CGAL users discussion list

[Asher Kamiraze <>]

Mariette, Sebastien,

Thank you, it works well. I am now facing another problem. I would like to iterate over the ordered boundary vertices of the computer alpha-shape, but I cannot figure how to do it.

You recently answered a similar question, but I do not understand your solution:

"Iterate over REGULAR edges and build the graph of vertices.
From this you should be able to get the contour. You might also
might consider SINGULAR edges to get antennas".

How do you build the graph of vertices? What if I want to build a 3D polyline with the boundary vertices?

Hope you could help.

Regards,

Asher

2011/5/19 Sebastien Loriot (GeometryFactory) <sloriot.ml@gmail.com>

Sorry this is this that I posted in my first
mail + setting Infinity to -1 and UNDEFINED to -2
(that you are missing here).

S.


Asher Kamiraze wrote:

Thank you Sebastien

I finally did that and it seems to work:

 Alpha_shape_2(Dt& dt, NT alpha = 0, Mode m = REGULARIZED)
     :_alpha(alpha), _mode(m),       use_vertex_cache(false), use_edge_cache(false)
 {
     Dt::swap(dt);
     if (dimension() == 2)
     {
         // Compute the associated _interval_face_map
         initialize_interval_face_map();

         // Compute the associated _interval_edge_map
         initialize_interval_edge_map();

         // Compute the associated _interval_vertex_map
         initialize_interval_vertex_map();

         // merge the two maps
         initialize_alpha_spectrum();
     }
 }

Thank you for your help.

Regards,

Asher

2011/5/19 Sebastien Loriot (GeometryFactory) <sloriot.ml <http://sloriot.ml>@gmail.com <http://gmail.com>>

   Asher Kamiraze wrote:

       OK. May this simple hack inspired from the 3D version will work?


   That's what I did too.


   S.


        Alpha_shape_2(Dt& dt, NT alpha = 0, Mode m = REGULARIZED)
          :_alpha(alpha), _mode(m),     use_vertex_cache(false),
       use_facet_cache(false)
          {
            Dt::swap(dt);
            if (dimension() == 2) initialize_alpha();
          }

       Regards,

       Asher

       2011/5/19 Mariette Yvinec <
       <mailto:>

       <mailto: <mailto:>>>

          I see ...
          In 3D you may construct  the Alpha_shape from the Triangulation
          so that you can enter your vertices wih their info
          in the triangulation and then build the Alpha_shape from the
          triangulation.

          In 2D, such a constructor does not seem to exist.
          Obviously, we should add it.

          Regards,
          Mariette


          Le 19/05/11 15:52, Asher Kamiraze a écrit :

              Hi,

              2011/5/19 Mariette Yvinec <
           <mailto:>
              <mailto:
           <mailto:>>>


                  push_back should be private too.


              Yes, that's what I thought.
                                 If you use push_back,
                   the triangulation will be updated but not the
           alpha_shapes...

                  I don't see any problem in what you propose.


              Since I can use neither push_back nor insert, how can I
           attach
              info to a vertex?
              If it is a triangulation, I add a point and get the returned
              vertex, then attach the info on it. With alpha shapes, I
           have to
              give as input a range of points, and do not know how to
           attach
              info of the resulting vertices.

              Regards,

              Asher



                  Le 19/05/11 12:46, Asher Kamiraze a écrit :

                      Hi all,

                      Is there a way to build an alpha_shape_2 with info?
                      Something similar to a triangulation with info.
                      I have also noticed that insert() is private in
                      alpha_shape_2, but, push_back is not. Isn't it a
           problem?

                      Here is what I am trying to do:


                         typedef
                      CGAL::Exact_predicates_exact_constructions_kernel
                                            K;

                         struct vertex_info { Point point3d; };
                         struct face_info {};

                         typedef
                                 CGAL::Triangulation_vertex_base_with_info_2<vertex_info,K>
                                   Vb_2;
                         typedef
                                 CGAL::Constrained_triangulation_face_base_with_info_2<face_info,K>
                           Fb_2;

                         typedef CGAL::Alpha_shape_vertex_base_2<K,
           Vb_2> Vb;
                         typedef CGAL::Alpha_shape_face_base_2<K, Fb_2>
            Fb;

                         typedef
           CGAL::Triangulation_data_structure_2<Vb,Fb>                                               TDS_2;
                         typedef CGAL::Delaunay_triangulation_2<K  ,
           TDS_2>                                         DT_2;

                         typedef CGAL::Alpha_shape_2<DT_2> Alpha_shape_2;


                                 Alpha_shape_2 alpha_shape(0.5);
                                 // How to insert points in alpha_shape
           with
                      attached info?

                      Hope you could help

                      Best regards

                      Asher


                  --         Mariette Yvinec
                  Geometrica project team
                  INRIA  Sophia-Antipolis




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          --     Mariette Yvinec
          Geometrica project team
          INRIA  Sophia-Antipolis  




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