shading normals

SeeSharp/Integrators/Bidir/BidirBase.cs

@@ -395,6 +395,11 @@ void ConnectLightVertexToCamera(in PathVertex vertex, in PathVertex ancestor, Ve
         if (bsdfValue == RgbColor.Black)
             return;
 
+        // The shading cosine only cancels out with the Jacobian if the geometry aligns with the shading geometry
+        bsdfValue *=
+            float.Abs(Vector3.Dot(vertex.Point.ShadingNormal, dirToAncestor)) /
+            float.Abs(Vector3.Dot(vertex.Point.Normal, dirToAncestor));
+
         // Compute the surface area pdf of sampling the previous vertex instead
         var (pdfReverse, _) = shader.Pdf(dirToAncestor);
         if (ancestor.Point.Mesh != null)
@@ -468,8 +473,11 @@ RgbColor Connect(in SurfaceShader shader, PathVertex vertex, PathVertex ancestor
         var dirFromCamToLight = Vector3.Normalize(vertex.Point.Position - shader.Point.Position);
 
         SurfaceShader lightShader = new(vertex.Point, dirToAncestor, true);
+        var bsdfWeightLight = lightShader.Evaluate(-dirFromCamToLight) * float.Abs(Vector3.Dot(vertex.Point.Normal, -dirFromCamToLight));
+        bsdfWeightLight *=
+            float.Abs(Vector3.Dot(vertex.Point.ShadingNormal, dirToAncestor)) /
+            float.Abs(Vector3.Dot(vertex.Point.Normal, dirToAncestor));
 
-        var bsdfWeightLight = lightShader.EvaluateWithCosine(-dirFromCamToLight);
         var bsdfWeightCam = shader.EvaluateWithCosine(dirFromCamToLight);
 
         if (bsdfWeightCam == RgbColor.Black || bsdfWeightLight == RgbColor.Black)

SeeSharp/Integrators/Bidir/VertexConnectionAndMerging.cs

@@ -271,16 +271,23 @@ protected virtual RgbColor Merge(in CameraPath path, float cameraJacobian, in Su
         if (depth > MaxDepth || depth < MinDepth)
             return RgbColor.Black;
 
+        // Discard photons on (almost) perpendicular surfaces. This avoids outliers and somewhat reduces
+        // light leaks, but slightly amplifies darkening from kernel estimation bias.
+        if (float.Abs(Vector3.Dot(shader.Point.Normal, photon.Point.Normal)) < 0.4f) {
+            return RgbColor.Black;
+        }
+
         // Compute the contribution of the photon
         var ancestor = LightPaths[idx.pathIdx, idx.vertexIdx - 1];
         var dirToAncestor = Vector3.Normalize(ancestor.Point.Position - shader.Point.Position);
         var bsdfValue = shader.Evaluate(dirToAncestor);
+        bsdfValue *=
+            float.Abs(Vector3.Dot(shader.Point.ShadingNormal, dirToAncestor)) /
+            float.Abs(Vector3.Dot(photon.Point.Normal, dirToAncestor));
         var photonContrib = photon.Weight * bsdfValue / NumLightPaths.Value;
 
         // Early exit + prevent NaN / Inf
         if (photonContrib == RgbColor.Black) return RgbColor.Black;
-        // Prevent outliers due to numerical issues with photons arriving almost parallel to the surface
-        if (Math.Abs(Vector3.Dot(dirToAncestor, shader.Point.Normal)) < 1e-4f) return RgbColor.Black;
 
         // Compute the missing pdf terms and the MIS weight
         var (pdfLightReverse, pdfCameraReverse) = shader.Pdf(dirToAncestor);

SeeSharp/Integrators/Common/RandomWalk.cs

@@ -191,6 +191,20 @@ RgbColor ContinueWalk(Ray ray, SurfacePoint previousPoint, float pdfDirection, R
             if (dirSample.PdfForward == 0 || dirSample.Weight == RgbColor.Black)
                 break;
 
+            if (isOnLightSubpath) {
+                // The direction sample is multiplied by the shading cosine, but we need the geometric one
+                dirSample.Weight *=
+                    float.Abs(Vector3.Dot(hit.Normal, dirSample.Direction)) /
+                    float.Abs(Vector3.Dot(hit.ShadingNormal, dirSample.Direction));
+
+                // Rendering equation cosine cancels with the Jacobian, but only if geometry and shading geometry align
+                dirSample.Weight *=
+                    float.Abs(Vector3.Dot(hit.ShadingNormal, -ray.Direction)) /
+                    float.Abs(Vector3.Dot(hit.Normal, -ray.Direction));
+
+                SanityChecks.IsNormalized(ray.Direction);
+            }
+
             // Continue the path with the next ray
             prefixWeight *= dirSample.Weight / survivalProb;
             depth++;