[petsc-users] dmplex face normals orientation

Matthew Knepley knepley at gmail.com
Fri Apr 14 04:00:31 CDT 2017

On Wed, Apr 12, 2017 at 10:52 AM, Ingo Gaertner <ingogaertner.tus at gmail.com>

> Hello,
> I have problems determining the orientation of the face normals of a
> DMPlex.
> I create a DMPlex, for example with DMPlexCreateHexBoxMesh().
> Next, I get the face normals using DMPlexComputeGeometryFVM(DM dm, Vec
> *cellgeom, Vec *facegeom). facegeom gives the correct normals, but I don't
> know how the inside/outside is defined with respect to the adjacant cells?

The normal should be outward, unless the face has orientation < 0.

> Finally, I iterate over all cells. For each cell I iterate over the
> bounding faces (obtained from DMPlexGetCone) and try to obtain their
> orientation with respect to the current cell using
> DMPlexGetConeOrientation(). However, the six integers for the orientation
> are the same for each cell. I expect them to flip between neighbour cells,
> because if a face normal is pointing outside for any cell, the same normal
> is pointing inside for its neighbour. Apparently I have a misunderstanding
> here.

I see the orientations changing sign for adjacent cells. Want to send a
simple code? You should see this
for examples. You can run SNES ex12 with -dm_view ::ascii_info_detail to
see the change in sign.



> How can I make use of the face normals in facegeom and the orientation
> values from DMPlexGetConeOrientation() to get the outside face normals for
> each cell?
> Thank you
> Ingo
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