Fix T46521: Python: bvh.ray_cast doesn't find a plane facing in the other direction under certain circumstances

The issue was caused by wrong sign check. It originally came from more optimized
Cycles code where because of other reasons it wasn't visible yet. But in fact it
should be solved there as well.

1 files changed, 9 insertions, 10 deletions

diff --git a/source/blender/blenlib/intern/math_geom.c b/source/blender/blenlib/intern/math_geom.c
index dad2a2fc288..e0058091447 100644
--- a/source/blender/blenlib/intern/math_geom.c
+++ b/source/blender/blenlib/intern/math_geom.c
@@ -1384,16 +1384,14 @@ bool isect_ray_tri_watertight_v3(
 	const float cy = c_ky - sy * c_kz;
 
 	/* Calculate scaled barycentric coordinates. */
-	float u = cx * by - cy * bx;
-	int sign_mask = (float_as_int(u) & (int)0x80000000);
-	float v = ax * cy - ay * cx;
-	float w, det;
+	const float u = cx * by - cy * bx;
+	const float v = ax * cy - ay * cx;
+	const float w = bx * ay - by * ax;
+	float det;
 
-	if (sign_mask != (float_as_int(v) & (int)0x80000000)) {
-		return false;
-	}
-	w = bx * ay - by * ax;
-	if (sign_mask != (float_as_int(w) & (int)0x80000000)) {
+	if ((u < 0.0f || v < 0.0f || w < 0.0f) &&
+	    (u > 0.0f || v > 0.0f || w > 0.0f))
+	{
 		return false;
 	}
 
@@ -1406,8 +1404,9 @@ bool isect_ray_tri_watertight_v3(
 		/* Calculate scaled z-coordinates of vertices and use them to calculate
 		 * the hit distance.
 		 */
+		const int sign_det = (float_as_int(det) & (int)0x80000000);
 		const float t = (u * a_kz + v * b_kz + w * c_kz) * sz;
-		const float sign_t = xor_fl(t, sign_mask);
+		const float sign_t = xor_fl(t, sign_det);
 		if ((sign_t < 0.0f)
 		    /* differ from Cycles, don't read r_lambda's original value
 		     * otherwise we won't match any of the other intersect functions here...