Trouble finding union of MultiPolygons

  • Ross

    Ross - 2012-02-21


    I'm trying to take the union of two multipolygons, and then store it in a third multipolygon. However, for some reason not specified in the Javadoc, the Polygon2DUtils.union method returns a SimplePolygon2D instead of just a Polygon2D. Obviously I can't caste this to MultiPolygon2D, so the union operation is unable to handle polygons with holes.

    Please help/fix this.

            double s1x = {105, 97, 89, 97};
            double s1y = {100, 102, 100, 98};

            double s2x = {113, 123, 133, 123};
            double s2y = {100, 102, 100, 98};

            LinearRing2D s1 = new LinearRing2D(s1x, s1y);
            LinearRing2D s2 = new LinearRing2D(s2x, s2y);

            MultiPolygon2D mp1 = new MultiPolygon2D(s1);
            MultiPolygon2D mp2 = new MultiPolygon2D(s2);

            double hole1x = {102, 97, 92, 97};
            double hole1y = {100, 101, 100, 99};
            LinearRing2D holeRing = new LinearRing2D(hole1x, hole1y);

            AffineTransform2D sca1 = AffineTransform2D.createScaling(mp1.getCentroid(),10, 10);
            AffineTransform2D sca2 = AffineTransform2D.createScaling(mp2.getCentroid(),10, 10);

            mp1 = mp1.transform(sca1);
            mp2 = mp2.transform(sca2);

            MultiPolygon2D intersect = (MultiPolygon2D)Polygon2DUtils.union(mp1, mp2);

    Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException: math.geom2d.polygon.SimplePolygon2D cannot be cast to math.geom2d.polygon.MultiPolygon2D

  • David

    David - 2012-02-22


    when computing a boolean operation on polygons, the result is converted to a simple- or multi-polygon, depending if there are holes or multi components.

    SimplePolygon2D and MultiPolygon2D are implementation classes, but both are implementation of the Polygon2D interface. I would suggest using the type "Polygon2D" for storing the result, this should work in all cases.

    If you really need to use a MultiPolygon as result, you can create a new one, using a syntax similar to:
    MultiPolygon mpoly = new MultiPolygon2D(Polygon2DUtils.union(mp1, mp2).getRings());

    hope this helps ?



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