id,summary,reporter,owner,description,type,status,milestone,component,version,severity,resolution,keywords,cc
8376,difference of non-closed polygons returns closed polygon,Volker Schöch <vschoech@…>,Barend Gehrels,"The output multi-polygon from boost::geometry::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.

''Note: The sym_difference algorithm has the same problem which I filed as a separate ticket #8375.''

This is the data used as input to difference:

{{{
_TPolygon<int> const polygon =
	_TPolygon<int>(""MULTIPOLYGON(((1920 1462,3720 1462,3720 3262,1920 3262)))"")
	- _TPolygon<int>(""MULTIPOLYGON(((1918 1957,1918 1657,2218 2189)),((3718 1957,3360 2233,3718 1561)),((3360 2233,2818 3253,2218 2189,2818 2653)))"");
}}}

This is the resulting multi-polygon, with the first contained polygon being closed:

{{{
MULTIPOLYGON(((1920 1660,1920 1462,3720 1462,3720 3262,1920 3262,1920 1958,2218 2189,1920 1660),(3718 1561,3360 2233,3718 1957),(2818 2653,2218 2189,2818 3253,3360 2233)))
}}}

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
{
	// should not be necessary
	//if( boost::geometry::area(geometry)==0 ) return *this;

	_TPolygon< T > polygonOut;
	boost::geometry::difference(*this, geometry, polygonOut);

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

	return polygonOut;
}
}}}",Bugs,closed,Boost 1.56.0,geometry,Boost 1.55.0,Problem,fixed,"difference, multi-polygon, polygon, closed, open, correct",mkaravel