BlueRoseNote/03-UnrealEngine/Rendering/RenderingPipeline/Lighting/Lighting.md

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title	date	excerpt	tags	rating
Untitled	2025-02-11 11:30:34			⭐

FSortedLightSetSceneInfo

有序的光源集合相关定义：

/** Data for a simple dynamic light. */  
class FSimpleLightEntry  
{  
public:  
    FVector3f Color;  
    float Radius;  
    float Exponent;  
    float InverseExposureBlend = 0.0f;  
    float VolumetricScatteringIntensity;  
    bool bAffectTranslucency;  
};

struct FSortedLightSceneInfo  
{  
    union  
    {  
       struct  
       {  
          // Note: the order of these members controls the light sort order!  
          // Currently bHandledByLumen is the MSB and LightType is LSB          /** The type of light. */          uint32 LightType : LightType_NumBits;  
          /** Whether the light has a texture profile. */  
          uint32 bTextureProfile : 1;  
          /** Whether the light uses a light function. */  
          uint32 bLightFunction : 1;  
          /** Whether the light uses lighting channels. */  
          uint32 bUsesLightingChannels : 1;  
          /** Whether the light casts shadows. */  
          uint32 bShadowed : 1;  
          /** Whether the light is NOT a simple light - they always support tiled/clustered but may want to be selected separately. */  
          uint32 bIsNotSimpleLight : 1;  
          /* We want to sort the lights that write into the packed shadow mask (when enabled) to the front of the list so we don't waste slots in the packed shadow mask. */  
          uint32 bDoesNotWriteIntoPackedShadowMask : 1;  
          /**   
           * True if the light doesn't support clustered deferred, logic is inverted so that lights that DO support clustered deferred will sort first in list   
           * Super-set of lights supporting tiled, so the tiled lights will end up in the first part of this range.  
           */          
           uint32 bClusteredDeferredNotSupported : 1;  
          /** Whether the light should be handled by Lumen's Final Gather, these will be sorted to the end so they can be skipped */  
          uint32 bHandledByLumen : 1;  
       } Fields;  
       /** Sort key bits packed into an integer. */  
       int32 Packed;  
    } SortKey;  
  
    const FLightSceneInfo* LightSceneInfo;  
    int32 SimpleLightIndex;  
  
    /** Initialization constructor. */  
    explicit FSortedLightSceneInfo(const FLightSceneInfo* InLightSceneInfo)  
       : LightSceneInfo(InLightSceneInfo),  
       SimpleLightIndex(-1)  
    {       
	    SortKey.Packed = 0;  
		SortKey.Fields.bIsNotSimpleLight = 1;  
    }  
    explicit FSortedLightSceneInfo(int32 InSimpleLightIndex)  
       : LightSceneInfo(nullptr),  
       SimpleLightIndex(InSimpleLightIndex)  
    {   
        SortKey.Packed = 0;  
		SortKey.Fields.bIsNotSimpleLight = 0;  
    }};  
  
/**   
 * Stores info about sorted lights and ranges.   
 * The sort-key in FSortedLightSceneInfo gives rise to the following order:  
 *  [SimpleLights,Clustered,UnbatchedLights,LumenLights] * Note that some shadowed lights can be included in the clustered pass when virtual shadow maps and one pass projection are used. */struct FSortedLightSetSceneInfo  
{  
    int32 SimpleLightsEnd;  
    int32 ClusteredSupportedEnd;  
  
    /** First light with shadow map or */  
    int32 UnbatchedLightStart;  
  
    int32 LumenLightStart;  
  
    FSimpleLightArray SimpleLights;  
    TArray<FSortedLightSceneInfo, SceneRenderingAllocator> SortedLights;  
};

开始获取有序光源集合

UE的光源分配由FDeferredShadingSceneRenderer::Render内的bComputeLightGrid变量决定的，bComputeLightGrid的赋值逻辑如下：

void FDeferredShadingSceneRenderer::Render(FRHICommandListImmediate& RHICmdList) {
...
	bool bComputeLightGrid = false;

	if (RendererOutput == ERendererOutput::FinalSceneColor)
	{
		if (bUseVirtualTexturing)
		{
			// Note, should happen after the GPU-Scene update to ensure rendering to runtime virtual textures is using the correctly updated scene
			FVirtualTextureSystem::Get().EndUpdate(GraphBuilder, MoveTemp(VirtualTextureUpdater), FeatureLevel);
		}

#if RHI_RAYTRACING
		GatherRayTracingWorldInstancesForView(GraphBuilder, ReferenceView, RayTracingScene, InitViewTaskDatas.RayTracingRelevantPrimitives);
#endif // RHI_RAYTRACING

		bool bAnyLumenEnabled = false;

		{
			if (bUseGBuffer)
			{
				bComputeLightGrid = bRenderDeferredLighting;
			}
			else
			{
				bComputeLightGrid = ViewFamily.EngineShowFlags.Lighting;
			}

			for (int32 ViewIndex = 0; ViewIndex < Views.Num(); ViewIndex++)
			{
				FViewInfo& View = Views[ViewIndex];
				bAnyLumenEnabled = bAnyLumenEnabled
					|| GetViewPipelineState(View).DiffuseIndirectMethod == EDiffuseIndirectMethod::Lumen
					|| GetViewPipelineState(View).ReflectionsMethod == EReflectionsMethod::Lumen;
			}

			bComputeLightGrid |= (
				ShouldRenderVolumetricFog() ||
				VolumetricCloudWantsToSampleLocalLights(Scene, ViewFamily.EngineShowFlags) ||
				ViewFamily.ViewMode != VMI_Lit ||
				bAnyLumenEnabled ||
				VirtualShadowMapArray.IsEnabled() ||
				ShouldVisualizeLightGrid());
		}
	}
...
}	

获取有序的光源集合

void FDeferredShadingSceneRenderer::Render(FRHICommandListImmediate& RHICmdList) {
...
	// 有序的光源集合.
	FSortedLightSetSceneInfo& SortedLightSet = *GraphBuilder.AllocObject<FSortedLightSetSceneInfo>();  
	{  
	    RDG_CSV_STAT_EXCLUSIVE_SCOPE(GraphBuilder, SortLights);  
	    RDG_GPU_STAT_SCOPE(GraphBuilder, SortLights);  
	    ComputeLightGridOutput = GatherLightsAndComputeLightGrid(GraphBuilder, bComputeLightGrid, SortedLightSet);  
	}
...
}	

PS. 简单光源都可以被分块或分簇渲染，但对于非简单光源，只有满足以下条件的光源才可被分块或分簇渲染：

• 没有使用光源的附加特性（TextureProfile、LightFunction、LightingChannel）。
• 没有开启阴影。
• 非平行光或矩形光。

另外，是否支持分块渲染，还需要光源场景代理的IsTiledDeferredLightingSupported返回true，长度为0的点光源才支持分块渲染。

GatherLightsAndComputeLightGrid

FComputeLightGridOutput FDeferredShadingSceneRenderer::GatherLightsAndComputeLightGrid(FRDGBuilder& GraphBuilder, bool bNeedLightGrid, FSortedLightSetSceneInfo& SortedLightSet)
{
	SCOPED_NAMED_EVENT(GatherLightsAndComputeLightGrid, FColor::Emerald);
	FComputeLightGridOutput Result = {};

	bool bShadowedLightsInClustered = ShouldUseClusteredDeferredShading()
		&& CVarVirtualShadowOnePassProjection.GetValueOnRenderThread()
		&& VirtualShadowMapArray.IsEnabled();

	const bool bUseLumenDirectLighting = ShouldRenderLumenDirectLighting(Scene, Views[0]);

	GatherAndSortLights(SortedLightSet, bShadowedLightsInClustered, bUseLumenDirectLighting);
	
	if (!bNeedLightGrid)
	{
		SetDummyForwardLightUniformBufferOnViews(GraphBuilder, ShaderPlatform, Views);
		return Result;
	}

	bool bAnyViewUsesForwardLighting = false;
	bool bAnyViewUsesLumen = false;
	for (int32 ViewIndex = 0; ViewIndex < Views.Num(); ViewIndex++)
	{
		const FViewInfo& View = Views[ViewIndex];
		bAnyViewUsesForwardLighting |= View.bTranslucentSurfaceLighting || ShouldRenderVolumetricFog() || View.bHasSingleLayerWaterMaterial || VolumetricCloudWantsToSampleLocalLights(Scene, ViewFamily.EngineShowFlags) || ShouldVisualizeLightGrid();
		bAnyViewUsesLumen |= GetViewPipelineState(View).DiffuseIndirectMethod == EDiffuseIndirectMethod::Lumen || GetViewPipelineState(View).ReflectionsMethod == EReflectionsMethod::Lumen;
	}
	
	const bool bCullLightsToGrid = GLightCullingQuality 
		&& (IsForwardShadingEnabled(ShaderPlatform) || bAnyViewUsesForwardLighting || IsRayTracingEnabled() || ShouldUseClusteredDeferredShading() ||
			bAnyViewUsesLumen || ViewFamily.EngineShowFlags.VisualizeMeshDistanceFields || VirtualShadowMapArray.IsEnabled());

	// Store this flag if lights are injected in the grids, check with 'AreLightsInLightGrid()'
	bAreLightsInLightGrid = bCullLightsToGrid;
	
	Result = ComputeLightGrid(GraphBuilder, bCullLightsToGrid, SortedLightSet);

	return Result;
}

• GatherAndSortLights：收集与排序当前场景中所有的可见光源（当前View）。
• ComputeLightGrid：是在锥体空间（frustum space）裁剪局部光源和反射探针到3D格子中，构建每个视图相关的光源列表和格子。

RenderLights() -> RenderLight()

InternalRenderLight()