CGAL users discussion list

i found a possible bug in CGAL::intersection when i try to intersect two 
objects of type Triangle_2. in the case where the intersection should be a 
triangle something doesn't work properly, the result is that i the function 
returns no object.

i'm using CGAL version 3.1, but as far as i can see from the list of changes 
this problem hasn't been resolved in version 3.3

i use microsoft visual c++ compiler.

the following code should return a Triangle_2 when i try intersecting the 
triangles {(0,0),(1,0),(0,1)} and {(0.1,0.1),(1.1,0.1),(0.1,1)}.



#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>
#include <CGAL/intersections.h>
#include <CGAL/Polygon_2.h>

#include <fstream>

struct K : CGAL::Exact_predicates_inexact_constructions_kernel {};

typedef K::Point_2   Point;

int main()
{
        CGAL::Triangle_2<K> aTri,aTri2,inters_tri;
        CGAL::Object result;
        Point   inters_pt;
        CGAL::Segment_2<K> inters_seg;
        CGAL::Polygon_2<K> aPoly;
        std::vector<Point>      intersections,inters_pts;
                                        
        aTri = CGAL::Triangle_2<K>(Point(0,0),Point(1,0),Point(0,1));
        aTri2 = 
CGAL::Triangle_2<K>(Point(0.1,0.1),Point(1.1,0.1),Point(0.1,1));
        std::cout<<"Triangle 1: "<<aTri<<"  Triangle 2: "<<aTri2<<std::endl;
        result = CGAL::intersection(aTri2,aTri);
        if (CGAL::assign(inters_pt, result)){
                std::cout<<"Point\n";
        }else if (CGAL::assign(inters_pts,result)){
                std::cout<<"multiple Points\n";
        }else if (CGAL::assign(inters_seg,result)){
                std::cout<<"Segment\n";
        }else if (CGAL::assign(inters_tri,result)){
                std::cout<<"Triangle\n";
        }else{
                std::cout<<"No intersection\n";
        }

  return 0;
}


as a fix i made a change to the file triangle_2_triangle_2_intersection.c 
such that the case when a triangle should be returned is treated as if it 
would be a polygon (in which case a vector of points is returned). around 
line 380 i made the following change:


    case is_t::TRIANGLE: {
 /*       typename K::Triangle_2 itr;
        ispair.intersection(itr);
        return make_object(itr);
*/        typedef std::vector<typename K::Point_2> Container;
        Container points(ispair.vertex_count());
        for (int i =0; i < ispair.vertex_count(); i++) {
            points[i] = ispair.vertex(i);
        }
        return make_object(points);
    }

unless this issue is a result of a problem in my code (e.g. type of 
'inters_tri') i believe this is a bug in version 3.1.

best regards
matthias preisig