Lumen:代码层面全流程

Lumen离线预处理

MeshCard生成

目前从代码来看，MeshCard生成，必须要在Editor上才可以，运行时不行。后面如果运行时加载vrscene，这个会是难点。

具体代码如下：

Engine/Source/Runtime/Engine/Private/MeshCardRepresentation.cpp

void FAsyncCardRepresentationTaskWorker::DoWork()
{
	// Put on background thread to avoid interfering with game-thread bound tasks
	FQueuedThreadPoolTaskGraphWrapper TaskGraphWrapper(ENamedThreads::AnyBackgroundThreadNormalTask);
	GCardRepresentationAsyncQueue->Build(&Task, TaskGraphWrapper);
}
 
void FCardRepresentationAsyncQueue::Build(FAsyncCardRepresentationTask* Task, FQueuedThreadPool& BuildThreadPool)
{
#if WITH_EDITOR
	
	// Editor 'force delete' can null any UObject pointers which are seen by reference collecting (eg UProperty or serialized)
	if (Task->StaticMesh && Task->GenerateSource)
	{
		TRACE_CPUPROFILER_EVENT_SCOPE(FCardRepresentationAsyncQueue::Build);
 
		const FStaticMeshLODResources& LODModel = Task->GenerateSource->GetRenderData()->LODResources[0];
 
		Task->bSuccess = MeshUtilities->GenerateCardRepresentationData(
			Task->StaticMesh->GetName(),
			Task->SourceMeshData,
			LODModel,
			BuildThreadPool,
			Task->MaterialBlendModes,
			Task->GenerateSource->GetRenderData()->Bounds,
			Task->GenerateSource->GetRenderData()->LODResources[0].DistanceFieldData,
			Task->MaxLumenMeshCards,
			Task->bGenerateDistanceFieldAsIfTwoSided,
			*Task->GeneratedCardRepresentation);
	}
#endif
}

Engine/Source/Developer/MeshUtilities/Private/MeshCardRepresentationUtilities.cpp

主要函数：

• FMeshUtilities::GenerateCardRepresentationData
• MeshRepresentation::SetupEmbreeScene
• BuildMeshCards

MeshDistanceField生成

在导入模型时，异步生成。

Lumen运行时流程

GlobalDistanceField生成

Engine/Source/Runtime/Renderer/Private/DeferredShadingRenderer.cpp

Render：PrepareDistanceFieldScene，在RenderPrePass之前。

• 具体执行：Engine/Source/Runtime/Renderer/Private/GlobalDistanceField.cpp
  • UpdateGlobalDistanceFieldVolume（）

LumenScene Init

Render: BeginUpdateLumenSceneTasks -> UpdateSurfaceCachePrimitives(AddMeshCards-> AddMeshCardsFromBuildData) -> UpdateSurfaceCacheMeshCards -> ProcessLumenSurfaceCacheRequests -> AddCardCaptureDraws

具体代码：

• LumenSceneData 初始化 [1]-CSDN博客
• LumenSceneData 初始化 [2]-CSDN博客

LumenScene Update

材质相关，生成SurfaceCache。

BuildRenderingCommands → ClearCardCapture → MeshCardCapture → CopyCardsToSurfaceCache.

具体代码：

• https://blog.csdn.net/sh15285118586/article/details/132211067

LumenScene Lighting

BuildCardUpdateContext -> RenderDirectLightingForLumenScene -> RenderRadiosityForLumenScene

具体代码：

• Lumen:Radiosity-CSDN博客
• Lumen DirectLighting:CullTiles-CSDN博客
• Lumen Directlighting: CombineLighting-CSDN博客

文章参考：

• 游戏引擎随笔 0x31：UE5 Lumen 源码解析（三）光影篇 - 知乎
• 游戏引擎随笔 0x32：UE5 Lumen 源码解析（四）Radiosity 篇 - 知乎

LumenScene ProbeGather

UniformPlacement Probe -> AdaptivePlacement Probe -> GenerateBRDF_PDF -> UpdateRadianceCache → ComputeLightingPDF(重要性采样) → TraceScreenProbes → FilterScreenProbes → InterpolateAndIntegrate

具体代码：

• CSDN

文章参考：

• 游戏引擎随笔 0x29：UE5 Lumen 源码解析（一）原理篇 - 知乎
• 游戏引擎随笔 0x34：UE5 Lumen 源码解析（六）Importance Sampling 篇 - 知乎

Reference

• UE5 Lumen实现分析 - 知乎
• UE5 Lumen原理介绍 - 知乎
• UE5 Lumen GI 实现分析 - 知乎