Geometry Boost List: Re: [geometry] self intersection doesn't find collinear intersection types

Subject: Re: [geometry] self intersection doesn't find collinear intersection types
From: Adam Wulkiewicz (adam.wulkiewicz_at_[hidden])
Date: 2016-05-11 06:19:31

Hi Sergei,

Sergei Benkovich wrote:
> Hey,
> i have an issue where i find self intersections in a polygon.but it 
> doesn't seem to find the method_collinear type.
>
> this is the piece of code:
>
>
>   namespace bg = boost::geometry;
>   using namespace std;
>   typedef bg::model::d2::point_xy<double> point_2d;
>   typedef 
> bg::model::polygon<boost::geometry::model::d2::point_xy<double> > Polygon;
>   Polygon poly;
>   poly.outer().push_back(point_2d(0, 10));
>   poly.outer().push_back(point_2d(10, 0));
>

Have in mind that this polygon is invalid (too small number of points) 
so it may even be ignored (I don't remember).
But of course if you had a valid polygon like:

Polygon poly{ { {0, 0},{0, 10}, {10, 10}, {10, 0}, {0, 0} } };

There would be no self intersections returned because the polygon is not 
self-intersecting.
However if you had a (invalid) Polygons like this:

Polygon poly{ { {0, 0},{0, 10}, {10, 10}, {10, 0}, {0, 0},{ 0, 10 },{ 
10, 10 },{ 10, 0 },{ 0, 0 } } };
Polygon poly{ { {0, 0},{0, 10}, {10, 10}, {10, 0}, { 10, 10 },{ 0, 10 
},{ 0, 0 } } };

you'd get collinear and blocking self-intersection turns respectively.

Regards,
Adam