UnrealEngineUWP/Engine/Shaders/Private/VirtualShadowMaps/VirtualShadowMapBuildPerPageDrawCommands.usf

// Copyright Epic Games, Inc. All Rights Reserved.

/*=============================================================================
	VirtualShadowMapBuildPerPageDrawCommands.usf: 
=============================================================================*/

#include "../Common.ush"
#include "VirtualShadowMapPageOverlap.ush"
#include "VirtualShadowMapProjectionCommon.ush"
#include "../Nanite/NaniteCullingCommon.ush"
#include "../Nanite/NaniteDataDecode.ush"
#include "../InstanceCulling/InstanceCullingCommon.ush"
#include "../InstanceCulling/InstanceCullingLoadBalancer.ush"
#include "../WaveOpUtil.ush"
#include "/Engine/Shared/VirtualShadowMapDefinitions.h"
#include "VirtualShadowMapStats.ush"
#include "VirtualShadowMapPageCacheCommon.ush"

RWStructuredBuffer<FVSMVisibleInstanceCmd> VisibleInstancesOut;
RWStructuredBuffer<uint> VisibleInstanceCountBufferOut;
uint TotalPrimaryViews;
uint VisibleInstancesBufferNum;

#if ENABLE_BATCH_MODE
StructuredBuffer<FVSMCullingBatchInfo> VSMCullingBatchInfos;
#else // !ENABLE_BATCH_MODE
uint FirstPrimaryView;
uint NumPrimaryViews;
uint DynamicInstanceIdOffset;
uint DynamicInstanceIdMax;
#endif // ENABLE_BATCH_MODE

StructuredBuffer<uint> DrawCommandDescs;

RWBuffer<uint> DrawIndirectArgsBufferOut;

StructuredBuffer<uint> PrimitiveRevealedMask;
uint PrimitiveRevealedNum;

RWStructuredBuffer<uint> OutDirtyPageFlags;

#if USE_HZB_OCCLUSION
// Mode 1 == previous frame HZB, 
// Mode 2 == this frame HZB (Nanite only).
uint HZBMode;
#endif

// TODO: Move to common header
// Get the area of an "inclusive" rect (which means that the max is inside the rect), also guards against negative area (where min > max)
uint GetInclusiveRectArea(uint4 Rect)
{
	if (all(Rect.zw >= Rect.xy))
	{
		uint2 Size = Rect.zw - Rect.xy;
		return (Size.x + 1) * (Size.y + 1);
	}
	return 0;
}

bool WasPrimitiveRevealed(FVSMCullingBatchInfo VSMCullingBatchInfo, uint PersistentPrimitiveId)
{
	// Use to filter out transient primitives as well as opting out of the test (if e.g., caching is off)
	if (PersistentPrimitiveId >= VSMCullingBatchInfo.PrimitiveRevealedNum)
	{
		return false;
	}

	uint PrimitiveIdWordOffset = VSMCullingBatchInfo.PrimitiveRevealedOffset + PersistentPrimitiveId / 32U;
	uint PrimitiveIdWordMask = 1U << (PersistentPrimitiveId % 32U);

	return (PrimitiveRevealedMask[PrimitiveIdWordOffset] & PrimitiveIdWordMask) != 0U;
}

void WriteCmd(uint MipViewId, uint InstanceId, uint IndirectArgIndex, uint CullingFlags, bool bStaticPage)
{
	FPageInfo PageInfo;
	PageInfo.ViewId = MipViewId;
	PageInfo.bStaticPage = bStaticPage;

	FVSMVisibleInstanceCmd VisibleInstanceCmd;
	VisibleInstanceCmd.PackedPageInfo = PackPageInfo(PageInfo);
	VisibleInstanceCmd.InstanceIdAndFlags = (CullingFlags << INSTANCE_ID_NUM_BITS) | InstanceId;
	VisibleInstanceCmd.IndirectArgIndex = IndirectArgIndex;

	uint VisibleInstanceOutputOffset = 0U;
	WaveInterlockedAddScalar_(VisibleInstanceCountBufferOut[0], 1U, VisibleInstanceOutputOffset);
	if (VisibleInstanceOutputOffset < VisibleInstancesBufferNum)
	{
		// TODO: Flag & feedback on overflow.
		VisibleInstancesOut[VisibleInstanceOutputOffset] = VisibleInstanceCmd;
	}
}

#define MAX_SINGLE_THREAD_MARKING_AREA (8U)
#define MARKING_JOB_QUEUE_SIZE (NUM_THREADS_PER_GROUP * 2U)

struct FSharedMarkingJob
{
	uint4 RectPages;
	uint VirtualShadowMapId;
	uint MipLevel;
	bool bMaterialInvalidates;
	bool bShouldCacheAsStatic;
	bool bHasMoved;
	bool bIsViewUncached;
};

groupshared uint NumSharedMarkingJobs;
groupshared uint2 SharedMarkingJobs[MARKING_JOB_QUEUE_SIZE];

uint2 PackJob(FSharedMarkingJob Job)
{
	uint2 Packed;
	Packed.x = (Job.RectPages.x << 24U) | (Job.RectPages.y << 16U) | (Job.RectPages.z << 8U) | Job.RectPages.w;
	Packed.y = (Job.VirtualShadowMapId << 7U) 
		| (Job.MipLevel << 4U) 
		| (Job.bIsViewUncached ?      (1U << 3) : 0U)
		| (Job.bHasMoved ?            (1U << 2) : 0U) 
		| (Job.bMaterialInvalidates ? (1U << 1) : 0U) 
		| (Job.bShouldCacheAsStatic ?  1U : 0U);
	return Packed;
}

FSharedMarkingJob UnpackJob(uint2 Packed)
{
	FSharedMarkingJob Job;
	Job.RectPages.x = Packed.x >> 24U;
	Job.RectPages.y = (Packed.x >> 16U) & 0xFF;
	Job.RectPages.z = (Packed.x >> 8U) & 0xFF;
	Job.RectPages.w = Packed.x & 0xFF;

	Job.VirtualShadowMapId = Packed.y >> 7U;
	Job.MipLevel = (Packed.y >> 4U) & 0x7;
	Job.bIsViewUncached = (Packed.y & (1U << 3)) != 0U;
	Job.bHasMoved = (Packed.y & (1U << 2)) != 0U;
	Job.bMaterialInvalidates = (Packed.y & (1U << 1)) != 0U;
	Job.bShouldCacheAsStatic = (Packed.y & 0x1) != 0U;

	return Job;
}

bool MarkPageDirty(FVirtualSMLevelOffset PageTableLevelOffset, uint2 vPage, uint MipLevel, bool bMaterialInvalidates, bool bShouldCacheAsStatic, bool bIsViewUncached, uint PageFlagMask)
{
	uint PageFlagOffset = CalcPageOffset(PageTableLevelOffset, MipLevel, vPage);
	uint PageFlag = VirtualShadowMap.PageFlags[PageFlagOffset];
	// TODO: store page flags in the page table entry and only fetch that.
	if ((PageFlag & PageFlagMask) == PageFlagMask)
	{
		FShadowPhysicalPage PhysPage = ShadowGetPhysicalPage(PageFlagOffset);
		if (PhysPage.bThisLODValid)
		{
			// Mark the page dirty so we regenerate HZB, etc.
			uint PhysPageIndex = VSMPhysicalPageAddressToIndex(PhysPage.PhysicalAddress);
			if (bShouldCacheAsStatic || bIsViewUncached)
			{
				OutDirtyPageFlags[PhysPageIndex] = 1U;
			}
			if (bMaterialInvalidates)
			{
				uint Offset = VirtualShadowMap.MaxPhysicalPages * (bShouldCacheAsStatic ? 2U : 1U);
				// Store invalidation flags after the dirty flags.
				OutDirtyPageFlags[Offset + PhysPageIndex] = 1U;
			}
			return true;
		}
	}
	return false;
}

#if VSM_GENERATE_STATS
uint NumPageAreaDiagnosticSlots;
uint LargeInstancePageAreaThreshold;
#endif


[numthreads(NUM_THREADS_PER_GROUP, 1, 1)]
void CullPerPageDrawCommandsCs(uint3 GroupId : SV_GroupID, int GroupThreadIndex : SV_GroupIndex)
{
	uint DispatchGroupId = GetUnWrappedDispatchGroupId(GroupId);

	if (DispatchGroupId >= InstanceCullingLoadBalancer_GetNumBatches())
	{
		return;
	}
	
	if (GroupThreadIndex == 0)
	{
		NumSharedMarkingJobs = 0U;
	}
	GroupMemoryBarrierWithGroupSync();

#if ENABLE_BATCH_MODE
	// Load Instance culling context batch info, indirection per group
	FContextBatchInfo BatchInfo = BatchInfos[BatchInds[DispatchGroupId]];
	FVSMCullingBatchInfo VSMCullingBatchInfo = VSMCullingBatchInfos[BatchInds[DispatchGroupId]];
#else // !ENABLE_BATCH_MODE
	// Single Instance culling context batch in the call, set up batch from the kernel parameters
	FContextBatchInfo BatchInfo = (FContextBatchInfo)0;
	BatchInfo.DynamicInstanceIdOffset = DynamicInstanceIdOffset;
	BatchInfo.DynamicInstanceIdMax = DynamicInstanceIdMax;

	FVSMCullingBatchInfo VSMCullingBatchInfo;
	VSMCullingBatchInfo.FirstPrimaryView = FirstPrimaryView;
	VSMCullingBatchInfo.NumPrimaryViews = NumPrimaryViews;
	VSMCullingBatchInfo.PrimitiveRevealedOffset = 0U;
	VSMCullingBatchInfo.PrimitiveRevealedNum = PrimitiveRevealedNum;

#endif // ENABLE_BATCH_MODE

	uint CurrentBatchProcessingMode = 0U;
	FInstanceWorkSetup WorkSetup = InstanceCullingLoadBalancer_Setup(GroupId, GroupThreadIndex, GetItemDataOffset(BatchInfo, CurrentBatchProcessingMode));
	if (WorkSetup.bValid)
	{
		const FInstanceCullingPayload Payload = LoadInstanceCullingPayload(WorkSetup.Item.Payload, BatchInfo);

		uint InstanceDataOffset = WorkSetup.Item.InstanceDataOffset;

		if (Payload.bDynamicInstanceDataOffset)
		{
			InstanceDataOffset += BatchInfo.DynamicInstanceIdOffset;
			checkSlow(InstanceDataOffset + uint(WorkSetup.LocalItemIndex) < BatchInfo.DynamicInstanceIdMax);
		}

		uint InstanceId = InstanceDataOffset + uint(WorkSetup.LocalItemIndex);
		FDrawCommandDesc DrawCommandDesc = UnpackDrawCommandDesc(DrawCommandDescs[Payload.IndirectArgIndex]);

		// Load relevant instance data
		FInstanceSceneData InstanceData = GetInstanceSceneData(InstanceId, Scene.GPUScene.InstanceDataSOAStride);
		FPrimitiveSceneData PrimitiveData = GetPrimitiveData(InstanceData.PrimitiveId);

		bool bMaterialInvalidatesAnyView = false;
		bool bShouldCacheAsStaticForAnyView = false;
		uint ThreadTotalForAllViews = 0;
#if VSM_GENERATE_STATS
		uint TotalPageArea = 0U;
		float NumVisibleLevels = 0.0f;
#endif
		// Loop over views and output visible instance (i.e., those that overlap a valid page)
		for (uint PrimaryViewId = VSMCullingBatchInfo.FirstPrimaryView; PrimaryViewId < VSMCullingBatchInfo.FirstPrimaryView + VSMCullingBatchInfo.NumPrimaryViews; ++PrimaryViewId)
		{
			FNaniteView NaniteView = GetNaniteView(PrimaryViewId);

			uint CullingFlags = INSTANCE_CULLING_FLAGS_DEFAULT;
			bool bEnableWPO = DrawCommandDesc.bMaterialUsesWorldPositionOffset;
			
			FInstanceDynamicData DynamicData = CalculateInstanceDynamicData(NaniteView, InstanceData);

			FBoxCull Cull;
			Cull.Init(
				NaniteView,
				InstanceData.LocalBoundsCenter,
				InstanceData.LocalBoundsExtent,
				InstanceData.NonUniformScale.xyz,
				DynamicData.LocalToTranslatedWorld,
				DynamicData.PrevLocalToTranslatedWorld );

			Cull.bSkipWPODisableDistance |= DrawCommandDesc.bMaterialAlwaysEvaluatesWorldPositionOffset;

			bEnableWPO = Cull.Distance( PrimitiveData ) && bEnableWPO;
			if (!bEnableWPO)
			{
				// Disable the Evaluate WPO culling flag if WPO was disabled
				CullingFlags &= ~INSTANCE_CULLING_FLAG_EVALUATE_WPO;
			}

			const bool bMaterialInvalidates = bEnableWPO && ((PrimitiveData.Flags & PRIMITIVE_SCENE_DATA_FLAG_DISABLE_MATERIAL_INVALIDATIONS) == 0u);

			bMaterialInvalidatesAnyView |= bMaterialInvalidates;

			Cull.GlobalClipPlane();

			// A thing is considered to have moved (which causes it to render into cached pages) if:
			// It was uploaded this frame, OR
			const bool bHasMoved = GetGPUSceneFrameNumber() == InstanceData.LastUpdateSceneFrameNumber
				// it has a material that uses WPO OR
				|| bMaterialInvalidates
				// the primitive was "revealed" this frame - i.e., transitioned from culled -> visible on CPU
				|| WasPrimitiveRevealed(VSMCullingBatchInfo, PrimitiveData.PersistentPrimitiveIndex);

			FFrustumCullData FrustumCull = (FFrustumCullData)0;
			BRANCH
			if( Cull.bIsVisible )
			{
				FrustumCull = Cull.Frustum();
			}

			StatsBufferInterlockedAdd(VSM_STAT_NON_NANITE_INSTANCES_TOTAL, NaniteView.TargetNumMipLevels);

			BRANCH
			if (Cull.bIsVisible)
			{
				// Compute estimated footprint in the VSM base level - note that we don't use the actual footprint because it changes under rotation and also with edge clipping which makes it unstable when used for culling.
				float PixelEstRadius = CalcClipSpaceRadiusEstimate(Cull.bIsOrtho, InstanceData, Cull.LocalToTranslatedWorld, NaniteView.ViewToClip) * float(VSM_VIRTUAL_MAX_RESOLUTION_XY);

				bool bShouldCacheAsStatic = ShouldCacheInstanceAsStatic(InstanceData, Cull.bIsViewUncached);
				uint BaseFlagMask = GetPageFlagMaskForRendering(InstanceData, NaniteView, bHasMoved);
				bShouldCacheAsStaticForAnyView = bShouldCacheAsStaticForAnyView || bShouldCacheAsStatic;

				// Loop over mip levels and count number of output visible instances
				for (uint MipLevel = 0U; MipLevel < uint(NaniteView.TargetNumMipLevels); ++MipLevel)
				{
					uint MipViewId = MipLevel * TotalPrimaryViews + PrimaryViewId;
					FNaniteView MipView = GetNaniteView(MipViewId);
					uint VirtualShadowMapId = uint(MipView.TargetLayerIndex);

					FScreenRect Rect = GetScreenRect(MipView.ViewRect, FrustumCull, 4);

					uint FlagMask = BaseFlagMask | GetDetailGeometryFlagMaskForCulling(bShouldCacheAsStatic, false, PixelEstRadius);
					// Update for next clip level
					PixelEstRadius *= 0.5f;

					uint4 RectPages = GetPageRect(Rect, VirtualShadowMapId, MipLevel);

					if (OverlapsAnyValidPage(VirtualShadowMapId, MipLevel, RectPages, FlagMask))
					{
#if VSM_GENERATE_STATS
						TotalPageArea += float(GetInclusiveRectArea(RectPages));
						NumVisibleLevels += 1.0f;
#endif
#if USE_HZB_OCCLUSION
						if (Cull.bViewHZB)
						{
							// Assume:
							//  1. Virtual address space has not changed (if so it'd have been invalidated)
							//  2. Physical pages that are chached have not changed (guaranteed by persistent caching)
							//  OR (mode 2): Using this frame's HZB from Nanite shadow passes.
							// Then we can:
							//  1. Iterate the clipped page rect
							//  2. Test against only valid pages which enables
							//  3. rejecting any that don't pass HZB at least once (or touches an uncached page)

							// In Mode 2 we are using the current-frame HZB and can use any page whether it is cached or not.
							// TODO: Figure out which flag(s) we actually want here. Also need to check the non-nanite one?
							// TODO: for mode 1, we need to use the previous view transform and rect.
							uint VisiblePageMask = HZBMode == 1 ? (VSM_DYNAMIC_UNCACHED_FLAG | VSM_STATIC_UNCACHED_FLAG) : 0U;
							if (!IsVisibleMaskedHZB(HZBMode == 1 ? MipView.TargetPrevLayerIndex : VirtualShadowMapId, MipLevel, Rect, true, HZBMode == 2, VisiblePageMask))

							{
								StatsBufferInterlockedInc(VSM_STAT_NON_NANITE_INSTANCES_HZB_CULLED);
								continue;
							}
						}
#endif // USE_HZB_OCCLUSION

						uint NumMappedPages = 0U;
						{

							uint2 RectPagesSize = (RectPages.zw + 1u) - RectPages.xy;
							if (RectPagesSize.x * RectPagesSize.y <= MAX_SINGLE_THREAD_MARKING_AREA)
							{
								FVirtualSMLevelOffset PageTableLevelOffset = CalcPageTableLevelOffset(VirtualShadowMapId, MipLevel);

								for (uint Y = RectPages.y; Y <= RectPages.w; ++Y)
								{
									for (uint X = RectPages.x; X <= RectPages.z; ++X)
									{
										if (MarkPageDirty(PageTableLevelOffset, uint2(X, Y), MipLevel, bMaterialInvalidates, bShouldCacheAsStatic, Cull.bIsViewUncached, FlagMask))
										{
											// Count pages if we're touching all of them anyway
											++NumMappedPages;
										}
									}
								}
							}
							else
							{
								// defer the work and use all threads in group
								uint JobIndex = 0U;
								InterlockedAdd(NumSharedMarkingJobs, 1U, JobIndex);
								FSharedMarkingJob Job;
								Job.RectPages = RectPages;
								Job.VirtualShadowMapId = VirtualShadowMapId;
								Job.MipLevel = MipLevel;
								Job.bMaterialInvalidates = bMaterialInvalidates;
								Job.bShouldCacheAsStatic = bShouldCacheAsStatic;
								Job.bIsViewUncached = Cull.bIsViewUncached;
								Job.bHasMoved = bHasMoved;
								if (JobIndex < MARKING_JOB_QUEUE_SIZE)
								{
									SharedMarkingJobs[JobIndex] = PackJob(Job);
								}
								// Must assume we have mapped pages (or defer the cmd write also...)
								NumMappedPages = 1U;
							}

							// 
						}

						if (NumMappedPages > 0U)
						{
							++ThreadTotalForAllViews;
							WriteCmd(MipViewId, InstanceId, Payload.IndirectArgIndex, CullingFlags, bShouldCacheAsStatic);
						}
					}
					else
					{
						StatsBufferInterlockedInc(VSM_STAT_NON_NANITE_INSTANCES_PAGE_MASK_CULLED);
					}
				}
			}
			else
			{
				StatsBufferInterlockedAdd(VSM_STAT_NON_NANITE_INSTANCES_FRUSTUM_CULLED, NaniteView.TargetNumMipLevels);
			}
		}

		// If the instance has WPO or similar and invalidated the static, we want to move the primitive to be treated as dynamic.
		if (ThreadTotalForAllViews > 0U && bMaterialInvalidatesAnyView && bShouldCacheAsStaticForAnyView)
		{
			RecordStaticPrimitiveInvalidation(PrimitiveData.PersistentPrimitiveIndex);
		}

		StatsBufferInterlockedAdd(VSM_STAT_NON_NANITE_INSTANCES_DRAWN, ThreadTotalForAllViews);

		// Accumulate total number of instances for each indirect argument, is also used to allocate space and output compact range of instances later
		InterlockedAdd(DrawIndirectArgsBufferOut[Payload.IndirectArgIndex * INDIRECT_ARGS_NUM_WORDS + 1], ThreadTotalForAllViews);

#if VSM_GENERATE_STATS
		if (NumPageAreaDiagnosticSlots > 0U && TotalPageArea > LargeInstancePageAreaThreshold)
		{
			for (uint Index = 0U; Index < NumPageAreaDiagnosticSlots; ++Index)
			{
				uint PrevArea = 0U;
				// Store these after the stats slots
				InterlockedMax(OutStatsBuffer[VSM_NUM_STATS + 2U * Index], TotalPageArea, PrevArea);
				// If the area was greater then we store the persistent Primitive ID after the area.
				// This obviously has a data race, so numbers & IDs will only be approximately correct, could use 64-bit atomics instead...
				if (PrevArea < TotalPageArea)
				{
					OutStatsBuffer[VSM_NUM_STATS + 2U * Index + 1U] = PrimitiveData.PersistentPrimitiveIndex;
					break;
				}
			}
		}
#endif
	}
	GroupMemoryBarrierWithGroupSync();
	uint NumMarkingJobs = NumSharedMarkingJobs;
	for (uint JobIndex = 0U; JobIndex < min(MARKING_JOB_QUEUE_SIZE, NumMarkingJobs); ++JobIndex)
	{
		FSharedMarkingJob Job = UnpackJob(SharedMarkingJobs[JobIndex]);
		FVirtualSMLevelOffset PageTableLevelOffset = CalcPageTableLevelOffset(Job.VirtualShadowMapId, Job.MipLevel);
		uint2 RectPagesSize = (Job.RectPages.zw + 1u) - Job.RectPages.xy;
		for (uint Index = GroupThreadIndex; Index < RectPagesSize.x * RectPagesSize.y; Index += NUM_THREADS_PER_GROUP)
		{
			uint LocalPageY = uint(floor((float(Index) + 0.5f) * rcp(RectPagesSize.x)));
			uint2 vPage = Job.RectPages.xy + uint2(Index - RectPagesSize.x * LocalPageY, LocalPageY);
			uint FlagMask = GetPageFlagMaskForRendering(Job.bShouldCacheAsStatic, Job.bHasMoved);

			// We don't need to add the correct flag mask here, as we don't look at the VSM_DETAIL_GEOMETRY_FLAG for invalidaiton processing
			MarkPageDirty(PageTableLevelOffset, vPage, Job.MipLevel, Job.bMaterialInvalidates, Job.bShouldCacheAsStatic, Job.bIsViewUncached, FlagMask);
		}
	}
}

Buffer<uint> DrawIndirectArgsBuffer;
RWBuffer<uint> InstanceIdOffsetBufferOut;
RWStructuredBuffer<uint> OutputOffsetBufferOut;
RWStructuredBuffer<uint> TmpInstanceIdOffsetBufferOut;
uint NumIndirectArgs;

/**
 * Separate pass to allocate space, needs to run once the final space requirements are known. We buffer the page/instance-draw info and reshuffle later.
 * TODO: Possibly just re-run the culling process in the output pass, saves storing stuff, but may cost more and runs the risk of the passes disagreeing e.g., due to rounding or whatever.
 */
[numthreads(NUM_THREADS_PER_GROUP, 1, 1)]
void AllocateCommandInstanceOutputSpaceCs(uint IndirectArgIndex : SV_DispatchThreadID)
{
	if (IndirectArgIndex < NumIndirectArgs)
	{
		uint CommandInstanceCount = DrawIndirectArgsBuffer[IndirectArgIndex * INDIRECT_ARGS_NUM_WORDS + 1];
		uint CommandInstanceOffset = 0U;
		if (CommandInstanceCount > 0U)
		{
			InterlockedAdd(OutputOffsetBufferOut[0], CommandInstanceCount, CommandInstanceOffset);
		}
		InstanceIdOffsetBufferOut[IndirectArgIndex] = CommandInstanceOffset;
		// Store second copy for use during output pass (as we need the first offset buffer during the actual rendering)
		TmpInstanceIdOffsetBufferOut[IndirectArgIndex] = CommandInstanceOffset;
	}

	// Also set up indirect dispatch args for the output pass (OutputCommandInstanceLists)
	//if (IndirectArgIndex == 0)
	//{
	//	uint NumVisibleInstances = VisibleInstanceCountBuffer[0];
	//	// ...dispatch args to process all the visible instances
	//}
}

StructuredBuffer<FVSMVisibleInstanceCmd> VisibleInstances;
StructuredBuffer <uint> VisibleInstanceCountBuffer;
//RWStructuredBuffer<uint> TmpInstanceIdOffsetBufferOut;
RWStructuredBuffer<uint> InstanceIdsBufferOut;
RWStructuredBuffer<uint> PageInfoBufferOut;


[numthreads(NUM_THREADS_PER_GROUP, 1, 1)]
void OutputCommandInstanceListsCs(uint VisibleInstanceIndex : SV_DispatchThreadID)
{
	uint NumVisibleInstances = VisibleInstanceCountBuffer[0];

	if (VisibleInstanceIndex < NumVisibleInstances)
	{
		FVSMVisibleInstanceCmd VisibleInstanceCmd = VisibleInstances[VisibleInstanceIndex];
		const uint InstanceId = VisibleInstanceCmd.InstanceIdAndFlags & INSTANCE_ID_MASK;
		const uint CullingFlags = VisibleInstanceCmd.InstanceIdAndFlags >> INSTANCE_ID_NUM_BITS;

		// Scatter the instance ID & other data.
		uint InstanceIdOutputOffset = 0;
		InterlockedAdd(TmpInstanceIdOffsetBufferOut[VisibleInstanceCmd.IndirectArgIndex], 1U, InstanceIdOutputOffset);
		// TODO: maybe repack as uint2 since that might be better for these type of presumably scalar loads.
		InstanceIdsBufferOut[InstanceIdOutputOffset] = PackInstanceCullingOutput(InstanceId, 0u, CullingFlags);
		PageInfoBufferOut[InstanceIdOutputOffset] = VisibleInstanceCmd.PackedPageInfo;
	}
}