From ccea6fdde90d0c2e8cc6a15381266399ce3bdce8 Mon Sep 17 00:00:00 2001
From: Marius Kintel <marius@kintel.net>
Date: Sun, 7 Dec 2014 20:09:45 -0500
Subject: [PATCH] Bugfix: 2D scale or mirror sometimes caused flipped normals
 which caused larger issues downstream. Fixes #1055

---
 src/GeometryEvaluator.cc | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/GeometryEvaluator.cc b/src/GeometryEvaluator.cc
index 71b52a81..40250c68 100644
--- a/src/GeometryEvaluator.cc
+++ b/src/GeometryEvaluator.cc
@@ -547,7 +547,12 @@ Response GeometryEvaluator::visit(State &state, const TransformNode &node)
 							node.matrix(1,0), node.matrix(1,1), node.matrix(1,3),
 							node.matrix(3,0), node.matrix(3,1), node.matrix(3,3);
 						newpoly->transform(mat2);
-						geom = newpoly;
+						// A 2D transformation may flip the winding order of a polygon.
+						// If that happens with a sanitized polygon, we need to reverse
+						// the winding order for it to be correct.
+						if (newpoly->isSanitized() && mat2.matrix().determinant() <= 0) {
+							geom.reset(ClipperUtils::sanitize(*newpoly));
+						}
 					}
 					else if (geom->getDimension() == 3) {
 						shared_ptr<const PolySet> ps = dynamic_pointer_cast<const PolySet>(geom);