[Gmsh] PostView format and background mesh

[michael.asam at infineon.com]

Hi Juan,

the parsed .pos format (as used e.g. in bgmesh.pos) documentation is a bit hidden in the Gmsh 
manual. You'll find it in chapter 8.1 Post-processing commands (-> View "string" ...)
Please note also the given hint:
"However this "parsed format" is read by Gmsh's script parser, which makes it
inefficient if there are many elements in the dataset. Also, there is no connectivity
information in parsed views and all the elements are independent (all fields
can be discontinuous), so a lot of information can be duplicated. For large
datasets, you should thus use the mesh-based post-processing file format described
in Chapter 9 [File formats], page 87, or use one of the standard formats
like MED."

I hope this is of some help.

Cheers,
Michael





-----Original Message-----
From: gmsh-bounces at ace20.montefiore.ulg.ac.be [mailto:gmsh-bounces at ace20.montefiore.ulg.ac.be] On Behalf Of J S
Sent: Tuesday, July 30, 2013 8:30 AM
To: Geordie McBain
Cc: gmsh at geuz.org
Subject: Re: [Gmsh] PostView format and background mesh

On 07/30/2013 01:10 AM, Geordie McBain wrote:
> 2013/7/30 J S <j.s4403 at gmail.com>:
>>>> The missing pieces are:
>>>> 4. write out a characteristic length in a format understood by gmsh.
>>> Write out the characteristic length in the .msh format.  Then let
>>> "gmsh -0" convert that to .pos, as described.
What is the format?  It is not documented as far as a I can tell?


>> How do you specify the characteristic length in .msh format?  I don't see it
>> in the reference manual?
> The format is described at
> http://geuz.org/gmsh/doc/texinfo/gmsh.html#MSH-ASCII-file-format.  The
> characteristic length can be output as either NodeData or ElementData,
> depending on how you calculate it (which of course will be very much
> dependent on your application).
>     As a simple example, you might decide that the magnitude of the
> gradient of the solution of your partial differential equation on the
> first mesh might be a useful indicator.  This will be P0 if your
> solution is P1, so you'd output it as ElementData, or rather its
> reciprocal, since you want something that's smaller where the gradient
> is steeper to get smaller elements there.
>    A more sophisticated tool is Pascal Frey's mshmet
> <http://www.ann.jussieu.fr/~frey/software.html>.  There's an interface
> to this in FreeFem++.  It computes an indicator field on nodes, so
> that should be written as NodeData.
>
>> bgmesh.pos has 3 numbers after the 9 numbers corresponding to triangle
>> coordinates.  Which one is the characteristic length?
> I don't know.  I haven't investigated the .pos format at all.  I just
> let "gmsh -0" do its job and it does appear to.
>
> If I had to guess, I'd say that they might be the values at the
> vertices, but really I don't know.
>
>>   How do I specify this in 1d and 3d?
> The "gmsh -0" conversion certainly works in three dimensions.  I
> haven't tried one dimension, but I would think it would work there
> too.


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