CGAL users discussion list

[Sylvain Pion <>]

Jens K. Becker wrote:
> Hi all,
>
> I am very happy to give any information you need to solve the problem 
> (and am
> quite sure that in the end I did something wrong somewhere). Here is 
> what I am
> trying to do:
>
> 1. Make a polyhedron
> 2. Delete all halfedges that separate facets that are in one plane
>   - check if the facet halfedge->facet() and 
> halfedge->opposite()->facet() are
> coplanar
>   - if they are, use join_facet(halfedge) to delete the corresponding 
> edge and
> therewith merge the two facets
> 3. Retriangulate the polyhedron so that each facet is a triangle
>
> all of that using:
> typedef CGAL::Exact_predicates_inexact_constructions_kernel K;
> (me and Gmpq do not work together very well so I try with doubles)
>
> This works OUTSIDE of valgrind only if the polyhedra are cubes. If they are
> anything else, it works only INSIDE of valgrind.
>
> If I start the program OUTSIDE of valgrind, it stops with:
>
> CGAL error: precondition violation!
> Expr: circulator_size(h->vertex_begin()) >= size_type(3)
> File: /home/jkbecker/Code/eKolos/src/eKolos/core/aPolyhedron_3.h
> Line: 1196
> Explanation:
> terminate called after throwing an instance of 
> 'CGAL::Precondition_exception'
>  what():  CGAL ERROR: precondition violation!
> Expr: circulator_size(h->vertex_begin()) >= size_type(3)
> File: /home/jkbecker/Code/eKolos/src/eKolos/core/aPolyhedron_3.h
> Line: 1196
>
> CGAL probably produces antennas and at a later stage bails out because 
> of it.
>
> INSIDE of valgrind, it works fine.
>
> The polyhedra check out ok using p.is_valid(), they fail the higher tests
> (p.is_valid(true,1)), but I am not sure about the real meaning of this 
> yet (I
> will send output if needed).

Depending on how the coplanarity test is performed, you may internally use
geometric constructions.  I'm not that familiar with the Polyhedron, but I
believe it stores a plane in each facet, and constructing a plane from
the vertices is a construction.  Therefore you may run into a robustness
issue that involves constructions, requiring exact constructions, so
either Cartesian<Gmpq>, or the hopefully faster
Exact_predicates_exact_constructions_kernel (EPECK).
Note that converting between Gmpq (or the exact number type of the EPECK
kernel) to double can easily be done with the CGAL::to_double function.

Another approach may be to not rely on the plane, and perform coplanarity
tests based only on points, which uses only a predicate, and therefore the
Exact_predicates_inexact_constructions_kernel (EPICK) might be enough to
ensure robustness.

--
Sylvain Pion
INRIA Sophia-Antipolis
Geometrica Project-Team
CGAL, http://cgal.org/

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