dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Mon, May 30, 2011 at 10:33:29PM +0200, Marie E. Rognes wrote:
> On 05/30/2011 07:36 PM, Anders Logg wrote:
> >On Mon, May 30, 2011 at 10:24:48AM -0700, Johan Hake wrote:
> >>On Monday May 30 2011 04:33:29 Martin Sandve Alnæs wrote:
> >>>On 30 May 2011 12:51, Marie E. Rognes<meg@xxxxxxxxx>  wrote:
> >>>>On 05/30/2011 11:26 AM, Martin Sandve Alnæs wrote:
> >>>>>Since this feature implementation relies on modifying immutable objects,
> >>>>>I'm not the least surprised you're getting problems. The bug is not that
> >>>>>dolfin subdomains are not passed with forms, but that they are allowed
> >>>>>to be attached in the first place on an existing and assumed immutable
> >>>>>form object.
> >>>>
> >>>>Yes...
> >>>>
> >>>>>The short term solution to this bug is to revert pydolfin back to
> >>>>>providing subdomains as arguments to assemble and variationalproblem
> >>>>>where they belong, instead of attaching them to forms. I think this
> >>>>>should be done for fenics 1.0 if this bug is a problem.
> >>>>>
> >>>>>Improvements to the language for expressing subdomains of various kinds
> >>>>>is in the design stage, but that won't happen before the summer.
> >>>>
> >>>>Specifications of subdomain does not belong as arguments to assemble and
> >>>>variational problem. If you have a form
> >>>>
> >>>>        L = g*v*dG
> >>>>
> >>>>where G is a part of a boundary (In semi-math, semi-UFL notation), G
> >>>>should be related to the form. Not to the matrix resulting from the
> >>>>assembly of the form.
> >>>>
> >>>>(cc to DOLFIN since the below involves DOLFIN mainly)
> >>>>
> >>>>The interface to VariationalProblem
> >>>>
> >>>>        VariationalProblem(., ., bcs, exterior_facet_domains,
> >>>>interior_facet_domains, cell_facet_domains)
> >>>>
> >>>>was rather suboptimal because it assumed implicitly that the bilinear and
> >>>>the linear form were defined over the same subdomains. That in,
> >>>>combination with dx = dx(0) etc, is highly bugprone.
> >>>>
> >>>>I care of course because if you want to use the same patent for an
> >>>>variational problem with automatic adaptivity, and take care of the
> >>>>above, the required  input will look something like this
> >>>>
> >>>>        VariationalProblem(., ., bcs,
> >>>>                        primal_bilinear_exterior_facet_domains,
> >>>>                        primal_bilinear_interior_facet_domains,
> >>>>                        primal_bilinear_cell_domains,
> >>>>                        primal_linear_exterior_facet_domains,
> >>>>                        primal_linear_interior_facet_domains,
> >>>>                        primal_linear_cell_domains,
> >>>>                        goal_exterior_facet_domains,
> >>>>                        goal_interior_facet_domains,
> >>>>                        goal_cell_domains)
> >>>>
> >>>>which I can't live with.
> >>>>
> >>>>The Coefficient/Function magic must involve some of the same issues as
> >>>>this. I imagine that a similar way of fixing it should be possible.
> >>>
> >>>I'm not saying it can't be implemented, only that it can't
> >>>be implemented well within the FEniCS 1.0 timeframe, and
> >>>that fixing the current solution will lead down a bad path.
> >>>I'm hoping for a much better solution later this year, but
> >>>that will require some design work first.
> >>>
> >>>An alternative short term approach could be to attach the data to the
> >>>measures.
> >>>
> >>>dxp = dx(cell_domains=primal_cell_domains)
> >>>dsp = ds(exterior_facet_domains=primal_exterior_facet_domains)
> >>>L = g*dxp(1) + f*dsp(0)
> >>>
> >>>and making sure that this data follows measure objects around.
> >>>They can then be collected in ufl preprocess just like functions
> >>>and function spaces. Then the connection between the
> >>>meshfunction and the dx(i) index looks more explicit as well.
> >>
> >>I like that syntax. I guess we only allow one type of cell integral within one
> >>form?
>
> I don't see how this last guess would follow from the above. Why?
>
>
> >>
> >>My conserns regards changing the subdomains after creating the form, as this
> >>can be a convinient way of reusing a compiled form. But I guess letting domain
> >>arguments in assemble override domains in the form should fix that.
> >
> >I like the current syntax and would not like to change it again. An
> >important point is that it allows the same syntax to be used in both
> >Python and C++:
> >
> >a.exterior_facet_domains = exterior_facet_domains
> >a.exterior_facet_domains = exterior_facet_domains;
> >
> >This won't work with dx etc.
> >
>
> The same syntax is not used in C++ and Python for coefficients. With
> that argument, we should not allow
>
> 	f = Function(V)
> 	a = f*ds
>
> but force
>
> 	a.f = f
>
> in Python also. (Which I of course do not advocate, but also makes
> me not buy the argument.)

It's a valid argument: we try to make the interfaces similar where
possible. The two interfaces are similar now and a change would make
the interfaces different. But see below...

> >I suggest we keep the current syntax and find an improved
> >implementation later.
> >
>
>
> I'll throw in the following suggestion in the mix, which is based on
> same idea as Martin's, but a bit terser:
>
> -------------------------------------------------------------------
> Defining sub-domains by mesh functions using boundary domains as an
> example
> -------------------------------------------------------------------
>
> Python
> ********************************************************************
>
> boundaries = FacetFunction("uint", mesh)
> ds = ds(boundaries)
> a = u*v*ds(0) + f*v*ds(1)
>
> C++
> ********************************************************************
>
> UFL:
> --------------------------------------------------------------------
> a = u*v*ds(0) + f*v*ds(1)
>
> main:
> --------------------------------------------------------------------
>
> Form a(V);
> FacetFunction<uint> boundaries(mesh);
> a.ds = boundaries;
>
> Advantages:
>
> * Backwards compatible (except last 2 releases or so)
> * Very similar construction to Functions/Coefficients
> * ds is way shorter than exterior_facet_domains
> * Retains desired link between form and domains of integration
> * Does not mess with UFL immutability
> * Can implement with ufl.Measure.metadata or revamp Measure further

I like a.ds = exterior_facet_domains!

--
Anders