CGAL users discussion list

[Nicolau Andres Thio <>]

Hello,

I am new to CGAL and have been trying to add polygon intersection to my
code. I have been baffled as to why my code is giving me the results it is;
after doing some extensive search online I have found no mention of a bug
but I am not sure what else it could be (barring a simple mistake I am just
not seeing). I have come up with a very simple example to check the polygon
intersection code is doing what I think it should. Here is my code:

#include <CGAL/Cartesian.h>
#include <CGAL/Polygon_2.h>
#include <CGAL/Polygon_with_holes_2.h>
#include <CGAL/General_polygon_2.h>
#include <CGAL/Exact_predicates_exact_constructions_kernel.h>
#include <CGAL/Boolean_set_operations_2.h>




typedef CGAL::Exact_predicates_exact_constructions_kernel       Rat_Kernel;
typedef Rat_Kernel::Point_2                                             
Rat_Point;
typedef Rat_Kernel::Segment_2                                           
Rat_Segment;
typedef CGAL::Polygon_2<Rat_Kernel>                                     
Rat_Polygon;
typedef CGAL::Polygon_with_holes_2<Rat_Kernel>                          
Rat_Polygon_Holes;

int main(int argc, char** argv){


    Rat_Polygon p, q, result;
    std::list<Rat_Polygon_Holes> polyIterator;

    p.push_back(Rat_Point(1, 1));
    p.push_back(Rat_Point(1, 0));
    p.push_back(Rat_Point(0, 0));
    p.push_back(Rat_Point(0, 1));
    q.push_back(Rat_Point(-1, 0.5));
    q.push_back(Rat_Point(2, 0.5));
    q.push_back(Rat_Point(0, 2));

    CGAL::intersection(q, p, std::back_inserter(polyIterator));
    result = polyIterator.begin()->outer_boundary();
    printf("Intersection polygon vertices: ");
        for (auto vi = result.vertices_begin(); vi != result.vertices_end(); 
++vi){
                printf("(%.2f,%.2f) ", CGAL::to_double((*vi).x()),
CGAL::to_double((*vi).y()));
    }
    printf("\nIntersection polygon edges ");
        for (auto ei = result.edges_begin(); ei != result.edges_end(); ++ei){
                        printf("(%.2f,%.2f) - (%.2f,%.2f) :: ", 
                                CGAL::to_double((*ei).source().x()), 
                                CGAL::to_double((*ei).source().y()),
                                CGAL::to_double((*ei).target().x()), 
                                CGAL::to_double((*ei).target().y()));
        }
    printf("\n\n");
}


The output is not the correct intersection however:

Intersection polygon vertices: (0.00,1.00) (1.00,0.50) (1.00,0.50)
(1.00,1.00) 
Intersection polygon edges (0.00,1.00) - (1.00,0.50) :: (1.00,0.50) -
(1.00,0.50) :: (1.00,0.50) - (1.00,1.00) :: (1.00,1.00) - (0.00,1.00) ::



 The correct intersection polygon should have vertices (0,0.5), (1,0.5),
(1,1) and (0,1). Am I missing something or is this an actual bug?

Thank you very much in advance for your help.

Regards,

Nico



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