[petsc-users] Inquiry regarding DMAdaptLabel function

[Matthew Knepley]

On Sat, Feb 18, 2023 at 2:25 AM Zongze Yang <yangzongze at gmail.com> wrote:

> Dear PETSc Group,
>
> I am writing to inquire about the function DMAdaptLabel in PETSc.
> I am trying to use it coarse a mesh, but the resulting mesh is refined.
>
> In the following code, all of the `adpat` label values were set to 2
> (DM_ADAPT_COARSEN).
> There must be something wrong. Could you give some suggestions?
>

Sorry for the late reply. You are right, I need to put in error messages
for this. Here is what is happening.
PETSc tries to fallback if you do not have certain packages. In this case,
you are not using DMForest,
which responds to both coarsen and refine, so the
mesh generator interprets all markers as refine (they
cannot coarsen). I will add a check that fails on the coarsen marker.

Coarsening is much more difficult in the presence of boundaries, which is
why it is not implemented in
most packages. For unstructured coarsening, I do not think there is any
choice but MMG.

  Thanks,

     Matt

```python
> from firedrake import *
> from firedrake.petsc import PETSc
>
> def mark_all_cells(mesh):
>     plex = mesh.topology_dm
>     with PETSc.Log.Event("ADD_ADAPT_LABEL"):
>         plex.createLabel('adapt')
>         cs, ce = plex.getHeightStratum(0)
>         for i in range(cs, ce):
>             plex.setLabelValue('adapt', i, 2)
>
>     return plex
>
> mesh = RectangleMesh(10, 10, 1, 1)
>
> x = SpatialCoordinate(mesh)
> V = FunctionSpace(mesh, 'CG', 1)
> f = Function(V).interpolate(10 + 10*sin(x[0]))
> triplot(mesh)
>
> plex = mark_all_cells(mesh)
> new_plex = plex.adaptLabel('adapt')
> mesh = Mesh(new_plex)
> triplot(mesh)
> ```
>
> Thank you very much for your time.
>
> Best wishes,
> Zongze
>


-- 
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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