[Gmsh] negative volume tets in MEDIT .mesh output

[Christophe Geuzaine]

Geordie McBain wrote:
> Hello.  I'm sometimes finding I get negative volume tetrahedral
> elements when saving in MEDIT .mesh format; i.e. the vertices are
> sometimes in the wrong order.  I can demonstrate this most simply by
> appending the following lines to t2.geo; e.g. by saving them in
> myt2.geo and calling the lot with "gmsh t2.geo myt2.geo":
> 
> Mesh 3;
> Mesh.Format = 30;
> Save "first.mesh";
> Mesh.Format = 1;
> Save "myt2.msh";
> Mesh.Format = 30;
> Save "myt2.mesh";
> Exit;
> 
> This produces two .mesh files and one .msh.  The two .mesh files are
> identical in every respect except that the first and second vertices
> of each tetrahedron have been swapped.  Thus those of the first .mesh
> file have negative volume.  (I believe the MEDIT .mesh file format has
> the same tetrahedron node-numbering convention as Gmsh.  This is
> supported by integrating over the volumes in FreeFem++, which reads
> .mesh files.  Meshes with negative-volume elements don't work well in
> finite element solutions in FreeFem++.)  The lines 1989--1993 of the
> two files are:
> 
> Tetrahedra
>  8429
>  1488 423 1489 1577 119
>  448 1029 374 398 119
>  1313 1312 1272 1311 119
> 
> Tetrahedra
>  8429
>  423 1488 1489 1577 119
>  1029 448 374 398 119
>  1312 1313 1272 1311 119
> 
> The second ("myt2.mesh"), is correct, and corresponds closely to
> 2042--2044 of the myt2.msh file:
> 
> 185 4 3 1 119 0 423 1488 1489 1577
> 186 4 3 1 119 0 1029 448 374 398
> 187 4 3 1 119 0 1312 1313 1272 1311
> 
>>From the command line ("gmsh -3 -format mesh t2.geo"), I get different
> results (a different number of vertices and tetrahedra), but again
> negative volumes.
> 
> Am I doing something wrong here?  How can I ensure my .mesh file has a
> consistent node-numbering of its tetrahedra?

Hi Geordie - Indeed, we forgot to call setVolumePositive() in the I/O 
routine for the "mesh" format!

This should be fixed in tomorrow's snapshot.

Thanks for the report,

Christophe




> 
> The above results are with Gmsh 2.3.1 compiled from source on openSUSE
> 10.2.  If I run on Ubuntu Hardy using the gmsh package (v. 2.2.5), I
> get different results: for first.mesh there are a mixture of positive
> and negative volume tetrahedra, but myt2.mesh comes out consistently
> positive again.
> 
> Geordie McBain
> http://gdmcbain.freeshell.org
> 
> 
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> 
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-- 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science
http://www.montefiore.ulg.ac.be/~geuzaine