[petsc-users] Finding which cell an arbitrary point belongs to in DMPlex

[Hapla Vaclav]

There is also DMPlexFindVertices() which finds the nearest vertex to the given coords in the given radius.

You can then get support or its transitive closure for that vertex.

I wrote it some time ago mainly for debug purposes. It uses just brute force. I'm not sure it deserves to exist :-) Maybe we should somehow merge these functionalities.

Thanks,

Vaclav

On 16 Sep 2020, at 01:44, Matthew Knepley <knepley at gmail.com<mailto:knepley at gmail.com>> wrote:

On Tue, Sep 15, 2020 at 6:18 PM Jeremy Theler <jeremy at seamplex.com<mailto:jeremy at seamplex.com>> wrote:
On Mon, 2020-09-14 at 20:28 -0400, Matthew Knepley wrote:
> On Mon, Sep 14, 2020 at 6:15 PM Jeremy Theler <jeremy at seamplex.com<mailto:jeremy at seamplex.com>>
> wrote:
> > Hello all
> >
> > Say I have a fully-interpolated 3D DMPlex and a point with
> > arbitrary
> > coordinates x,y,z. What's the most efficient way to know which cell
> > this point belongs to in parallel? Cells can be either tets or
> > hexes.
>
> I should make a tutorial on this, but have not had time so far.

Thank you very much for this mini-tutorial.

>
> The intention is that you use
>
>
> https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/DM/DMLocatePoints.html
>
> This will just brute force search unless you also give
>
>   -dm_plex_hash_location

Well, for a 3D DMplex PETSc (and git blame) tells me that you "have
only coded this for 2D." :-)

Crap. I need to do 3D. It's not hard, just work.

> which builds a grid hash to accelerate it. I should probably expose
>
>   DMPlexLocatePoint_Internal()
>
> which handles the single cell queries. If you just had one point,
> that might make it simpler,
> although you would still write your own loop.

I see that DMLocatePoints() loops over all the cells until it finds the
right one. I was thinking about finding first the nearest vertex to the
point and then sweeping over all the cells that share this vertex
testing for DMPlexLocatePoint_Internal(). The nearest node ought to be
found using an octree or similar. Any direction regarding this idea?

So you can imagine both a topological search and a geometric search. Generally, people want geometric.
The geometric hash we use is just to bin elements on a regular grid.

>  If your intention is to interpolate a field at these
> locations, I created
>
>
> https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/SNES/DMInterpolationCreate.html
>
> which no one but me uses so far, but I think it is convenient.

Any other example apart from src/snes/tutorials/ex63.c?

That is the only one in PETSc. The PyLith code uses this to interpolate to seismic stations.

  Thanks,

     Matt

Thank you.

>
>   Thanks,
>
>     Matt
>
> > Regards
> > --
> > jeremy theler
> > www.seamplex.com<http://www.seamplex.com/>
> >
> >
>
>
>



--
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

https://www.cse.buffalo.edu/~knepley/<http://www.cse.buffalo.edu/~knepley/>

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