Re: [math] Polygon intersection vertices outside original polygon

[Curtis Jensen]

On Mon, Jul 4, 2011 at 2:00 PM, Curtis Jensen <cur...@the-jensens.org>wrote:

> I'm using the RegonFactory.intersection method to get the intersection of
> polygons.  However, I'm getting points that are outside of one of the
> original polygons.  See example below.  Am I misinterpreting what the
> intersection method does, miss-using it, or is this a bug?
>
>
>      Vector2D[][] vertices1 = new Vector2D[][] {
>     new Vector2D[] {
>                 new Vector2D(-25.8907, 53.6079),
>                 new Vector2D(-25.3586, 53.5214),
>                 new Vector2D(-25.6256, 53.1507),
>                 new Vector2D(-26.0395, 53.2562)
>             }
>         };
>         PolygonsSet set1 = buildSet(vertices1);
>         Vector2D[][] vertices2 = new Vector2D[][] {
>             new Vector2D[] {
>                 new Vector2D(-25.7455, 53.3656),
>                 new Vector2D(-25.3007, 53.2765),
>                 new Vector2D(-25.4181, 52.9993),
>                 new Vector2D(-25.9476, 53.0366)
>             }
>         };
>         PolygonsSet set2 = buildSet(vertices2);
>         PolygonsSet intersectionSet  = (PolygonsSet) new
> RegionFactory<Euclidean2D>().intersection(set1.copySelf(), set2.copySelf());
>
>         Vector2D[][] intersectionVerts = intersectionSet.getVertices();
>         for (Vector2D[] set : intersectionVerts) {
>         for (Vector2D vertex : set) {
>         System.out.println(vertex);
>         }
>         }
>
>
> OUTPUT:
> {-26.04; 53.26}
> {-25.89; 53.61}
> {-25.36; 53.52}
> {-25.51; 53.32}
> {-25.3; 53.28}
> {-25.42; 53}                  <- OUTSIDE polygon A
> {-25.72; 53.02}             <- OUTSIDE polygon A
> {-25.95; 53.04}             <- OUTSIDE polygon A
> {-25.84; 53.21}
>


I should add that the buildSet function is the same as that in the test suit
of commons math:
    private PolygonsSet buildSet(Vector2D[][] vertices) {
        ArrayList<SubHyperplane<Euclidean2D>> edges = new
ArrayList<SubHyperplane<Euclidean2D>>();
        for (int i = 0; i < vertices.length; ++i) {
            int l = vertices[i].length;
            for (int j = 0; j < l; ++j) {
                edges.add(buildSegment(vertices[i][j], vertices[i][(j + 1) %
l]));
            }
        }
        return new PolygonsSet(edges);
    }