要保留深度内容，决定它的代码：
bKeepDepthContent =
bRequiresMultiPass ||
bForceDepthResolve ||
bRequiresPixelProjectedPlanarRelfectionPass ||
bSeparateTranslucencyActive ||
Views[0].bIsReflectionCapture ||
(bDeferredShading && bPostProcessUsesSceneDepth) ||
bShouldRenderVelocities ||
bIsFullPrepassEnabled;
// 带MSAA的深度从不保留.
bKeepDepthContent = (NumMSAASamples > 1 ? false : bKeepDepthContent);
上述代码还揭示了平面反射在移动端的一种特殊渲染方式：Pixel Projected Reflection（PPR）。它的实现原理类似于SSR，但需要的数据更少，只需要场景颜色、深度缓冲和反射区域。它的核心步骤：
- 计算场景颜色的所有像素在反射平面的镜像位置。
- 测试像素的反射是否在反射区域内。
- 光线投射到镜像像素位置。
- 测试交点是否在反射区域内。
- 如果找到相交点，计算像素在屏幕的镜像位置。
- 在交点处写入镜像像素的颜色。
- 如果反射区域内的交点被其它物体遮挡，则剔除此位置的反射。
PPR效果一览。
PPR可以在工程配置中设置：
12.3.3 RenderDeferred
UE在4.26在移动端渲染管线增加了延迟渲染分支，并在4.27做了改进和优化。移动端是否开启延迟着色的特性由以下代码决定：
// Engine\Source\Runtime\RenderCore\Private\RenderUtils.cpp
bool IsMobileDeferredShadingEnabled(const FStaticShaderPlatform Platform)
{
// 禁用OpenGL的延迟着色.
if (IsOpenGLPlatform(Platform))
{
// needs MRT framebuffer fetch or PLS
return false;
}
// 控制台变量"r.Mobile.ShadingPath"要为1.
static auto* MobileShadingPathCvar = IConsoleManager::Get().FindTConsoleVariableDataInt(TEXT("r.Mobile.ShadingPath"));
return MobileShadingPathCvar->GetValueOnAnyThread() == 1;
}
简单地说就是非OpenGL图形API且控制台变量r.Mobile.ShadingPath设为1。
r.Mobile.ShadingPath不可在编辑器动态设置值，只能在项目工程根目录/Config/DefaultEngine.ini增加以下字段来开启：[/Script/Engine.RendererSettings]
r.Mobile.ShadingPath=1
添加以上字段后，重启UE编辑器，等待shader编译完成即可预览移动端延迟着色效果。
以下是延迟渲染分支FMobileSceneRenderer::RenderDeferred的代码和解析：
FRHITexture* FMobileSceneRenderer::RenderDeferred(FRHICommandListImmediate& RHICmdList, const TArrayView<const FViewInfo*> ViewList, const FSortedLightSetSceneInfo& SortedLightSet)
{
FSceneRenderTargets& SceneContext = FSceneRenderTargets::Get(RHICmdList);
// 准备GBuffer.
FRHITexture* ColorTargets[4] = {
SceneContext.GetSceneColorSurface(),
SceneContext.GetGBufferATexture().GetReference(),
SceneContext.GetGBufferBTexture().GetReference(),
SceneContext.GetGBufferCTexture().GetReference()
};
// RHI是否需要将GBuffer存储到GPU的系统内存中，并在单独的渲染通道中进行着色.
ERenderTargetActions GBufferAction = bRequiresMultiPass ? ERenderTargetActions::Clear_Store : ERenderTargetActions::Clear_DontStore;
EDepthStencilTargetActions DepthAction = bKeepDepthContent ? EDepthStencilTargetActions::ClearDepthStencil_StoreDepthStencil : EDepthStencilTargetActions::ClearDepthStencil_DontStoreDepthStencil;
// RT的load/store动作.
ERenderTargetActions ColorTargetsAction[4] = {ERenderTargetActions::Clear_Store, GBufferAction, GBufferAction, GBufferAction};
if (bIsFullPrepassEnabled)
{
ERenderTargetActions DepthTarget = MakeRenderTargetActions(ERenderTargetLoadAction::ELoad, GetStoreAction(GetDepthActions(DepthAction)));
ERenderTargetActions StencilTarget = MakeRenderTargetActions(ERenderTargetLoadAction::ELoad, GetStoreAction(GetStencilActions(DepthAction)));
DepthAction = MakeDepthStencilTargetActions(DepthTarget, StencilTarget);
}
FRHIRenderPassInfo BasePassInfo = FRHIRenderPassInfo();
int32 ColorTargetIndex = 0;
for (; ColorTargetIndex < UE_ARRAY_COUNT(ColorTargets); ++ColorTargetIndex)
{
BasePassInfo.ColorRenderTargets[ColorTargetIndex].RenderTarget = ColorTargets[ColorTargetIndex];
BasePassInfo.ColorRenderTargets[ColorTargetIndex].ResolveTarget = nullptr;
BasePassInfo.ColorRenderTargets[ColorTargetIndex].ArraySlice = -1;
BasePassInfo.ColorRenderTargets[ColorTargetIndex].MipIndex = 0;
BasePassInfo.ColorRenderTargets[ColorTargetIndex].Action = ColorTargetsAction[ColorTargetIndex];
}
if (MobileRequiresSceneDepthAux(ShaderPlatform))
{
BasePassInfo.ColorRenderTargets[ColorTargetIndex].RenderTarget = SceneContext.SceneDepthAux->GetRenderTargetItem().ShaderResourceTexture.GetReference();
BasePassInfo.ColorRenderTargets[ColorTargetIndex].ResolveTarget = nullptr;
BasePassInfo.ColorRenderTargets[ColorTargetIndex].ArraySlice = -1;
BasePassInfo.ColorRenderTargets[ColorTargetIndex].MipIndex = 0;
BasePassInfo.ColorRenderTargets[ColorTargetIndex].Action = GBufferAction;
ColorTargetIndex++;
}
BasePassInfo.DepthStencilRenderTarget.DepthStencilTarget = SceneContext.GetSceneDepthSurface();
BasePassInfo.DepthStencilRenderTarget.ResolveTarget = nullptr;
BasePassInfo.DepthStencilRenderTarget.Action = DepthAction;
BasePassInfo.DepthStencilRenderTarget.ExclusiveDepthStencil = FExclusiveDepthStencil::DepthWrite_StencilWrite;
BasePassInfo.SubpassHint = ESubpassHint::DeferredShadingSubpass;
if (!bIsFullPrepassEnabled)
{
BasePassInfo.NumOcclusionQueries = ComputeNumOcclusionQueriesToBatch();
BasePassInfo.bOcclusionQueries = BasePassInfo.NumOcclusionQueries != 0;
}
BasePassInfo.ShadingRateTexture = nullptr;
BasePassInfo.bIsMSAA = false;
BasePassInfo.MultiViewCount = 0;
RHICmdList.BeginRenderPass(BasePassInfo, TEXT("BasePassRendering"));
if (GIsEditor && !Views[0].bIsSceneCapture)
{
DrawClearQuad(RHICmdList, Views[0].BackgroundColor);
}
// 深度PrePass
if (!bIsFullPrepassEnabled)
{
RHICmdList.SetCurrentStat(GET_STATID(STAT_CLM_MobilePrePass));
// Depth pre-pass
RenderPrePass(RHICmdList);
}
// BasePass: 不透明和镂空物体.
RHICmdList.SetCurrentStat(GET_STATID(STAT_CLMM_Opaque));
RenderMobileBasePass(RHICmdList, ViewList);
RHICmdList.ImmediateFlush(EImmediateFlushType::DispatchToRHIThread);
// 遮挡剔除.
if (!bIsFullPrepassEnabled)
{
// Issue occlusion queries
RHICmdList.SetCurrentStat(GET_STATID(STAT_CLMM_Occlusion));
RenderOcclusion(RHICmdList);
RHICmdList.ImmediateFlush(EImmediateFlushType::DispatchToRHIThread);
}
// 非多Pass模式
if (!bRequiresMultiPass)
{
// 下个子Pass: SSceneColor + GBuffer写入, SceneDepth只读.
RHICmdList.NextSubpass();
// 渲染贴花.
if (ViewFamily.EngineShowFlags.Decals)
{
CSV_SCOPED_TIMING_STAT_EXCLUSIVE(RenderDecals);
RenderDecals(RHICmdList);
}
// 下个子Pass: SceneColor写入, SceneDepth只读
RHICmdList.NextSubpass();
// 延迟光照着色.
MobileDeferredShadingPass(RHICmdList, *Scene, ViewList, SortedLightSet);
// 绘制半透明.
if (ViewFamily.EngineShowFlags.Translucency)
{
CSV_SCOPED_TIMING_STAT_EXCLUSIVE(RenderTranslucency);
SCOPE_CYCLE_COUNTER(STAT_TranslucencyDrawTime);
RenderTranslucency(RHICmdList, ViewList);
FRHICommandListExecutor::GetImmediateCommandList().PollOcclusionQueries();
RHICmdList.ImmediateFlush(EImmediateFlushType::DispatchToRHIThread);
}
// 结束渲染Pass.
RHICmdList.EndRenderPass();
}
// 多Pass模式(PC设备模拟的移动端).
else
{
// 结束子pass.
RHICmdList.NextSubpass();
RHICmdList.NextSubpass();
RHICmdList.EndRenderPass();
// SceneColor + GBuffer write, SceneDepth is read only
{
for (int32 Index = 0; Index < UE_ARRAY_COUNT(ColorTargets); ++Index)
{
BasePassInfo.ColorRenderTargets[Index].Action = ERenderTargetActions::Load_Store;
}
BasePassInfo.DepthStencilRenderTarget.Action = EDepthStencilTargetActions::LoadDepthStencil_StoreDepthStencil;
BasePassInfo.DepthStencilRenderTarget.ExclusiveDepthStencil = FExclusiveDepthStencil::DepthRead_StencilRead;
BasePassInfo.SubpassHint = ESubpassHint::None;
BasePassInfo.NumOcclusionQueries = 0;
BasePassInfo.bOcclusionQueries = false;
RHICmdList.BeginRenderPass(BasePassInfo, TEXT("AfterBasePass"));
// 渲染贴花.
if (ViewFamily.EngineShowFlags.Decals)
{
CSV_SCOPED_TIMING_STAT_EXCLUSIVE(RenderDecals);
RenderDecals(RHICmdList);
}
RHICmdList.EndRenderPass();
}
// SceneColor write, SceneDepth is read only
{
FRHIRenderPassInfo ShadingPassInfo(
SceneContext.GetSceneColorSurface(),
ERenderTargetActions::Load_Store,
nullptr,
SceneContext.GetSceneDepthSurface(),
EDepthStencilTargetActions::LoadDepthStencil_StoreDepthStencil,
nullptr,
nullptr,
VRSRB_Passthrough,
FExclusiveDepthStencil::DepthRead_StencilWrite
);
ShadingPassInfo.NumOcclusionQueries = 0;
ShadingPassInfo.bOcclusionQueries = false;
RHICmdList.BeginRenderPass(ShadingPassInfo, TEXT("MobileShadingPass"));
// 延迟光照着色.
MobileDeferredShadingPass(RHICmdList, *Scene, ViewList, SortedLightSet);
// 绘制半透明.
if (ViewFamily.EngineShowFlags.Translucency)
{
CSV_SCOPED_TIMING_STAT_EXCLUSIVE(RenderTranslucency);
SCOPE_CYCLE_COUNTER(STAT_TranslucencyDrawTime);
RenderTranslucency(RHICmdList, ViewList);
FRHICommandListExecutor::GetImmediateCommandList().PollOcclusionQueries();
RHICmdList.ImmediateFlush(EImmediateFlushType::DispatchToRHIThread);
}
RHICmdList.EndRenderPass();
}
}
return ColorTargets[0];
}
由上面可知，移动端的延迟渲染管线和PC比较类似，先渲染BasePass，获得GBuffer几何信息，再执行光照计算。它们的流程图如下：
stateDiagram-v2
[*] --> DrawClearQuad*
DrawClearQuad* --> RenderPrePass*
RenderPrePass* --> RenderMobileBasePass
RenderMobileBasePass --> RenderOcclusion*
RenderOcclusion* --> RenderDecals*
RenderDecals* --> MobileDeferredShadingPass*
MobileDeferredShadingPass* --> RenderTranslucency*
RenderTranslucency* --> PreTonemapMSAA*
PreTonemapMSAA* --> [*]
当然，也有和PC不一样的地方，最明显的是移动端使用了适配TB(D)R架构的SubPass渲染，使得移动端在渲染PrePass深度、BasePass、光照计算时，让场景颜色、深度、GBuffer等信息一直在On-Chip的缓冲区中，提升渲染效率，降低设备能耗。
12.3.3.1 MobileDeferredShadingPass
延迟渲染光照的过程由MobileDeferredShadingPass担当：
void MobileDeferredShadingPass(
FRHICommandListImmediate& RHICmdList,
const FScene& Scene,
const TArrayView<const FViewInfo*> PassViews,
const FSortedLightSetSceneInfo &SortedLightSet)
{
SCOPED_DRAW_EVENT(RHICmdList, MobileDeferredShading);
const FViewInfo& View0 = *PassViews[0];
FSceneRenderTargets& SceneContext = FSceneRenderTargets::Get(RHICmdList);
// 创建Uniform Buffer.
FUniformBufferRHIRef PassUniformBuffer = CreateMobileSceneTextureUniformBuffer(RHICmdList);
FUniformBufferStaticBindings GlobalUniformBuffers(PassUniformBuffer);
SCOPED_UNIFORM_BUFFER_GLOBAL_BINDINGS(RHICmdList, GlobalUniformBuffers);
// 设置视口.
RHICmdList.SetViewport(View0.ViewRect.Min.X, View0.ViewRect.Min.Y, 0.0f, View0.ViewRect.Max.X, View0.ViewRect.Max.Y, 1.0f);
// 光照的默认材质.
FCachedLightMaterial DefaultMaterial;
DefaultMaterial.MaterialProxy = UMaterial::GetDefaultMaterial(MD_LightFunction)->GetRenderProxy();
DefaultMaterial.Material = DefaultMaterial.MaterialProxy->GetMaterialNoFallback(ERHIFeatureLevel::ES3_1);
check(DefaultMaterial.Material);
// 绘制平行光.
RenderDirectLight(RHICmdList, Scene, View0, DefaultMaterial);
if (GMobileUseClusteredDeferredShading == 0)
{
// 渲染非分簇的简单光源.
RenderSimpleLights(RHICmdList, Scene, PassViews, SortedLightSet, DefaultMaterial);
}
// 渲染非分簇的局部光源.
int32 NumLights = SortedLightSet.SortedLights.Num();
int32 StandardDeferredStart = SortedLightSet.SimpleLightsEnd;
if (GMobileUseClusteredDeferredShading != 0)
{
StandardDeferredStart = SortedLightSet.ClusteredSupportedEnd;
}
// 渲染局部光源.
for (int32 LightIdx = StandardDeferredStart; LightIdx < NumLights; ++LightIdx)
{
const FSortedLightSceneInfo& SortedLight = SortedLightSet.SortedLights[LightIdx];
const FLightSceneInfo& LightSceneInfo = *SortedLight.LightSceneInfo;
RenderLocalLight(RHICmdList, Scene, View0, LightSceneInfo, DefaultMaterial);
}
}
下面继续分析渲染不同类型光源的接口：
// Engine\Source\Runtime\Renderer\Private\MobileDeferredShadingPass.cpp
// 渲染平行光
static void RenderDirectLight(FRHICommandListImmediate& RHICmdList, const FScene& Scene, const FViewInfo& View, const FCachedLightMaterial& DefaultLightMaterial)
{
FSceneRenderTargets& SceneContext = FSceneRenderTargets::Get(RHICmdList);
// 查找第一个平行光.
FLightSceneInfo* DirectionalLight = nullptr;
for (int32 ChannelIdx = 0; ChannelIdx < UE_ARRAY_COUNT(Scene.MobileDirectionalLights) && !DirectionalLight; ChannelIdx++)
{
DirectionalLight = Scene.MobileDirectionalLights[ChannelIdx];
}
// 渲染状态.
FGraphicsPipelineStateInitializer GraphicsPSOInit;
RHICmdList.ApplyCachedRenderTargets(GraphicsPSOInit);
// 增加自发光到SceneColor.
GraphicsPSOInit.BlendState = TStaticBlendState<CW_RGB, BO_Add, BF_One, BF_One>::GetRHI();
GraphicsPSOInit.RasterizerState = TStaticRasterizerState<>::GetRHI();
// 只绘制默认光照模型(MSM_DefaultLit)的像素.
uint8 StencilRef = GET_STENCIL_MOBILE_SM_MASK(MSM_DefaultLit);
GraphicsPSOInit.DepthStencilState = TStaticDepthStencilState<
false, CF_Always,
true, CF_Equal, SO_Keep, SO_Keep, SO_Keep,
false, CF_Always, SO_Keep, SO_Keep, SO_Keep,
GET_STENCIL_MOBILE_SM_MASK(0x7), 0x00>::GetRHI(); // 4 bits for shading models
// 处理VS.
TShaderMapRef<FPostProcessVS> VertexShader(View.ShaderMap);
const FMaterialRenderProxy* LightFunctionMaterialProxy = nullptr;
if (View.Family->EngineShowFlags.LightFunctions && DirectionalLight)
{
LightFunctionMaterialProxy = DirectionalLight->Proxy->GetLightFunctionMaterial();
}
FMobileDirectLightFunctionPS::FPermutationDomain PermutationVector = FMobileDirectLightFunctionPS::BuildPermutationVector(View, DirectionalLight != nullptr);
FCachedLightMaterial LightMaterial;
TShaderRef<FMobileDirectLightFunctionPS> PixelShader;
GetLightMaterial(DefaultLightMaterial, LightFunctionMaterialProxy, PermutationVector.ToDimensionValueId(), LightMaterial, PixelShader);
GraphicsPSOInit.BoundShaderState.VertexDeclarationRHI = GFilterVertexDeclaration.VertexDeclarationRHI;
GraphicsPSOInit.BoundShaderState.VertexShaderRHI = VertexShader.GetVertexShader();
GraphicsPSOInit.BoundShaderState.PixelShaderRHI = PixelShader.GetPixelShader();
GraphicsPSOInit.PrimitiveType = PT_TriangleList;
SetGraphicsPipelineState(RHICmdList, GraphicsPSOInit);
// 处理PS.
FMobileDirectLightFunctionPS::FParameters PassParameters;
PassParameters.Forward = View.ForwardLightingResources->ForwardLightDataUniformBuffer;
PassParameters.MobileDirectionalLight = Scene.UniformBuffers.MobileDirectionalLightUniformBuffers[1];
PassParameters.ReflectionCaptureData = Scene.UniformBuffers.ReflectionCaptureUniformBuffer;
FReflectionUniformParameters ReflectionUniformParameters;
SetupReflectionUniformParameters(View, ReflectionUniformParameters);
PassParameters.ReflectionsParameters = CreateUniformBufferImmediate(ReflectionUniformParameters, UniformBuffer_SingleDraw);
PassParameters.LightFunctionParameters = FVector4(1.0f, 1.0f, 0.0f, 0.0f);
if (DirectionalLight)
{
const bool bUseMovableLight = DirectionalLight && !DirectionalLight->Proxy->HasStaticShadowing();
PassParameters.LightFunctionParameters2 = FVector(DirectionalLight->Proxy->GetLightFunctionFadeDistance(), DirectionalLight->Proxy->GetLightFunctionDisabledBrightness(), bUseMovableLight ? 1.0f : 0.0f);
const FVector Scale = DirectionalLight->Proxy->GetLightFunctionScale();
// Switch x and z so that z of the user specified scale affects the distance along the light direction
const FVector InverseScale = FVector(1.f / Scale.Z, 1.f / Scale.Y, 1.f / Scale.X);
PassParameters.WorldToLight = DirectionalLight->Proxy->GetWorldToLight() * FScaleMatrix(FVector(InverseScale));
}
FMobileDirectLightFunctionPS::SetParameters(RHICmdList, PixelShader, View, LightMaterial.MaterialProxy, *LightMaterial.Material, PassParameters);
RHICmdList.SetStencilRef(StencilRef);
const FIntPoint TargetSize = SceneContext.GetBufferSizeXY();
// 用全屏幕的矩形绘制.
DrawRectangle(
RHICmdList,
0, 0,
View.ViewRect.Width(), View.ViewRect.Height(),
View.ViewRect.Min.X, View.ViewRect.Min.Y,
View.ViewRect.Width(), View.ViewRect.Height(),
FIntPoint(View.ViewRect.Width(), View.ViewRect.Height()),
TargetSize,
VertexShader);
}
// 渲染非分簇模式的简单光源.
static void RenderSimpleLights(
FRHICommandListImmediate& RHICmdList,
const FScene& Scene,
const TArrayView<const FViewInfo*> PassViews,
const FSortedLightSetSceneInfo &SortedLightSet,
const FCachedLightMaterial& DefaultMaterial)
{
const FSimpleLightArray& SimpleLights = SortedLightSet.SimpleLights;
const int32 NumViews = PassViews.Num();
const FViewInfo& View0 = *PassViews[0];
// 处理VS.
TShaderMapRef<TDeferredLightVS<true>> VertexShader(View0.ShaderMap);
TShaderRef<FMobileRadialLightFunctionPS> PixelShaders[2];
{
const FMaterialShaderMap* MaterialShaderMap = DefaultMaterial.Material->GetRenderingThreadShaderMap();
FMobileRadialLightFunctionPS::FPermutationDomain PermutationVector;
PermutationVector.Set<FMobileRadialLightFunctionPS::FSpotLightDim>(false);
PermutationVector.Set<FMobileRadialLightFunctionPS::FIESProfileDim>(false);
PermutationVector.Set<FMobileRadialLightFunctionPS::FInverseSquaredDim>(false);
PixelShaders[0] = MaterialShaderMap->GetShader<FMobileRadialLightFunctionPS>(PermutationVector);
PermutationVector.Set<FMobileRadialLightFunctionPS::FInverseSquaredDim>(true);
PixelShaders[1] = MaterialShaderMap->GetShader<FMobileRadialLightFunctionPS>(PermutationVector);
}
// 设置PSO.
FGraphicsPipelineStateInitializer GraphicsPSOLight[2];
{
SetupSimpleLightPSO(RHICmdList, View0, VertexShader, PixelShaders[0], GraphicsPSOLight[0]);
SetupSimpleLightPSO(RHICmdList, View0, VertexShader, PixelShaders[1], GraphicsPSOLight[1]);
}
// 设置模板缓冲.
FGraphicsPipelineStateInitializer GraphicsPSOLightMask;
{
RHICmdList.ApplyCachedRenderTargets(GraphicsPSOLightMask);
GraphicsPSOLightMask.PrimitiveType = PT_TriangleList;
GraphicsPSOLightMask.BlendState = TStaticBlendStateWriteMask<CW_NONE, CW_NONE, CW_NONE, CW_NONE, CW_NONE, CW_NONE, CW_NONE, CW_NONE>::GetRHI();
GraphicsPSOLightMask.RasterizerState = View0.bReverseCulling ? TStaticRasterizerState<FM_Solid, CM_CCW>::GetRHI() : TStaticRasterizerState<FM_Solid, CM_CW>::GetRHI();
// set stencil to 1 where depth test fails
GraphicsPSOLightMask.DepthStencilState = TStaticDepthStencilState<
false, CF_DepthNearOrEqual,
true, CF_Always, SO_Keep, SO_Replace, SO_Keep,
false, CF_Always, SO_Keep, SO_Keep, SO_Keep,
0x00, STENCIL_SANDBOX_MASK>::GetRHI();
GraphicsPSOLightMask.BoundShaderState.VertexDeclarationRHI = GetVertexDeclarationFVector4();
GraphicsPSOLightMask.BoundShaderState.VertexShaderRHI = VertexShader.GetVertexShader();
GraphicsPSOLightMask.BoundShaderState.PixelShaderRHI = nullptr;
}
// 遍历所有简单光源列表, 执行着色计算.
for (int32 LightIndex = 0; LightIndex < SimpleLights.InstanceData.Num(); LightIndex++)
{
const FSimpleLightEntry& SimpleLight = SimpleLights.InstanceData[LightIndex];
for (int32 ViewIndex = 0; ViewIndex < NumViews; ViewIndex++)
{
const FViewInfo& View = *PassViews[ViewIndex];
const FSimpleLightPerViewEntry& SimpleLightPerViewData = SimpleLights.GetViewDependentData(LightIndex, ViewIndex, NumViews);
const FSphere LightBounds(SimpleLightPerViewData.Position, SimpleLight.Radius);
if (NumViews > 1)
{
// set viewports only we we have more than one
// otherwise it is set at the start of the pass
RHICmdList.SetViewport(View.ViewRect.Min.X, View.ViewRect.Min.Y, 0.0f, View.ViewRect.Max.X, View.ViewRect.Max.Y, 1.0f);
}
// 渲染光源遮罩.
SetGraphicsPipelineState(RHICmdList, GraphicsPSOLightMask);
VertexShader->SetSimpleLightParameters(RHICmdList, View, LightBounds);
RHICmdList.SetStencilRef(1);
StencilingGeometry::DrawSphere(RHICmdList);
// 渲染光源.
FMobileRadialLightFunctionPS::FParameters PassParameters;
FDeferredLightUniformStruct DeferredLightUniformsValue;
SetupSimpleDeferredLightParameters(SimpleLight, SimpleLightPerViewData, DeferredLightUniformsValue);
PassParameters.DeferredLightUniforms = TUniformBufferRef<FDeferredLightUniformStruct>::CreateUniformBufferImmediate(DeferredLightUniformsValue, EUniformBufferUsage::UniformBuffer_SingleFrame);
PassParameters.IESTexture = GWhiteTexture->TextureRHI;
PassParameters.IESTextureSampler = GWhiteTexture->SamplerStateRHI;
if (SimpleLight.Exponent == 0)
{
SetGraphicsPipelineState(RHICmdList, GraphicsPSOLight[1]);
FMobileRadialLightFunctionPS::SetParameters(RHICmdList, PixelShaders[1], View, DefaultMaterial.MaterialProxy, *DefaultMaterial.Material, PassParameters);
}
else
{
SetGraphicsPipelineState(RHICmdList, GraphicsPSOLight[0]);
FMobileRadialLightFunctionPS::SetParameters(RHICmdList, PixelShaders[0], View, DefaultMaterial.MaterialProxy, *DefaultMaterial.Material, PassParameters);
}
VertexShader->SetSimpleLightParameters(RHICmdList, View, LightBounds);
// 只绘制默认光照模型(MSM_DefaultLit)的像素.
uint8 StencilRef = GET_STENCIL_MOBILE_SM_MASK(MSM_DefaultLit);
RHICmdList.SetStencilRef(StencilRef);
// 用球体渲染光源(点光源和聚光灯)， 以快速剔除光源影响之外的像素.
StencilingGeometry::DrawSphere(RHICmdList);
}
}
}
// 渲染局部光源.
static void RenderLocalLight(
FRHICommandListImmediate& RHICmdList,
const FScene& Scene,
const FViewInfo& View,
const FLightSceneInfo& LightSceneInfo,
const FCachedLightMaterial& DefaultLightMaterial)
{
if (!LightSceneInfo.ShouldRenderLight(View))
{
return;
}
// 忽略非局部光源(光源和聚光灯之外的光源).
const uint8 LightType = LightSceneInfo.Proxy->GetLightType();
const bool bIsSpotLight = LightType == LightType_Spot;
const bool bIsPointLight = LightType == LightType_Point;
if (!bIsSpotLight && !bIsPointLight)
{
return;
}
// 绘制光源模板.
if (GMobileUseLightStencilCulling != 0)
{
RenderLocalLight_StencilMask(RHICmdList, Scene, View, LightSceneInfo);
}
// 处理IES光照.
bool bUseIESTexture = false;
FTexture* IESTextureResource = GWhiteTexture;
if (View.Family->EngineShowFlags.TexturedLightProfiles && LightSceneInfo.Proxy->GetIESTextureResource())
{
IESTextureResource = LightSceneInfo.Proxy->GetIESTextureResource();
bUseIESTexture = true;
}
FGraphicsPipelineStateInitializer GraphicsPSOInit;
RHICmdList.ApplyCachedRenderTargets(GraphicsPSOInit);
GraphicsPSOInit.BlendState = TStaticBlendState<CW_RGBA, BO_Add, BF_One, BF_One, BO_Add, BF_One, BF_One>::GetRHI();
GraphicsPSOInit.PrimitiveType = PT_TriangleList;
const FSphere LightBounds = LightSceneInfo.Proxy->GetBoundingSphere();
// 设置光源光栅化和深度状态.
if (GMobileUseLightStencilCulling != 0)
{
SetLocalLightRasterizerAndDepthState_StencilMask(GraphicsPSOInit, View);
}
else
{
SetLocalLightRasterizerAndDepthState(GraphicsPSOInit, View, LightBounds);
}
// 设置VS
TShaderMapRef<TDeferredLightVS<true>> VertexShader(View.ShaderMap);
const FMaterialRenderProxy* LightFunctionMaterialProxy = nullptr;
if (View.Family->EngineShowFlags.LightFunctions)
{
LightFunctionMaterialProxy = LightSceneInfo.Proxy->GetLightFunctionMaterial();
}
FMobileRadialLightFunctionPS::FPermutationDomain PermutationVector;
PermutationVector.Set<FMobileRadialLightFunctionPS::FSpotLightDim>(bIsSpotLight);
PermutationVector.Set<FMobileRadialLightFunctionPS::FInverseSquaredDim>(LightSceneInfo.Proxy->IsInverseSquared());
PermutationVector.Set<FMobileRadialLightFunctionPS::FIESProfileDim>(bUseIESTexture);
FCachedLightMaterial LightMaterial;
TShaderRef<FMobileRadialLightFunctionPS> PixelShader;
GetLightMaterial(DefaultLightMaterial, LightFunctionMaterialProxy, PermutationVector.ToDimensionValueId(), LightMaterial, PixelShader);
GraphicsPSOInit.BoundShaderState.VertexDeclarationRHI = GetVertexDeclarationFVector4();
GraphicsPSOInit.BoundShaderState.VertexShaderRHI = VertexShader.GetVertexShader();
GraphicsPSOInit.BoundShaderState.PixelShaderRHI = PixelShader.GetPixelShader();
SetGraphicsPipelineState(RHICmdList, GraphicsPSOInit);
VertexShader->SetParameters(RHICmdList, View, &LightSceneInfo);
// 设置PS.
FMobileRadialLightFunctionPS::FParameters PassParameters;
PassParameters.DeferredLightUniforms = TUniformBufferRef<FDeferredLightUniformStruct>::CreateUniformBufferImmediate(GetDeferredLightParameters(View, LightSceneInfo), EUniformBufferUsage::UniformBuffer_SingleFrame);
PassParameters.IESTexture = IESTextureResource->TextureRHI;
PassParameters.IESTextureSampler = IESTextureResource->SamplerStateRHI;
const float TanOuterAngle = bIsSpotLight ? FMath::Tan(LightSceneInfo.Proxy->GetOuterConeAngle()) : 1.0f;
PassParameters.LightFunctionParameters = FVector4(TanOuterAngle, 1.0f /*ShadowFadeFraction*/, bIsSpotLight ? 1.0f : 0.0f, bIsPointLight ? 1.0f : 0.0f);
PassParameters.LightFunctionParameters2 = FVector(LightSceneInfo.Proxy->GetLightFunctionFadeDistance(), LightSceneInfo.Proxy->GetLightFunctionDisabledBrightness(), 0.0f);
const FVector Scale = LightSceneInfo.Proxy->GetLightFunctionScale();
// Switch x and z so that z of the user specified scale affects the distance along the light direction
const FVector InverseScale = FVector(1.f / Scale.Z, 1.f / Scale.Y, 1.f / Scale.X);
PassParameters.WorldToLight = LightSceneInfo.Proxy->GetWorldToLight() * FScaleMatrix(FVector(InverseScale));
FMobileRadialLightFunctionPS::SetParameters(RHICmdList, PixelShader, View, LightMaterial.MaterialProxy, *LightMaterial.Material, PassParameters);
// 只绘制默认光照模型(MSM_DefaultLit)的像素.
uint8 StencilRef = GET_STENCIL_MOBILE_SM_MASK(MSM_DefaultLit);
RHICmdList.SetStencilRef(StencilRef);
// 点光源用球体绘制.
if (LightType == LightType_Point)
{
StencilingGeometry::DrawSphere(RHICmdList);
}
// 聚光灯用锥体绘制.
else // LightType_Spot
{
StencilingGeometry::DrawCone(RHICmdList);
}
}
绘制光源时，按光源类型划分为三个步骤：平行光、非分簇简单光源、局部光源（点光源和聚光灯）。需要注意的是，移动端只支持默认光照模型(MSM_DefaultLit)的计算，其它高级光照模型（头发、次表面散射、清漆、眼睛、布料等）暂不支持。
绘制平行光时，最多只能绘制1个，采用的是全屏幕矩形绘制，支持若干级CSM阴影。
绘制非分簇简单光源时，无论是点光源还是聚光灯，都采用球体绘制，不支持阴影。
绘制局部光源时，会复杂许多，先绘制局部光源模板缓冲，再设置光栅化和深度状态，最后才绘制光源。其中点光源采用球体绘制，不支持阴影；聚光灯采用锥体绘制，可以支持阴影，默认情况下，聚光灯不支持动态光影计算，需要在工程配置中开启：
此外，是否开启模板剔除光源不相交的像素由GMobileUseLightStencilCulling决定，而GMobileUseLightStencilCulling又由r.Mobile.UseLightStencilCulling决定，默认为1（即开启状态）。渲染光源的模板缓冲代码如下：
static void RenderLocalLight_StencilMask(FRHICommandListImmediate& RHICmdList, const FScene& Scene, const FViewInfo& View, const FLightSceneInfo& LightSceneInfo)
{
const uint8 LightType = LightSceneInfo.Proxy->GetLightType();
FGraphicsPipelineStateInitializer GraphicsPSOInit;
// 应用缓存好的RT(颜色/深度等).
RHICmdList.ApplyCachedRenderTargets(GraphicsPSOInit);
GraphicsPSOInit.PrimitiveType = PT_TriangleList;
// 禁用所有RT的写操作.
GraphicsPSOInit.BlendState = TStaticBlendStateWriteMask<CW_NONE, CW_NONE, CW_NONE, CW_NONE, CW_NONE, CW_NONE, CW_NONE, CW_NONE>::GetRHI();
GraphicsPSOInit.RasterizerState = View.bReverseCulling ? TStaticRasterizerState<FM_Solid, CM_CCW>::GetRHI() : TStaticRasterizerState<FM_Solid, CM_CW>::GetRHI();
// 如果深度测试失败, 则写入模板缓冲值为1.
GraphicsPSOInit.DepthStencilState = TStaticDepthStencilState<
false, CF_DepthNearOrEqual,
true, CF_Always, SO_Keep, SO_Replace, SO_Keep,
false, CF_Always, SO_Keep, SO_Keep, SO_Keep,
0x00,
// 注意只写入Pass专用的沙盒(SANBOX)位, 即模板缓冲的索引为0的位.
STENCIL_SANDBOX_MASK>::GetRHI();
// 绘制光源模板的VS是TDeferredLightVS.
TShaderMapRef<TDeferredLightVS<true> > VertexShader(View.ShaderMap);
GraphicsPSOInit.BoundShaderState.VertexDeclarationRHI = GetVertexDeclarationFVector4();
GraphicsPSOInit.BoundShaderState.VertexShaderRHI = VertexShader.GetVertexShader();
// PS为空.
GraphicsPSOInit.BoundShaderState.PixelShaderRHI = nullptr;
SetGraphicsPipelineState(RHICmdList, GraphicsPSOInit);
VertexShader->SetParameters(RHICmdList, View, &LightSceneInfo);
// 模板值为1.
RHICmdList.SetStencilRef(1);
// 根据不同光源用不同形状绘制.
if (LightType == LightType_Point)
{
StencilingGeometry::DrawSphere(RHICmdList);
}
else // LightType_Spot
{
StencilingGeometry::DrawCone(RHICmdList);
}
}
每个局部光源首先绘制光源范围内的Mask，再计算通过了Stencil测试（Early-Z）的像素的光照。具体的剖析过程以下图的聚光灯为例：
上：场景中一盏等待渲染的聚光灯；中：利用模板Pass绘制出的模板Mask（白色区域），标记了屏幕空间中和聚光灯形状重叠且深度更近的像素 ；下：对有效像素进行光照计算后的效果。
对有效像素进行光照计算时，使用的DepthStencil状态如下：
翻译成文字就是，执行光照的像素必须在光源形状体之内，光源形状之外的像素会被剔除。模板Pass标记的是比光源形状深度更近的像素（光源形状体之外的像素），光源绘制Pass通过模板测试剔除模板Pass标记的像素，然后再通过深度测试找出在光源形状体内的像素，从而提升光照计算效率。
移动端的这种光源模板裁剪（Light Stencil Culling）技术和Siggraph2020的Unity演讲Deferred Shading in Unity URP提及的基于模板的光照计算相似（思想一致，但做法可能不完全一样）。该论文还提出了更加契合光源形状的几何体模拟：
以及对比了各种光源计算方法在PC和移动端的性能，下面是Mali GPU的对比图：
Mali Gpu在使用不同光照渲染技术的性能对比，可见在移动端，基于模板裁剪的光照算法要优于常规和分块算法。
值得一提的是，光源模板裁剪技术结合GPU的Early-Z技术，将极大提升光照渲染性能。而当前主流的移动端GPU都支持Early-Z技术，也为光源模板裁剪的应用奠定了基础。
UE目前实现的光源裁剪算法兴许还有改进的空间，比如背向光源的像素（下图红框所示）其实也是可以不计算的。（但如何快速有效地找到背向光源的像素又是一个问题）
12.3.3.2 MobileBasePassShader
本节主要阐述移动端BasePass涉及的shader，包括VS和PS。先看VS：
// Engine\Shaders\Private\MobileBasePassVertexShader.usf
(......)
struct FMobileShadingBasePassVSToPS
{
FVertexFactoryInterpolantsVSToPS FactoryInterpolants;
FMobileBasePassInterpolantsVSToPS BasePassInterpolants;
float4 Position : SV_POSITION;
};
#define FMobileShadingBasePassVSOutput FMobileShadingBasePassVSToPS
#define VertexFactoryGetInterpolants VertexFactoryGetInterpolantsVSToPS
// VS主入口.
void Main(
FVertexFactoryInput Input
, out FMobileShadingBasePassVSOutput Output
#if INSTANCED_STEREO
, uint InstanceId : SV_InstanceID
, out uint LayerIndex : SV_RenderTargetArrayIndex
#elif MOBILE_MULTI_VIEW
, in uint ViewId : SV_ViewID
#endif
)
{
// 立体视图模式.
#if INSTANCED_STEREO
const uint EyeIndex = GetEyeIndex(InstanceId);
ResolvedView = ResolveView(EyeIndex);
LayerIndex = EyeIndex;
Output.BasePassInterpolants.MultiViewId = float(EyeIndex);
// 多视图模式.
#elif MOBILE_MULTI_VIEW
#if COMPILER_GLSL_ES3_1
const int MultiViewId = int(ViewId);
ResolvedView = ResolveView(uint(MultiViewId));
Output.BasePassInterpolants.MultiViewId = float(MultiViewId);
#else
ResolvedView = ResolveView(ViewId);
Output.BasePassInterpolants.MultiViewId = float(ViewId);
#endif
#else
ResolvedView = ResolveView();
#endif
// 初始化打包的插值数据.
#if PACK_INTERPOLANTS
float4 PackedInterps[NUM_VF_PACKED_INTERPOLANTS];
UNROLL
for(int i = 0; i < NUM_VF_PACKED_INTERPOLANTS; ++i)
{
PackedInterps[i] = 0;
}
#endif
// 处理顶点工厂数据.
FVertexFactoryIntermediates VFIntermediates = GetVertexFactoryIntermediates(Input);
float4 WorldPositionExcludingWPO = VertexFactoryGetWorldPosition(Input, VFIntermediates);
float4 WorldPosition = WorldPositionExcludingWPO;
// 获取材质的顶点数据, 处理坐标等.
half3x3 TangentToLocal = VertexFactoryGetTangentToLocal(Input, VFIntermediates);
FMaterialVertexParameters VertexParameters = GetMaterialVertexParameters(Input, VFIntermediates, WorldPosition.xyz, TangentToLocal);
half3 WorldPositionOffset = GetMaterialWorldPositionOffset(VertexParameters);
WorldPosition.xyz += WorldPositionOffset;
float4 RasterizedWorldPosition = VertexFactoryGetRasterizedWorldPosition(Input, VFIntermediates, WorldPosition);
Output.Position = mul(RasterizedWorldPosition, ResolvedView.TranslatedWorldToClip);
Output.BasePassInterpolants.PixelPosition = WorldPosition;
#if USE_WORLD_POSITION_EXCLUDING_SHADER_OFFSETS
Output.BasePassInterpolants.PixelPositionExcludingWPO = WorldPositionExcludingWPO.xyz;
#endif
// 裁剪面.
#if USE_PS_CLIP_PLANE
Output.BasePassInterpolants.OutClipDistance = dot(ResolvedView.GlobalClippingPlane, float4(WorldPosition.xyz - ResolvedView.PreViewTranslation.xyz, 1));
#endif
// 顶点雾.
#if USE_VERTEX_FOG
float4 VertexFog = CalculateHeightFog(WorldPosition.xyz - ResolvedView.TranslatedWorldCameraOrigin);
#if PROJECT_SUPPORT_SKY_ATMOSPHERE && MATERIAL_IS_SKY==0 // Do not apply aerial perpsective on sky materials
if (ResolvedView.SkyAtmosphereApplyCameraAerialPerspectiveVolume > 0.0f)
{
const float OneOverPreExposure = USE_PREEXPOSURE ? ResolvedView.OneOverPreExposure : 1.0f;
// Sample the aerial perspective (AP). It is also blended under the VertexFog parameter.
VertexFog = GetAerialPerspectiveLuminanceTransmittanceWithFogOver(
ResolvedView.RealTimeReflectionCapture, ResolvedView.SkyAtmosphereCameraAerialPerspectiveVolumeSizeAndInvSize,
Output.Position, WorldPosition.xyz*CM_TO_SKY_UNIT, ResolvedView.TranslatedWorldCameraOrigin*CM_TO_SKY_UNIT,
View.CameraAerialPerspectiveVolume, View.CameraAerialPerspectiveVolumeSampler,
ResolvedView.SkyAtmosphereCameraAerialPerspectiveVolumeDepthResolutionInv,
ResolvedView.SkyAtmosphereCameraAerialPerspectiveVolumeDepthResolution,
ResolvedView.SkyAtmosphereAerialPerspectiveStartDepthKm,
ResolvedView.SkyAtmosphereCameraAerialPerspectiveVolumeDepthSliceLengthKm,
ResolvedView.SkyAtmosphereCameraAerialPerspectiveVolumeDepthSliceLengthKmInv,
OneOverPreExposure, VertexFog);
}
#endif
#if PACK_INTERPOLANTS
PackedInterps[0] = VertexFog;
#else
Output.BasePassInterpolants.VertexFog = VertexFog;
#endif // PACK_INTERPOLANTS
#endif // USE_VERTEX_FOG
(......)
// 获取待插值的数据.
Output.FactoryInterpolants = VertexFactoryGetInterpolants(Input, VFIntermediates, VertexParameters);
Output.BasePassInterpolants.PixelPosition.w = Output.Position.w;
// 打包插值数据.
#if PACK_INTERPOLANTS
VertexFactoryPackInterpolants(Output.FactoryInterpolants, PackedInterps);
#endif // PACK_INTERPOLANTS
#if !OUTPUT_MOBILE_HDR && COMPILER_GLSL_ES3_1
Output.Position.y *= -1;
#endif
}
以上可知，视图实例会根据立体绘制、多视图和普通模式不同而不同处理。支持顶点雾，但默认是关闭的，需要在工程配置内开启。
存在打包插值模式，为了压缩VS到PS之间的插值消耗和带宽。是否开启由宏PACK_INTERPOLANTS决定，它的定义如下：
// Engine\Shaders\Private\MobileBasePassCommon.ush
#define PACK_INTERPOLANTS (USE_VERTEX_FOG && NUM_VF_PACKED_INTERPOLANTS > 0 && (ES3_1_PROFILE))
也就是说，只有开启顶点雾、存在顶点工厂打包插值数据且是OpenGLES3.1着色平台才开启打包插值的特性。相比PC端的BasePass的VS，移动端的做了大量的简化，可以简单地认为只是PC端的一个很小的子集。下面继续分析PS：
// Engine\Shaders\Private\MobileBasePassPixelShader.usf
#include "Common.ush"
// 各类宏定义.
#define MobileSceneTextures MobileBasePass.SceneTextures
#define EyeAdaptationStruct MobileBasePass
(......)
// 最接近被渲染对象的场景的预归一化捕获(完全粗糙材质不支持)
#if !FULLY_ROUGH
#if HQ_REFLECTIONS
#define MAX_HQ_REFLECTIONS 3
TextureCube ReflectionCubemap0;
SamplerState ReflectionCubemapSampler0;
TextureCube ReflectionCubemap1;
SamplerState ReflectionCubemapSampler1;
TextureCube ReflectionCubemap2;
SamplerState ReflectionCubemapSampler2;
// x,y,z - inverted average brightness for 0, 1, 2; w - sky cube texture max mips.
float4 ReflectionAverageBrigtness;
float4 ReflectanceMaxValueRGBM;
float4 ReflectionPositionsAndRadii[MAX_HQ_REFLECTIONS];
#if ALLOW_CUBE_REFLECTIONS
float4x4 CaptureBoxTransformArray[MAX_HQ_REFLECTIONS];
float4 CaptureBoxScalesArray[MAX_HQ_REFLECTIONS];
#endif
#endif
#endif
// 反射球/IBL等接口.
half4 GetPlanarReflection(float3 WorldPosition, half3 WorldNormal, half Roughness);
half MobileComputeMixingWeight(half IndirectIrradiance, half AverageBrightness, half Roughness);
half3 GetLookupVectorForBoxCaptureMobile(half3 ReflectionVector, ...);
half3 GetLookupVectorForSphereCaptureMobile(half3 ReflectionVector, ...);
void GatherSpecularIBL(FMaterialPixelParameters MaterialParameters, ...);
void BlendReflectionCaptures(FMaterialPixelParameters MaterialParameters, ...)
half3 GetImageBasedReflectionLighting(FMaterialPixelParameters MaterialParameters, ...);
// 其它接口.
half3 FrameBufferBlendOp(half4 Source);
bool UseCSM();
void ApplyPixelDepthOffsetForMobileBasePass(inout FMaterialPixelParameters MaterialParameters, FPixelMaterialInputs PixelMaterialInputs, out float OutDepth);
// 累积动态点光源.
#if MAX_DYNAMIC_POINT_LIGHTS > 0
void AccumulateLightingOfDynamicPointLight(
FMaterialPixelParameters MaterialParameters,
FMobileShadingModelContext ShadingModelContext,
FGBufferData GBuffer,
float4 LightPositionAndInvRadius,
float4 LightColorAndFalloffExponent,
float4 SpotLightDirectionAndSpecularScale,
float4 SpotLightAnglesAndSoftTransitionScaleAndLightShadowType,
#if SUPPORT_SPOTLIGHTS_SHADOW
FPCFSamplerSettings Settings,
float4 SpotLightShadowSharpenAndShadowFadeFraction,
float4 SpotLightShadowmapMinMax,
float4x4 SpotLightShadowWorldToShadowMatrix,
#endif
inout half3 Color)
{
uint LightShadowType = SpotLightAnglesAndSoftTransitionScaleAndLightShadowType.w;
float FadedShadow = 1.0f;
// 计算聚光灯阴影.
#if SUPPORT_SPOTLIGHTS_SHADOW
if ((LightShadowType & LightShadowType_Shadow) == LightShadowType_Shadow)
{
float4 HomogeneousShadowPosition = mul(float4(MaterialParameters.AbsoluteWorldPosition, 1), SpotLightShadowWorldToShadowMatrix);
float2 ShadowUVs = HomogeneousShadowPosition.xy / HomogeneousShadowPosition.w;
if (all(ShadowUVs >= SpotLightShadowmapMinMax.xy && ShadowUVs <= SpotLightShadowmapMinMax.zw))
{
// Clamp pixel depth in light space for shadowing opaque, because areas of the shadow depth buffer that weren't rendered to will have been cleared to 1
// We want to force the shadow comparison to result in 'unshadowed' in that case, regardless of whether the pixel being shaded is in front or behind that plane
float LightSpacePixelDepthForOpaque = min(HomogeneousShadowPosition.z, 0.99999f);
Settings.SceneDepth = LightSpacePixelDepthForOpaque;
Settings.TransitionScale = SpotLightAnglesAndSoftTransitionScaleAndLightShadowType.z;
half Shadow = MobileShadowPCF(ShadowUVs, Settings);
Shadow = saturate((Shadow - 0.5) * SpotLightShadowSharpenAndShadowFadeFraction.x + 0.5);
FadedShadow = lerp(1.0f, Square(Shadow), SpotLightShadowSharpenAndShadowFadeFraction.y);
}
}
#endif
// 计算光照.
if ((LightShadowType & ValidLightType) != 0)
{
float3 ToLight = LightPositionAndInvRadius.xyz - MaterialParameters.AbsoluteWorldPosition;
float DistanceSqr = dot(ToLight, ToLight);
float3 L = ToLight * rsqrt(DistanceSqr);
half3 PointH = normalize(MaterialParameters.CameraVector + L);
half PointNoL = max(0, dot(MaterialParameters.WorldNormal, L));
half PointNoH = max(0, dot(MaterialParameters.WorldNormal, PointH));
// 计算光源的衰减.
float Attenuation;
if (LightColorAndFalloffExponent.w == 0)
{
// Sphere falloff (technically just 1/d2 but this avoids inf)
Attenuation = 1 / (DistanceSqr + 1);
float LightRadiusMask = Square(saturate(1 - Square(DistanceSqr * (LightPositionAndInvRadius.w * LightPositionAndInvRadius.w))));
Attenuation *= LightRadiusMask;
}
else
{
Attenuation = RadialAttenuation(ToLight * LightPositionAndInvRadius.w, LightColorAndFalloffExponent.w);
}
#if PROJECT_MOBILE_ENABLE_MOVABLE_SPOTLIGHTS
if ((LightShadowType & LightShadowType_SpotLight) == LightShadowType_SpotLight)
{
Attenuation *= SpotAttenuation(L, -SpotLightDirectionAndSpecularScale.xyz, SpotLightAnglesAndSoftTransitionScaleAndLightShadowType.xy) * FadedShadow;
}
#endif
// 累加光照结果.
#if !FULLY_ROUGH
FMobileDirectLighting Lighting = MobileIntegrateBxDF(ShadingModelContext, GBuffer, PointNoL, MaterialParameters.CameraVector, PointH, PointNoH);
Color += min(65000.0, (Attenuation) * LightColorAndFalloffExponent.rgb * (1.0 / PI) * (Lighting.Diffuse + Lighting.Specular * SpotLightDirectionAndSpecularScale.w));
#else
Color += (Attenuation * PointNoL) * LightColorAndFalloffExponent.rgb * (1.0 / PI) * ShadingModelContext.DiffuseColor;
#endif
}
}
#endif
(......)
// 计算非直接光照.
half ComputeIndirect(VTPageTableResult LightmapVTPageTableResult, FVertexFactoryInterpolantsVSToPS Interpolants, float3 DiffuseDir, FMobileShadingModelContext ShadingModelContext, out half IndirectIrradiance, out half3 Color)
{
//To keep IndirectLightingCache conherence with PC, initialize the IndirectIrradiance to zero.
IndirectIrradiance = 0;
Color = 0;
// 非直接漫反射.
#if LQ_TEXTURE_LIGHTMAP
float2 LightmapUV0, LightmapUV1;
uint LightmapDataIndex;
GetLightMapCoordinates(Interpolants, LightmapUV0, LightmapUV1, LightmapDataIndex);
half4 LightmapColor = GetLightMapColorLQ(LightmapVTPageTableResult, LightmapUV0, LightmapUV1, LightmapDataIndex, DiffuseDir);
Color += LightmapColor.rgb * ShadingModelContext.DiffuseColor * View.IndirectLightingColorScale;
IndirectIrradiance = LightmapColor.a;
#elif CACHED_POINT_INDIRECT_LIGHTING
#if MATERIALBLENDING_MASKED || MATERIALBLENDING_SOLID
// 将法线应用到半透明物体.
FThreeBandSHVectorRGB PointIndirectLighting;
PointIndirectLighting.R.V0 = IndirectLightingCache.IndirectLightingSHCoefficients0[0];
PointIndirectLighting.R.V1 = IndirectLightingCache.IndirectLightingSHCoefficients1[0];
PointIndirectLighting.R.V2 = IndirectLightingCache.IndirectLightingSHCoefficients2[0];
PointIndirectLighting.G.V0 = IndirectLightingCache.IndirectLightingSHCoefficients0[1];
PointIndirectLighting.G.V1 = IndirectLightingCache.IndirectLightingSHCoefficients1[1];
PointIndirectLighting.G.V2 = IndirectLightingCache.IndirectLightingSHCoefficients2[1];
PointIndirectLighting.B.V0 = IndirectLightingCache.IndirectLightingSHCoefficients0[2];
PointIndirectLighting.B.V1 = IndirectLightingCache.IndirectLightingSHCoefficients1[2];
PointIndirectLighting.B.V2 = IndirectLightingCache.IndirectLightingSHCoefficients2[2];
FThreeBandSHVector DiffuseTransferSH = CalcDiffuseTransferSH3(DiffuseDir, 1);
// 计算加入了法线影响的漫反射光照.
half3 DiffuseGI = max(half3(0, 0, 0), DotSH3(PointIndirectLighting, DiffuseTransferSH));
IndirectIrradiance = Luminance(DiffuseGI);
Color += ShadingModelContext.DiffuseColor * DiffuseGI * View.IndirectLightingColorScale;
#else
// 半透明使用无方向(Non-directional), 漫反射被打包在xyz, 已经在cpu端除了PI和SH漫反射.
half3 PointIndirectLighting = IndirectLightingCache.IndirectLightingSHSingleCoefficient.rgb;
half3 DiffuseGI = PointIndirectLighting;
IndirectIrradiance = Luminance(DiffuseGI);
Color += ShadingModelContext.DiffuseColor * DiffuseGI * View.IndirectLightingColorScale;
#endif
#endif
return IndirectIrradiance;
}
// PS主入口.
PIXELSHADER_EARLYDEPTHSTENCIL
void Main(
FVertexFactoryInterpolantsVSToPS Interpolants
, FMobileBasePassInterpolantsVSToPS BasePassInterpolants
, in float4 SvPosition : SV_Position
OPTIONAL_IsFrontFace
, out half4 OutColor : SV_Target0
#if DEFERRED_SHADING_PATH
, out half4 OutGBufferA : SV_Target1
, out half4 OutGBufferB : SV_Target2
, out half4 OutGBufferC : SV_Target3
#endif
#if USE_SCENE_DEPTH_AUX
, out float OutSceneDepthAux : SV_Target4
#endif
#if OUTPUT_PIXEL_DEPTH_OFFSET
, out float OutDepth : SV_Depth
#endif
)
{
#if MOBILE_MULTI_VIEW
ResolvedView = ResolveView(uint(BasePassInterpolants.MultiViewId));
#else
ResolvedView = ResolveView();
#endif
#if USE_PS_CLIP_PLANE
clip(BasePassInterpolants.OutClipDistance);
#endif
// 解压打包的插值数据.
#if PACK_INTERPOLANTS
float4 PackedInterpolants[NUM_VF_PACKED_INTERPOLANTS];
VertexFactoryUnpackInterpolants(Interpolants, PackedInterpolants);
#endif
#if COMPILER_GLSL_ES3_1 && !OUTPUT_MOBILE_HDR && !MOBILE_EMULATION
// LDR Mobile needs screen vertical flipped
SvPosition.y = ResolvedView.BufferSizeAndInvSize.y - SvPosition.y - 1;
#endif
// 获取材质的像素属性.
FMaterialPixelParameters MaterialParameters = GetMaterialPixelParameters(Interpolants, SvPosition);
FPixelMaterialInputs PixelMaterialInputs;
{
float4 ScreenPosition = SvPositionToResolvedScreenPosition(SvPosition);
float3 WorldPosition = BasePassInterpolants.PixelPosition.xyz;
float3 WorldPositionExcludingWPO = BasePassInterpolants.PixelPosition.xyz;
#if USE_WORLD_POSITION_EXCLUDING_SHADER_OFFSETS
WorldPositionExcludingWPO = BasePassInterpolants.PixelPositionExcludingWPO;
#endif
CalcMaterialParametersEx(MaterialParameters, PixelMaterialInputs, SvPosition, ScreenPosition, bIsFrontFace, WorldPosition, WorldPositionExcludingWPO);
#if FORCE_VERTEX_NORMAL
// Quality level override of material's normal calculation, can be used to avoid normal map reads etc.
MaterialParameters.WorldNormal = MaterialParameters.TangentToWorld[2];
MaterialParameters.ReflectionVector = ReflectionAboutCustomWorldNormal(MaterialParameters, MaterialParameters.WorldNormal, false);
#endif
}
// 像素深度偏移.
#if OUTPUT_PIXEL_DEPTH_OFFSET
ApplyPixelDepthOffsetForMobileBasePass(MaterialParameters, PixelMaterialInputs, OutDepth);
#endif
// Mask材质.
#if !EARLY_Z_PASS_ONLY_MATERIAL_MASKING
//Clip if the blend mode requires it.
GetMaterialCoverageAndClipping(MaterialParameters, PixelMaterialInputs);
#endif
// 计算并缓存GBuffer数据, 防止后续多次采用纹理.
FGBufferData GBuffer = (FGBufferData)0;
GBuffer.WorldNormal = MaterialParameters.WorldNormal;
GBuffer.BaseColor = GetMaterialBaseColor(PixelMaterialInputs);
GBuffer.Metallic = GetMaterialMetallic(PixelMaterialInputs);
GBuffer.Specular = GetMaterialSpecular(PixelMaterialInputs);
GBuffer.Roughness = GetMaterialRoughness(PixelMaterialInputs);
GBuffer.ShadingModelID = GetMaterialShadingModel(PixelMaterialInputs);
half MaterialAO = GetMaterialAmbientOcclusion(PixelMaterialInputs);
// 应用AO.
#if APPLY_AO
half4 GatheredAmbientOcclusion = Texture2DSample(AmbientOcclusionTexture, AmbientOcclusionSampler, SvPositionToBufferUV(SvPosition));
MaterialAO *= GatheredAmbientOcclusion.r;
#endif
GBuffer.GBufferAO = MaterialAO;
// 由于IEEE 754 (FP16)可表示的最小标准值是2^-24 = 5.96e-8, 而后面的粗糙度涉及到1.0 / Roughness^4的计算, 所以为了防止除零错误, 需保证Roughness^4 >= 5.96e-8, 此处直接Clamp粗糙度到0.015625(0.015625^4 = 5.96e-8).
// 另外, 为了匹配PC端的延迟渲染(粗糙度存储在8位的值), 因此也自动Clamp到1.0.
GBuffer.Roughness = max(0.015625, GetMaterialRoughness(PixelMaterialInputs));
// 初始化移动端着色模型上下文FMobileShadingModelContext.
FMobileShadingModelContext ShadingModelContext = (FMobileShadingModelContext)0;
ShadingModelContext.Opacity = GetMaterialOpacity(PixelMaterialInputs);
// 薄层透明度物
#if MATERIAL_SHADINGMODEL_THIN_TRANSLUCENT
(......)
#endif
half3 Color = 0;
// 自定义数据.
half CustomData0 = GetMaterialCustomData0(MaterialParameters);
half CustomData1 = GetMaterialCustomData1(MaterialParameters);
InitShadingModelContext(ShadingModelContext, GBuffer, MaterialParameters.SvPosition, MaterialParameters.CameraVector, CustomData0, CustomData1);
float3 DiffuseDir = MaterialParameters.WorldNormal;
// 头发模型.
#if MATERIAL_SHADINGMODEL_HAIR
(......)
#endif
// 光照图虚拟纹理.
VTPageTableResult LightmapVTPageTableResult = (VTPageTableResult)0.0f;
#if LIGHTMAP_VT_ENABLED
{
float2 LightmapUV0, LightmapUV1;
uint LightmapDataIndex;
GetLightMapCoordinates(Interpolants, LightmapUV0, LightmapUV1, LightmapDataIndex);
LightmapVTPageTableResult = LightmapGetVTSampleInfo(LightmapUV0, LightmapDataIndex, SvPosition.xy);
}
#endif
#if LIGHTMAP_VT_ENABLED
// This must occur after CalcMaterialParameters(), which is required to initialize the VT feedback mechanism
// Lightmap request is always the first VT sample in the shader
StoreVirtualTextureFeedback(MaterialParameters.VirtualTextureFeedback, 0, LightmapVTPageTableResult.PackedRequest);
#endif
// 计算非直接光.
half IndirectIrradiance;
half3 IndirectColor;
ComputeIndirect(LightmapVTPageTableResult, Interpolants, DiffuseDir, ShadingModelContext, IndirectIrradiance, IndirectColor);
Color += IndirectColor;
// 预计算的阴影图.
half Shadow = GetPrimaryPrecomputedShadowMask(LightmapVTPageTableResult, Interpolants).r;
#if DEFERRED_SHADING_PATH
float4 OutGBufferD;
float4 OutGBufferE;
float4 OutGBufferF;
float4 OutGBufferVelocity = 0;
GBuffer.IndirectIrradiance = IndirectIrradiance;
GBuffer.PrecomputedShadowFactors.r = Shadow;
// 编码GBuffer数据.
EncodeGBuffer(GBuffer, OutGBufferA, OutGBufferB, OutGBufferC, OutGBufferD, OutGBufferE, OutGBufferF, OutGBufferVelocity);
#else
#if !MATERIAL_SHADINGMODEL_UNLIT
// 天光.
#if ENABLE_SKY_LIGHT
half3 SkyDiffuseLighting = GetSkySHDiffuseSimple(MaterialParameters.WorldNormal);
half3 DiffuseLookup = SkyDiffuseLighting * ResolvedView.SkyLightColor.rgb;
IndirectIrradiance += Luminance(DiffuseLookup);
#endif
Color *= MaterialAO;
IndirectIrradiance *= MaterialAO;
float ShadowPositionZ = 0;
#if DIRECTIONAL_LIGHT_CSM && !MATERIAL_SHADINGMODEL_SINGLELAYERWATER
// CSM阴影.
if (UseCSM())
{
half ShadowMap = MobileDirectionalLightCSM(MaterialParameters.ScreenPosition.xy, MaterialParameters.ScreenPosition.w, ShadowPositionZ);
#if ALLOW_STATIC_LIGHTING
Shadow = min(ShadowMap, Shadow);
#else
Shadow = ShadowMap;
#endif
}
#endif /* DIRECTIONAL_LIGHT_CSM */
// 距离场阴影.
#if APPLY_DISTANCE_FIELD
if (ShadowPositionZ == 0)
{
Shadow = Texture2DSample(MobileBasePass.ScreenSpaceShadowMaskTexture, MobileBasePass.ScreenSpaceShadowMaskSampler, SvPositionToBufferUV(SvPosition)).x;
}
#endif
half NoL = max(0, dot(MaterialParameters.WorldNormal, MobileDirectionalLight.DirectionalLightDirectionAndShadowTransition.xyz));
half3 H = normalize(MaterialParameters.CameraVector + MobileDirectionalLight.DirectionalLightDirectionAndShadowTransition.xyz);
half NoH = max(0, dot(MaterialParameters.WorldNormal, H));
// 平行光 + IBL
#if FULLY_ROUGH
Color += (Shadow * NoL) * MobileDirectionalLight.DirectionalLightColor.rgb * ShadingModelContext.DiffuseColor;
#else
FMobileDirectLighting Lighting = MobileIntegrateBxDF(ShadingModelContext, GBuffer, NoL, MaterialParameters.CameraVector, H, NoH);
// MobileDirectionalLight.DirectionalLightDistanceFadeMADAndSpecularScale.z保存了平行光的SpecularScale.
Color += (Shadow) * MobileDirectionalLight.DirectionalLightColor.rgb * (Lighting.Diffuse + Lighting.Specular * MobileDirectionalLight.DirectionalLightDistanceFadeMADAndSpecularScale.z);
// 头发着色.
#if !(MATERIAL_SINGLE_SHADINGMODEL && MATERIAL_SHADINGMODEL_HAIR)
(......)
#endif
#endif /* FULLY_ROUGH */
// 局部光源, 最多4个.
#if MAX_DYNAMIC_POINT_LIGHTS > 0 && !MATERIAL_SHADINGMODEL_SINGLELAYERWATER
if(NumDynamicPointLights > 0)
{
#if SUPPORT_SPOTLIGHTS_SHADOW
FPCFSamplerSettings Settings;
Settings.ShadowDepthTexture = DynamicSpotLightShadowTexture;
Settings.ShadowDepthTextureSampler = DynamicSpotLightShadowSampler;
Settings.ShadowBufferSize = DynamicSpotLightShadowBufferSize;
Settings.bSubsurface = false;
Settings.bTreatMaxDepthUnshadowed = false;
Settings.DensityMulConstant = 0;
Settings.ProjectionDepthBiasParameters = 0;
#endif
AccumulateLightingOfDynamicPointLight(MaterialParameters, ...);
if (MAX_DYNAMIC_POINT_LIGHTS > 1 && NumDynamicPointLights > 1)
{
AccumulateLightingOfDynamicPointLight(MaterialParameters, ...);
if (MAX_DYNAMIC_POINT_LIGHTS > 2 && NumDynamicPointLights > 2)
{
AccumulateLightingOfDynamicPointLight(MaterialParameters, ...);
if (MAX_DYNAMIC_POINT_LIGHTS > 3 && NumDynamicPointLights > 3)
{
AccumulateLightingOfDynamicPointLight(MaterialParameters, ...);
}
}
}
}
#endif
// 天空光.
#if ENABLE_SKY_LIGHT
#if MATERIAL_TWOSIDED && LQ_TEXTURE_LIGHTMAP
if (NoL == 0)
{
#endif
#if MATERIAL_SHADINGMODEL_SINGLELAYERWATER
ShadingModelContext.WaterDiffuseIndirectLuminance += SkyDiffuseLighting;
#endif
Color += SkyDiffuseLighting * half3(ResolvedView.SkyLightColor.rgb) * ShadingModelContext.DiffuseColor * MaterialAO;
#if MATERIAL_TWOSIDED && LQ_TEXTURE_LIGHTMAP
}
#endif
#endif
#endif /* !MATERIAL_SHADINGMODEL_UNLIT */
#if MATERIAL_SHADINGMODEL_SINGLELAYERWATER
(......)
#endif // MATERIAL_SHADINGMODEL_SINGLELAYERWATER
#endif// DEFERRED_SHADING_PATH
// 处理顶点雾.
half4 VertexFog = half4(0, 0, 0, 1);
#if USE_VERTEX_FOG
#if PACK_INTERPOLANTS
VertexFog = PackedInterpolants[0];
#else
VertexFog = BasePassInterpolants.VertexFog;
#endif
#endif
// 自发光.
half3 Emissive = GetMaterialEmissive(PixelMaterialInputs);
#if MATERIAL_SHADINGMODEL_THIN_TRANSLUCENT
Emissive *= TopMaterialCoverage;
#endif
Color += Emissive;
#if !MATERIAL_SHADINGMODEL_UNLIT && MOBILE_EMULATION
Color = lerp(Color, ShadingModelContext.DiffuseColor, ResolvedView.UnlitViewmodeMask);
#endif
// 组合雾颜色到输出颜色.
#if MATERIALBLENDING_ALPHACOMPOSITE || MATERIAL_SHADINGMODEL_SINGLELAYERWATER
OutColor = half4(Color * VertexFog.a + VertexFog.rgb * ShadingModelContext.Opacity, ShadingModelContext.Opacity);
#elif MATERIALBLENDING_ALPHAHOLDOUT
// not implemented for holdout
OutColor = half4(Color * VertexFog.a + VertexFog.rgb * ShadingModelContext.Opacity, ShadingModelContext.Opacity);
#elif MATERIALBLENDING_TRANSLUCENT
OutColor = half4(Color * VertexFog.a + VertexFog.rgb, ShadingModelContext.Opacity);
#elif MATERIALBLENDING_ADDITIVE
OutColor = half4(Color * (VertexFog.a * ShadingModelContext.Opacity.x), 0.0f);
#elif MATERIALBLENDING_MODULATE
half3 FoggedColor = lerp(half3(1, 1, 1), Color, VertexFog.aaa * VertexFog.aaa);
OutColor = half4(FoggedColor, ShadingModelContext.Opacity);
#else
OutColor.rgb = Color * VertexFog.a + VertexFog.rgb;
#if !MATERIAL_USE_ALPHA_TO_COVERAGE
// Scene color alpha is not used yet so we set it to 1
OutColor.a = 1.0;
#if OUTPUT_MOBILE_HDR
// Store depth in FP16 alpha. This depth value can be fetched during translucency or sampled in post-processing
OutColor.a = SvPosition.z;
#endif
#else
half MaterialOpacityMask = GetMaterialMaskInputRaw(PixelMaterialInputs);
OutColor.a = GetMaterialMask(PixelMaterialInputs) / max(abs(ddx(MaterialOpacityMask)) + abs(ddy(MaterialOpacityMask)), 0.0001f) + 0.5f;
#endif
#endif
#if !MATERIALBLENDING_MODULATE && USE_PREEXPOSURE
OutColor.rgb *= ResolvedView.PreExposure;
#endif
#if MATERIAL_IS_SKY
OutColor.rgb = min(OutColor.rgb, Max10BitsFloat.xxx * 0.5f);
#endif
#if USE_SCENE_DEPTH_AUX
OutSceneDepthAux = SvPosition.z;
#endif
// 处理颜色的alpha.
#if USE_EDITOR_COMPOSITING && (MOBILE_EMULATION)
// Editor primitive depth testing
OutColor.a = 1.0;
#if MATERIALBLENDING_MASKED
// some material might have an opacity value
OutColor.a = GetMaterialMaskInputRaw(PixelMaterialInputs);
#endif
clip(OutColor.a - GetMaterialOpacityMaskClipValue());
#else
#if OUTPUT_GAMMA_SPACE
OutColor.rgb = sqrt(OutColor.rgb);
#endif
#endif
#if NUM_VIRTUALTEXTURE_SAMPLES || LIGHTMAP_VT_ENABLED
FinalizeVirtualTextureFeedback(
MaterialParameters.VirtualTextureFeedback,
MaterialParameters.SvPosition,
ShadingModelContext.Opacity,
View.FrameNumber,
View.VTFeedbackBuffer
);
#endif
}
移动端的BasePassPS的处理过程比较复杂，步骤繁多，主要有：解压插值数据，获取并计算材质属性，计算并缓村GBuffer，处理或调整GBuffer数据，计算前向渲染分支的光照（平行光、局部光），计算距离场、CSM等阴影，计算天空光，处理静态光照、非直接光和IBL，计算雾效，以及处理水体、头发、薄层透明度等特殊着色模型。
由于标准16位浮点数（FP16）可表示的最小值是\(\cfrac{1.0}{2^{24}} = 5.96 \cdot 10^{-8}\)，而后续的光照计算涉及到粗糙度的4次方运算（\(\cfrac{1.0}{\text{Roughness}^4}\)），为了防止除零错误，需要将粗糙度截取到\(0.015625\)（\(0.015625^4 = 5.96 \cdot 10^{-8}\)）。
GBuffer.Roughness = max(0.015625, GetMaterialRoughness(PixelMaterialInputs));
这也警示我们在开发移动端的渲染特性时，需要格外注意和把控数据精度，否则在低端设备经常由于数据精度不足而出现各种奇葩的画面异常。
虽然上面的代码较多，但由很多宏控制着，实际渲染单个材质所需的代码可能只是其中的很小的一个子集。比如说，默认支持4个局部光源，但如果在工程配置（下图）中可以设为2或更少，则实际执行的光源指令少了很多。
如果是前向渲染分支，则GBuffer的很多处理将被忽略；如果是延迟渲染分支，则平行光、局部光源的计算将被忽略，由延迟渲染Pass的shader执行。
下面对重要接口EncodeGBuffer做剖析：
void EncodeGBuffer(
FGBufferData GBuffer,
out float4 OutGBufferA,
out float4 OutGBufferB,
out float4 OutGBufferC,
out float4 OutGBufferD,
out float4 OutGBufferE,
out float4 OutGBufferVelocity,
float QuantizationBias = 0 // -0.5 to 0.5 random float. Used to bias quantization.
)
{
if (GBuffer.ShadingModelID == SHADINGMODELID_UNLIT)
{
OutGBufferA = 0;
SetGBufferForUnlit(OutGBufferB);
OutGBufferC = 0;
OutGBufferD = 0;
OutGBufferE = 0;
}
else
{
// GBufferA: 八面体压缩后的法线, 预计算阴影因子, 逐物体数据.
#if MOBILE_DEFERRED_SHADING
OutGBufferA.rg = UnitVectorToOctahedron( normalize(GBuffer.WorldNormal) ) * 0.5f + 0.5f;
OutGBufferA.b = GBuffer.PrecomputedShadowFactors.x;
OutGBufferA.a = GBuffer.PerObjectGBufferData;
#else
(......)
#endif
// GBufferB: 金属度, 高光度, 粗糙度, 着色模型, 其它Mask.
OutGBufferB.r = GBuffer.Metallic;
OutGBufferB.g = GBuffer.Specular;
OutGBufferB.b = GBuffer.Roughness;
OutGBufferB.a = EncodeShadingModelIdAndSelectiveOutputMask(GBuffer.ShadingModelID, GBuffer.SelectiveOutputMask);
// GBufferC: 基础色, AO或非直接光.
OutGBufferC.rgb = EncodeBaseColor( GBuffer.BaseColor );
#if ALLOW_STATIC_LIGHTING
// No space for AO. Multiply IndirectIrradiance by AO instead of storing.
OutGBufferC.a = EncodeIndirectIrradiance(GBuffer.IndirectIrradiance * GBuffer.GBufferAO) + QuantizationBias * (1.0 / 255.0);
#else
OutGBufferC.a = GBuffer.GBufferAO;
#endif
OutGBufferD = GBuffer.CustomData;
OutGBufferE = GBuffer.PrecomputedShadowFactors;
}
#if WRITES_VELOCITY_TO_GBUFFER
OutGBufferVelocity = GBuffer.Velocity;
#else
OutGBufferVelocity = 0;
#endif
}
在默认光照模型(DefaultLit)下，BasePass输出的结果有以下几种纹理：
12.3.3.3 MobileDeferredShading
移动端延迟光照的VS和PC端是一样的，都是DeferredLightVertexShaders.usf，但PS不一样，用的是MobileDeferredShading.usf。由于VS和PC一样，且没有特殊的操作，此处就忽略，如果有兴趣的同学可以看第五篇的小节5.5.3.1 DeferredLightVertexShader。
下面直接分析PS代码：
// Engine\Shaders\Private\MobileDeferredShading.usf
(......)
// 移动端光源数据结构体.
struct FMobileLightData
{
float3 Position;
float InvRadius;
float3 Color;
float FalloffExponent;
float3 Direction;
float2 SpotAngles;
float SourceRadius;
float SpecularScale;
bool bInverseSquared;
bool bSpotLight;
};
// 获取GBuffer数据.
void FetchGBuffer(in float2 UV, out float4 GBufferA, out float4 GBufferB, out float4 GBufferC, out float4 GBufferD, out float SceneDepth)
{
// Vulkan的子pass获取数据.
#if VULKAN_PROFILE
GBufferA = VulkanSubpassFetch1();
GBufferB = VulkanSubpassFetch2();
GBufferC = VulkanSubpassFetch3();
GBufferD = 0;
SceneDepth = ConvertFromDeviceZ(VulkanSubpassDepthFetch());
// Metal的子pass获取数据.
#elif METAL_PROFILE
GBufferA = SubpassFetchRGBA_1();
GBufferB = SubpassFetchRGBA_2();
GBufferC = SubpassFetchRGBA_3();
GBufferD = 0;
SceneDepth = ConvertFromDeviceZ(SubpassFetchR_4());
// 其它平台(DX, OpenGL)的子pass获取数据.
#else
GBufferA = Texture2DSampleLevel(MobileSceneTextures.GBufferATexture, MobileSceneTextures.GBufferATextureSampler, UV, 0);
GBufferB = Texture2DSampleLevel(MobileSceneTextures.GBufferBTexture, MobileSceneTextures.GBufferBTextureSampler, UV, 0);
GBufferC = Texture2DSampleLevel(MobileSceneTextures.GBufferCTexture, MobileSceneTextures.GBufferCTextureSampler, UV, 0);
GBufferD = 0;
SceneDepth = ConvertFromDeviceZ(Texture2DSampleLevel(MobileSceneTextures.SceneDepthTexture, MobileSceneTextures.SceneDepthTextureSampler, UV, 0).r);
#endif
}
// 解压GBuffer数据.
FGBufferData DecodeGBufferMobile(
float4 InGBufferA,
float4 InGBufferB,
float4 InGBufferC,
float4 InGBufferD)
{
FGBufferData GBuffer;
GBuffer.WorldNormal = OctahedronToUnitVector( InGBufferA.xy * 2.0f - 1.0f );
GBuffer.PrecomputedShadowFactors = InGBufferA.z;
GBuffer.PerObjectGBufferData = InGBufferA.a;
GBuffer.Metallic = InGBufferB.r;
GBuffer.Specular = InGBufferB.g;
GBuffer.Roughness = max(0.015625, InGBufferB.b);
// Note: must match GetShadingModelId standalone function logic
// Also Note: SimpleElementPixelShader directly sets SV_Target2 ( GBufferB ) to indicate unlit.
// An update there will be required if this layout changes.
GBuffer.ShadingModelID = DecodeShadingModelId(InGBufferB.a);
GBuffer.SelectiveOutputMask = DecodeSelectiveOutputMask(InGBufferB.a);
GBuffer.BaseColor = DecodeBaseColor(InGBufferC.rgb);
#if ALLOW_STATIC_LIGHTING
GBuffer.GBufferAO = 1;
GBuffer.IndirectIrradiance = DecodeIndirectIrradiance(InGBufferC.a);
#else
GBuffer.GBufferAO = InGBufferC.a;
GBuffer.IndirectIrradiance = 1;
#endif
GBuffer.CustomData = HasCustomGBufferData(GBuffer.ShadingModelID) ? InGBufferD : 0;
return GBuffer;
}
// 直接光照.
half3 GetDirectLighting(
FMobileLightData LightData,
FMobileShadingModelContext ShadingModelContext,
FGBufferData GBuffer,
float3 WorldPosition,
half3 CameraVector)
{
half3 DirectLighting = 0;
float3 ToLight = LightData.Position - WorldPosition;
float DistanceSqr = dot(ToLight, ToLight);
float3 L = ToLight * rsqrt(DistanceSqr);
// 光源衰减.
float Attenuation = 0.0;
if (LightData.bInverseSquared)
{
// Sphere falloff (technically just 1/d2 but this avoids inf)
Attenuation = 1.0f / (DistanceSqr + 1.0f);
Attenuation *= Square(saturate(1 - Square(DistanceSqr * Square(LightData.InvRadius))));
}
else
{
Attenuation = RadialAttenuation(ToLight * LightData.InvRadius, LightData.FalloffExponent);
}
// 聚光灯衰减.
if (LightData.bSpotLight)
{
Attenuation *= SpotAttenuation(L, -LightData.Direction, LightData.SpotAngles);
}
// 如果衰减不为0, 则计算直接光照.
if (Attenuation > 0.0)
{
half3 H = normalize(CameraVector + L);
half NoL = max(0.0, dot(GBuffer.WorldNormal, L));
half NoH = max(0.0, dot(GBuffer.WorldNormal, H));
FMobileDirectLighting Lighting = MobileIntegrateBxDF(ShadingModelContext, GBuffer, NoL, CameraVector, H, NoH);
DirectLighting = (Lighting.Diffuse + Lighting.Specular * LightData.SpecularScale) * (LightData.Color * (1.0 / PI) * Attenuation);
}
return DirectLighting;
}
// 光照函数.
half ComputeLightFunctionMultiplier(float3 WorldPosition);
// 使用光网格添加局部光照, 不支持动态阴影, 因为需要逐光源阴影图.
half3 GetLightGridLocalLighting(const FCulledLightsGridData InLightGridData, ...);
// 平行光的PS主入口.
void MobileDirectLightPS(
noperspective float4 UVAndScreenPos : TEXCOORD0,
float4 SvPosition : SV_POSITION,
out half4 OutColor : SV_Target0)
{
// 恢复(读取)GBuffer数据.
FGBufferData GBuffer = (FGBufferData)0;
float SceneDepth = 0;
{
float4 GBufferA = 0;
float4 GBufferB = 0;
float4 GBufferC = 0;
float4 GBufferD = 0;
FetchGBuffer(UVAndScreenPos.xy, GBufferA, GBufferB, GBufferC, GBufferD, SceneDepth);
GBuffer = DecodeGBufferMobile(GBufferA, GBufferB, GBufferC, GBufferD);
}
// 计算基础向量.
float2 ScreenPos = UVAndScreenPos.zw;
float3 WorldPosition = mul(float4(ScreenPos * SceneDepth, SceneDepth, 1), View.ScreenToWorld).xyz;
half3 CameraVector = normalize(View.WorldCameraOrigin - WorldPosition);
half NoV = max(0, dot(GBuffer.WorldNormal, CameraVector));
half3 ReflectionVector = GBuffer.WorldNormal * (NoV * 2.0) - CameraVector;
half3 Color = 0;
// Check movable light param to determine if we should be using precomputed shadows
half Shadow = LightFunctionParameters2.z > 0.0f ? 1.0f : GBuffer.PrecomputedShadowFactors.r;
// CSM阴影.
#if APPLY_CSM
float ShadowPositionZ = 0;
float4 ScreenPosition = SvPositionToScreenPosition(float4(SvPosition.xyz,SceneDepth));
float ShadowMap = MobileDirectionalLightCSM(ScreenPosition.xy, SceneDepth, ShadowPositionZ);
Shadow = min(ShadowMap, Shadow);
#endif
// 着色模型上下文.
FMobileShadingModelContext ShadingModelContext = (FMobileShadingModelContext)0;
{
half DielectricSpecular = 0.08 * GBuffer.Specular;
ShadingModelContext.DiffuseColor = GBuffer.BaseColor - GBuffer.BaseColor * GBuffer.Metallic; // 1 mad
ShadingModelContext.SpecularColor = (DielectricSpecular - DielectricSpecular * GBuffer.Metallic) + GBuffer.BaseColor * GBuffer.Metallic; // 2 mad
// 计算环境的BRDF.
ShadingModelContext.SpecularColor = GetEnvBRDF(ShadingModelContext.SpecularColor, GBuffer.Roughness, NoV);
}
// 局部光源.
float2 LocalPosition = SvPosition.xy - View.ViewRectMin.xy;
uint GridIndex = ComputeLightGridCellIndex(uint2(LocalPosition.x, LocalPosition.y), SceneDepth);
// 分簇光源
#if USE_CLUSTERED
{
const uint EyeIndex = 0;
const FCulledLightsGridData CulledLightGridData = GetCulledLightsGrid(GridIndex, EyeIndex);
Color += GetLightGridLocalLighting(CulledLightGridData, ShadingModelContext, GBuffer, WorldPosition, CameraVector, EyeIndex, 0);
}
#endif
// 计算平行光.
half NoL = max(0, dot(GBuffer.WorldNormal, MobileDirectionalLight.DirectionalLightDirectionAndShadowTransition.xyz));
half3 H = normalize(CameraVector + MobileDirectionalLight.DirectionalLightDirectionAndShadowTransition.xyz);
half NoH = max(0, dot(GBuffer.WorldNormal, H));
FMobileDirectLighting Lighting;
Lighting.Specular = ShadingModelContext.SpecularColor * CalcSpecular(GBuffer.Roughness, NoH);
Lighting.Diffuse = ShadingModelContext.DiffuseColor;
Color += (Shadow * NoL) * MobileDirectionalLight.DirectionalLightColor.rgb * (Lighting.Diffuse + Lighting.Specular * MobileDirectionalLight.DirectionalLightDistanceFadeMADAndSpecularScale.z);
// 处理反射(IBL, 反射捕捉器).
#if APPLY_REFLECTION
uint NumCulledEntryIndex = (ForwardLightData.NumGridCells + GridIndex) * NUM_CULLED_LIGHTS_GRID_STRIDE;
uint NumLocalReflectionCaptures = min(ForwardLightData.NumCulledLightsGrid[NumCulledEntryIndex + 0], ForwardLightData.NumReflectionCaptures);
uint DataStartIndex = ForwardLightData.NumCulledLightsGrid[NumCulledEntryIndex + 1];
float3 SpecularIBL = CompositeReflectionCapturesAndSkylight(
1.0f,
WorldPosition,
ReflectionVector,//RayDirection,
GBuffer.Roughness,
GBuffer.IndirectIrradiance,
1.0f,
0.0f,
NumLocalReflectionCaptures,
DataStartIndex,
0,
true);
Color += SpecularIBL * ShadingModelContext.SpecularColor;
#elif APPLY_SKY_REFLECTION
float SkyAverageBrightness = 1.0f;
float3 SpecularIBL = GetSkyLightReflection(ReflectionVector, GBuffer.Roughness, SkyAverageBrightness);
SpecularIBL *= ComputeMixingWeight(GBuffer.IndirectIrradiance, SkyAverageBrightness, GBuffer.Roughness);
Color += SpecularIBL * ShadingModelContext.SpecularColor;
#endif
// 天空光漫反射.
half3 SkyDiffuseLighting = GetSkySHDiffuseSimple(GBuffer.WorldNormal);
Color+= SkyDiffuseLighting * half3(View.SkyLightColor.rgb) * ShadingModelContext.DiffuseColor * GBuffer.GBufferAO;
half LightAttenuation = ComputeLightFunctionMultiplier(WorldPosition);
#if USE_PREEXPOSURE
// MobileHDR applies PreExposure in tonemapper
LightAttenuation *= View.PreExposure;
#endif
OutColor.rgb = Color.rgb * LightAttenuation;
OutColor.a = 1;
}
// 局部光源的PS主入口.
void MobileRadialLightPS(
float4 InScreenPosition : TEXCOORD0,
float4 SVPos : SV_POSITION,
out half4 OutColor : SV_Target0
)
{
FGBufferData GBuffer = (FGBufferData)0;
float SceneDepth = 0;
{
float2 ScreenUV = InScreenPosition.xy / InScreenPosition.w * View.ScreenPositionScaleBias.xy + View.ScreenPositionScaleBias.wz;
float4 GBufferA = 0;
float4 GBufferB = 0;
float4 GBufferC = 0;
float4 GBufferD = 0;
FetchGBuffer(ScreenUV, GBufferA, GBufferB, GBufferC, GBufferD, SceneDepth);
GBuffer = DecodeGBufferMobile(GBufferA, GBufferB, GBufferC, GBufferD);
}
// With a perspective projection, the clip space position is NDC * Clip.w
// With an orthographic projection, clip space is the same as NDC
float2 ClipPosition = InScreenPosition.xy / InScreenPosition.w * (View.ViewToClip[3][3] < 1.0f ? SceneDepth : 1.0f);
float3 WorldPosition = mul(float4(ClipPosition, SceneDepth, 1), View.ScreenToWorld).xyz;
half3 CameraVector = normalize(View.WorldCameraOrigin - WorldPosition);
half NoV = max(0, dot(GBuffer.WorldNormal, CameraVector));
// 组装光源数据结构体.
FMobileLightData LightData = (FMobileLightData)0;
{
LightData.Position = DeferredLightUniforms.Position;
LightData.InvRadius = DeferredLightUniforms.InvRadius;
LightData.Color = DeferredLightUniforms.Color;
LightData.FalloffExponent = DeferredLightUniforms.FalloffExponent;
LightData.Direction = DeferredLightUniforms.Direction;
LightData.SpotAngles = DeferredLightUniforms.SpotAngles;
LightData.SpecularScale = 1.0;
LightData.bInverseSquared = INVERSE_SQUARED_FALLOFF;
LightData.bSpotLight = IS_SPOT_LIGHT;
}
FMobileShadingModelContext ShadingModelContext = (FMobileShadingModelContext)0;
{
half DielectricSpecular = 0.08 * GBuffer.Specular;
ShadingModelContext.DiffuseColor = GBuffer.BaseColor - GBuffer.BaseColor * GBuffer.Metallic; // 1 mad
ShadingModelContext.SpecularColor = (DielectricSpecular - DielectricSpecular * GBuffer.Metallic) + GBuffer.BaseColor * GBuffer.Metallic; // 2 mad
// 计算环境BRDF.
ShadingModelContext.SpecularColor = GetEnvBRDF(ShadingModelContext.SpecularColor, GBuffer.Roughness, NoV);
}
// 计算直接光.
half3 Color = GetDirectLighting(LightData, ShadingModelContext, GBuffer, WorldPosition, CameraVector);
// IES, 光照函数.
half LightAttenuation = ComputeLightProfileMultiplier(WorldPosition, DeferredLightUniforms.Position, -DeferredLightUniforms.Direction, DeferredLightUniforms.Tangent);
LightAttenuation*= ComputeLightFunctionMultiplier(WorldPosition);
#if USE_PREEXPOSURE
// MobileHDR applies PreExposure in tonemapper
LightAttenuation*= View.PreExposure;
#endif
OutColor.rgb = Color * LightAttenuation;
OutColor.a = 1;
}
以上可知，平行光和局部光源的PS是不同的入口，主要是因为两者的区别较大，平行光直接在主入口计算光照，附带计算了反射（IBL、捕捉器）、天空光漫反射；而局部光源会构建一个光源结构体，进入直接光计算函数，最后处理局部光源特有的IES和光照函数。
另外，获取GBuffer时，采用了SubPass特有的读取模式，不同的着色平台有所不同：
// Vulkan
[[vk::input_attachment_index(1)]]
SubpassInput<float4> GENERATED_SubpassFetchAttachment0;
#define VulkanSubpassFetch0() GENERATED_SubpassFetchAttachment0.SubpassLoad()
// Metal
Texture2D<float4> gl_LastFragDataRGBA_1;
#define SubpassFetchRGBA_1() gl_LastFragDataRGBA_1.Load(uint3(0, 0, 0), 0)
// DX / OpenGL
Texture2DSampleLevel(GBufferATexture, GBufferATextureSampler, UV, 0);
团队招员
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- UE逻辑开发。
- UE引擎程序。
- UE图形渲染。
- TA（技术向、美术向）。
要求：对技术有热情，扎实的技术基础，良好的沟通和合作能力，有UE使用经验或移动端开发经验者更佳。
有意向或想了解更多的请添加博主微信：81079389（注明博客园求职），或者发简历到博主邮箱：81079389#qq.com（#换成@）。
静待各路英雄豪杰相会。
特别说明
- Part 1结束，Part 2的内容有：
- 移动端渲染技术
- 移动端优化技巧
- 感谢所有参考文献的作者，部分图片来自参考文献和网络，侵删。
- 本系列文章为笔者原创，只发表在博客园上，欢迎分享本文链接，但未经同意，不允许转载！
- 系列文章，未完待续，完整目录请戳内容纲目。
- 系列文章，未完待续，完整目录请戳内容纲目。
- 系列文章，未完待续，完整目录请戳内容纲目。
参考文献
- Unreal Engine Source
- Rendering and Graphics
- Materials
- Graphics Programming
- Mobile Rendering
- Qualcomm® Adreno™ GPU
- PowerVR Developer Documentation
- Arm Mali GPU Best Practices Developer Guide
- Arm Mali GPU Graphics and Gaming Development
- Moving Mobile Graphics
- GDC Vault
- Siggraph Conference Content
- GameDev Best Practices
- Accelerating Mobile XR
- Frequently Asked Questions
- Google Developer Contributes Universal Bandwidth Compression To Freedreno Driver
- Using pipeline barriers efficiently
- Optimized pixel-projected reflections for planar reflectors
- UE4画面表现移动端较PC端差异及最小化差异的分享
- Deferred Shading in Unity URP
- 移动游戏性能优化通用技法
- 深入GPU硬件架构及运行机制
- Adaptive Performance in Call of Duty Mobile
- Jet Set Vulkan : Reflecting on the move to Vulkan
- Vulkan Best Practices - Memory limits with Vulkan on Mali GPUs
- A Year in a Fortnite
- The Challenges of Porting Traha to Vulkan
- L2M - Binding and Format Optimization
- Adreno Best Practices
- 移动设备GPU架构知识汇总
- Mali GPU Architectures
- Cyclic Redundancy Check
- Arm Guide for Unreal Engine
- Arm Virtual Reality
- Best Practices for VR on Unreal Engine
- Optimizing Assets for Mobile VR
- Arm® Guide for Unreal Engine 4 Optimizing Mobile Gaming Graphics
- Adaptive Scalable Texture Compression
- Tile-Based Rendering
- Understanding Render Passes
- Intro to Moving Mobile Graphics
- Mobile Graphics 101
- Intro to Moving Mobile Graphics
- Mobile Graphics 101
- Vulkan API
- Best Practices for Shaders
