Fix for crash in Freestyle with sketchy chaining and Bezier Curve geometry modifier.

When the sketchy chaining is used, stroke geometry may contain a 180-degree U-turn.
If the 'error' parameter of the Bezier Curve geometry modifier is small (e.g., 10),
Bezier curve fitting will recursively split the original stroke into two pieces.
This splitting may take place at a U-turn point, causing a numerical singularity issue
that leads to a crash.

Problem report by edna in the BA Freestyle thread, with an example .blend to reproduce
the problem.  Thanks a lot!

1 files changed, 6 insertions, 0 deletions

diff --git a/source/blender/freestyle/intern/geometry/FitCurve.cpp b/source/blender/freestyle/intern/geometry/FitCurve.cpp
index cc21ba05f3d..a5701ea05e5 100644
--- a/source/blender/freestyle/intern/geometry/FitCurve.cpp
+++ b/source/blender/freestyle/intern/geometry/FitCurve.cpp
@@ -376,6 +376,12 @@ static Vector2 ComputeCenterTangent(Vector2 *d, int center)
 	tHatCenter[0] = (V1[0] + V2[0]) / 2.0;
 	tHatCenter[1] = (V1[1] + V2[1]) / 2.0;
 	tHatCenter = *V2Normalize(&tHatCenter);
+
+	/* avoid numerical singularity in the special case when V1 == -V2 */
+	if (V2Length(&tHatCenter) < M_EPSILON) {
+		tHatCenter = *V2Normalize(&V1);
+	}
+
 	return tHatCenter;
 }