CGAL users discussion list

[Sylvain Pion <>]

Matthijs Sypkens Smit a écrit :
> Hi,
>
> I'm wondering about the following: I know the construction of a 
> circumcenter (of 4 points) is only possible when the points are not 
> collinear. However, when using a 
> Exact_predicates_inexact_constructions_kernel I encounter cases where:
> - coplanar_orientation() returns COLLINEAR, but the construction does not 
> fail and returns a point,
> - coplanar_orientation() does not return COLLINEAR, but the circumcenter 
> construction does end in an assertion failure.
>
> Are both these cases to be expected? In particular the last surprised me, 
> since I was hoping to 'catch' the assertion failure, by testing in 
> advance, but now it seems that this does not generally work.
>
> Is it just plain impossible to construct circumcenters without getting 
> assertion failures using my choice of kernel, or are there ways around 
> this (by testing beforehand)?

As Camille said, it is to be expected.  Usually it happens when you
insist on computing the circumcenters of slivers.  What we use in this
case is something like Robust_circumcenter_traits_3, which computes the
circumcenter approximateluy, but under some control: actually, it
computes it exactly under some Lazy_exact number type, then ask a double
approximation (which precision can be controlled, but the default is
usually fine).

> btw;I have tried to check that using a 
> Exact_predicates_exact_constructions_kernel prevents the problem from 
> happening, but I have little experience with this kernel as of yet. I'm 
> getting
>   CGAL error: assertion violation!
>   Expr: !(i>s)
>   File: /user/CGAL-3.3/include/CGAL/Interval_nt.h
>   Line: 77
>   Explanation: Variable used before being initialized (or CGAL bug)
> but so far I cannot seem to find the cause of the problem. I do believe 
> that all variables involved have actually been initialized.

This most probably indicates a bug in your code, such as the use of
coordinates before intializing/assigning them.

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