[petsc-users] DMPlex tetrahedra facets orientation

[Nicolas Barral]

On 07/03/2021 16:54, Matthew Knepley wrote:
> On Sun, Mar 7, 2021 at 8:52 AM Nicolas Barral 
> <nicolas.barral at math.u-bordeaux.fr 
> <mailto:nicolas.barral at math.u-bordeaux.fr>> wrote:
> 
>     Matt,
> 
>     Thanks for your answer.
> 
>     However, DMPlexComputeCellGeometryFVM does not compute what I need
>     (normals of height 1 entities). I can't find any function doing
>     that, is
>     there one ?
> 
> 
> The normal[] in DMPlexComputeCellGeometryFVM() is exactly what you want. 
> What does not look right to you?


So it turns out it's not what I want because I need non-normalized 
normals. It doesn't seem like I can easily retrieve the norm, can I?

If not, I'll fallback to computing them by hand for now. Is the 
following assumption safe or do I have to use DMPlexGetOrientedFace?
 >  if I call P0P1P2P3 a tet and note x the cross product,
 >  P3P2xP3P1 is the outward normal to face P1P2P3
 >  P0P2xP0P3              "                P0P2P3
 >  P3P1xP3P0              "                P0P1P3
 >  P0P1xP0P2              "                P0P1P2

Thanks

-- 
Nicolas
> 
>    Thanks,
> 
>      Matt
> 
>     So far I've been doing it by hand, and after a lot of experimenting the
>     past weeks, it seems that if I call P0P1P2P3 a tetrahedron and note x
>     the cross product,
>     P3P2xP3P1 is the outward normal to face P1P2P3
>     P0P2xP0P3              "                P0P2P3
>     P3P1xP3P0              "                P0P1P3
>     P0P1xP0P2              "                P0P1P2
>     Have I been lucky but can't expect it to be true ?
> 
>     (Alternatively, there is a link between the normals and the element
>     Jacobian, but I don't know the formula and can  find them)
> 
> 
>     Thanks,
> 
>     -- 
>     Nicolas
> 
>     On 08/02/2021 15:19, Matthew Knepley wrote:
>      > On Mon, Feb 8, 2021 at 6:01 AM Nicolas Barral
>      > <nicolas.barral at math.u-bordeaux.fr
>     <mailto:nicolas.barral at math.u-bordeaux.fr>
>      > <mailto:nicolas.barral at math.u-bordeaux.fr
>     <mailto:nicolas.barral at math.u-bordeaux.fr>>> wrote:
>      >
>      >     Hi all,
>      >
>      >     Can I make any assumption on the orientation of triangular
>     facets in a
>      >     tetrahedral plex ? I need the inward facet normals. Do I need
>     to use
>      >     DMPlexGetOrientedFace or can I rely on either the tet vertices
>      >     ordering,
>      >     or the faces ordering ? Could DMPlexGetRawFaces_Internal be
>     enough ?
>      >
>      >
>      > You can do it by hand, but you have to account for the face
>     orientation
>      > relative to the cell. That is what
>      > DMPlexGetOrientedFace() does. I think it would be easier to use the
>      > function below.
>      >
>      >     Alternatively, is there a function that computes the normals
>     - without
>      >     bringing out the big guns ?
>      >
>      >
>      > This will compute the normals
>      >
>      >
>     https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/DMPLEX/DMPlexComputeCellGeometryFVM.html
>      > Should not be too heavy weight.
>      >
>      >    THanks,
>      >
>      >      Matt
>      >
>      >     Thanks
>      >
>      >     --
>      >     Nicolas
>      >
>      >
>      >
>      > --
>      > 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
>      >
>      > https://www.cse.buffalo.edu/~knepley/
>     <http://www.cse.buffalo.edu/~knepley/>
> 
> 
> 
> -- 
> 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
> 
> https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>