polygon_data_set::insert creates unwanted polygon point

[todeschini-felipe]

I want to represent a donut shape with boost::polygon, such as the figure below, with the following points:

10 polygon : (0, 0), (10, 0), (10, 10), (8, 10), (8, 2), (2, 2), (2, 8), (8, 8), (8, 10), (0, 10),

Once I include the polygon into a polygon set data, however, it creates an additional unwanted point (8,8) from (8,10)->(8,2). Even boost::polygon::simplify leaves this point. Is that the expected behavior?

12 insert : (10, 10), (8, 10), (8, 8), (8, 2), (2, 2), (2, 8), (8, 8), (8, 10), (0, 10), (0, 0), (10, 0), (10, 10),
12 simplf : (10, 10), (8, 10), (8, 8), (8, 2), (2, 2), (2, 8), (8, 8), (8, 10), (0, 10), (0, 0), (10, 0), (10, 10), 

#include <iostream>

#include <boost/geometry.hpp>
#include <boost/geometry/geometries/linestring.hpp>
#include <boost/geometry/geometries/point_xy.hpp>
#include <boost/polygon/polygon.hpp>

#include <boost/assign.hpp>

using namespace boost::assign;
namespace gtl = boost::polygon;
using namespace boost::polygon::operators;

//--------------------------------------------------------------------------------
// CPoint
//--------------------------------------------------------------------------------
struct CPoint {
  //CPoint() : x(0), y(0) {}
  //CPoint(int a, int b) : x(a), y(b) {}

  int x;
  int y;
};

namespace boost::polygon {
template <>
struct geometry_concept<CPoint> { typedef point_concept type; };
template <>
struct point_traits<CPoint> {
  typedef int coordinate_type;

  static inline coordinate_type get(const CPoint& point, orientation_2d orient) {
    return ((orient == HORIZONTAL) ? point.x : point.y);
  }
};

template <>
struct point_mutable_traits<CPoint> {
  typedef int coordinate_type;

  static inline void set(CPoint& point, orientation_2d orient, int value) {
    if(orient == HORIZONTAL)
      point.x = value;
    else
      point.y = value;
  }
  static inline CPoint construct(int x_value, int y_value) {
    CPoint retval;
    retval.x = x_value;
    retval.y = y_value;
    return retval;
  }
};
}

//--------------------------------------------------------------------------------
// CPolygon
//--------------------------------------------------------------------------------
typedef std::list<CPoint> CPolygon;

namespace boost::polygon {
template <>
struct geometry_concept<CPolygon>{ typedef polygon_concept type; };

template <>
struct polygon_traits<CPolygon> {
  typedef int coordinate_type;
  typedef CPolygon::const_iterator iterator_type;
  typedef CPoint point_type;

  static inline iterator_type begin_points(const CPolygon& t) { return t.begin(); }
  static inline iterator_type end_points(const CPolygon& t) { return t.end(); }
  static inline std::size_t size(const CPolygon& t) { return t.size(); }
  static inline winding_direction winding(const CPolygon& t) { return unknown_winding; }
};

template <>
struct polygon_mutable_traits<CPolygon> {
  template <typename iT>
  static inline CPolygon& set_points(CPolygon& t, iT input_begin, iT input_end) {
    t.clear();
    while(input_begin != input_end) {
      t.push_back(CPoint());
      gtl::assign(t.back(), *input_begin);
      ++input_begin;
    }
    return t;
  }
};
}

std::ostream &operator<<(std::ostream &os, const CPoint &p) {
  return os << "(" << p.x << ", " << p.y << ")";
}

std::ostream &operator<<(std::ostream &os, const CPolygon &p) {
  for (const auto point : p)
    os << point << ", ";
  return os;
}


int main()
{
  CPolygon donut{
    CPoint{0, 0}, CPoint{10,0}, CPoint{10, 10},
    CPoint{8, 10}, CPoint{8, 2},
    CPoint{2, 2}, CPoint{2, 8}, CPoint{8, 8},
   CPoint{8,10}, CPoint{0,10} }; 

  std::cout << donut.size() << "  donut : " << donut << std::endl;

  boost::polygon::polygon_set_data<int> ps;
  std::vector<CPolygon> polygons;
  ps.insert(donut);
  ps.get(polygons);
  for (const auto &pl : polygons)
    std::cout << pl.size() << " insert : " << pl << std::endl;

  polygons.clear();
  boost::polygon::simplify(ps, 1e-3);
  ps.get(polygons);
  for (const auto &pl : polygons)
    std::cout << pl.size() << " simplf : " << pl << std::endl;

	return 0;
}

[by408]

ps.get(polygons); function applies a union operation to the polygons. This is by definition. If you add overlapping polygons and call get function you will get merged results, it won't match your original input.