[petsc-users] Face coloring in DMPlex

[Matthew Knepley]

On Sun, Mar 9, 2025 at 8:04 PM Onur via petsc-users <petsc-users at mcs.anl.gov>
wrote:

> Hi,
>
> I am building a solver and for mesh handling, I use DMPlex. In my 3D
> mesh, I need to color faces. The adjacency information appears correct
> based on my checks(But I tried setting adjacency and creating new sections
> too):
> for (PetscInt f = fStart; f < fEnd; ++f) {
>     PetscInt adjSize = PETSC_DETERMINE;
>     PetscInt *adj = NULL;
>     PetscCallVoid(DMPlexGetAdjacency(dm_, f, &adjSize, &adj));
>     PetscCallVoid(PetscPrintf(PETSC_COMM_WORLD, "[%4d]", f));
>     PetscInt count = 0;
>     for (int i = 0; i < adjSize; ++i) {
>         if (adj[i] >= fStart && adj[i] < fEnd) {
>             count++;
>             PetscCallVoid(PetscPrintf(PETSC_COMM_WORLD, " %4d", adj[i]));
>         }
>     }
>     PetscCallVoid(PetscPrintf(PETSC_COMM_WORLD, " | %d\n", count));
>     PetscCallVoid(PetscFree(adj));
> }
>
> I am testing this on a mesh consisting of quadrilateral elements. This
> code correctly outputs 7 adjacent faces for interior faces and 4 for
> boundary faces (including the face itself).
>
> However, when I call DMCreateColoring, I get the following error:
>
> [0]PETSC ERROR: No support for this operation for this object type
> [0]PETSC ERROR: No method getcoloring for DM of type plex
>
> What is the way to perform face coloring using DMPlex?
>
> You can see at the bottom of this page (
https://urldefense.us/v3/__https://petsc.org/main/manualpages/DM/DMCreateColoring/__;!!G_uCfscf7eWS!Ye71m4cv18ArH6KNma4l3K6Dap0gpNdURt6Rr0xaC8a8xT0ZlCHGYf_082sga_F8wArt0_KklHPfxzx6WRgM$ ) that there are no
Plex-specific implementations of DMCreateColorjng. This is because I do not
know of any algorithms for unstructured meshes that work better than
coloring the nonzero structure. Thus I have always used the greedy coloring
on the matrix. Could you use that?

  Thanks,

     Matt

> Thank you!
>
> Onur
>
>
>

-- 
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

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