dolfin team mailing list archive

[Matthew Knepley <knepley@xxxxxxxxx>]

On Tue, Apr 28, 2009 at 3:32 PM, Anders Logg <logg@xxxxxxxxx> wrote:

> On Tue, Apr 28, 2009 at 03:25:18PM -0500, Matthew Knepley wrote:
> > On Tue, Apr 28, 2009 at 3:05 PM, Robert Kirby <robert.c.kirby@xxxxxxxxx>
> wrote:
> >
> >     Hi all,
> >     it's my understanding that the way ufc ordering works is for adjacent
> cells
> >     to
> >     alternate orientation so that they traverse edges (and faces in 3d)
> the
> >     same global
> >     way.  It seems that in 2d this is equivalent to two-coloring a graph
> (each
> >     cell is either
> >     clockwise or counterclockwise)
> >
> >     Has there been any thought to issues at imposing ufc ordering in
> parallel,
> >     where each
> >     process has to assign an orientation to the first local cell, and
> different
> >     processors
> >     might disagree?
> >
> >
> > How would this disagreement come about? Maybe I do not understand what
> UFC is
> > doing
> > here is how I do it:
> >
> >   1) Values are associated with Sieve points. If there are multiple
> values on
> > an edge, these
> >        values are ordered here.
> >
> >   2) Every Sieve arrow has an orientation. This orientation produced by a
> > traversal concatenates
> >       these orientations, and is relative to orientation to the initial
> > orientation in the Sieve.
> >
> >   3) I do not see the parallel problem because all data is just traded
> between
> > shared sieve
> >       points, and thus has identical orientations. However, I use the
> > traversals to construct
> >       ordered arrays of data. Maybe UFC does something else.
> >
> >   Matt
>
> With UFC, the orientation is never stored but implied by the vertices
> of the entity, going from lower index to higher index. So an edge from
> vertex 15 to 25 will be directed 15 --> 25 which means two cells
> sharing a common edge from 15 to 25 will both assign the same
> direction (since they have a common numbering for the vertices).


How does this work for higher dimensions?

   Matt


>
> --
> Anders
>
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-- 
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