dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Fri, Apr 29, 2011 at 09:15:51AM +0100, Garth N. Wells wrote:
>
>
> On 29/04/11 08:58, Martin Sandve Alnæs wrote:
> > On 29 April 2011 09:35, Anders Logg <logg@xxxxxxxxx> wrote:
> >> 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.
> >
>
> I don't see the relevance of this. FFC can just create Lagrange elements
> of a given degree to pass to UFL. We don't need to discuss the element
> type at this stage.

It's harder than that since we may have both scalar and vector-valued
elements.

Would it be enough for now to extract the following to send to UFL:

  common_cell
  common_family
  common_degree

?

--
Anders