[Bf-blender-cvs] [2fb639d] master: Fix T45778: Objects scaled to 0 cause black artifacts with Static BVH

[Sergey Sharybin]

Commit: 2fb639deedbdb0b0a821eb04ef69a7dd0080ea94
Author: Sergey Sharybin
Date:   Mon Aug 24 21:21:30 2015 +0200
Branches: master
https://developer.blender.org/rB2fb639deedbdb0b0a821eb04ef69a7dd0080ea94

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.

===================================================================

M	intern/cycles/kernel/geom/geom_triangle_intersect.h

===================================================================

diff --git a/intern/cycles/kernel/geom/geom_triangle_intersect.h b/intern/cycles/kernel/geom/geom_triangle_intersect.h
index 3ef918d..7ca046f 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;