id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
10713	Boost Geometry dissolve	jdh@…	Barend Gehrels	"Dear Boost Geometry Contributors

I was testing out the new 1.57 beta therefore also downloaded the latest 
\geometry\extensions\algorithms\dissolve.hpp file.

Testing that I stumbled upon a strange thing which I thought would be interesting for you.

Using the below example:

using namespace boost::geometry;
using namespace boost::geometry::model;
using namespace boost::geometry::model::d2;

int _tmain(int argc, _TCHAR* argv[])
{
  polygon<point_xy<float>> Poly;
  read_wkt(""POLYGON((-0.7189743518829346 4.1308121681213379, 0.0831791982054710 4.1034231185913086, 0.1004156470298767 4.1107301712036133, 0.1044322624802589 4.1026973724365234, 0.0831791982054710 4.1034231185913086, -0.7711903452873230 3.7412264347076416, -0.7189743518829346 4.1308121681213379))"", Poly);

  multi_polygon<polygon<point_xy<float>>> Dissolved;
  boost::geometry::dissolve(Poly, Dissolved);
  return 0;
}

I find that dissolve gives me 2 polygons back with 5 outline points in each. However the input polygon is like two triangles touching each other so I was expecting an output of two polygons with each 4 points.

I then looked at the dissolve.hpp and wondered if it could be because of the no_rescale_policy() specified in the dissolve function. Modified the function to use the rescale policy found in other of your geometry functions:

inline void dissolve(Geometry const& geometry, Collection& output_collection)
{
  concept::check<Geometry const>();
		
  typedef typename boost::range_value<Collection>::type geometry_out;
		
  concept::check<geometry_out>();

  typedef typename geometry::point_type<Geometry>::type point_type;
  typedef typename geometry::rescale_policy_type<point_type>::type  rescale_policy_type;

  rescale_policy_type robust_policy = geometry::get_rescale_policy<rescale_policy_type>(geometry);

  dispatch::dissolve
  <
    typename tag<Geometry>::type,
    typename tag<geometry_out>::type,
    Geometry,
    geometry_out
  >::apply(geometry, robust_policy, std::back_inserter(output_collection));
}

Doing so made the dissolve function return two polygons with each 4 points. 

I then ran a lot of different other tests and none of those failed for me.

I therefore wonder if there is a reason for the no_rescale_policy() or if it is better to use the “robust_policy”

Best regards and thanks a lot for this great library

Johan Doré
"	Bugs	closed	To Be Determined	geometry	Boost 1.57.0	Problem	fixed