CGAL users discussion list

["Wayne C. Gramlich" <>]

Chris Hamilton wrote:
> > The .off file format can not represent all Nef3 polyhedra
> > objects.  In particular, if you have a surface with one
> > or more holes in it, .off format can not represent it.
>
> Makes sense.  However, the intersection of any two closed and bounded
> polyhedra should itself be a closed and bounded polyhedron, which should
> be representable by the .off file format.  Whatever the degeneracy
> implied by "intersecting constraints", it should be representable in
> Polyhedron_3 objects (and hence .off files) somehow, no?

I'm not sure what you say is true.  I can easily come up with two
.off files which when intersected would not be representable in
.off format.

> My actual question is this: what does 'intersecting constraints' mean,
> and why is not representable by Polyhedron_3 objects (and hence .off
> files)?  Is this something that is an error, is this something that is
> possible but not yet implemented, or is this something that can't be
> implemented for some reason?

Good question.  Alas, I do not know the answer.

> > There is a file format called .nef3 that can represent all
> > .nef3 objects.  The demo program ../dem/Nef_3/nef_3_homgeneous
> > can read .nef3 objects.  It used to be able to write .nef3
> > objects but that code was removed for some reason.  I have
> > a snippet of code that adds back the "savenef3" command
> > to the demo program if you are interested.
>
> Are these files viewable somehow, or is only the internals of CGAL that
> can make use of them?

Yes.  The unmodified nef_3_homgeneous program can read,
intersect, and display both .off and .nef3 files.  The only
thing it can not do (currently) is write .nef3 files.  For
example:

  nef_3_homogeneous loadoff file1.off loadoff file2.off int reg vis

will read in file1.off, followed by file2.off, compute the
intersection, regularize it, and then display the result in
a visualization window.

-Wayne