[Gmsh] "Perfect Match Failed" Problem with Blossom

[Christophe Geuzaine]

Hi Johannes,

Indeed: the problem comes from the "Line in Surface" command, which is still a bit experimental.

For your geometry you should really define two surfaces ; that would be more "natural". Attached is an example.

Christophe

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On 13 Mar 2013, at 15:30, Johannes Reinhardt <johannes.reinhardt at unifr.ch> wrote:

> Hi,
> 
> sorry, that was unfortunate: I had accidentally attached the case for dist = 2.1, which works also here. For larger dists (e.g. 3.5) it fails, see new attachment.
> 
> It fails with all the triangulation methods, and the resulting triangulation always shows weird artifacts (see attached screenshot), but at different places depending on the triangulation method.
> 
> Thank you for your rapid answer
> 
> Johannes
> 
> 
> On 03/13/2013 02:11 PM, Christophe Geuzaine wrote:
>> 
>> Hi Johannes,
>> 
>> That's weird - it seems to work fine over here (cf screenshot). I will test it on our Linux machine when I have the time.
>> 
>> Does it happen with all the triangulation methods (MeshAdapt, Delaunay, Frontal, Delaunay for quads)?
>> 
>> 
>> Christophe
>> <Mail Attachment.png>
>> 
>> On 13 Mar 2013, at 13:32, Johannes Reinhardt <johannes.reinhardt at unifr.ch> wrote:
>> 
>>> Hello everybody,
>>> 
>>> first I want to thank everybody involved in the development of gmsh. It is a really great tool and has helped me a lot in my research.
>>> 
>>> However I run into a probem when I try to create a 2D quad mesh with the outline of two superimposed disks embedded. However Blossom almost always fails with the message:
>>> 
>>> Warning : Perfect Match Failed in Quadrangulation, Applying Graph Splits
>>> 
>>> printed 10 times. I attached a simple geometry file that shows this problem.
>>> 
>>> For small distances (e.g. dist=2.1) this problem does not occur, but for larger distances (and these are the ones that I want to study), I always get this error.
>>> 
>>> For other geometries where I had this problem, playing a bit with the mesh element sizes helped, but I could not find good numbers for this case. I also tried both DelQuad and MeshAdapt as Meshing Algorithms, but I always get the error above.
>>> 
>>> I briefly looked at the Blossom Quad publication, but could not find any explanation for this behaviour. My best guess is, that the embedded curve conflicts with a perfect matching, and the extra edges mechanism fails in this case.
>>> 
>>> I tested both gmsh 2.6.1 compiled from source and the precompiled gmsh 2.7.0 Linux 64bit binary on Ubuntu 11.10, both show the same behaviour.
>>> 
>>> Has anybody an idea how to reformulate the geometry description to avoid this problem?
>>> 
>>> Thanks in advance and greetings
>>> 
>>> Johannes
>>> <tip.geo>_______________________________________________
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>> 
>> -- 
>> Prof. Christophe Geuzaine
>> University of Liege, Electrical Engineering and Computer Science 
>> http://www.montefiore.ulg.ac.be/~geuzaine
>> 
>> 
>> 
> 
> <Screenshot at 2013-03-13 15:22:24.png><tip.geo>_______________________________________________
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-- 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science 
http://www.montefiore.ulg.ac.be/~geuzaine