[Gmsh] PostView format and background mesh

[j s]

On 7/30/13 9:41 PM, j s wrote:
>
>
> 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?
>
It looks like I got it working.  It looks like I need to use an element 
instead of a point, and it needs to be big enough to have an effect.

Thanks,

Juan

> 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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>