[HDRP] [Path Tracing] Added proper computation of geometric normal

com.unity.render-pipelines.high-definition/CHANGELOG.md

@@ -21,6 +21,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Fixed couple samplers that had the wrong name in raytracing code
 - VFX : Debug material view were rendering pink for albedo. (case 1290752)
 - VFX: Fixed LPPV with lit particles in deferred (case 1293608)
+- Fixed computation of geometric normal in path tracing (case 1293029).
 
 ### Changed
 - Removed the material pass probe volumes evaluation mode.

com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/Raytracing/Shaders/RaytracingIntersection.hlsl

@@ -174,4 +174,15 @@ void GetCurrentIntersectionVertex(AttributeData attributeData, out IntersectionV
 #endif
 }
 
+// Compute the proper world space geometric normal from the intersected triangle
+void GetCurrentIntersectionGeometricNormal(AttributeData attributeData, out float3 geomNormalWS)
+{
+    uint3 triangleIndices = UnityRayTracingFetchTriangleIndices(PrimitiveIndex());
+    float3 p0 = UnityRayTracingFetchVertexAttribute3(triangleIndices.x, kVertexAttributePosition);
+    float3 p1 = UnityRayTracingFetchVertexAttribute3(triangleIndices.y, kVertexAttributePosition);
+    float3 p2 = UnityRayTracingFetchVertexAttribute3(triangleIndices.z, kVertexAttributePosition);
+
+    geomNormalWS = normalize(mul(cross(p1 - p0, p2 - p0), (float3x3)WorldToObject3x4()));
+}
+
 #endif // UNITY_RAYTRACING_INTERSECTION_INCLUDED

com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/ShaderPass/ShaderPassPathTracing.hlsl

@@ -54,6 +54,11 @@ void ComputeSurfaceScattering(inout PathIntersection pathIntersection : SV_RayPa
     posInput.positionWS = fragInput.positionRWS;
     posInput.positionSS = pathIntersection.pixelCoord;
 
+    // For path tracing, we want the front-facing test to be performed on the actual geometric normal
+    float3 geomNormal;
+    GetCurrentIntersectionGeometricNormal(attributeData, geomNormal);
+    fragInput.isFrontFace = dot(WorldRayDirection(), geomNormal) < 0.0;
+
     // Build the surfacedata and builtindata
     SurfaceData surfaceData;
     BuiltinData builtinData;
@@ -75,7 +80,11 @@ void ComputeSurfaceScattering(inout PathIntersection pathIntersection : SV_RayPa
 
 #ifndef SHADER_UNLIT
 
-    // Let's compute the world space position (the non-camera relative one if camera relative rendering is enabled)
+    // Override the geometric normal (otherwise, it is merely the non-mapped smooth normal)
+    // Also make sure that it is in the same hemisphere as the shading normal (which may have been flipped)
+    bsdfData.geomNormalWS = dot(bsdfData.normalWS, geomNormal) > 0.0 ? geomNormal : -geomNormal;
+
+    // Compute the world space position (the non-camera relative one if camera relative rendering is enabled)
     float3 shadingPosition = fragInput.positionRWS;
 
     // Get current path throughput