[Gmsh] [FORGED] Re: Windows, Ubuntu give different meshes

[Christophe Geuzaine]

> On 1 Jul 2019, at 13:29, Gib Bogle <g.bogle at auckland.ac.nz> wrote:
> 
> I'm interested in knowing how the differences arise.  I can understand how different machines, compilers can give numerical results that differ at the limits of the selected precision, but it's not obvious how this leads to the generation of different meshes.
> 

Unstructured mesh generation is an iterative process, where adding a new mesh entity (e.g. a node) depends on the location of previous entities through geometrical calculations. It's thus very nonlinear, as a small perturbation on one node can lead to discrete decisions (e.g. swapping an edge) that will lead to substantially different final meshes.

Christophe



> Gib
> From: gmsh <gmsh-bounces at ace20.montefiore.ulg.ac.be> on behalf of moritz braun <moritz.braun at gmail.com>
> Sent: Monday, 1 July 2019 10:28 p.m.
> To: Christophe Geuzaine
> Cc: gmsh at onelab.info; gmsh at geuz.org
> Subject: [FORGED] Re: [Gmsh] Windows, Ubuntu give different meshes
>  
> Dear Christophe
> 
> I assume  identical  geometries leading to different meshes
> between different versions of  redhat 7.4 to 7.6 
> can be expected. 
> I was quite surprised because my self consitent code behaved differently depending
> on the redhat version and now I understand!
> 
> Regards
> 
> Moritz 
> 
> 
> On Fri, Jun 28, 2019 at 8:04 AM Christophe Geuzaine <cgeuzaine at uliege.be> wrote:
> 
> 
> > On 27 Jun 2019, at 03:33, Gib Bogle <g.bogle at auckland.ac.nz> wrote:
> > 
> > I have been testing with the demo file cube.geo:
> > 
> > lc = 0.3;
> > Point(1) = {0.0,0.0,0.0,lc};
> > Point(2) = {1,0.0,0.0,lc};
> > Point(3) = {1,1,0.0,lc};
> > Point(4) = {0,1,0.0,lc};
> > Line(1) = {4,3};
> > Line(2) = {3,2};
> > Line(3) = {2,1};
> > Line(4) = {1,4};
> > Line Loop(5) = {2,3,4,1};
> > Plane Surface(6) = {5};
> > tmp[] = Extrude {0,0.0,1} {
> >   Surface{6};
> > };
> > Physical Volume(1) = tmp[1];
> > 
> > I generate the mesh file at the command line like this:
> > 
> > gmsh -3 cube.geo
> > 
> > on a Windows 7 machine and on Ubuntu 16.04.  The gmsh version is 4.3.0.
> > 
> > I find that the meshes created on the two systems are a bit different.  On Windows the mesh has 137 nodes and 372 elements, while on Ubuntu it has 136 nodes and 373 elements.
> > 
> > Should I be concerned about this?
> 
> No, on the same machine the meshes should be identical, but across OSes (and different compilers) small variations are normal.
> 
> Christophe
> 
> 
> > 
> > Thanks
> > Gib
> > _______________________________________________
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> 
> — 
> Prof. Christophe Geuzaine
> University of Liege, Electrical Engineering and Computer Science 
> http://www.montefiore.ulg.ac.be/~geuzaine
> 
> 
> 
> 
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> 
> -- 
> Prof M Braun         Tel.:27-12-4298006/8027
> Physics Department  Fax.: 27-12-4293643
> University of South Africa (UNISA)      
> moritz.braun at gmail.com
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— 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science 
http://www.montefiore.ulg.ac.be/~geuzaine