[petsc-users] Input arguments of DMPlexCreateFromDAG

[Matthew Knepley]

On Fri, Jan 31, 2014 at 5:55 AM, Cedric Doucet <cedric.doucet at inria.fr>wrote:

>
> Hello,
>
> DMPlexInterpolate is very interesting!
> Is it slow?
>

It is as fast as I know how to make it. It uses hashing to find the matches.


> If I want to call DMplexInterpolate, just after DMPlexCreateFromDAG call,
> for the example of two triangles sharing an edge, I only have to specify
> cells and vertices like this in DMPlexCreateFromDAG:
>
> PetscInt    numPoints[3]             = {4, 0, 2};
> PetscInt    coneSize[11]              = {3, 3, 0, 0, 0, 0, 0, 0, 0, 0, 0};
> PetscScalar vertexCoords[8]      = {-0.5, 0.5, 0.0, 0.0, 0.0, 1.0, 0.5,
> 0.5};
>
> and let cones and coneOrientations be empty?
>

cones[] would be the vertices in each cell, and coneOrientations would be
all zeros.

   Matt


> Cédric
>
> ------------------------------
>
> *De: *"Matthew Knepley" <knepley at gmail.com>
> *À: *"Cedric Doucet" <cedric.doucet at inria.fr>
> *Cc: *petsc-users at mcs.anl.gov
> *Envoyé: *Vendredi 31 Janvier 2014 12:21:40
> *Objet: *Re: [petsc-users] Input arguments of DMPlexCreateFromDAG
>
> On Fri, Jan 31, 2014 at 3:23 AM, Cedric Doucet <cedric.doucet at inria.fr>wrote:
>
>>
>> Thank you for your answer.
>> I understand well the reason why coneSize begins with faces of cells and
>> vertices first.
>> However, I am not sure to understand how cone points are numbered in the
>> definition of <start>.
>> For example, in CreateSimplex2D in ex5.c (two triangles sharing an edge),
>> we have:
>>
>> PetscInt    numPoints[3]             = {4, 5, 2};
>> PetscInt    coneSize[11]              = {3, 3, 0, 0, 0, 0, 2, 2, 2, 2, 2};
>> PetscInt    cones[16]                   = {6, 7, 8,  9, 7, 10,  2, 3,  3,
>> 4,  4, 2,  5, 4,  3, 5};
>> PetscInt    coneOrientations[16] = {0, 0, 0,  0, -2, 0,  0, 0,  0, 0,  0,
>> 0,  0, 0,  0, 0};
>> PetscScalar vertexCoords[8]      = {-0.5, 0.5, 0.0, 0.0, 0.0, 1.0, 0.5,
>> 0.5};
>>
>> The fact that coneOrientations[4] equals -2 means that edge cones[4]=7
>> must be oriented in reverse order starting at vertex 1.
>> However, edge 7 is defined by vertices (3,4). Does it mean that a local
>> numbering is used here: (3,4)~(v0,v1)?
>>
>
> Yes, exactly the numbering from cones[8-9].
>
>
>> In algebraic topology, a convenient way to orient simplices is to number
>> their vertices in ascendant order.
>> For example, an edge (i,j) is oriented so that i<j, a face (i,j,k) is
>> oriented so that i<j<k, etc.
>> Can we adopt the same convention here and does it simplify something?
>>
>
> No, because we do not have exclusively simplices.
>
> You can simplify by only specifying cells and vertices, which need no
> orientations, and then calling DMPlexInterpolate().
>
>   Matt
>
>
>> Best regards,
>>
>> Cédric Doucet
>>
>>
>> ------------------------------
>>
>> *De: *"Matthew Knepley" <knepley at gmail.com>
>> *À: *"Cedric Doucet" <cedric.doucet at inria.fr>
>> *Cc: *petsc-users at mcs.anl.gov
>> *Envoyé: *Jeudi 30 Janvier 2014 19:26:36
>> *Objet: *Re: [petsc-users] Input arguments of DMPlexCreateFromDAG
>>
>> 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
>>
>>
>>
>
>
> --
> 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
>
>
>


-- 
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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20140131/e89161c9/attachment.html>