sym_difference of non-closed polygons returns closed polygon

[Volker Schöch <vschoech@…>]

The output multi-polygon from boost::geometry::sym_difference should have the same orientation, closed-ness and underlying scalar data type, as the input (multi-)polygons. My polygon type is oriented counter-clockwise and not closed, my point type is based on int. Please find below some code that results in a closed polygon.

This is the data used as input to sym_difference:

_TPolygon<int> const polygonB =
	_TPolygon<int>("MULTIPOLYGON(((813 277,1341 319,1863 361,2391 571,2913 361,3441 319,3963 277,3963 32767,813 32767)))")
	^ _TPolygon<int>("MULTIPOLYGON(((813 277,1341 319,1863 571,2391 571,2913 571,3441 319,3963 277,3963 32767,813 32767)))");

This is the resulting multi-polygon, consisting of two closed polygons:

polygonB: MULTIPOLYGON(((2391 571,1863 571,1341 319,1863 361,2391 571)),((2391 571,2913 361,3441 319,2913 571,2391 571)))

This is my code that wraps boost::geometry to implement the operator used above:

template<typename T>
template<typename Geometry>
_TPolygon< T > _TPolygon< T >::operator^(Geometry const& geometry) const
{
       _TPolygon< T > polygonOut;
       boost::geometry::sym_difference(*this, geometry, polygonOut);

       // should not be necessary
       //boost::geometry::correct( polygonOut );

       return polygonOut;
}

[Volker Schöch <vschoech@…>]

Our conversation on the geometry mailing list regarding this issue in 1.48.0: ​http://lists.boost.org/geometry/2012/02/1857.php

[Volker Schöch <vschoech@…>]

The difference algorithm has the same problem which I filed as a separate ticket #8376.

[anonymous]

Still reproducible with boost 1.55.0.

[vschoech@…]

Sorry for "anonymous" in the previous comment. That was me.

[mkaravel]

See attached test program and output, as well as comments in ticket #9828

[mkaravel]

stand-along program for reproducing ticket #8375

[mkaravel]

output of test program for ticket #8375 (corrected)