CGAL users discussion list

[Andreas Fabri <>]

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 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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