Fix T45778: Objects scaled to 0 cause black artifacts with Static BVH

The issue was caused by some numeric instability in triangle intersection which
was visible on avx2 CPUs and GPUs (at least sm_20 here) but maybe some others
too.

Committing rather a workaround for now to be safe for the release, still need
some investigation.

From tests with grass field from Gooseberry project didn't see measurable
slowdown.

1 files changed, 3 insertions, 0 deletions

diff --git a/intern/cycles/kernel/geom/geom_triangle_intersect.h b/intern/cycles/kernel/geom/geom_triangle_intersect.h
index 3ef918dc842..7ca046f565b 100644
--- a/intern/cycles/kernel/geom/geom_triangle_intersect.h
+++ b/intern/cycles/kernel/geom/geom_triangle_intersect.h
@@ -173,6 +173,9 @@ ccl_device_inline bool triangle_intersect(KernelGlobals *kg,
 	if(kernel_tex_fetch(__prim_visibility, triAddr) & visibility)
 #endif
 	{
+		if(len_squared(cross(tri_a, tri_b)) < 1e-14f) {
+			return false;
+		}
 		/* Normalize U, V, W, and T. */
 		const float inv_det = 1.0f / det;
 		isect->prim = triAddr;