Geometry Boost List: Re: [geometry] Difference problem using boost geometry in 1.47

Subject: Re: [geometry] Difference problem using boost geometry in 1.47
From: Barend Gehrels (barend_at_[hidden])
Date: 2013-10-26 05:54:13

Hi Dan,

Macumber, Daniel wrote On 25-10-2013 23:50:
>
> I am using boost geometry in 1.47 and am pretty happy with it.  
> However, I am having a problem performing an intersection that I can't 
> figure out how to solve.  When I difference two geometries, I am 
> sometimes left with a result that uses reverse winding to subtract a 
> piece of the result rather than providing a "clean" polygon with 
> winding in one direction only.  I've posted an image here that 
> demonstrates this:
>
> http://oi42.tinypic.com/2elfp0o.jpg
>
> I get the feeling that if I were to use the right models this might 
> somehow be fixed?  If not is there an algorithm that can clean up the 
> result?  Here is the code I am using:
>
> typedef boost::geometry::model::d2::point_xy<double> BoostPoint;
>
> typedef boost::geometry::model::polygon<BoostPoint> BoostPolygon;
>
> typedef boost::geometry::model::ring<BoostPoint> BoostRing;
>
> BoostRing facePolygon;
>
> BoostRing otherFacePolygon;
>
> std::vector<BoostPolygon> faceDifferenceResult;
>
> ....
>
> boost::geometry::difference(facePolygon, otherFacePolygon, 
> faceDifferenceResult);
>
>

This should be handled correctly. The usage seems good. Do you have them 
clockwise ordered? You might call boost::geometry::correct first.

Do you have the coordinates  (preferably WKT) of your input polygons? I 
can then better check the case.

Thanks, Barend