boost::geometry::intersection returns input polygon when the result should be empty.

[Nick <7415963@…>]

The result of an intersection is sometimes equal to one of the input polygons, when the result should be empty. For example:

    typedef boost::geometry::model::d2::point_xy<double> Point;
    typedef boost::geometry::model::polygon<Point> Polygon;
    Polygon pg1;
    Polygon pg2;
    boost::geometry::model::multi_polygon<Polygon> result;
   
    boost::geometry::append(pg1, Point(-6.0, 3.2500000000000013));
    boost::geometry::append(pg1, Point(-9.0, 3.2500000000000022));
    boost::geometry::append(pg1, Point(-9.0, 6.5));
    boost::geometry::append(pg1, Point(-6.0, 6.5));
    boost::geometry::append(pg1, Point(-6.0, 3.2500000000000013));

    boost::geometry::append(pg2, Point(-3.0, 0.0));
    boost::geometry::append(pg2, Point(-6.0, 0.0));
    boost::geometry::append(pg2, Point(-6.0, 3.2500000000000013));
    boost::geometry::append(pg2, Point(-3.0, 3.2500000000000009));
    boost::geometry::append(pg2, Point(-3.0, 0.0));

    boost::geometry::intersection(pg1, pg2, result);

After the intersection, result contains a polygon equal to pg1. We're using boost 1.55 and also tried the rescale_to_integer branch, which gives the same results.

The problem seems to be with boost::geometry::within. We've previously had problems with within that we resolved by using a different strategy. Changing the strategy of the within calls in the function update_selection_map in select_rings.hpp makes intersection return the correct result for this case.