Mark twist-free linear extrusions of convex polygons convex

src/GeometryEvaluator.cc

@@ -633,7 +633,8 @@ static void add_slice(PolySet *ps, const Polygon2d &poly,
 */
 static Geometry *extrudePolygon(const LinearExtrudeNode &node, const Polygon2d &poly)
 {
-	PolySet *ps = new PolySet(3);
+	bool cvx = poly.is_convex();
+	PolySet *ps = new PolySet(3, !cvx ? boost::tribool(false) : node.twist == 0 ? boost::tribool(true) : unknown);
 	ps->setConvexity(node.convexity);
 	if (node.height <= 0) return ps;
 

src/Polygon2d.cc

@@ -92,3 +92,20 @@ void Polygon2d::resize(Vector2d newsize, const Eigen::Matrix<bool,2,1> &autosize
 	this->transform(t);
 }
 
+bool Polygon2d::is_convex() const {
+	if (theoutlines.size() > 1) return false;
+	if (theoutlines.empty()) return true;
+
+	std::vector<Vector2d> const& pts = theoutlines[0].vertices;
+	int N = pts.size();
+
+	// Check for a right turn. This assumes the polygon is simple.
+	for (int i = 0; i < N; i++) {
+		Vector2d d1 = pts[(i+1)%N] - pts[i];
+		Vector2d d2 = pts[(i+2)%N] - pts[(i+1)%N];
+		double zcross = d1[0]*d2[1]-d1[1]*d2[0];
+		if (zcross < 0) return false;
+	}
+	return true;
+}
+

src/Polygon2d.h

@@ -36,6 +36,7 @@ public:
 
 	bool isSanitized() const { return this->sanitized; }
 	void setSanitized(bool s) { this->sanitized = s; }
+	bool is_convex() const;
 private:
 	Outlines2d theoutlines;
 	bool sanitized;