dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Thu, Apr 28, 2011 at 10:48:28PM +0100, Garth N. Wells wrote:
>
>
> On 28/04/11 22:17, Anders Logg wrote:
> > On Thu, Apr 28, 2011 at 12:55:02PM +0200, Martin Sandve Alnæs wrote:
> >> 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.
> >
> > The functionality is already there since this is already extracted in
> > the preprocess function.
> >
>
> We need a function to do this before preprocess in order to pass the
> cell and element types to the preprocess function.

Yes, but as I just said, we already do that inside preprocess (by
calling extract_sub_elements) so the functionality is already there to
be used.

> >>> 3. Add 'unspecified_elements=[]' (perhaps a dict?) to the argument list
> >>> of ufl.algorithms.preprocess.
> >
> > Not sure if this is needed.
> >
> >>> 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.
> >
> > I think all we need to do (in FFC) is to
> >
> > 1. extract the minimal amount of data we need to decide the undecided
> > degree and cell (essentially building the list of elements)
> >
> > 2. select the degree and cell (as we do today)
> >
> > 3. pass the degree and cell to compute_form_data (and thus to
> > preprocess)
> >
>
> Related to my point 3, to keep UFL general, I think that the element
> should be passed, and not just the degree. It is conceivable that
> something other than continuous Lagrange elements could be used, which
> is why an element rather than a degree should be supplied.

I'd prefer if we just got the current functionality in place first
(without using the now banned set_foo functions) before we make any
such extensions. I can have a shot at this. If we want to extend it to
other types of elements, we need to have a long discussion on how to
choose the element type before we proceed.

--
Anders