[petsc-users] DMPlex Interpolation

[Lawrence Mitchell]

On Mon, 29 Aug 2022 at 14:55, Matthew Knepley <knepley at gmail.com> wrote:
>
> On Sun, Aug 28, 2022 at 7:17 PM Matthew Knepley <knepley at gmail.com> wrote:
>>
>> On Sun, Aug 28, 2022 at 5:36 PM Mike Michell <mi.mike1021 at gmail.com> wrote:
>>>
>>> Thank you for the reply.
>>>
>>> I think it can be more helpful for me if the attached sample code (DMInterpolation_Mod.tar) could be checked by you.
>>
>>
>> Okay, you are right. I will run it tomorrow.
>
>
> The problem was somewhat conceptual. When you are restricting state, meaning a function you want to use directly, rather than
> residuals, meaning some average or integral, you want to preserve scale. This is what the 'vscale' vector is for. I added the
> VecPointwiseMult to your code (attached) to do this. I also changed to a more general mehs creation. To get your run, use

The right answer is to use DMCreateInjection
(https://petsc.org/release/docs/manualpages/DM/DMCreateInjection.html)
to move fine grid state to coarse grid state (rather than
DMCreateInterpolation and then VecPointwiseMult, which doesn't work
for most discretisations).

Matt: the interface to DMCreateInjection is now correct (it returns a
Mat, rather than a VecScatter) so DMPlexComputeInjectorFEM
(https://petsc.org/release/docs/manualpages/DMPLEX/DMPlexComputeInjectorFEM.html)
should be updated to take advantage.

Lawrence