CGAL users discussion list

["Sebastien Loriot (GeometryFactory)" <>]

sander wrote:
> Dear cgal!
> I have an application where I need to compute the intersections of small
> polyhedra, mainly tetrahedra and hexahedra.  I have implemented a solution
> using Polyhedron_3 and Nef_Polyhedron_3, and it works like a charm.  The
> problem is that I have to compute many such intersections (think hundreds of
> thousands, these polyhedra are elements in finite element grids). 
> Therefore, speed is important, and the price of exact number types is too
> high.  On the other hand, perfect precision is not needed, and therefore I
> want to use the builtin 'double' type.  I was hoping that if there is a
> stability problem in a degenerate case that would throw an exception (is
> that true). I would catch that and use an exact type then.
>
> The problem is that the code doesn't compile with the kernel
> CGAL::Cartesian.  Nef_Polyhedron expects the number type to be a quotient. 
> This appears to be a known issue, at least it is mentioned here:
> http://cgal-discuss.949826.n4.nabble.com/Operator-Overloading-problem-for-Vector-in-Gmpzf-Kernel-td2548286.html
> Therefore I don't submit a formal bug report (yet).
>
> - is there a workaround?  Some trick that I can do to get Nef_Polyhedra with
> the speed of 'double'?
> - Is this considered a bug that should be fixed?
> - I am using cgal 3.6.1, the one in current Debian.  Is it maybe even fixed
> in later versions?
>
> Thank you for your help
> Oliver Sander
> FU Berlin
>
> --
> View this message in context: 
> http://cgal-discuss.949826.n4.nabble.com/Using-Nef-polyhedra-with-builtin-double-number-type-tp3382392p3382392.html
> Sent from the cgal-discuss mailing list archive at Nabble.com.
try with Simple_cartesian< Quotient< Interval_nt<true> > > and catch 
CGAL::Uncertain_conversion_exception.

Note that if at some point you have a non triangular face (even in the 
intersection) it will highly probably fail as coplanarity test are often
done.

see documentation of Interval_nt here:
http://www.cgal.org/Manual/latest/doc_html/cgal_manual/NumberTypeSupport_ref/Class_Interval_nt.html

S.