CGAL users discussion list

[Chris Hamilton <>]

>> 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.

Yeah, a little bit of a rash statement on my part.  Definitely possible
to come up with counter-examples.  However, the whole point of
Nef_polyhedra is to get around these ugly cases (ie: where the
intersection are or contain surfaces, etc).  Is it possible for two
nef_polyhedron_3 without holes to produce an intersection with holes?

>> 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.

Anybody on this list have any input here?

>>> 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.

Excellent.  Thanks for this.

Chris