CGAL users discussion list

[Stephen Wong <>]

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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