[Gmsh] PostView format and background mesh

[j s]

On 7/30/13 2:12 AM, michael.asam at infineon.com wrote:
> 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 created one point with an assigned characteristic length and it did 
not affect the mesh.  I also tried one triangle with 3 characteristic 
lengths on my 2d example.  Are there any additional rules besides 
merging the file and applying the view as a background mesh?

Regards,

Juan
>
> 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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