dolfin team mailing list archive

[Martin Sandve Alnæs <martinal@xxxxxxxxx>]

On 28 April 2011 11:45, Garth N. Wells <gnw20@xxxxxxxxx> wrote:
>
>
> On 27/04/11 20:50, Johan Hake wrote:
>> On Wednesday April 27 2011 12:45:46 Martin Sandve Alnæs wrote:
>>> On 27 April 2011 21:34, Anders Logg <logg@xxxxxxxxx> wrote:
>>>> On Wed, Apr 27, 2011 at 09:30:08PM +0200, Martin Sandve Alnæs wrote:
>>>>> 2011/4/27 Johan Hake <johan.hake@xxxxxxxxx>
>>>>>
>>>>>     On Wednesday April 27 2011 12:03:56 Martin Sandve Aln s wrote:
>>>>>     > On 27 April 2011 19:07, Garth N. Wells <gnw20@xxxxxxxxx> wrote:
>>>>>     > > I'm starting here a new thread on how to deal with the recent
>>>>
>>>> change in
>>>>
>>>>>     > > UFL that has broken a good number of DOLFIN demos. The previous
>>>>
>>>> thread
>>>>
>>>>>     > > meandered and got side-tracked.
>>>>>     > >
>>>>>     > > The framework in we need to operate is:
>>>>>     > >
>>>>>     > > A. UFL will not allow forms to be modified post-construction.
>>>>>     > >
>>>>>     > > B. It should be relatively easy to replace ufl.Coefficients in
>>>>>     > > a
>>>>
>>>> form
>>>>
>>>>>     > > and return a new form.
>>>>>     > >
>>>>>     > > C. The issue with replacing ufl.Coefficients is that we lose
>>>>
>>>> DOLFIN
>>>>
>>>>>     data
>>>>>
>>>>>     > > (like the eval() functions) associated with the removed
>>>>
>>>> coefficients.
>>>>
>>>>>     > > I'll kick off with the obvious solution:
>>>>>     > >
>>>>>     > > 1. Require that all DOLFIN Expressions are associated with a
>>>>>     > > ufl.FiniteElement.
>>>>>     > >
>>>>>     > > Other solutions?
>>>>>     > >
>>>>>     > > Garth
>>>>>     >
>>>>>     > 2.  At the stage when ffc calls ufl.preprocess, or even in
>>>>>
>>>>>     ufl.preprocess,
>>>>>
>>>>>     > let the preprocessed form contain ufl Coefficients with new
>>>>
>>>> elements in
>>>>
>>>>>     > place of the dolfin.Expressions. This is similar to the
>>>>
>>>> replacements done
>>>>
>>>>>     > for renumbering of Coefficients, and could either be done
>>>>
>>>> simultaneously
>>>>
>>>>>     or
>>>>>
>>>>>     > as an additional step. The original Form and Expression objects
>>>>
>>>> will be
>>>>
>>>>>     > untouched, and the preprocessed form will be fine.
>>>>>
>>>>>     +
>>>>>
>>>>>     However, setting cell and degree is done during analysis and relies
>>>>
>>>> on
>>>>
>>>>>     form_data. The form is also preprocessed when the form_data is
>>>>
>>>> extracted.
>>>>
>>>>>     This
>>>>>     means that for the preprocessed form to get correct signature, cell
>>>>
>>>> and
>>>>
>>>>>     degrees being set, we need to break up the logic.
>>>>>
>>>>>      1) extract form_data
>>>>>      2) set degree and cell
>>>>>      3) genererate preprocessed form
>>>>>
>>>>> Lets figure out the exact algorithm if we need it. It could perhaps be
>>>>> integrated better with preprocess. Or it might be better to extract
>>>>> just
>>>>
>>>> the
>>>>
>>>>> information needed to determine degree and cell first, and pass the
>>>>
>>>> element
>>>>
>>>>> replacements to preprocess.
>>>>
>>>> That's what I suggested in an earlier mail. Preprocess already gets
>>>> common_cell. We could also figure out common_degree before calling
>>>> preprocess but that requires getting the data stored in
>>>> form_data.sub_elements.
>>>
>>> Extracting all sub elements from a form before preprocessing should be easy
>>> and efficient.
>>>
>>> I assume it's still possible to construct an Expression with a specific
>>> FunctionSpace?
>>
>> Yes.
>>
>
> So it seems we've reached a solution that won't require any changes to
> DOLFIN, and only minimal changes to FFC. The story is:
>
> 1. UFL will permit elements without a cell and without a degree. The
> will leads an error for some operations, like grad and div.
>
> 2. Add a function to UFL to extract all sub-elements from a form.
>
> 3. Add 'unspecified_elements=[]' (perhaps a dict?) to the argument list
> of ufl.algorithms.preprocess.
>
> 4. For coefficients with incomplete elements, preprocess will replace
> these with coefficients based on elements from the list
> 'unspecified_elements'. The new form will be the 'preprocessed form'.
>
> Is that it? Anything else?
>
> Garth

I think that should be all.

Martin