[petsc-users] Input arguments of DMPlexCreateFromDAG

[Matthew Knepley]

On Thu, Jan 30, 2014 at 12:19 PM, Cedric Doucet <cedric.doucet at inria.fr>wrote:

> Hello,
> I tried to use DMPlexCreateFromDAG function to create a DM structure from
> a hybrid mesh.
> To understand how it works, I look at ex5.c file.
> Unfortunately, there are some things that I do not understand.
> 1. Why does coneSize lists faces of cells first, then faces of vertices
> and finally faces of edges (in 2D)? Listing faces of vertices, then faces
> of edges and finally faces of cells is not simpler?
>

This is really about what order you number points. I wanted to support
meshes with just cells and vertices, as well as
those with face and edges. I also wanted to be able to convert between
them. Thus it made sense to leave the cell
and vertex numbers invariant under this change. I still think this is the
best pragmatic alternative.


> 2  What does coneOrientations contain? For two counterclockwise oriented
> triangles sharing an edge e={v0,v1}, I understand that {v0,v1} is the right
> oriented edge for the second triangle (and e for the first one) but what
> is the meaning of value -2? I read that it is -(o+1) with o=1 but why does
> o equal 1 in this case?
>

Right now, orientation o = <sign><start> means:

  <sign>: + means traverse in cone order
                 - means traverse in the reverse of cone order

  <start>: cone point to start iteration on
                if <sign> is -, then start on point -(<start>+1)

Thus -2 means start on point 1 and go in reverse order, so
for an edge that would be {1, 0} which is what you want.

What we should really have is <start> identify a group element from the
symmetry
group of the point, and <sign> indicate inversion. However, that will be a
big rewrite
and needs to be motivated by applications.

   Matt

Thank you very much for your help!
> Best regards,
> Cédric Doucet
>
>


-- 
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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