Path tracer: added support for transparency on the Unlit shader.

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

@@ -135,6 +135,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Added CustomPassUtils API to simplify Blur, Copy and DrawRenderers custom passes.
 - Added an API in HDRP to override the camera within the rendering of a frame (mainly for custom pass).
 - Added more custom pass API functions, mainly to render objects from another camera.
+- Added support for transparent Unlit in path tracing.
 
 ### Fixed
 - Fix when rescale probe all direction below zero (1219246)

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

@@ -223,9 +223,30 @@ void ClosestHit(inout PathIntersection pathIntersection : SV_RayPayload, Attribu
     }
 
 #else // HAS_LIGHTLOOP
+
     pathIntersection.value = (!currentDepth || computeDirect) ? bsdfData.color * GetInverseCurrentExposureMultiplier() + builtinData.emissiveColor : 0.0;
+
+// Simulate opacity blending by simply continuing along the current ray
+#ifdef _SURFACE_TYPE_TRANSPARENT
+    if (builtinData.opacity < 1.0)
+    {
+        RayDesc rayDescriptor;
+        rayDescriptor.Origin = GetAbsolutePositionWS(fragInput.positionRWS) - fragInput.tangentToWorld[2] * _RaytracingRayBias;
+        rayDescriptor.Direction = WorldRayDirection();
+        rayDescriptor.TMin = 0.0;
+        rayDescriptor.TMax = FLT_INF;
+
+        PathIntersection nextPathIntersection = pathIntersection;
+        nextPathIntersection.remainingDepth--;
+
+        TraceRay(_RaytracingAccelerationStructure, RAY_FLAG_CULL_BACK_FACING_TRIANGLES, RAYTRACINGRENDERERFLAG_PATH_TRACING, 0, 1, 2, rayDescriptor, nextPathIntersection);
+
+        pathIntersection.value = lerp(nextPathIntersection.value, pathIntersection.value, builtinData.opacity);
+    }
 #endif
 
+#endif // HAS_LIGHTLOOP
+
     ApplyFogAttenuation(WorldRayOrigin(), WorldRayDirection(), pathIntersection.t, pathIntersection.value, computeDirect);
 
     if (currentDepth)