Clipper does not create hole when making a union

Question

I have the following code:

class Program {
    static void Main(string[] args) {

        Polygon a = new Polygon();
        a.Add(new IntPoint(0,0));
        a.Add(new IntPoint(2,0));
        a.Add(new IntPoint(2,1));
        a.Add(new IntPoint(1,1));
        a.Add(new IntPoint(1,2));
        a.Add(new IntPoint(2,2));
        a.Add(new IntPoint(2,3));
        a.Add(new IntPoint(0,3));

        Polygon b = new Polygon();
        b.Add(new IntPoint(2,0));
        b.Add(new IntPoint(3,0));
        b.Add(new IntPoint(3,3));
        b.Add(new IntPoint(2,3));

        PolyTree solution = new PolyTree();

        Clipper c = new Clipper();
        c.AddPolygon(a,PolyType.ptSubject);
        c.AddPolygon(b,PolyType.ptSubject);
        c.Execute(ClipType.ctUnion,solution);


        printPolygonTree(solution);

        Console.ReadLine();
    }

    static void printPolygonTree(PolyNode tree) {
        Console.WriteLine((tree.IsHole?"Hole":"Polygon")+":");
        foreach(IntPoint point in tree.Contour) {
            Console.WriteLine(point.X+"/"+point.Y);
        }
        foreach(PolyNode node in tree.Childs) {
            printPolygonTree(node);
        }
    }
}

It should unify the polygons a and b, which should result in a big square that contains a small square as a hole. But I get one polygon instead, that has a cut to connect the inner and outer polygon into one polygon.

Is there a way to unify the 2 polygons as expected?

Graphic:

Answer 1

There's a new version of Clipper that's about to be released that addresses this issue together with a number of other improvements.

You can download a preview of Clipper ver 6 from the SF trunk here:

See also: the SF discussion about this here