CGAL users discussion list

[Menelaos Karavelas <>]

In order to determine if this is really a bug in the code or not, you need to 
provide an input instance that creates the problem. It would also make sense 
to send the program that you have written that creates this kind of problems.

Regarding the choice of the kernel: Filtered_kernel<Simple_cartesian<double> 
> is a bad choice. You should use the filtered traits instead. 
CGAL::Cartesian<CGAL::Gmpq> and 
CGAL::Exact_predicates_exact_constructions_kernel should work.

One final issue: how do you know that your non-convex polygon does not have 
self-intersections? Could it be that your polygon is not really simple (i.e., 
its edges strongly intersect), while you consider it to be simple?
Have you tried building the segment Delaunay graph with a traits class that 
supports segment intersections?

- m.

On 5 Jul 2012, at 20:56, Cornelia Auer wrote:

> Thank you very much for your response!
> 
> However using the CGAL::Exact_predicates_exact_constructions_kernel the 
> insertion of segments breaks even earlier. I end up in:
> 
> CGAL error: assertion violation!
> Expression : !(i>s)
> File       : 
> c:\Users\bzfauerc\MyAmiraVC9\product\include\arch-Win64VC9\CGAL/In Line     
>   : 89
> Explanation:  Variable used before being initialized (or CGAL bug)
> 
> All I do is:
> 
> p1=Point_2(to_exact(myDoublePoint1.x), to_exact(myDoublePoint1.y) );
> p2=Point_2(to_exact(myDoublePoint2.x), to_exact(myDoublePoint2.y) );
> v_h=sdg.insert(p1,p2, v_h);
> 
> I haven't tried the CGAL::Simple_cartesian<CGAL::Gmpq> Kernel yet, as I 
> have trouble in my working enviroment to get it properly included.
> 
> However if you see something obviously wrong the way I'm proceeding above I 
> would be very happy if you let me know. Or could it really be a CGAL bug?
> 
> Thanks in advance for your answer!
> Best,
> Cornelia Auer
> 
> 
> 
> Am 05.07.2012 07:56, schrieb Sebastien Loriot (GeometryFactory):
>> On 07/04/2012 07:46 PM, Cornelia Auer wrote:
>>> Dear CGAL users & developers,
>>> 
>>> with the help of the segment_delaunay_graph_2 I try to compute the
>>> medial axis for a given *non*-convex polygon.
>>> To build the segment delaunay graph (SDG) I am adding edge by edge of
>>> the initial polygon. This should give me only weakly intersecting
>>> segments.
>>> 
>>> Now, this works perfectly for most of my polygons, however sometimes the
>>> construction of the SDG crashes:
>>> 
>>> I try to insert a new segment by
>>> v_h=sdg.insert(p1,p2, v_h);
>>> however within the insert routine the check SDG.is_valid() fails and the
>>> procedure crashes. But for no obvious reason (looking at the polygon).
>>> Due to the high grade of templatization the code is really hard to
>>> debug/step through for me. I also tried to find the answer by reading
>>> the paper of Karavelas, but without success.
>>> 
>>> I would be extremly thankful for any information how degenerate cases
>>> (!SDG.is_valid()) arise for weakly intersecting input segments.
>>> 
>>> Or if this could be a numerical issue, as I use the
>>> Filtered_kernel<Simple_cartesian<double> >.
>> Indeed, you should try with a kernel with exact constructions
>> (CGAL::Simple_cartesian<CGAL::Gmpq> or
>> CGAL::Exact_predicates_exact_constructions_kernel).
>> 
>> Sebastien.
>> 
>>> 
>>> 
>>> Thanks in advance for your help
>>> Best,
>>> Cornelia Auer
>>> 
>>> 
>>> 
>>> 
>>> 
>> 
>> 
> 
> 
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