difference algorithm returning invalid geometry

[Volker Schöch <vschoech@…>]

My "tc::geo::polygon" type is actually a multi-polygon, using a polygon type that is based on int, oriented counter-clockwise and open (not closed). Please consider the following example:

tc::geo::polygon<int> polygonA;
boost::geometry::read_wkt("MULTIPOLYGON(((1920 1462,3720 1462,3720 3262,1920 3262,1920 1462)))", polygonA);
boost::geometry::is_valid(polygonA); // returns true

tc::geo::polygon<int> polygonB;
boost::geometry::read_wkt("MULTIPOLYGON(((1918 1957,1918 1657,2218 2189,1918 1957)),((3718 1957,3360 2233,3718 1561,3718 1957)),((3360 2233,2818 3253,2218 2189,2818 2653,3360 2233)))", polygonB);
boost::geometry::is_valid(polygonB); // returns true

tc::geo::polygon<int> polygonC;
boost::geometry::difference(polygonA, polygonB, polygonC);
// polygonC: MULTIPOLYGON(((1920 1660,1920 1462,3720 1462,3720 3262,1920 3262,1920 1959,2218 2189,1920 1660),(3718 1561,3360 2233,3718 1957,3718 1561),(2818 2653,2218 2189,2818 3253,3360 2233,2818 2653)))
boost::geometry::is_valid(polygonC); // returns true

tc::geo::polygon<int> polygonD;
boost::geometry::read_wkt("MULTIPOLYGON(((1918 2155,1918 1957,2818 2653,3718 1957,3718 2154,2818 3055,1918 2155)))", polygonD);
boost::geometry::is_valid(polygonD); // returns true

tc::geo::polygon<int> polygonE;
boost::geometry::difference(polygonC, polygonD, polygonE);
// polygonE: MULTIPOLYGON(((2218 2189,1920 1660,1920 1462,3720 1462,3720 3262,1920 3262,1920 2157,2562 2799,2818 3253,3043 2829,3718 2154,3718 1957,3360 2234,3360 2233,2818 2653,2218 2189),(3718 1957,3718 1561,3360 2233,3718 1957)))
boost::geometry::is_valid(polygonD); // returns false!

The difference of two valid multi-polygons yields an invalid multi-polygon.

May be related to tickets #10661, #11674, #11676.

[Barend Gehrels]

Unfortunately not yet fixed

[Volker Schöch <vschoech@…>]

Still present in boost 1.66.0.

[Volker Schöch <vschoech@…>]

No longer reproducible in 1.67.0.

[Barend Gehrels]

Thanks for verifying!