Cycles: Fix rays parallel to the surface in the triangle refine and MultiGGX code

In the triangle intersection refinement code, rays that are parallel to the triangle caused a divide by zero.
These rays might initially hit the triangle due to the watertight intersection test, but are very rare - therefore, just skipping the refinement for them works fine.

Also, a few remaining issues in the MultiGGX code are fixed that were caused by rays parallel to the surface (which happened more often there due to smooth shading).

1 files changed, 20 insertions, 6 deletions

diff --git a/intern/cycles/kernel/geom/geom_triangle_intersect.h b/intern/cycles/kernel/geom/geom_triangle_intersect.h
index caa6c9d9a5b..eb0decc800b 100644
--- a/intern/cycles/kernel/geom/geom_triangle_intersect.h
+++ b/intern/cycles/kernel/geom/geom_triangle_intersect.h
@@ -342,9 +342,16 @@ ccl_device_inline float3 triangle_refine(KernelGlobals *kg,
 	float3 tvec = make_float3(P.x - tri_c.x, P.y - tri_c.y, P.z - tri_c.z);
 	float3 qvec = cross(tvec, edge1);
 	float3 pvec = cross(D, edge2);
-	float rt = dot(edge2, qvec) / dot(edge1, pvec);
-
-	P = P + D*rt;
+	float det = dot(edge1, pvec);
+	if(det != 0.0f) {
+		/* If determinant is zero it means ray lies in the plane of
+		 * the triangle. It is possible in theory due to watertight
+		 * nature of triangle intersection. For suc hcases we simply
+		 * don't refine intersection hoping it'll go all fine.
+		 */
+		float rt = dot(edge2, qvec) / det;
+		P = P + D*rt;
+	}
 
 	if(isect->object != OBJECT_NONE) {
 #  ifdef __OBJECT_MOTION__
@@ -400,9 +407,16 @@ ccl_device_inline float3 triangle_refine_subsurface(KernelGlobals *kg,
 	float3 tvec = make_float3(P.x - tri_c.x, P.y - tri_c.y, P.z - tri_c.z);
 	float3 qvec = cross(tvec, edge1);
 	float3 pvec = cross(D, edge2);
-	float rt = dot(edge2, qvec) / dot(edge1, pvec);
-
-	P = P + D*rt;
+	float det = dot(edge1, pvec);
+	if(det != 0.0f) {
+		/* If determinant is zero it means ray lies in the plane of
+		 * the triangle. It is possible in theory due to watertight
+		 * nature of triangle intersection. For such cases we simply
+		 * don't refine intersection hoping it'll go all fine.
+		 */
+		float rt = dot(edge2, qvec) / det;
+		P = P + D*rt;
+	}
 #endif  /* __INTERSECTION_REFINE__ */
 
 	if(isect->object != OBJECT_NONE) {