UnrealEngine/Engine/Shaders/Private/Nanite/NaniteRayTrace.ush

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

#pragma once

#include "../Common.ush"

#define NANITE_USE_RAYTRACING_UNIFORM_BUFFER 1
#define NANITE_NUM_TEXCOORDS_TO_DECODE 0

#include "/Engine/Private/Nanite/NaniteDataDecode.ush"
#include "/Engine/Private/Nanite/NaniteAttributeDecode.ush"

void GetNaniteTriangleEdges(uint TriangleDataOffset, out float3 LocalEdge0, out float3 LocalEdge1)
{
	const uint PackedTriangleData = RayTracingDataBuffer[TriangleDataOffset];

	const uint PageIndex = PackedTriangleData & NANITE_MAX_GPU_PAGES_MASK;
	const uint ClusterIndex = (PackedTriangleData >> NANITE_MAX_GPU_PAGES_BITS) & NANITE_MAX_CLUSTERS_PER_PAGE_MASK;
	const uint TriIndex = (PackedTriangleData >> (NANITE_MAX_GPU_PAGES_BITS + NANITE_MAX_CLUSTERS_PER_PAGE_BITS)) & NANITE_MAX_CLUSTER_TRIANGLES_MASK;

	FCluster Cluster = GetCluster(PageIndex, ClusterIndex);

	const uint3 TriIndices = DecodeTriangleIndices(Cluster, TriIndex);

	// TODO: Nanite-Assemblies: These need to be transformed by assembly transform (encode the index in PackedTriangleData?)
	const float3 PointLocalNoWPO[3] =
	{
		DecodePosition(TriIndices.x, Cluster),
		DecodePosition(TriIndices.y, Cluster),
		DecodePosition(TriIndices.z, Cluster)
	};
	
	LocalEdge0 = PointLocalNoWPO[1] - PointLocalNoWPO[0];
	LocalEdge1 = PointLocalNoWPO[2] - PointLocalNoWPO[0];
}

#ifndef NANITE_PROCEDURAL_RAY_TRACING
#define NANITE_PROCEDURAL_RAY_TRACING	0
#endif

#if NANITE_PROCEDURAL_RAY_TRACING

#include "/Engine/Shared/ThirdParty/RayTriangleIntersection.h"

#include "../RayTracing/RayTracingCommon.ush"
#include "NaniteVertexFetch.ush"

#define INTERSECT_NANITE_TYPE_NAIVE (0)
#define INTERSECT_NANITE_TYPE INTERSECT_NANITE_TYPE_NAIVE

#define INTERSECT_TRIANGLE_TYPE_MOLLER_TRUMBORE (0)
#define INTERSECT_TRIANGLE_TYPE_WATERTIGHT (1)
#define INTERSECT_TRIANGLE_TYPE INTERSECT_TRIANGLE_TYPE_WATERTIGHT

#define BACKFACE_CULLING (0) // TODO
#define INTERSECTION_LOCAL_SPACE (1)
#define LOCAL_SPACE_WITHOUT_MATRIX_MUL (1)

#define TEST_BBOX (1 && INTERSECTION_LOCAL_SPACE)

struct FNaniteIntersectionResult
{
	float2 Barycentrics;
	uint PageIndex;
	uint ClusterIndex;
	uint TriIndex;

	uint NodeIntersectionCount;
	uint ClusterIntersectionCount;
	uint TriangleIntersectionCount;
};

#define STACK_SIZE 32
struct FSimpleStack
{
	void Init()
	{
		DataOffset = 0;
	}

	void Push(uint Value)
	{
		Data[DataOffset++] = Value;
	}

	uint Pop()
	{
		return Data[--DataOffset];
	}

	bool Empty() 
	{
		return DataOffset == 0;
	}

	uint Data[STACK_SIZE];
	uint DataOffset;
};

bool RayTriangleIntersection(float3 RayOrigin, float3 RayDirection, float3 Vertex0, float3 Vertex1, float3 Vertex2, out float IntersectionT, out float2 Barycentrics)
{
#if INTERSECT_TRIANGLE_TYPE == INTERSECT_TRIANGLE_TYPE_WATERTIGHT
	return RayTriangleIntersectionWatertight(RayOrigin, RayDirection, Vertex0, Vertex1, Vertex2, IntersectionT, Barycentrics);
#elif INTERSECT_TRIANGLE_TYPE == INTERSECT_TRIANGLE_TYPE_MOLLER_TRUMBORE
	return RayTriangleIntersectionMollerTrumbore(RayOrigin, RayDirection, Vertex0, Vertex1, Vertex2, IntersectionT, Barycentrics);
#else
#error Unknown INTERSECTION_TRIANGLE_TYPE
#endif
}

bool RayAABBIntersection(float3 RayOrigin, float3 RayDirection, float3 BoxCenter, float3 BoxExtent, float TMin, float RayTCurrent)
{
	const float3 BoxMin = BoxCenter - BoxExtent;
	const float3 BoxMax = BoxCenter + BoxExtent;
	const float3 InvRayDirection = rcp(RayDirection);

	const float3 FirstPlaneIntersections = (BoxMin - RayOrigin) * InvRayDirection;
	const float3 SecondPlaneIntersections = (BoxMax - RayOrigin) * InvRayDirection;
	const float3 ClosestPlaneIntersections = min(FirstPlaneIntersections, SecondPlaneIntersections);
	const float3 FurthestPlaneIntersections = max(FirstPlaneIntersections, SecondPlaneIntersections);
	const float ClosestDistance = max3(ClosestPlaneIntersections.x, ClosestPlaneIntersections.y, ClosestPlaneIntersections.z);
	const float FurthestDistance = min3(FurthestPlaneIntersections.x, FurthestPlaneIntersections.y, FurthestPlaneIntersections.z);
	if (ClosestDistance > FurthestDistance || TMin > FurthestDistance || RayTCurrent < ClosestDistance)
	{
		return false;
	}

	return true;
}

bool NaniteClustersIntersection(
	FInstanceSceneData InstanceData, uint PageIndex, uint BaseClusterIndex, uint NumClusters, uint AssemblyTransformIndex,
	float CutError,	float3 Origin, float3 Direction, float4x4 LocalToTranslatedWorld, float3 InvScale, float3 BoundsCenter,
	float MaxWPOExtent,	float TMin, inout float RayTCurrent, inout FNaniteIntersectionResult NaniteIntersectionResult)
{
	bool bIntersection = false;
	for (int ClusterIndex = 0; ClusterIndex < NumClusters; ++ClusterIndex)
	{
		FCluster Cluster = GetCluster(PageIndex, BaseClusterIndex + ClusterIndex);

		bool bSmallEnoughToDraw = CutError > Cluster.LODError;

#if TEST_BBOX
		// TODO: Take AbsMaxMaterialDisplacement + MaxWPOExtent when RT supports displacement
		float3 BoxBoundsCenter = (Cluster.BoxBoundsCenter - BoundsCenter) * InvScale;
		float3 BoxBoundsExtent = (Cluster.BoxBoundsExtent + MaxWPOExtent) * InvScale;

		const bool bBoxIntersection = RayAABBIntersection(Origin, Direction, BoxBoundsCenter, BoxBoundsExtent, TMin, RayTCurrent);
#else
		const bool bBoxIntersection = true;
#endif

		++NaniteIntersectionResult.ClusterIntersectionCount;

		if ((!bSmallEnoughToDraw && !(Cluster.Flags & NANITE_CLUSTER_FLAG_STREAMING_LEAF)) || !bBoxIntersection)
			continue;

		NaniteIntersectionResult.TriangleIntersectionCount += Cluster.NumTris;

		for (int TriIndex = 0; TriIndex < Cluster.NumTris; ++TriIndex)
		{
			const uint3 TriIndices = DecodeTriangleIndices(Cluster, TriIndex);

			float3 PointLocal[3];
			FetchLocalNaniteTrianglePositions(InstanceData, Cluster, AssemblyTransformIndex, TriIndices, PointLocal);

			float IntersectionT = RayTCurrent;
			float2 Barycentrics = 0.0;
#if INTERSECTION_LOCAL_SPACE
			PointLocal[0] -= BoundsCenter;
			PointLocal[1] -= BoundsCenter;
			PointLocal[2] -= BoundsCenter;

			PointLocal[0] *= InvScale;
			PointLocal[1] *= InvScale;
			PointLocal[2] *= InvScale;

			bool bHit = RayTriangleIntersection(Origin, Direction, PointLocal[0], PointLocal[1], PointLocal[2], IntersectionT, Barycentrics);
#else
			const float3 PointWorld0 = mul(float4(PointLocal[0], 1), LocalToTranslatedWorld).xyz;
			const float3 PointWorld1 = mul(float4(PointLocal[1], 1), LocalToTranslatedWorld).xyz;
			const float3 PointWorld2 = mul(float4(PointLocal[2], 1), LocalToTranslatedWorld).xyz;

			bool bHit = RayTriangleIntersection(Origin, Direction, PointWorld0, PointWorld1, PointWorld2, IntersectionT, Barycentrics);
#endif
			
			if (bHit && IntersectionT >= TMin && IntersectionT <= RayTCurrent)
			{
				NaniteIntersectionResult.Barycentrics = Barycentrics;
				NaniteIntersectionResult.TriIndex = TriIndex;
				NaniteIntersectionResult.PageIndex = PageIndex;
				NaniteIntersectionResult.ClusterIndex = BaseClusterIndex + ClusterIndex;

				bIntersection = true;
				RayTCurrent = IntersectionT;
			}
		}
	}

	return bIntersection;
}

bool NaniteIntersection(float3 Origin, float3 Direction, FInstanceSceneData InstanceData, FPrimitiveSceneData PrimitiveData, float CutError, float TMin, inout float RayTCurrent, out FNaniteIntersectionResult NaniteIntersectionResult)
{
	float4x4 LocalToTranslatedWorld = DFHackToFloat(InstanceData.LocalToWorld);
	float4x4 TranslatedWorldToLocal = DFHackToFloat(InstanceData.WorldToLocal);

#if LOCAL_SPACE_WITHOUT_MATRIX_MUL
	const float3 InvScale = 0.5f / PrimitiveData.InstanceLocalBoundsExtent;
	const float3 BoundsCenter = PrimitiveData.InstanceLocalBoundsCenter;
#else
	const float3 InvScale = 1.0f;
	const float3 BoundsCenter = 0.0f;
#endif

#if INTERSECTION_LOCAL_SPACE && !LOCAL_SPACE_WITHOUT_MATRIX_MUL
	Origin = mul(float4(Origin, 1), TranslatedWorldToLocal).xyz;
	Direction = mul(float4(Direction, 0), TranslatedWorldToLocal).xyz;
#endif

	const int HierarchyOffset = InstanceData.NaniteHierarchyOffset;
	const uint RuntimeResourceID = InstanceData.NaniteRuntimeResourceID;

	bool bIntersection = false;

	NaniteIntersectionResult.NodeIntersectionCount = 0;
	NaniteIntersectionResult.ClusterIntersectionCount = 0;
	NaniteIntersectionResult.TriangleIntersectionCount = 0;

	FSimpleStack Stack;
	Stack.Init();
	Stack.Push(0);

	while (!Stack.Empty())
	{		
		const uint NodeIndex = Stack.Pop();

		for (uint ChildIndex = 0; ChildIndex < NANITE_MAX_BVH_NODE_FANOUT; ++ChildIndex)
		{
			const FHierarchyNodeSlice HierarchyNodeSlice = GetHierarchyNodeSlice(GetHierarchyNodeOffset(HierarchyOffset, NodeIndex), ChildIndex);

#if TEST_BBOX
			// TODO: Take AbsMaxMaterialDisplacement + MaxWPOExtent when RT supports displacement
			float3 BoxBoundsCenter = (HierarchyNodeSlice.BoxBoundsCenter - BoundsCenter) * InvScale;
			float3 BoxBoundsExtent = (HierarchyNodeSlice.BoxBoundsExtent + PrimitiveData.MaxWPOExtent) * InvScale;

			bool bBoxIntersection = RayAABBIntersection(Origin, Direction, BoxBoundsCenter, BoxBoundsExtent, TMin, RayTCurrent);
#else
			const bool bBoxIntersection = true;
#endif

			++NaniteIntersectionResult.NodeIntersectionCount;

			bool bVisible = HierarchyNodeSlice.bEnabled;
			bool bLoaded = HierarchyNodeSlice.bLoaded;
			bool bShouldVisitChild = CutError < HierarchyNodeSlice.MaxParentLODError;

			if (!bVisible || !bLoaded || !bShouldVisitChild || !bBoxIntersection)
				continue;

			if (HierarchyNodeSlice.bLeaf)
			{
				const uint NumClusters = HierarchyNodeSlice.NumChildren;
				const uint PageIndex = HierarchyNodeSlice.ChildStartReference >> NANITE_MAX_CLUSTERS_PER_PAGE_BITS;
				const uint BaseClusterIndex = HierarchyNodeSlice.ChildStartReference & NANITE_MAX_CLUSTERS_PER_PAGE_MASK;
				const uint AssemblyTransformIndex = HierarchyNodeSlice.AssemblyTransformIndex;

				// TODO: Take AbsMaxMaterialDisplacement + MaxWPOExtent when RT supports displacement
				if (NaniteClustersIntersection(
						InstanceData, PageIndex, BaseClusterIndex, NumClusters, AssemblyTransformIndex,
						CutError, Origin, Direction, LocalToTranslatedWorld, InvScale, BoundsCenter,
						PrimitiveData.MaxWPOExtent, TMin, RayTCurrent, NaniteIntersectionResult))
				{
					bIntersection = true;
				}
			}
			else
			{
				Stack.Push(HierarchyNodeSlice.ChildStartReference);
			}
		}
	}
	return bIntersection;
}

#endif // NANITE_PROCEDURAL_RAY_TRACING