difference of non-closed polygons returns closed polygon

[Volker Schöch <vschoech@…>]

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;
}

[Volker Schöch <vschoech@…>]

Our conversation on the geometry mailing list regarding this issue in 1.48.0: ​http://lists.boost.org/geometry/2012/02/1829.php

[vschoech@…]

Still reproducible in 1.55.

[mkaravel]

stand-alone test program for ticket #8376

[mkaravel]

output of test program for ticket #8376

[mkaravel]

Please see attached test program and output. See also comments for ticket #9828.

[mkaravel]

Replace irrelevant attachment by an empty file

[mkaravel]

The attachment with file name ticket_8375.cpp was uploaded by error, and to avoid any confusion is has been replaced by an empty file.