[petsc-users] DMGetCoordinatesLocal and DMPlexGetCellCoordinates in PETSc > 3.18

[Matthew Knepley]

On Wed, May 17, 2023 at 10:21 AM Berend van Wachem <berend.vanwachem at ovgu.de>
wrote:

> Dear Matt,
>
> Thanks for getting back to me so quickly.
>
> If I scale each of the coordinates of the mesh (say, I want to cube each
> co-ordinate), and I do this for both:
>
> DMGetCoordinatesLocal();
> DMGetCellCoordinatesLocal();
>
> How do I know I am not cubing one coordinate multiple times?
>

Good question. Right now, the only connection between the two sets of
coordinates is DMLocalizeCoordinates(). Since sometimes people want to do
non-trivial things to
coordinates, I prefer not to push in an API for "just" scaling, but I could
be convinced
the other way.

  Thanks,

     Matt


> Thanks, Berend.
>
> On 5/17/23 16:10, Matthew Knepley wrote:
> > On Wed, May 17, 2023 at 10:02 AM Berend van Wachem
> > <berend.vanwachem at ovgu.de <mailto:berend.vanwachem at ovgu.de>> wrote:
> >
> >     Dear PETSc Team,
> >
> >     We are using DMPlex, and we create a mesh using
> >
> >     DMPlexCreateBoxMesh (.... );
> >
> >     and get a uniform mesh. The mesh is periodic.
> >
> >     We typically want to "scale" the coordinates (vertices) of the mesh,
> >     and
> >     to achieve this, we call
> >
> >     DMGetCoordinatesLocal(dm, &coordinates);
> >
> >     and scale the entries in the Vector coordinates appropriately.
> >
> >     and then
> >
> >     DMSetCoordinatesLocal(dm, coordinates);
> >
> >
> >     After this, we localise the coordinates by calling
> >
> >     DMLocalizeCoordinates(dm);
> >
> >     This worked fine up to PETSc 3.18, but with versions after this, the
> >     coordinates we get from the call
> >
> >     DMPlexGetCellCoordinates(dm, CellID, &isDG, &CoordSize,
> >     &ArrayCoordinates, &Coordinates);
> >
> >     are no longer correct if the mesh is periodic. A number of the
> >     coordinates returned from calling DMPlexGetCellCoordinates are wrong.
> >
> >     I think, this is because DMLocalizeCoordinates is now automatically
> >     called within the routine DMPlexCreateBoxMesh.
> >
> >     So, my question is: How should we scale the coordinates from a
> periodic
> >     DMPlex mesh so that they are reflected correctly when calling both
> >     DMGetCoordinatesLocal and DMPlexGetCellCoordinates, with PETSc
> versions
> >       >= 3.18?
> >
> >
> > I think we might have to add an API function. For now, when you scale
> > the coordinates,
> > can you scale both copies?
> >
> >    DMGetCoordinatesLocal()
> >    DMGetCellCoordinatesLocal();
> >
> > and then set them back.
> >
> >    Thanks,
> >
> >       Matt
> >
> >     Many thanks, Berend.
> >
> > --
> > 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/>
>


-- 
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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