331 lines
17 KiB
Markdown
331 lines
17 KiB
Markdown
---
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title: UE5RayTracing渲染管线笔记——(1)
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date: 2022-08-09 13:55:15
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tags: RayTracing
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rating: ⭐️⭐️
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---
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## 任务
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- [ ] 查看这个如何针对各个View构建场景
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## 渲染事件
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RayTracingScene位于LumenSceneUpdate之后。
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## 收集场景信息
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### 场景信息
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在FScene中定了这2个变量来存储RayTracing专用的场景信息：
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```c#
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FRayTracingScene RayTracingScene;
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TArray<FLightSceneInfo*, TInlineAllocator<4>> RayTracedLights;
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```
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FRayTracingScene还存储着FRayTracingGeometryInstance数组、TArray<const FRayTracingGeometry*> GeometriesToBuild、RayTracingSceneBuffer、RayTracingSceneSRV。
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### 收集过程
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主要的逻辑位于GatherRayTracingWorldInstancesForView()中，通过RayTracingCollector来收集场景中的图元。在FSceneRenderer定义了MeshCollector与RayTracingCollector，其中MeshCollector的GatherDynamicMeshElements()在计算可见性阶段被调用()。
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```c#
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// Gather mesh instances, shaders, resources, parameters, etc. and build ray tracing acceleration structure
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FRayTracingScene& RayTracingScene = Scene->RayTracingScene;
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RayTracingScene.Reset(); // Resets the internal arrays, but does not release any resources.
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const int32 ReferenceViewIndex = 0;
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FViewInfo& ReferenceView = Views[ReferenceViewIndex];
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// Prepare the scene for rendering this frame.
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GatherRayTracingWorldInstancesForView(GraphBuilder, ReferenceView, RayTracingScene);
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```
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- GatherRayTracingWorldInstancesForView()
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- FGPUScenePrimitiveCollector DummyDynamicPrimitiveCollector;
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- 给RayTracingCollector的内部变量赋值：RayTracingCollector.AddViewMeshArrays(&View,&View.RayTracedDynamicMeshElements,&View.SimpleElementCollector,&DummyDynamicPrimitiveCollector,ViewFamily.GetFeatureLevel(),&DynamicIndexBufferForInitViews,&DynamicVertexBufferForInitViews,&DynamicReadBufferForInitViews);
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- 创建Mesh资源收集器：`View.RayTracingMeshResourceCollector = MakeUnique<FRayTracingMeshResourceCollector>(...);`
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- 初始化Rtx裁剪变量：View.RayTracingCullingParameters.Init(View);
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- 创建FRayTracingMaterialGatheringContext MaterialGatheringContext{Scene,&View,ViewFamily,GraphBuilder,*View.RayTracingMeshResourceCollector};
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- 声明FRelevantPrimitive结构体，实现InstancingKey()用于返回图元类型掩码。并定义FRelevantPrimitive数组，长度为场景图元总数。
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- 遍历所有图元，
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### 加速结构构建
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>RayTracingGem中有提到了加速结构的Rebuild与Refit概念。
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该步骤会在BasePass()之前调用。DispatchRayTracingWorldUpdates()的注释说：
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>异步构建可能会与BasePass重合。 Async AS builds can potentially overlap with BasePass
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GeometriesToBuild在GatherRayTracingWorldInstancesForView()被填充，之后在DispatchRayTracingWorldUpdates()中通过**GRayTracingGeometryManager.ForceBuildIfPending(GraphBuilder.RHICmdList, RayTracingScene.GeometriesToBuild);**更新。
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- FRayTracingGeometryManager GRayTracingGeometryManager：全局的场景管理类。
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- ForceBuildIfPending()：添加需要强制构建的多边形。
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- ProcessBuildRequests()：在排序请求后，调用**InCmdList.BuildAccelerationStructures(BuildParams);**构建加速结构。Render() 2634 =>DispatchRayTracingWorldUpdates()=>ProcessBuildRequests()=>InCmdList.BuildAccelerationStructures(BuildParams);
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加速结构存在一个UAV上，以FRayTracingGeometryBuildParams为单位。里面存储了`FRayTracingGeometryRHIRef Geometry、BuildMode、以及TArrayView<const FRayTracingGeometrySegment> Segments;`
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### FRayTracingScene
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使用这个类来管理Rtx场景。
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## RayTracingCommon.h
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UE使用宏来简化RayTracingShader的编写。 比如RayTracingShader入口函数：
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```c++
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#ifndef RAY_TRACING_ENTRY_RAYGEN
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#define RAY_TRACING_ENTRY_RAYGEN(name)\
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[shader("raygeneration")] void name()
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#endif // RAY_TRACING_ENTRY_RAYGEN
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#ifndef RAY_TRACING_ENTRY_INTERSECTION
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#define RAY_TRACING_ENTRY_INTERSECTION(name)\
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[shader("intersection")] void name()
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#endif //RAY_TRACING_ENTRY_INTERSECTION
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#ifndef RAY_TRACING_ENTRY_CLOSEST_HIT
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#define RAY_TRACING_ENTRY_CLOSEST_HIT(name, payload_type, payload_name, attributes_type, attributes_name)\
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[shader("closesthit")] void name(inout payload_type payload_name, in attributes_type attributes_name)
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#endif //RAY_TRACING_ENTRY_CLOSEST_HIT
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#ifndef RAY_TRACING_ENTRY_ANY_HIT
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#define RAY_TRACING_ENTRY_ANY_HIT(name, payload_type, payload_name, attributes_type, attributes_name)\
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[shader("anyhit")] void name(inout payload_type payload_name, in attributes_type attributes_name)
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#endif // RAY_TRACING_ENTRY_ANY_HIT
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#ifndef RAY_TRACING_ENTRY_MISS
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#define RAY_TRACING_ENTRY_MISS(name, payload_type, payload_name)\
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[shader("miss")] void name(inout payload_type payload_name)
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#endif //RAY_TRACING_ENTRY_MISS
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```
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所以Name需要与**IMPLEMENT_GLOBAL_SHADER**中定义的Shader入口函数名相同。
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RayTracing函数：
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- FMinimalPayload Payload=TraceVisibilityRay()
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- FMaterialClosestHitPayload Payload = TraceMaterialRay()
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以及其他工具函数：
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- DispatchRaysIndex()
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- GetPixelCoord()
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## RenderRayTracingReflections
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- SortedDeferred
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FRayTracingDeferredReflectionsRGS
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## RenderDiffuseIndirectAndAmbientOcclusion
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RenderRayTracingAmbientOcclusion()
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- 遍历每个View
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- 计算使用对应方式计算GI。将结果传递给FDiffuseIndirectInputs对象。
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- 通过AmbientOcclusionRGS()RayTracing降噪得到AmbientOcclusionMask并传递给FDiffuseIndirectInputs.AmbientOcclusionMask。
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- 如果有头发会多渲染头发的AO。
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- 调用FDiffuseIndirectCompositePS将之前的渲染结果与GI、AO效果合成在一起。
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FDiffuseIndirectCompositePS()
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### AmbientOcclusionRGS
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RayTracingAmbientOcclusionRGS.usf
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- 计算UV、当前像素的FGBufferData以及WorldPosition与CameraDirection
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- 对于非SHADINGMODELID_TWOSIDED_FOLIAGE并且开启CONFIG_SHOOT_WITH_GEOMETRIC_NORMAL，则重新计算法线：
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- 通过HalfFOV * WorldDepth * ViewInvSize.z，计算像素半径
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- 计算通过DDX与DDY计算几何法线。
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- 计算3个球形高斯分布（没看懂）。
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- 初始化RayTracing相关变量开始RayTracing。如果不开启追踪，将**Visibility = 1.0;RayCount = SamplesPerPixel;SamplesPerPixelLocal = 0.0;**
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- 使用RandomSequence生成随机样本。之后调用GenerateCosineNormalRay()，生成Ray。
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1. 调用RandomSequence_GenerateSample2D()取得2维随机样本。默认使用Sobol低差异序列，其他还有Halton与Hash随机(https://github.com/skeeto/hash-prospector)
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2. 进行余弦-半球采样并转换局部坐标为世界空间。
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3. 完成Ray的初始化。
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- 调用ApplyCameraRelativeDepthBias()，对Ray的起点进行摄像机->像素坐标方向的偏移一个ε，以解决浮点数不精确的问题。
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- 计算光线采样权重， **max(dot(WorldNormal, Ray.Direction), 0.05) / max(RayPDF, 0.05);**
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- 调用TraceVisibilityRay()进行RayTracing。
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- 累加采样结果。Tracing范围内没有遮挡物Visibility就是1，否则就是1-IntensityLocal。该值为后处理空间里设定的AO亮度值RayTracingAOIntensity。如果Ray Hit还会设置新的ClosestRayHitDistance值。
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- 输出结果。 OcclusionMask=ShadingDotGeometric * (Visibility / RayCount);HitDistance=ClosestRayHitDistance;（这两个UAV都是屏幕空间降噪器的贴图变量，Shader处理完之后会进行降噪处理）
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- 否则直接使用世界法线，ShadingDotGeometric=1.0
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## RenderRayTracingSkyLight
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- 初始化FPathTracingSkylight SkylightParameters与FSkyLightData SkyLightData，如果天光功能未开启则返回黑色OutSkyLightTexture与OutHitDistanceTexture。
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- 使用CVarRayTracingSkyLightScreenPercentage计算ResolutionFraction。
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- 创建RDG Texture资源RayTracingSkylight与RayTracingSkyLightHitDistance。
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- 调用GenerateSkyLightVisibilityRays()生成Ray样本集（256*256），格式为`RWStructuredBuffer<SkyLightVisibilityRays>`，`SkyLightVisibilityRays为方向与PDF值，float4 DirectionAndPdf;`。
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- FGenerateSkyLightVisibilityRaysCS的流程
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1. 计算坐标UAV坐标与SkyLightSamplingStrategyPdf（会使用SkylightPdf）。
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- 大概率是在PrepareSkyTexture()中进行了资源绑定SkylightParameters->SkylightPdf = GraphBuilder.RegisterExternalTexture(Scene->PathTracingSkylightPdf, TEXT("PathTracer.SkylightPdf"));
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2. 计算每个像素数据。
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1. 生成随机序列，使用Hilbert curve算法: https://github.com/hcs0/Hackers-Delight/blob/master/hilbert/hil_s_from_xy.c.txt
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2. 使用Sobol算子采样来得到样本。
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3. 使用样本来计算天光采样结果,这一步会根据上下半球进行区分。
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- ```FSkyLightSample {float3 Direction;float3 Radiance;float Pdf;};```
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4. 计算最终的半球混合PDF：float MisWeightOverPdf = 1.0 / lerp(UniformPdf, SkyLightPdf, SkyLightSamplingStrategyPdf);
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5. 计算Ray的Index，并将结果写入。
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- 创建用于输出结果的UAV对象OutSkyLightTexture、OutHitDistanceTexture，并且取得SceneTextures。
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- 遍历所有View计算天光结果。
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- 填充FRayTracingSkyLightRGS::FParameters。如果视口内有头发，将会额外绑定HairStrandsVoxelUniformParameters。
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- 设置FRayTracingSkyLightRGS变体。
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- 计算FIntPoint RayTracingResolution = View.ViewRect.Size() / UpscaleFactor;
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- RayTraceDispatch()。
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- Denoising
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- 如果SceneViewState有效，返回SkyLightVisibilityRaysDimensions。
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- 合成SkyLight
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### FRayTracingSkyLightRGS
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FRayTracingSkyLightRGS是一个GlobalShader，但因为是一个RayTracing Shader，所以宏的类型为：
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```c++
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IMPLEMENT_GLOBAL_SHADER(FRayTracingSkyLightRGS, "/Engine/Private/Raytracing/RaytracingSkylightRGS.usf", "SkyLightRGS", SF_RayGen);
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```
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TLAS数据位于通过Scene->RayTracingScene->RayTracingSceneSRV。
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```c++
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PassParameters->TLAS = View.GetRayTracingSceneViewChecked();
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FRHIShaderResourceView* FViewInfo::GetRayTracingSceneViewChecked() const
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{
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FRHIShaderResourceView* Result = nullptr;
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check(Family);
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if (Family->Scene)
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{
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if (FScene* Scene = Family->Scene->GetRenderScene())
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{
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Result = Scene->RayTracingScene.GetShaderResourceViewChecked();
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}
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}
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checkf(Result, TEXT("Ray tracing scene SRV is expected to be created at this point."));
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return Result;
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}
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FRHIShaderResourceView* FRayTracingScene::GetShaderResourceViewChecked() const
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{
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checkf(RayTracingSceneSRV.IsValid(), TEXT("Ray tracing scene SRV was not created. Perhaps BeginCreate() was not called."));
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return RayTracingSceneSRV.GetReference();
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}
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```
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#### AddPass
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RayGem的AddPass()标记为ERDGPassFlags::Compute。RHICmdList.RayTraceDispatch()需要RayGem管线状态、Shader、RayTracingSceneRHI、RayTracing资源与分辨率。
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FRayTracingPipelineStateInitializer管线状态需要:
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- MaxPayloadSizeInBytes
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- RayGenShaderTable
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- HitGroupTable
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- bAllowHitGroupIndexing
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```c++
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FIntPoint RayTracingResolution = View.ViewRect.Size() / UpscaleFactor;
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GraphBuilder.AddPass(
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RDG_EVENT_NAME("SkyLightRayTracing %dx%d", RayTracingResolution.X, RayTracingResolution.Y),
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PassParameters,
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ERDGPassFlags::Compute,
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[PassParameters, this, &View, RayGenerationShader, RayTracingResolution](FRHIRayTracingCommandList& RHICmdList)
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{
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//资源绑定，将Texture、UniformStruct绑定的工具函数
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FRayTracingShaderBindingsWriter GlobalResources;
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SetShaderParameters(GlobalResources, RayGenerationShader, *PassParameters);
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//取得RayTracing管线状态
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FRayTracingPipelineState* Pipeline = View.RayTracingMaterialPipeline;
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//如果没开启RayTracing天光材质，则重新创建一个RayTracing管线状态。看得出主要需求RayGemShader与HitGroupTable
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if (CVarRayTracingSkyLightEnableMaterials.GetValueOnRenderThread() == 0)
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{
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// Declare default pipeline
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FRayTracingPipelineStateInitializer Initializer;
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Initializer.MaxPayloadSizeInBytes = RAY_TRACING_MAX_ALLOWED_PAYLOAD_SIZE; // sizeof(FPackedMaterialClosestHitPayload)
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FRHIRayTracingShader* RayGenShaderTable[] = { RayGenerationShader.GetRayTracingShader() };
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Initializer.SetRayGenShaderTable(RayGenShaderTable);
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FRHIRayTracingShader* HitGroupTable[] = { View.ShaderMap->GetShader<FOpaqueShadowHitGroup>().GetRayTracingShader() };
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Initializer.SetHitGroupTable(HitGroupTable);
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Initializer.bAllowHitGroupIndexing = false; // Use the same hit shader for all geometry in the scene by disabling SBT indexing.
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Pipeline = PipelineStateCache::GetAndOrCreateRayTracingPipelineState(RHICmdList, Initializer);
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}
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FRHIRayTracingScene* RayTracingSceneRHI = View.GetRayTracingSceneChecked();
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RHICmdList.RayTraceDispatch(Pipeline, RayGenerationShader.GetRayTracingShader(), RayTracingSceneRHI, GlobalResources, RayTracingResolution.X, RayTracingResolution.Y);
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});
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```
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#### Shader
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- 计算DispatchThreadId以及对应的屏幕UV。并且取得对应的FGBufferData。
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- 计算出WorldPosition以及CameraDirection。
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- 判断是否需要追踪光线: 是否是有限深度 && 当前像素的ShaderModel不是Unlit。如果需要追踪采样数为传入Shader的 SkyLight.SamplesPerPixel，否则为0。
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- 调用SkyLightEvaluate()，进行光追计算。
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- 初始化相关函数。
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- 计算天光采样PDF。
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- 采样循环
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- 根据bDecoupleSampleGeneration()，选择执行使用SkyLightVisibilityRays的样本 或者使用随机序列生成样本。
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- 如果当前像素的ShadingModel是Hair，需要重新计算CurrentWorldNormal。
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- 偏移当前光线的深度，并计算NoL。
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- 设置RayFlags，并且调用TraceVisibilityRay()进行光线追踪。返回FMinimalPayload（存光线命中距离信息）
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- 如果命中，累加RayDistance与HitCount。如没命中，累加BentNormal，并且计算FDirectLighting（光照计算，Hair会用另一套计算方式），最后累加ExitantRadiance、DiffuseThroughput、DiffuseExitantRadiance。
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- ExitantRadiance、DiffuseThroughput、DiffuseExitantRadiance除以样本数目（HitDistance = RayDistance / HitCount）。
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- 如果当前像素的ShadingModel是Hair，增加头发多重散射贡献值。
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- 合成估算结果。DiffuseExitantRadiance.r = Albedo.r > 0.0 ? DiffuseExitantRadiance.r / Albedo.r : DiffuseExitantRadiance.r;
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- 乘以曝光值。
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- 返回RWSkyOcclusionMaskUAV[DispatchThreadId]=float4(ClampToHalfFloatRange(DiffuseExitantRadiance.rgb), AmbientOcclusion);RWSkyOcclusionRayDistanceUAV[DispatchThreadId] = float2(HitDistance, SamplesPerPixel);
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#### 降噪（Denoising）
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- 调用IScreenSpaceDenoiser接口取得默认降噪器
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- 设置IScreenSpaceDenoiser::FDiffuseIndirectInputs的Color与RayHitDistance(OutSkyLightTexture、OutHitDistanceTexture)
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- 设置IScreenSpaceDenoiser::FAmbientOcclusionRayTracingConfig的ResolutionFraction与RayCountPerPixel(ResolutionFraction、GetSkyLightSamplesPerPixel(SkyLight))
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- 调用DenoiseSkyLight()输出降噪后的结果覆盖OutSkyLightTexture。
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```c++
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if (GRayTracingSkyLightDenoiser != 0)
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{
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//取得默认降噪器
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const IScreenSpaceDenoiser* DefaultDenoiser = IScreenSpaceDenoiser::GetDefaultDenoiser();
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const IScreenSpaceDenoiser* DenoiserToUse = DefaultDenoiser;// GRayTracingGlobalIlluminationDenoiser == 1 ? DefaultDenoiser : GScreenSpaceDenoiser;
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//降噪器变量结构体需要使用之前的渲染结果以及Hit距离结果
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IScreenSpaceDenoiser::FDiffuseIndirectInputs DenoiserInputs;
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DenoiserInputs.Color = OutSkyLightTexture;
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DenoiserInputs.RayHitDistance = OutHitDistanceTexture;
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{
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//初始化RayTracingConfig
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IScreenSpaceDenoiser::FAmbientOcclusionRayTracingConfig RayTracingConfig;
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RayTracingConfig.ResolutionFraction = ResolutionFraction;
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RayTracingConfig.RayCountPerPixel = GetSkyLightSamplesPerPixel(SkyLight);
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RDG_EVENT_SCOPE(GraphBuilder, "%s%s(SkyLight) %dx%d",
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DenoiserToUse != DefaultDenoiser ? TEXT("ThirdParty ") : TEXT(""),
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DenoiserToUse->GetDebugName(),
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View.ViewRect.Width(), View.ViewRect.Height());
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//降噪
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IScreenSpaceDenoiser::FDiffuseIndirectOutputs DenoiserOutputs = DenoiserToUse->DenoiseSkyLight(
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GraphBuilder,
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View,
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&View.PrevViewInfo,
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SceneTextures,
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DenoiserInputs,
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RayTracingConfig);
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//输出结果
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OutSkyLightTexture = DenoiserOutputs.Color;
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}
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}
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```
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## RenderRayTracingDebug
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位于渲染Fog与Translucency、VirtualTextureFeedbackEnd()之后
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```c++
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#if RHI_RAYTRACING
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if (IsRayTracingEnabled())
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{
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// Path tracer requires the full ray tracing pipeline support, as well as specialized extra shaders.
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// Most of the ray tracing debug visualizations also require the full pipeline, but some support inline mode.
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if (ViewFamily.EngineShowFlags.PathTracing
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&& FDataDrivenShaderPlatformInfo::GetSupportsPathTracing(Scene->GetShaderPlatform()))
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{
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for (const FViewInfo& View : Views)
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{
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RenderPathTracing(GraphBuilder, View, SceneTextures.UniformBuffer, SceneTextures.Color.Target);
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}
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}
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else if (ViewFamily.EngineShowFlags.RayTracingDebug)
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{
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for (const FViewInfo& View : Views)
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{
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RenderRayTracingDebug(GraphBuilder, View, SceneTextures.Color.Target);
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}
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}
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}
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#endif
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```
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DebugVisualizationMode具有3中模式：
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- TRAVERSAL：使用ComputeShader
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- RAY_TRACING_DEBUG_VIZ_PRIMARY_RAYS：使用SF_RayGen，FRayTracingPrimaryRaysRGS
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- DebugVisualizationMode == RAY_TRACING_DEBUG_VIZ_INSTANCES || DebugVisualizationMode == RAY_TRACING_DEBUG_VIZ_TRIANGLES;使用SF_RayGen，FRayTracingDebugRGS |