CGAL users discussion list

[Andreas Fabri <>]

Hello Martin,

If I understand it right, you would expect that the plane
equation gets recomputed when the off file is loaded.

I don't see that this is supported by the off file format: 
http://www.geomview.org/docs/oogltour.html

Also it would not make sense, that CGAL computes the plane
equation after loading, as you could not have computed
it when you wrote it, or have stored yet another plane.

One solution would be to have an eoff format
( I just come up with this name, with the 'e'
standing for extended), which writes/reads whatever
information added to vertices and faces to the stream.

best regards,

andreas

On 16/03/2010 11:43, Martin Baeker wrote:
> Dear all,
>
> I'm getting a bit desperate while trying to work with Polyhedra-stuff.
>
> The manual says that I can access the plane that belongs to a facet with
> Plane_3& f.plane () if Supports_facet_plane == CGAL::Tag_true
>
> However, I do not understand how this really works.
>
> When I create a Polyhedron, e.g., with the make_cube_3-routine that is
> provided in the examples, I can access the plane corresponding to each
> facet without problems.
> However, when I write this Polyhedron to an off-file and read it in
> again, I cannot access its planes anymore.
>
> Here is a simple test program to show this:
>
> void do_stuff(Polyhedron & pcube)
> {
> for (Halfedge_iterator h=pcube.halfedges_begin(); h !=
> pcube.halfedges_end(); ++h)
> {
> Facet face=*(h->facet());
> std::cout << face.plane().orthogonal_vector() << std::endl;
> }
> }
>
> int main() {
> Polyhedron pcube;
> make_cube_3(pcube);
> std::ofstream os1("poly2.off" ) ;
> os1 << pcube ;
> os1.close();
> do_stuff(pcube);
> std::cout << "\n";
> Polyhedron pcube2;
> std::ifstream is1("poly2.off" ) ;
> is1 >> pcube2;
> do_stuff(pcube2);
> }
>
> The output for pcube is correct; for pcube2, all orthogonal vectors are
> output as 0/1 0/1 0/1.
>
>
> Is there a way to make sure that I can always access the plane
> belonging to a facet? If not, why not? Could I do it by just taking
> three points from the facet and create a plane with them?
>
>
> Any help will be greatly appreciated,
>
> Martin.
>
>
>
>
>
> Priv.-Doz. Dr. Martin Bäker
> Institut für Werkstoffe
> Technische Universität Braunschweig
> Langer Kamp 8
> 38106 Braunschweig
> Germany
> Tel.: 00-49-531-391-3073
> Fax 00-49-531-391-3058
> e-mail 
> <>