dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

There are many more places in DOLFIN where we may iterate over the
vertices in a triangle, not just when we send data to the generated
UFC code. The topology must remain unchanged (it should still be
triangles) but the geometry may be modified for higher order mappings.

-- 
Anders


On Wed, Aug 20, 2008 at 02:32:34PM -0400, Shawn Walker wrote:
> I understand what you are saying, but the ordering of the vertices is 
> such that the first three vertices (0,1,2) are exactly the same as they 
> were before.  This would be done no matter what order polynomial map you 
> used. So, it seems to me it should be fine.
>
> However, to be sure, I can modify the ufc stuff such that the coordinates 
> variable only reads in the first three vertices.  So this would be  
> EXACTLY as it was before.  I would then create a new ufc::cell variable  
> called `map_coordinates' (if you have a better name, please suggest) and  
> this would read in the 6 vertices.  And in Tabulate_Tensor, if higher  
> order is desired, then the `map_coordinates' will be used to compute the  
> FEM matrix.
>
> This seems safe, though a little redundant.  Of course, one must be  
> careful when creating the higher order mesh .xml file and ensure that the 
> first three vertices correspond to the usual triangle vertices.
>
> I also want to make sure that the extra `map_coordinates' is available to 
> be modified with a loop in dolfin.  This would be necessary for an ALE  
> method when the mesh is deforming.
>
> - Shawn
>
> On Wed, 20 Aug 2008, Anders Logg wrote:
>
>> On Wed, Aug 20, 2008 at 07:15:06PM +0100, Garth N. Wells wrote:
>>>
>>>
>>> Anders Logg wrote:
>>>> I'm not sure this will work. If you attach 6 vertices to a triangle by
>>>>
>>>>   <triangle index="0" affine="false" v0="0" v1="1" v2="2" v3="4" v4="5" v5="6"/>
>>>>
>>>> then all sorts of things will break (I imagine). A triangle always has
>>>> three vertices.
>>>>
>>>
>>> I noticed this too. It would make some things troublesome.
>>>
>>>> The geometry of the triangles is separate.
>>>>
>>>> Maybe we could just add extra data which could be "control points" for
>>>> the cell facets? For P2 it would be the edge/face midpoints.
>>>>
>>>
>>> I like the idea of defining facet data which would contain the necessary
>>> info.
>>
>> A problem with defining facet data is that the facet numbering is not
>> known a priori. It depends on the algorithm used by DOLFIN to compute
>> the facets from the cells. So we can't store for example a mesh
>> function over the facets since the facet numbering may change.
>>
>> When we read input from VMTK, we need to read facet data (boundary
>> markers) and these are stored relative to the cell to which the facet
>> belongs and the local number of the facet relative to the cell (which
>> is unique).
>>
>> There is an example in data/meshes/aneurysm.xml.gz.
>>
>>

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