CGAL users discussion list

[Guillaume Damiand <>]

Le 01/10/2011 20:24, Josh Stratton a écrit :
> The first is a patch of four quads (say facet A, B, C, and D labeled
> from top-left, top-right, bottom-left, bottom-right).  This is a
> 2-manifold surface that can be represented in a polyehdron_3, correct?
> So if face B is removed, it is still a 2-manifold polyedron.

Yes.

> What would happen in CGAL if facet C is then removed?  Would that result in
> a run-time error?


No. In Polyhedron_3 the external face is always represented, thus you 
will obtain the "correct result", since the internal represented object 
has three faces (A, D and the external one) and thus is a 2-manifold.



> On Sat, Oct 1, 2011 at 9:35 AM, Guillaume Damiand
> <>
>   wrote:
> > Le 01/10/2011 00:05, Josh Stratton a écrit :
> > > I had a quick question regarding the half-edge data structure present
> > > in CGAL.  I've read over the documentation several times and
> > > understand that the mesh must be 2-manifold and doesn't support holes
> > > inside facets, which is fine.  What I couldn't find however, was
> > > whether the structure supports singular vertices, where one vertex is
> > > attached to multiple non-joining facets.
> > >
> > > *---*---*
> > > |     |    |
> > > *---*---*
> > > |     |    |
> > > *---*---*
> > >
> > >
> > >
> > > *---*
> > > |     |
> > > *---*---*
> > >        |    |
> > >        *---*
> > >
> > > For the vertex in the middle bridging the two facets, it will only
> > > point to an edge on one of the two faces.  Looking over the API, I
> > > don't see anything that allows lookup of the other face given just the
> > > vertex.  Is this not a supported mesh configuration in CGAL's
> > > half-edge implementation, or is it okay, but requires additional code
> > > to handle these cases?
> > I don't understand your first example.
> >
> > The second one is not a 2-manifold, thus it can not be represented by a
> > Polyhedron_3 (by definition).
> >
> > You can  add the surrounding face englobing your two squares =>  you obtain a
> > 2 manifold and thus one vertex (this is probably always the case for
> > Polyhedron_3 but I am not sure...)


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