CGAL users discussion list

[Peter Hachenberger <>]

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
> 
>