Geometry Boost List: Re: [geometry] Intersection bug

Subject: Re: [geometry] Intersection bug
From: Barend Gehrels (barend_at_[hidden])
Date: 2014-03-22 18:36:36

Hi,

741 5963 wrote On 21-3-2014 14:58:
> 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.
>

Thanks for your report, and for your suggestion for the fix. I can 
reproduce it indeed, also in the new approach (because it is not caused 
by that).

Can you create a ticket? (Otherwise I will create one).
https://svn.boost.org/trac/boost/

Regards, Barend