[Boost-bugs] [Boost C++ Libraries] #8376: difference of non-closed polygons returns closed polygon

Subject: [Boost-bugs] [Boost C++ Libraries] #8376: difference of non-closed polygons returns closed polygon
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2013-04-03 13:13:46

#8376: difference of non-closed polygons returns closed polygon
-----------------------------------------------------------------------+----
 Reporter:  Volker Schöch <vschoech@…>                                 |       Owner:  barendgehrels
     Type:  Bugs                                                       |      Status:  new          
Milestone:  To Be Determined                                           |   Component:  geometry     
  Version:  Boost 1.52.0                                               |    Severity:  Problem      
 Keywords:  difference, multi-polygon, polygon, closed, open, correct  |  
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 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;
 }
 }}}

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/8376>
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