dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Mon, Apr 27, 2009 at 09:21:58AM -0400, Shawn Walker wrote:
>
> On Mon, 27 Apr 2009, Anders Logg wrote:
>
>> On Mon, Apr 27, 2009 at 11:26:13AM +0200, Kent Andre wrote:
>>> On lø., 2009-04-25 at 00:14 +0200, Anders Logg wrote:
>>>> On Wed, Apr 22, 2009 at 05:28:30PM -0400, Shawn Walker wrote:
>>>>> Here is the changeset that adds a `higher_order_coordinates' variable for
>>>>> storing higher order mesh data.  This is a very minor change so please
>>>>> push this.
>>>>>
>>>>> A changeset for DOLFIN is coming immediately after this.
>>>>>
>>>>> - Shawn
>>>>
>>>> I'm not sure what to do about this. It's problematic to add
>>>> experimental work to UFC since it must be stable. In particular, any
>>>> small change to ufc.h means that all forms must be recompiled
>>>> everywhere for everyone.
>>>>
>>>> So before we make a change to UFC, we need to know exactly what we
>>>> need. Which also means I can't import your DOLFIN patch since it
>>>> depends on the UFC patch.
>>>>
>>>> I see you've added
>>>>
>>>>     double** higher_order_coordinates;
>>>>
>>>> to ufc::cell. This is analogous to what is now implemented in
>>>> MeshGeometry and the mesh XML format so I think it's good.
>>>>
>>>> The question is what other information we need. As it works now (for
>>>> the standard ufc::cell), UFC code generated by a form compiler knows
>>>> what to expect from for a ufc::cell argument. If higher order mappings
>>>> should work the same way, then the generated code and thus the form
>>>> compilers need to know which mapping should be used and also the
>>>> length of higher_order_coordinates. Is this what you were thinking?
>>>>
>>>> Before we do much more about it, more people need to weigh in on it as
>>>> it affects DOLFIN, UFC, SyFi and FFC.
>>>>
>>>
>>>
>>> But is there any other way around this. It would be nice with higher
>>> order meshes and UFC should not stop this.
>>>
>>> An alternative to changing the cell class would be to make a subclass
>>> of cell. Would this work ?
>>
>> How about just using the current ufc::cell data structure as it is but
>> let coordinates hold all the coordinates?
>>
>> This could also be the final solution. Then everything that's needed
>> is an extra argument to tabulate_tensor that tells the generated code
>> whether the cell is affinely mapped or not. The flag could simply be
>> an integer: 1 means affine, 2 means quadratic etc.
>
> But you still need to modify the ufc::cell code, I think.  There is also  
> an implicit assumption that the higher order coordinates should contain  
> the standard mesh vertex coordinates.  Of course, this is true for most 
> practical cases.  But for more fancy mappings, maybe this is not the  
> case.

It seems to me that a reasonable assumption would be to limit the
cases to P1, P2, P3, etc, that is, mappings that can be written down
using standard Lagrange bases so then the vertices will always be
included. They would also be first in the list meaning that the code
would actually work (but might not give accurate results) even if it
were generated for affine mappings.

> Also, in the ufc::cell code, you currently read in the cell coordinates  
> using info in MeshTopology.  However, the higher order coordinate info  
> resides in MeshGeometry (which is where it belongs).  So you would still  
> need to modify ufc.h.   Remember, there is higher order cell data that is 
> contained in MeshGeometry.

Where is MeshTopology used for this? I looked in UFCCell.h which is
where the coordinates are copied to ufc::cell and there MeshGeometry
is used.

> Is it really that hard to change ufc.h?  Other things have to be  
> recompiled, but isn't that automatic?

Yes, it's easy to change, but a main point with UFC is that we
shouldn't change it.

-- 
Anders

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