[petsc-users] DMPlex: Ghost points after DMRefine

[Matthew Knepley]

On Mon, Nov 30, 2015 at 7:01 AM, Morten Nobel-Jørgensen <mono at mek.dtu.dk>
wrote:

> I have a very simple unstructured mesh composed of two triangles (four
> vertices) with one shared edge using a DMPlex:
>
>  /|\
> / | \
> \ | /
>  \|/
>
> After distributing this mesh to two processes, each process owns a
> triangle. However one process owns tree vertices, while the last vertex is
> owned by the other process.
>
> The problem occurs after uniformly refining the dm. The mesh now looks
> like this:
>
>  /|\
> /\|/\
> \/|\/
>  \|/
>
> The new center vertex is now not listed as a ghost vertex but instead
> exists as two individual points.
>
> Is there any way that this new center vertex could be created as a ghost
> vertex during refinement?
>

This could be a bug with the l2g mapping. I do not recreate it when
refining, only the SF defining the mapping.
Here is an experiment: do not retrieve the mapping until after the
refinement. Do you get what you want? If so,
I can easily fix this by destroying the map when I refine.

  Thanks,

    Matt


> Kind regards,
> Morten
>
> Ps. Here are some code snippets for getting global point index and test of
> point is a ghost point:
>
> int localToGlobal(DM dm, PetscInt point){
>     const PetscInt* array;
>     ISLocalToGlobalMapping ltogm;
>     DMGetLocalToGlobalMapping(dm,&ltogm);
>     ISLocalToGlobalMappingGetIndices(ltogm, &array);
>     PetscInt res = array[point];
>     if (res < 0){ // if ghost
>         res = -res +1;
>     }
>     return res;
> }
>
> bool isGhost(DM dm, PetscInt point){
>     const PetscInt* array;
>     ISLocalToGlobalMapping ltogm;
>     DMGetLocalToGlobalMapping(dm,&ltogm);
>     ISLocalToGlobalMappingGetIndices(ltogm, &array);
>     return array[point]<0;
> }
>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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