[Boost-bugs] [Boost C++ Libraries] #9768: difference of multi-polygon and box returns multi-polygon with redundant points

Subject: [Boost-bugs] [Boost C++ Libraries] #9768: difference of multi-polygon and box returns multi-polygon with redundant points
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2014-03-10 15:09:31

#9768: difference of multi-polygon and box returns multi-polygon with redundant
points
-------------------------------------------------+-------------------------
 Reporter:  Volker Schöch <vschoech@…>           |      Owner:
     Type:  Bugs                                 |  barendgehrels
Milestone:  To Be Determined                     |     Status:  new
  Version:  Boost 1.55.0                         |  Component:  geometry
 Keywords:  difference, multi-polygon, polygon,  |   Severity:  Problem
  box, unique                                    |
-------------------------------------------------+-------------------------
 boost::geometry::difference should not add redundant points to a polygon.
 In the following example I subtract an empty box from a polygon, and the
 result ends up representing the empty box even though obviously it should
 not. My polygon type is '''oriented counter-clockwise''' and '''not
 closed''', my point type is based on '''int'''.

 This is the data used as input to difference:
 {{{
 _TPolygon<int> const polygon =
         _TPolygon<int>( "MULTIPOLYGON(((249 509,5103 509,5103 3566,249
 3566)))" )
         - _TRect<int>( "POLYGON((1220 1019,1220 1019,1220 1019,1220
 1019,1220 1019))" );
 }}}

 This is the resulting multi-polygon, with the additional/redundant empty
 box:
 {{{
 MULTIPOLYGON(((249 509,5103 509,5103 3566,249 3566),(1220 1019,1220
 1019,1220 1019)))
 }}}

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

         // the following line fixes the problem but should not be
 necessary
         //boost::geometry::unique( polygonOut );

         return polygonOut;
 }
 }}}

 These are the traits as defined to use _TRect<...> as a boost::geometry
 box:
 {{{
 namespace boost { namespace geometry { namespace traits {

         template<typename T>
         struct tag< _TRect<T> >
         {
                 typedef box_tag type;
         };

         template<typename T>
         struct point_type< _TRect<T> >
         {
                 typedef _TPoint<T> type;
         };

         template<typename T, std::size_t Dimension>
         struct indexed_access< _TRect<T>, min_corner, Dimension >
         {
                 typedef typename geometry::coordinate_type< _TPoint<T>
 >::type coordinate_type;

                 static inline coordinate_type get(_TRect<T> const& rect)
                 {
                         return geometry::get<Dimension>( rect.TopLeft() );
                 }

                 static inline void set(_TRect<T>& rect, coordinate_type
 const& value)
                 {
                         geometry::set<Dimension>( rect.TopLeft(), value );
                 }
         };

         template<typename T, std::size_t Dimension>
         struct indexed_access< _TRect<T>, max_corner, Dimension >
         {
                 typedef typename geometry::coordinate_type< _TPoint<T>
 >::type coordinate_type;

                 static inline coordinate_type get(_TRect<T> const& rect)
                 {
                         return geometry::get<Dimension>(
 rect.BottomRight() );
                 }

                 static inline void set(_TRect<T>& rect, coordinate_type
 const& value)
                 {
                         geometry::set<Dimension>( rect.BottomRight(),
 value );
                 }
         };

 } } } // namespace boost::geometry::traits
 }}}

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/9768>
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