CGAL users discussion list

[Peter Hachenberger <>]

This function converts one shell of the polyhedron into a Polyhedron_3.
So, if that helps you, you can convert the shells separately into
Polyhedron_3s and work on them.

Peter

On Tue, 2008-01-08 at 17:45 -0800, Stephen Wong wrote:
> Hi Peter,
> 
> Do you know anything about the undocumented function 
> convert_inner_shell_to_polyhedron() in Nef_polyhedron_3?  I am still 
> looking for a way to iterate through shells, and came across this function 
> in Nef_polyhedron_3.h.
> 
> Thank you again,
> 
> Stephen
> 
> -----Original Message-----
> From: Peter Hachenberger 
> [mailto:]
> Sent: January-08-08 1:31 PM
> To: 
> 
> Subject: RE: [cgal-discuss] Shell-like iteration in Polyhedra class
> 
> That's right. There is no notion of separate boundaries in Polyhedron_3.
> 
> On Tue, 2008-01-08 at 13:17 -0800, Stephen Wong wrote:
> > Hi Peter,
> >
> > Thanks for your response.
> >
> > Just to reaffirm what you have said - Polyhedron_3 has no way of 
> > differentiating between facets when there is more than one 2-manifold 
> > boundary (in a single Polyhedron_3).  Or, put in another way, 
> > Polyhedron_3 does not provide a means of iterating through facets of 
> > individual boundaries.
> >
> > Stephen
> >
> > -----Original Message-----
> > From: Peter Hachenberger 
> > [mailto:]
> > Sent: January-08-08 12:32 PM
> > To: 
> > 
> > Subject: Re: [cgal-discuss] Shell-like iteration in Polyhedra class
> >
> > Hi Stephen,
> >
> > the Polyhedron_3 data structure does not provide iteration over shells.
> > The reason is that Polyhedron_3 is designed to provide the typical mesh
> > operations and nothing else. From a mesh point of view shells and
> > volumes are not needed - you only think of boundaries (usally exactly
> > one two-manifold boundary). The advantage is, that the data structure of
> > Polyhedron_3 is not as complicated as the data structure of
> > Nef_polyhedron_3. It can be handled more efficiently.
> >
> > Peter
> >
> > On Tue, 2008-01-08 at 10:51 -0800, Stephen Wong wrote:
> > > Hello,
> > >
> > >
> > >
> > > Nef Polyhedra has a well-defined set of volume and shell iterators.
> > > However, after converting Nef Polyhedra back to B-rep Polyhedra using
> > > Nef_polyhedron_3<>::convert_to_Polyhedron() (after a Boolean
> > > operation, for example), the Polyhedron class does not have similar
> > > iterators.
> > >
> > >
> > >
> > > How can I iterate the faces between different “volumes” or “shells” in
> > > a Polyhedron?
> > >
> > >
> > >
> > > A concrete example:
> > >
> > > Nef polyhedron N1 is a box with lower left (0,0,0) and upper right
> > > (3,3,3)
> > >
> > > Nef polyhedron N2 is a box with lower left (1,1,1) and upper right
> > > (2,2,2)
> > >
> > > N1 -= N2 ; // difference boolean operator
> > >
> > > N1.convert_to_Polyhedron() ;
> > >
> > >
> > >
> > > This resultant Polyhedron has an outer shell and an inner shell
> > > bounding the void inside the cube.  I want to access facets for each
> > > shell of the Polyhedron.  The function facets_begin() provided in the
> > > Polyhedron class iterates through all facets in the polyhedron, i.e.
> > > all shells without distinction.
> > >
> > >
> > >
> > > Thanks,
> > >
> > >
> > >
> > > Stephen Wong
> > >
> > >
> >
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> >
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