On 23/09/2024 17:16, Matthew Knepley wrote:
On Mon, Sep 23, 2024 at 5:33 AM Matteo Semplice via petsc-users
<petsc-users@mcs.anl.gov> wrote:
Dear petsc,
I need to hand-code a jacobian and I can't figure out how to
translate the DMplex points/fields to matrix indices.
My DMPlex has a section with m fields per cell (which makes for
n>m dof
per cell since some fields are vector). Say I want to insert an nxn
block for the row corresponding to cell c and coloumn of its
neighbour
d. I guess that I should call either
MatSetValues/MatSetValuesLocal or
the blocked variants, but how do I find the row/col indices to
pass in
starting from the c/d dmplex points? And, while I am at it, which
MatSetValues version standard/Local standard/blocked is best?
I looked for a petsc example, but fails to find it: if there's
one, can
you just point me to it?
1. In FEM, the main unit of indices is usually the closure, so we provide
https://urldefense.us/v3/__https://petsc.org/main/manualpages/DMPlex/DMPlexGetClosureIndices/__;!!G_uCfscf7eWS!YwF8ebZtFAZdHXKPu98AWB7KPuScfxMu4dRoe8ZqVaPVoEO_nX_qJa-zS6pOWHnPUs2ME6GB7l_SlHBl8ypyOPOQXzbVSC9xXhd8tw$
which is what is used inside of
https://urldefense.us/v3/__https://petsc.org/main/manualpages/DMPlex/DMPlexMatSetClosure/__;!!G_uCfscf7eWS!YwF8ebZtFAZdHXKPu98AWB7KPuScfxMu4dRoe8ZqVaPVoEO_nX_qJa-zS6pOWHnPUs2ME6GB7l_SlHBl8ypyOPOQXzbVSC8i-x5AVA$
2. These indices are calculated using the global section, so you can just
DMGetGlobalSection(dm, &gs);
and then use PetscSectionGetDof() and PetscSectionGetOffset(), knowing
that
off-process values are encoded as -(off + 1), so you need to convert
those.
Does this make sense?
Yes! Thank you!
Matteo
