CGAL users discussion list

[Laure Guicherd <>]

Andreas Fabri wrote:
> Laure Guicherd wrote:
> > I think you are absolutely right.
> > Let me give you the context : I am working on an advanced mesh 
> > editor, and I want the final user to be able to give any mesh in 
> > parameter for the boolean operations, or, at least, prevent him from 
> > doing this operation if the entry mesh is not correct (i.e., if it 
> > has degenerate geometry, like null-length edges or self-intersections).
> >
> > So I guess I just have to check the integrity of my meshes. Is there 
> > any mean to do this in the Polyhedron or Nef Polyhedron structure ? 
> > Otherwise I'll just do it myself in my geometry structure :)
>
> Hi Laure,
>
> there is one chapter you might have a look at for testing whether a 
> polyhedral
> surface is self intersecting:  Intersecting Sequences of dD 
> Iso-oriented Boxes
>
> http://www.cgal.org/Manual/3.4/doc_html/cgal_manual/Box_intersection_d/Chapter_main.html 
>
>
> Note that it is not bound to the CGAL::Polyhedron, so you might use it on
> your own data structure.
>
> andreas
Thank you all for all your advices and help, I was finally able to get 
over all my issues :)
> > Thank you for your precious help,
> > Regards
> >
> > Laure
> >
> > Peter Hachenberger wrote:
> > > Fred says it perfectly. If you want to get a feasible solution, you 
> > > can only work
> > > with nef3 files to store intermediate results. Otherwise, you will 
> > > get errors.
> > >
> > > If you think that this is not the problem, then send me your 
> > > original tori
> > > files and I have test with those.
> > >
> > > Peter
> > >
> > > -----"Fred Dorosh" 
> > > <>
> > >  wrote: -----
> > >
> > >     To: 
> > >
> > >     From: "Fred Dorosh" 
> > > <>
> > >     Date: 06/15/2009 09:47PM
> > >     Subject: Re: [cgal-discuss] Crash on a Nef_polyhedron_3 boolean
> > >     operation
> > >
> > >     > I am getting really confused with this... because this mean 
> > > that the
> > >     > problem only occurs when I convert my result Nefs back to 
> > > Polyhedron
> > >     > then to my geometry, and then do the opposite process to get
> > >     back to nef
> > >     > structure.
> > >
> > >     Hi Laure,
> > >
> > >     This paragraph sets off warning bells for me - if you are
> > >     converting your
> > >     intermediate results back and forth from your own geometrical
> > >     representation, and that representation uses floating point
> > >     coordinates,
> > >     you're likely to run into trouble.
> > >
> > >     The reason is that very small features can be created as
> > >     byproducts of a
> > >     boolean operation. If you extract vertex coordinates from the 
> > > precise
> > >     geometry kernel and round them into e.g. 32-bit floating point, 
> > > these
> > >     small features often become corrupted and result in degenerate
> > >     geometry.
> > >     For instance, two nearby vertices may become merged and create a
> > >     zero-length edge. Other less detectable problems can also occur.
> > >     If you
> > >     keep the geometry in Nef representation until the final 
> > > operation, you
> > >     avoid this problem.
> > >
> > >     Regards,
> > >     Fred
> > >
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