Hdrp/pt sss diff mix

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

@@ -726,6 +726,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Fixed robustness issue with GetOddNegativeScale() in ray tracing, which was impacting normal mapping (1261160).
 - Fixed regression where moving face of the probe gizmo was not moving its position anymore.
 - Fixed XR single-pass macros in tessellation shaders.
+- Fixed path-traced subsurface scattering mixing with diffuse and specular BRDFs (1250601).
 
 ### Changed
 - Improve MIP selection for decals on Transparents

com.unity.render-pipelines.high-definition/Runtime/Material/Lit/LitPathTracing.hlsl

@@ -118,7 +118,7 @@ bool CreateMaterialData(PathIntersection pathIntersection, BuiltinData builtinDa
         // Otherwise, we just compute BSDFs as usual
         mtlData.subsurfaceWeightFactor = 1.0 - subsurfaceWeight;
 
-        mtlData.bsdfWeight[0] -= subsurfaceWeight;
+        mtlData.bsdfWeight[0] = max(mtlData.bsdfWeight[0] - subsurfaceWeight, BSDF_WEIGHT_EPSILON);
         mtlData.bsdfWeight /= mtlData.subsurfaceWeightFactor;
 
         sample -= subsurfaceWeight;
@@ -147,8 +147,7 @@ bool SampleMaterial(MaterialData mtlData, float3 inputSample, out float3 sampleD
         if (!BRDF::SampleLambert(mtlData, inputSample, sampleDir, result.diffValue, result.diffPdf))
             return false;
 
-        result.diffValue *= mtlData.bsdfData.ambientOcclusion * mtlData.bsdfData.subsurfaceMask * (1.0 - mtlData.bsdfData.transmittanceMask);
-        result.diffPdf *= mtlData.subsurfaceWeightFactor;
+        result.diffValue *= mtlData.bsdfData.ambientOcclusion * (1.0 - mtlData.bsdfData.transmittanceMask);
 
         return true;
     }
@@ -175,7 +174,7 @@ bool SampleMaterial(MaterialData mtlData, float3 inputSample, out float3 sampleD
                 result.specPdf += mtlData.bsdfWeight[1] * pdf;
             }
 
-            result.diffValue *= mtlData.bsdfData.ambientOcclusion * (1.0 - mtlData.bsdfData.subsurfaceMask) * (1.0 - mtlData.bsdfData.transmittanceMask) * (1.0 - fresnelClearCoat);
+            result.diffValue *= mtlData.bsdfData.ambientOcclusion * (1.0 - mtlData.bsdfData.transmittanceMask) * (1.0 - fresnelClearCoat);
 
             if (mtlData.bsdfWeight[2] > BSDF_WEIGHT_EPSILON)
             {
@@ -196,7 +195,7 @@ bool SampleMaterial(MaterialData mtlData, float3 inputSample, out float3 sampleD
             if (mtlData.bsdfWeight[0] > BSDF_WEIGHT_EPSILON)
             {
                 BRDF::EvaluateDiffuse(mtlData, sampleDir, result.diffValue, result.diffPdf);
-                result.diffValue *= mtlData.bsdfData.ambientOcclusion * (1.0 - mtlData.bsdfData.subsurfaceMask) * (1.0 - mtlData.bsdfData.transmittanceMask) * (1.0 - fresnelClearCoat);
+                result.diffValue *= mtlData.bsdfData.ambientOcclusion * (1.0 - mtlData.bsdfData.transmittanceMask) * (1.0 - fresnelClearCoat);
                 result.diffPdf *= mtlData.bsdfWeight[0];
             }
 
@@ -226,7 +225,7 @@ bool SampleMaterial(MaterialData mtlData, float3 inputSample, out float3 sampleD
             if (mtlData.bsdfWeight[0] > BSDF_WEIGHT_EPSILON)
             {
                 BRDF::EvaluateDiffuse(mtlData, sampleDir, result.diffValue, result.diffPdf);
-                result.diffValue *= mtlData.bsdfData.ambientOcclusion * (1.0 - mtlData.bsdfData.subsurfaceMask) * (1.0 - mtlData.bsdfData.transmittanceMask) * (1.0 - fresnelClearCoat);
+                result.diffValue *= mtlData.bsdfData.ambientOcclusion * (1.0 - mtlData.bsdfData.transmittanceMask) * (1.0 - fresnelClearCoat);
                 result.diffPdf *= mtlData.bsdfWeight[0];
             }
         }
@@ -248,8 +247,8 @@ bool SampleMaterial(MaterialData mtlData, float3 inputSample, out float3 sampleD
 #endif
 
 #ifdef _MATERIAL_FEATURE_SUBSURFACE_SCATTERING
-        result.diffPdf *= mtlData.subsurfaceWeightFactor;
-        result.specPdf *= mtlData.subsurfaceWeightFactor;
+        // We compensate for the fact that there is no spec when computing SSS
+        result.specValue /= mtlData.subsurfaceWeightFactor;
 #endif
     }
     else // Below
@@ -295,8 +294,7 @@ void EvaluateMaterial(MaterialData mtlData, float3 sampleDir, out MaterialResult
     if (mtlData.isSubsurface)
     {
         BRDF::EvaluateLambert(mtlData, sampleDir, result.diffValue, result.diffPdf);
-        result.diffValue *= mtlData.bsdfData.subsurfaceMask * (1.0 - mtlData.bsdfData.transmittanceMask); // AO purposedly ignored here
-        result.diffPdf *= mtlData.subsurfaceWeightFactor;
+        result.diffValue *= 1.0 - mtlData.bsdfData.transmittanceMask; // AO purposedly ignored here
 
         return;
     }
@@ -319,7 +317,7 @@ void EvaluateMaterial(MaterialData mtlData, float3 sampleDir, out MaterialResult
         if (mtlData.bsdfWeight[0] > BSDF_WEIGHT_EPSILON)
         {
             BRDF::EvaluateDiffuse(mtlData, sampleDir, result.diffValue, result.diffPdf);
-            result.diffValue *= (1.0 - mtlData.bsdfData.transmittanceMask) * (1.0 - mtlData.bsdfData.subsurfaceMask) * (1.0 - fresnelClearCoat); // AO purposedly ignored here
+            result.diffValue *= (1.0 - mtlData.bsdfData.transmittanceMask) * (1.0 - fresnelClearCoat); // AO purposedly ignored here
             result.diffPdf *= mtlData.bsdfWeight[0];
         }
 
@@ -331,8 +329,8 @@ void EvaluateMaterial(MaterialData mtlData, float3 sampleDir, out MaterialResult
         }
 
 #ifdef _MATERIAL_FEATURE_SUBSURFACE_SCATTERING
-        result.diffPdf *= mtlData.subsurfaceWeightFactor;
-        result.specPdf *= mtlData.subsurfaceWeightFactor;
+        // We compensate for the fact that there is no spec when computing SSS
+        result.specValue /= mtlData.subsurfaceWeightFactor;
 #endif
     }
 }

com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/PathTracing/Shaders/PathTracingBSDF.hlsl

@@ -478,7 +478,7 @@ bool RandomWalk(float3 position, float3 normal, float3 diffuseColor, float3 mean
         // Evaluate the length of our steps
         rayDesc.TMax = -log(1.0 - distSample) / sigmaT[channelIdx];
 
-        // Sample our next sepath segment direction
+        // Sample our next path segment direction
         rayDesc.Direction = walkIdx ?
             SampleSphereUniform(dirSample0, dirSample1) : SampleHemisphereCosine(dirSample0, dirSample1, -normal);
 
@@ -489,7 +489,7 @@ bool RandomWalk(float3 position, float3 normal, float3 diffuseColor, float3 mean
         TraceRay(_RaytracingAccelerationStructure, RAY_FLAG_FORCE_OPAQUE | RAY_FLAG_CULL_FRONT_FACING_TRIANGLES,
                  RAYTRACINGRENDERERFLAG_PATH_TRACING, 0, 1, 1, rayDesc, intersection);
 
-        // Define if we did a hit
+        // Check if we hit something
         hit = intersection.t > 0.0;
 
         // How much did the ray travel?
@@ -513,10 +513,24 @@ bool RandomWalk(float3 position, float3 normal, float3 diffuseColor, float3 mean
     while (!hit && walkIdx < MAX_WALK_STEPS);
 
     // Set the exit intersection position and normal
-    result.exitPosition = rayDesc.Origin;
-    result.exitNormal = intersection.value;
+    if (!hit)
+    {
+        result.exitPosition = position;
+        result.exitNormal = normal;
+        result.throughput = diffuseColor;
+
+        // By not returning false here, we default to a diffuse BRDF when an intersection is not found;
+        // this is physically wrong, but may prove more convenient for a user, as results will look
+        // like diffuse instead of getting slightly darker when the mean free path becomes shorter.
+        //return false;
+    }
+    else
+    {
+        result.exitPosition = rayDesc.Origin;
+        result.exitNormal = intersection.value;
+    }
 
-    return hit;
+    return true;
 }
 
 } // namespace SSS