CGAL users discussion list

[Luoyu <>]

Hi,
    I want to compute difference of the given polygons. And I found the
CGAL::difference() to do it. But in some cases, the difference return
incorrect result polygon.

Here is my test code and data.

#include <iostream>
#include "print_utils.h"

#include <CGAL/Exact_predicates_exact_constructions_kernel.h>
#include <CGAL/Boolean_set_operations_2.h>
#include <list>
typedef CGAL::Exact_predicates_exact_constructions_kernel Kernel;
typedef Kernel::Point_2                                   Point_2;
typedef CGAL::Polygon_2<Kernel>                           Polygon_2;
typedef CGAL::Polygon_with_holes_2<Kernel>               
Polygon_with_holes_2;
typedef std::list<Polygon_with_holes_2>                   Pwh_list_2;

Polygon_2 MakePolygon(const std::list<Point_2>& Polygon)
{
        Polygon_2 resultPolygon;
        for (std::list<Point_2>::const_iterator it = Polygon.cbegin(); it !=
Polygon.cend(); it++)
        {
                resultPolygon.push_back(*it);
        }

        return resultPolygon;
}



int main()
{
        Polygon_2 sourcePolygon;
        sourcePolygon.push_back(Point_2(-369.040009, 481.649994));
        sourcePolygon.push_back(Point_2(-369.040009, -439.420013));
        sourcePolygon.push_back(Point_2(220.240005, -439.420013));
        sourcePolygon.push_back(Point_2(220.240005, 481.649994));

        if (sourcePolygon.is_clockwise_oriented()) {
                sourcePolygon.reverse_orientation();
        }

        std::list<Point_2> hole_1{ Point_2(220.240005, -121.932671), 
                                                           
Point_2(220.240005, -439.420013), 
                                                           
Point_2(-369.040009, -439.420013)
        };

        std::list<Point_2> hole_2{ Point_2(-369.040009, 481.649994),
                                                           
Point_2(220.240005, 481.649994),
                                                           
Point_2(220.240005, 186.445908)
        };

        std::list<Point_2> hole_3{      Point_2(-369.040009, 481.649994),
                                                                
Point_2(-369.040009, -439.420013),
                                                                
Point_2(220.240005, -439.420013),
                                                                
Point_2(220.240005, 481.649994)
        };

        std::list<Polygon_2> allHoles;
        allHoles.push_back(MakePolygon(hole_1));
        allHoles.push_back(MakePolygon(hole_2));
        allHoles.push_back(MakePolygon(hole_3));

        for (Polygon_2 currentHole : allHoles)
        {
                if (currentHole.is_clockwise_oriented()) {
                        currentHole.reverse_orientation();
                }

                Pwh_list_2 difference;

                CGAL::difference(sourcePolygon, currentHole,
std::back_inserter(difference));
                if (difference.size() != 1) {
                        return -1;
                }

                sourcePolygon = difference.front().outer_boundary();
                if (sourcePolygon.is_clockwise_oriented()) {
                        sourcePolygon.reverse_orientation();
                }
        }

        print_polygon(sourcePolygon);

        return 0;
}



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