CGAL users discussion list

[Peter Hachenberger <>]

On Tue, 2008-02-05 at 22:41 +0800, Max wrote:
> >> I know nothing about indexed item before. But it works quite well.
> >
> >That's good to know. Than there is an alternative to
> >Exact_construction_excat_predicate_kernel. I'm curious about the
> >performance.
> 
> I've tried with both Exact_predicates_exact_constructions_kernel and
> Exact_predicates_inexact_constructions_kernel, with CGAL::SNC_indexed_items,
> but neither even passed compile (on VS2003/SP1 or VS2005/SP1).

That's not good. The SNC_indexed_items where written to make the use of
the Exact_predicates_exact_constructions_kernel efficient. What's the
compile error?

Don't use inexact kernels with Nef_3.

> >> One more question, the program output 'Constructor not available for 
> >> this Kernel'
> >> What does it mean? Could this message be ignored?
> >
> >This means that you called the constructor for creating a halfspace
> >while you use a standard kernel. This constructor has can not work
> >without an extended kernel.
> 
> Yes, after a deeper look, I realized that this message was issued by
> a statement
> 
> Nef_polyhedron_3 N1(pl, Nef_polyhedron_3::INCLUDED);
> 
> with
> typedef CGAL::Homogeneous<CGAL::Gmpzf>        Kernel;
> typedef CGAL::Nef_polyhedron_3<Kernel, CGAL::SNC_indexed_items> 
> Nef_polyhedron_3;
> 
> Yes, I just noticed the fact that all examples in the manual that use Nef_3
> constructor this way are all using the extended kernel.
> 
> 1) I changed the kernel from Homogeneous<CGAL::Gmpzf> to 
> Extended_homogeneous<CGAL::Gmpzf>
> and had another run. But this time I got a runtime error. It happened in 
> the main program 
> at this statement:
> 
>       Nef_polyhedron_3 N(P);

Both extended kernels do not work together with SNC_indexed_items, and
therefore also not with Gmpzf.

> 2) Despite of this problem, I recommend you add an explicit statement into 
> the doc
> regarding this requirement/restriction (if there was not). :-)

Ok, it misses in the reference manual, but if you read the user manual
on Nef_3 (the section on Infimaximal boxes), then you realize that the
constructor will not work.

Peter