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[Efi Fogel <>]

Yes.
The do_intersect() of polygons (or polygons with holes) function comes from the "2D Regularized Boolean Set-Operations".
It is consistent with the intersection() function that comes from the same package.
They both perform regularized operations.
The do_intersect() and intersection() functions of kernel objects, such as triangles, are provided by the "2D and 3D Geometry Kernel" package.
It's a bit unfortunate that they all reside in one namespace, namely cgal global namespace, as the latter 2 have different semantics.
We may fix this in the future by either moving the "regularized" functions to a different namespace or renaming them.

On Wed, Nov 14, 2012 at 6:00 PM, Stefan Schirra <> wrote:

On 14.11.2012 08:34, Ophir Setter wrote:

I checked the manually the also the "intentions" of the code.
The function do_intersect should return true if the two polygons "intersect in
their interior".

Ophir

OK, I see, the special case of (General_)Polygon(_with_holes)_2 and the general case of do_intersect(obj1, obj2) are handled differently, for whatever reasons.
So if you intersect triangles (like

  Polygon_2 P;
  P.push_back (Point_2 (0,0));
  P.push_back (Point_2 (1,0));
  P.push_back (Point_2 (0,1));

  Polygon_2 Q;
  Q.push_back(Point_2 (1,0));
  Q.push_back(Point_2 (1,1));
  Q.push_back(Point_2 (0,1));

) as triangles, you get different results?

On Wed, Nov 14, 2012 at 9:28 AM, Stefan Schirra

<-magdeburg.de <mailto:-magdeburg.de>> wrote:

    On 14.11.2012 04:11, Graviton wrote:

        I think that there is a bug with the method do_intersect.

        Test this sample:


        /*! \file do_intersect.cpp
           * Determining whether two triangles intersect.
           */

        #include <CGAL/Exact_predicates_exact___constructions_kernel.h>
        #include <CGAL/Boolean_set_operations___2.h>

        typedef CGAL::Exact_predicates_exact___constructions_kernel Kernel;

        typedef Kernel::Point_2                                   Point_2;
        typedef CGAL::Polygon_2<Kernel>                           Polygon_2;

        #include "print_utils.h"

        int main ()
        {
            Polygon_2 P;
            //P.push_back (Point_2 (-1,1));
            //P.push_back (Point_2 (0,-1));
            //P.push_back (Point_2 (1,1));

              P.push_back (Point_2 (13371696.6813195, -23135667.773556));

            P.push_back (Point_2 (13368240.604 <tel:%2813368240.604>, -23133412.8));
            P.push_back (Point_2 (13364140.624 <tel:%2813364140.624>,
        -23133192.28 <tel:23133192.28>));

              P.push_back (Point_2 (13368823.9405681, -23134726.6078103));
            std::cout << "P = "; print_polygon (P);

            Polygon_2 Q;
            //Q.push_back(Point_2 (-1,-1));
            //Q.push_back(Point_2 (1,-1));
            //Q.push_back(Point_2 (0,1));

              Q.push_back(Point_2 (13375139.672 <tel:%2813375139.672>,

        -23136795.756));
            Q.push_back(Point_2 (13371696.6813195, -23135667.773556));
            Q.push_back(Point_2 (13371696.682, -23135667.774));

                Q.push_back(Point_2 (13368823.9405681, -23134726.6078103));
            Q.push_back(Point_2 (13298835.453, -23111797.193));

            Q.push_back(Point_2 (13374018.867 <tel:%2813374018.867>,

        -23140216.837));
            std::cout << "Q = "; print_polygon (Q);

            if ((CGAL::do_intersect (P, Q)))
              std::cout << "The two polygons intersect in their interior." <<
        std::endl;
            else
              std::cout << "The two polygons do not intersect." << std::endl;

            return 0;
        }


        If you notice carefully, the P and Q are not intersecting each other ( two
        points of P and Q are the same, and the other two points of P are located on
        the RHS of Q).

        But from the above program, the CGAL:do_intersect returns a true. It should
        return a false.


    A quote from the manual:
    "do_intersect(obj1, obj2) checks whether obj1 and obj2 intersect. Two
    objects obj1 and obj2 intersect if there is a point p that is part of both
    obj1 and obj2..."

    So if P and Q have two common points, as you say, true is the correct answer.

    best regards

             Stefan



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