CGAL users discussion list

[Jean-Philippe Pons <>]

Thanks very much Sebastien. Your patch solves my problem. In particular 
it allows to implement a loader which is resilient to non-manifold 
polyhedra, with a user warning mentioning the number of discarded facets.

Best wishes,

Jean-Philippe

Sebastien Loriot (GeometryFactory) a écrit :
> Thanks for your bug report Jean-Philippe.
> Here is a patch and the modified file that should fix the problem you
> have encountered. Let me now whether you still have a problem.
>
> This bug will be fixed in the 3.6.1 bug fix release of CGAL.
>
> Best,
>
> S.
>
> Jean-Philippe Pons wrote:
> > Dear Sebastien,
> >
> > No. The case you mention may easily happen during the construction of a
> > manifold mesh, depending on the order of insertion of facets. As
> > expected, it is accepted by test_facet and do not cause the failure of
> > add_facet either.
> >
> > The case I have in mind is a little more complex: it happens when a
> > novel facet closes a cycle of facets while there already was another
> > facet incident to the central vertex of the created cycle.
> >
> > You can reproduce the aforementioned case with the piece of code given
> > below. The "Error in polyhedron construction" message should never
> > happen since all facets are tested before being inserted. But it happens
> > in practice when adding the fourth facet.
> >
> > Cheers,
> >
> > Jean-Philippe
> >
> > --------------
> > CGAL::Polyhedron_incremental_builder_3<HDS> B(hds, true);
> >
> > int nb_vertices = 6;
> > int f[] = {0,1,2, 0,2,3, 0,4,5, 0,3,1};
> > int nb_triangles = sizeof(f) / sizeof(int) / 3;
> >
> > B.begin_surface(nb_vertices, nb_triangles);
> >                for (int i=0; i<nb_vertices; i++)
> >    B.add_vertex(Point(0,0,0));
> >
> > for (int i=0; i<nb_triangles; i++)
> > {
> >    int *first = f+3*i, *beyond = first+3;
> >    if (B.test_facet(first, beyond))
> >    {
> >      cout << "Facet can be added" << endl;
> >      B.add_facet(first, beyond);
> >      if (B.error())
> >      {
> >        cout << "Error in polyhedron construction" << endl;
> >        break;
> >      }
> >    }
> >    else
> >      cout << "Facet cannot be added" << endl;
> > }
> >
> > B.end_surface();
> > -------------
> >
> > Sebastien Loriot (GeometryFactory) [via cgal-discuss] a écrit :
> >
> >  > Hello Jean-Philippe,
> >  >
> >  > Jean-Philippe Pons wrote:
> >  >
> >  >  > Dear CGAL developers,
> >  >  >
> >  >  > test_facet may return 'true' when testing a facet which creates a
> >  >  > non-manifold vertex. So that a subsequent call to add_facet will
> >  >  > unexpectedly fail.
> >  >  >
> >  >  > This is because test_facet only checks if one of the vertices of 
> > the
> >  > novel
> >  >  > facet already has a full cycle of facets, which is not 
> > sufficient. It
> >  > does
> >  >  > not check if the novel facet closes a cycle, while there is another
> >  >  > component of facets.
> >  > Do you mean for example:
> >  > the HDS contains only one triangle. Then we insert a new triangle
> >  > incident to only one vertex of the first triangle?
> >  >
> >  >  >
> >  >  > I have checked that the problem still exists in the latest version.
> >  >  >
> >  >  > Cheers,
> >  >  >
> >  >  > Jean-Philippe
> >  >  >
> >  >
> >  >
> >  > --
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> >  >
> >  >
> >  > 
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> > Polyhedron_incremental_builder_3::test_facet 
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