[petsc-users] coordinates of vertices of a cell in 3D DMPlex

[Matthew Knepley]

On Sat, Jul 25, 2020 at 4:10 AM Swarnava Ghosh <swarnava89 at gmail.com> wrote:

> Dear Petsc users,
>
> I had a trivial question about DMPlex. Suppose I have a 3D mesh of
> tetrahedrons. I want to find out the 3D coordinates of the vertices of a
> particular cell. What would be the function to do this?
>

Lots of good responses. You can see that it is taking some time for
canonical patterns to emerge as the "right way" to do something. The reason
we provide
multiple layers of interface is that user codes rely on different
abstractions and would like to interact with PETSc using different
assumptions.

If you just want the coordinates of each vertex in some cell with the
vertices in a canonical ordering, you can do as Mark suggested, with a
slight modification:

DM  plex, cdm;
Vec coordinates;

ierr = DMGetCoordinateDM(dm, &cdm);CHKERRQ(ierr);
ierr = DMGetCoordinatesLocal(dm, &coordinates);CHKERRQ(ierr);
ierr = DMConvert(cdm, DMPLEX, &plex);CHKERRQ(ierr);
ierr = DMPlexVecGetClosure(plex, NULL, coordinates, cell, NULL,
&coef);CHKERRQ(ierr);
....
ierr = DMPlexVecRestoreClosure(plex, NULL, coordinates, cell, NULL,
&coef);CHKERRQ(ierr);

We get a local coordinate vector, because local vectors are guaranteed to
store everything in the closure of anything in the Plex. Global vectors
are non-overlapping partitions, suitable for solvers, and might not have
some of the values.

  Thanks,

     Matt


> Thank you,
> SG
>


-- 
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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