poly2d/: automatically switch between clockwise and counter-clockwise polygons

The only functions that care about this at the moment are from CGAL,
and there we can determine from context what to do.

poly2d/README

@@ -16,23 +16,22 @@ operation in cameo. Later, poly2d could replace more parts of
 cameo.
 
 poly2d puts more emphasis on simplicity than on performance. Some
-functions expect clockwise closed polygons that don't self-intersect.
+functions expect closed polygons that don't self-intersect. For now,
+it doesn't matter whether polygons are clockwise or counter-clockwise. 
 The table below shows the capabilities
 
-Open     Counter-clockwise
+Open     Min. vertices
-|  Concave  Min. vertices
+|  Concave
 |  |  Self-intersect
-|  |  |  |  |
+|  |  |  |
-Y  Y  Y  Y  0		p2d_contains_poly(b), p2d_free, p2d_free_all,
+Y  Y  Y  0		p2d_contains_poly(b), p2d_copy, p2d_free, p2d_free_all,
-			p2d_is_closed, p2d_copy, p2d_reverse, p2d_vertices,
+			p2d_is_closed, p2d_no_intersect, p2d_reverse,
-			p2d_write_gnuplog, p2d_write_gnuplot_all
+			p2d_vertices, p2d_write_gnuplot, p2d_write_gnuplot_all
-Y  Y  Y  Y  1		p2d_read_gnuplot
+Y  Y  Y  1		p2d_read_gnuplot
--  Y  Y  Y  0		p2d_simplify
+-  Y  Y  0		p2d_simplify
--  Y  -  Y  3		p2d_is_cw
+-  Y  -  3		p2d_is_cw
--  Y  -  -  3		p2d_contains_point, p2d_contains_poly (a),
+-  Y  -  3		p2d_contains_point, p2d_contains_poly (a),
--  Y  -  #  3		p2d_area*, p2d_offset*
+-  Y  -  3		p2d_area*, p2d_offset*
-
-# CGAL uses ccw, poly2d uses cw. Need to switch.
 
 Not yet implemented:
 - p2d_simplify (low priority - the offsetting from CGAL already covers
@@ -43,8 +42,6 @@ Not yet specified:
 
 Other:
 - change the license from GPL to LGPL
-- change ccw to cw
-- make sure CGAL is only fed ccw ploygons and cw holes
 - transform CGAL's idea of outer polygons into something we can use
 - use more meaningful offset/overlap model for area fill
 - check for memory leaks

poly2d/cgal_helper.h

@@ -35,7 +35,7 @@ typedef CGAL::Exact_predicates_inexact_constructions_kernel K;
 typedef CGAL::Polygon_2<K> Polygon_2;
 
 
-static inline Polygon_2 p2d_to_P2(const struct p2d *p)
+static inline Polygon_2 p2d_to_P2(const struct p2d *p, int ccw)
 {
 	const struct v2d *v;
 	Polygon_2 np;
@@ -46,6 +46,8 @@ static inline Polygon_2 p2d_to_P2(const struct p2d *p)
 		v = v->next;
 	}
 	while (v != p->v);
+	if (p2d_is_cw(p) == ccw)
+		np.reverse_orientation();
 	return np;
 }
 

poly2d/p2d_area_holes.cpp

@@ -80,11 +80,11 @@ extern "C" void p2d_area_holes_append(const struct p2d *p,
 	const struct p2d *h;
 
 	assert(p2d_is_closed(p));
-	Polygon_with_holes poly(p2d_to_P2(p));
+	Polygon_with_holes poly(p2d_to_P2(p, 1));
 
 	for (h = holes; h; h = h->next) {
 		assert(p2d_is_closed(h));
-		poly.add_hole(p2d_to_P2(h));
+		poly.add_hole(p2d_to_P2(h, 0));
 	}
 
 	recurse_area(poly, offset, offset-overlap, last);

poly2d/p2d_offset.cpp

@@ -50,11 +50,11 @@ extern "C" struct p2d *p2d_offset_holes(const struct p2d *p,
 	struct p2d *res = NULL, **last = &res;
 
 	assert(p2d_is_closed(p));
-	Polygon_with_holes poly(p2d_to_P2(p));
+	Polygon_with_holes poly(p2d_to_P2(p, 1));
 
 	for (h = holes; h; h = h->next) {
 		assert(p2d_is_closed(h));
-		poly.add_hole(p2d_to_P2(h));
+		poly.add_hole(p2d_to_P2(h, 0));
 	}
 
 	PolygonPtrVector tmp = off > 0 ?

poly2d/test/offset

@@ -49,4 +49,28 @@ expect <<EOF
 
 EOF
 
+#------------------------------------------------------------------------------
+
+tst "inner offset, polygon cw" <<EOF
+struct p2d *p = p2d_new();
+struct p2d *q;
+
+p2d_append(p, v2d_new(0, 0));
+p2d_append(p, v2d_new(0, 1));
+p2d_append(p, v2d_new(2, 1));
+p2d_append(p, v2d_new(2, 0));
+p2d_close(p);
+q = p2d_offset(p, -0.1);
+p2d_write_gnuplot(stdout, q);
+EOF
+
+expect <<EOF
+0.1 0.1
+1.9 0.1
+1.9 0.9
+0.1 0.9
+0.1 0.1
+
+EOF
+
 ###############################################################################