
Geometry : 
Subject: [geometry] Intersection bug
From: 741 5963 (7415963_at_[hidden])
Date: 20140321 09:58:15
Hi,
There seems to be a bug in boost::geometry::intersection that we regularly
encounter. 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.
Regards
Geometry list run by mateusz at loskot.net