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Subject: [geometry] Issues with difference
From: Matthieu Beghin (matt.beghin_at_[hidden])
Date: 2019-03-07 10:44:47

Hi there,

I'm trying to solve my geometry problems with boost::geometry. I really
enjoy the API but I'm facing a few issues. Maybe the solutions are simple,
but I can't find my way. My context is: users are drawing shapes that are
combined using union / intersection / difference, I use "buffer" to have a
feathering effect around the shapes and in the end I triangulate the result
and render with OpenGL.

Situation 1: if I do a difference between two (valid) geometries that share
a common segment, the result is invalid (bg::is_valid return false, it find
some "turns"):

        namespace bg = boost::geometry; typedef
bg::model::d2::point_xy<double> point_xy; typedef
bg::model::polygon<point_xy> polygon_t; typedef
bg::model::multi_polygon<polygon_t> multiPolygon_t;
        multiPolygon_t poly1;
-0.713619134602,-1.00000012822 -0.493922219801,-0.598172925227
0.100631982002,-1.00000012886 -0.0624283708015,-1.00000011994
0.0862738908136,-0.440262107798 0.31341400405,-0.360828341246
0.292948255722,-0.357275641893 0.210997365241,-0.970143533681
-0.695818118925,-1.00000010731 -0.713619134602)))",poly1);

        multiPolygon_t poly2;
-0.493921978401,-0.909999987372 -0.360755621066,-0.909999996424
-0.91000000872,0.91000000872 -0.909999996424,0.909999996424
0.91000000872,-0.909999996424 0.91000000872,-0.909999911756
-0.0259065349961,-0.999999867625 -0.0624282647935,-1 1,1 1,1 -1,-1
-1,-0.999999965066 -0.493921978401)))",poly2);

        multiPolygon_t diff;

        bg::difference(poly1, poly2, diff);

  On this picture poly1 is in green, poly2 is in red.

[image: Screenshot 2019-03-07 at 11.31.29.png]

Situation 2: the difference between two valid geometries that have no
common edge is invalid. The subtracted geometry has an inner hole but I
suppose that should be supported...

        multiPolygon_t poly1; bg::read_wkt("MULTIPOLYGON(((-2 -2,-2
2,2 2,2 -2,-2 -2)))",poly1);

        multiPolygon_t poly2;
bg::read_wkt("MULTIPOLYGON(((-0.430323140092 -0.999999912736,-1 -1,-1
1,1 1,1 -1,0.431066411049 -0.999999901684,0.43106695576
-1.00000010168,-0.43032321787 -1.00000011274,-0.430323140092
-0.999999912736),(0.135529235005 0.316180944443,-0.949848473072
-0.124261498451,-0.47733861208 -0.590634822845,0.135529235005

        multiPolygon_t diff;

        bg::difference(poly1, poly2, diff);

On this picture poly1 is in green, poly2 is in red (the red triangle
is poly2 inner hole).

[image: Screenshot 2019-03-07 at 11.43.07.png]

Thanks for your attention!
Best Regards,
Matthieu Beghin


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