CGAL users discussion list

[Andreas Fabri <>]

Joel,

it would be helpful, if you saved the polyhedron just before the join 
operation fails.
Ideally, you provide a minimalistic example, and you tell us on which edge, 
that is
which two points, it fails.  Then we can try to have a look.

andreas


 wrote:
> Thanks for the help, the iteration works now.
> Sorry, I dont really know what an antenna is in the context of a polyhedron.
>
> I will try to clarify what I want to do:
> My input mesh is a triangle mesh, for example a cube.
> This cube consists of 12 triangles, 2 for each side.
> What I want to have is a polyhedron with 6 polygons (quads in this case).
>
> For the simple example of the cube it works well, but when I want to do the 
> same with a triangulated quad consisting of many triangles, there are many 
> edges which can not be joined via polyhedron.join_facet(...).
> I dont really know the reason for that, it would be nice if someone could 
> explain that.
>
> Joel
>
>
> -------- Original-Nachricht --------
> > Datum: Wed, 06 May 2009 21:48:52 +0200
> > Von: "Dr. J.K. Becker" 
> > <>
> > An: 
> >
> > Betreff: Re: [cgal-discuss] Joining faces in polyhedron
>
> > I am not sure if I really understood this Mathieu, but would that not be
> > an antenna which CGAL really does not like at all anyway (CGAL is happy
> > to produce one, but any further operations on a polyhedron with an
> > antenna will fail, at least it does for me)? And no, as far as I know,
> > there are no build in checks for antennas... 
> >
> > Anyway, polyhedra (or polygons) can have holes, no problem with that in
> > CGAL. 
> >
> > Joel, I am not sure I understood what you want to do correctly, but if
> > you want to join all incident facets of a 3D triangulation that have
> > the same normal, you will not join anything really (at least you will
> > most likely never end up with a closed polyhedron)?! Except if you
> > started of with a regular grid of points for the triangulation, but in
> > that case you dont really need to triangulate anything? I am trying to
> > figure out something very similar (and have not solved it) so maybe I am
> > completely wrong....
> >
> > Jens
> >  
> >
> > On Wed, 2009-05-06 at 18:08 +0200, Mathieu Brédif wrote:
> > > Hi,
> > >
> > > If I am not mistaken, polyhedral facets may not have holes in CGAL.
> > > This means that you can only remove edges if they are not adjacent to
> > > the same face on both sides.
> > >
> > > Mathieu
> > >
> > > On Wed, May 6, 2009 at 5:52 PM, Andreas Fabri
> > > <>
> > >  wrote:
> > > > Hi Joel,
> > > >
> > > > You must make a copy of the iterator, increment it and then work on
> > the copy
> > > > in the body of the loop.
> > > > This is because the halfedges are in an in-place list, your join
> > operation
> > > > removes them and the iterator
> > > > no longer points on something reasonable
> > > >
> > > > for ( Halfedge_iterator j = polyhedron.halfedges_begin(); j !=
> > > > polyhedron.halfedges_end(); ){
> > > >  Halfedge_iterator i = j;
> > > >  ++j;
> > > >  // and now your code again
> > > >  .
> > > >  .
> > > > }
> > > >
> > > >
> > > > I think you have even to increment j twice, as the twin halfedges are
> > stored
> > > > consecutively.
> > > > Alternatively you should use an Edge_iterator.
> > > >
> > > >
> > > > andreas
> > > >
> > > >
> > > >
> > > >  wrote:
> > > > > Hi!
> > > > >
> > > > > I am trying to convert a triangle mesh to a polyhedron, i.e. I want
> > to
> > > > > join all faces which are adjacent and have the same normal
> > > > >
> > > > > As an easy example I chose a flat patch consisting of triangles in
> > .off
> > > > > format. As a result I want to have a polyhedron consisting of a
> > single
> > > > > polygon.
> > > > >
> > > > > I was able to load the file, but the follwoing did not work:
> > > > >
> > > > > for ( Halfedge_iterator i = polyhedron.halfedges_begin(); i !=
> > > > > polyhedron.halfedges_end(); ++i) {
> > > > >    if( i->is_border_edge() )
> > > > >        continue;
> > > > >
> > > > >    Polyhedron::Facet_handle f1 = i->facet();
> > > > >    Polyhedron::Facet_handle f2 = i->opposite()->facet();
> > > > >        Halfedge_handle he1 = f1->halfedge();
> > > > >        Point_3 p0 = he1->vertex()->point();
> > > > >    Point_3 p1 = he1->next()->vertex()->point();
> > > > >    Point_3 p2 = he1->next()->next()->vertex()->point();
> > > > >        Kernel::Vector_3 n_i = CGAL::cross_product(p0-p2, p1-p2);
> > > > >    n_i = n_i / sqrt(n_i.squared_length());
> > > > >
> > > > >    Halfedge_handle he2 = f2->halfedge();
> > > > >    p0 = he2->vertex()->point();
> > > > >    p1 = he2->next()->vertex()->point();
> > > > >    p2 = he2->next()->next()->vertex()->point();
> > > > >
> > > > >    Kernel::Vector_3 n_j = CGAL::cross_product(p0-p2, p1-p2);
> > > > >    n_j = n_j / sqrt(n_j.squared_length());
> > > > >
> > > > >    if(n_i == n_j)
> > > > >    {
> > > > >        if( circulator_size(i->opposite()->vertex_begin()) >= 3 &&
> > > > > circulator_size(i->vertex_begin()) >= 3 )
> > > > >            polyhedron.join_facet(i);
> > > > >    }
> > > > > }
> > > > >
> > > > > There seem to be 2 problems: - after joining to faces the iterator
> > becomes
> > > > > invalid
> > > > > - some faces cannot be joined, but why and how can I change that?
> > > > >
> > > > > Can anyone help me please?
> > > > >
> > > > > Have a nice day,
> > > > > Joel
> > > > --
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