CGAL users discussion list

[BBB HHH <>]

Hi Ben,

The first version of source code (failing) is working on other lists of polygons but not on the list of polygons I already provided.  Before performing intersection of these polygons, i checked some conditions (CCW oriented, simple, and unbounded polygons).
If you examine the polygons (.off files for easy visualization or .dat files if you want to do it through programming) in the attachment of my previous e-mail (which are also attached to the current e-mail), you can see that the polygons are correct (CCW orientation, simple, bounded). Their correctness is also consolidated by the fact that the second version of the source code is well running.

I think the problem is not related to the -frounding-math version because of the successful execution of the second version of source code. Moreover, i am using exact number types.

Thnaks for your response
Hichem

---

De : Ben Supnik <>
À :
Envoyé le : Dimanche, 27 Septembre 2009, 16h23mn 55s
Objet : Re: Re : Re : [cgal-discuss] An error when trying to compute intersection of polygons

Hi Hichem,

The order of computation is different - the first (failing) case is I think more efficient because the code can use a merge-sort-like sequence.

You say that your polygons "seem" to be correct...did you use the CGAL tests for self-intersection, etc. or a different unrelated set of code to check the input data?

Also, I'm not sure what the MSVC version of -frounding-math is...if the low level rounding in CGAL is incorrect, polygon ops can become intermittently unreliable...I saw that when I had compiler options wrong, and the symptoms were similar.

cheers
Ben

BBB HHH wrote:
> Hello,
>
> I tried to compute the intersection of the polygons by inserting each polygon apart like this:
>
> std::vector<Polygon_2>        polygons;
> // Fill the list of polygons
> Initialize the polygon set with a polygon
> Polygon_set_2 S(first_polygon);
> // Instead of S.intersection(polygons.begin(), polygons.end());
> for (int i = 0; i != polygons.size(); ++i)
> {
>    S.intersection(polygons[i];
> }
>
> This variation of source code is working fine for the polygons I attached to the previous e-mail. I do not know what is the difference (perhaps in runtime or storage) between the first source code (which didn't work) and the second one?
>
> Thanks
> Hichem
>
> ------------------------------------------------------------------------
> *De :* BBB HHH <>
> *À :*
> *Envoyé le :* Samedi, 26 Septembre 2009, 18h25mn 19s
> *Objet :* Re : [cgal-discuss] An error when trying to compute intersection of polygons
>
> Hello,
>
> Attached to this e-mail, you can find the .dat files storing the polygns i am working with (file first_polygon.dat and files polygon_00.dat to polygon_15.dat). I also attached .off files storing the same polygons so you can visualize them.
>
> Thanks
> Hichem
>
> ------------------------------------------------------------------------
> *De :* "" <>
> *À :*
> *Envoyé le :* Samedi, 26 Septembre 2009, 16h24mn 57s
> *Objet :* Re: [cgal-discuss] An error when trying to compute intersection of polygons
>
> So you are trying to compute the intersection between 'first_polygon' and all the polygons in the range 'polygons.begin(), polygons.end()'. Could you please provide the data that causes the problem. It would be nice if you narrow it down first.
>
> Quoting "BBB HHH" < <mailto:>>:
>
>  > Hello,
>  >
>  > I am trying to compute the intersection of some polygons in 2D. For that purpose, i am using the 2D Regularized Boolean Set-Operations of CGAL 3.4. I am also using VS2005 under Windows XP SP3.
>  >
>  > For some polygons which seemto be correct (CCW orientation, non-degenerate, simple, bounded), i have the following error:
>  >
>  > CGAL error: precondition violation!
>  > _expression_ : cv.is_in_x_range (p)
>  > File      : c:\program files\cgal-3.4-beta1\include\cgal\arr_segment_traits_2.h
>  >
>  > Line      : 481
>  > Explanation:
>  > Refer to the bug-reporting instructions at http://www.cgal.org/bug_report.html
>  >
>  >
>  > I am using an exact kernel (CGAL::Simple_cartesian<CGAL::Lazy_exact_nt<Gmpq> >). The problematic line of code is that computing the intersection of some polygons
>  >
>  > std::vector<Polygon_2>        polygons;
>  > // Fill the list of polygons
>  > // Initialize the polygon set with a polygon
>  > Polygon_set_2 S(first_polygon);
>  > S.intersection(polygons.begin(), polygons.end());
>  >
>  > Thanks in advance for your help
>  > Hichem Barki
>  >
>  >
>  >
>  >      --
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>  >
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>
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