[petsc-users] how to get the vertices belongs to a control volume in 2D?

[Matthew Knepley]

On Fri, May 12, 2017 at 3:48 AM, Lawrence Mitchell <
lawrence.mitchell at imperial.ac.uk> wrote:

>
> > On 12 May 2017, at 04:57, 李季 <leejearl at 126.com> wrote:
> >
> > Hi developers:
> >     I have such a question that I want to get the vertices of a cell. I
> know I can get the points by
> >     1. Getting the faces of a cell such as "DMPlexGetCone(dm, c, &faces";
> >     2. Getting the vertices of every face of the cell such as
> "DMPlexGetCone(dm, f, &vertices)".
> >
> >     Then I can obtain the vertices belongs to a cell. Is there any
> concise routine which I can choose to get the
> > vertices of a cell directly?
>
> You should use the interface for the transitive closure.
>
> Find bounds of points that are vertices:
>
> DMPlexGetDepthStratum(dm, &vStart, &vEnd);
>
> ...
> DMPlexGetTransitiveClosure(dm, c, PETSC_TRUE, &nclosure, &closure);
> for (PetscInt i = 0; i < nclosure; i++) {
>     const PetscInt p = closure[2*i];
>     if (p >= vStart && p < vEnd) {
>         p is a vertex
>     }
> }
>
> This works regardless of the topological dimension of the "cell" point you
> are using (the same code is good to find the vertices in the closure of a
> facet, say).
>
> Matt's course notes (http://www.caam.rice.edu/~caam519/CSBook.pdf) have
> nice pictures that help understand this language in section 7.1.
>

Also note that this is fine for getting vertices if you want to do
topological things. However, if what you really want is
some function over the vertices (like coordinates), you should use


http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/DM/DMPlexVecGetClosure.html

  Thanks,

     Matt


> Cheers,
>
> Lawrence




-- 
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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