ffc team mailing list archive

[Johan Hake <hake@xxxxxxxxx>]

On Wednesday 19 August 2009 16:18:48 Kristian Oelgaard wrote:
> Quoting "Garth N. Wells" <gnw20@xxxxxxxxx>:
> > Johan Hake wrote:
> > > On Wednesday 19 August 2009 15:12:21 Garth N. Wells wrote:
> > >> Johan Hake wrote:
> > >>> On Wednesday 19 August 2009 14:57:20 Garth N. Wells wrote:
> > >>>> Johan Hake wrote:
> > >>>>> On Wednesday 19 August 2009 14:21:12 Garth N. Wells wrote:
> > >>>>>> Garth N. Wells wrote:
> > >>>>>>> Kent Andre wrote:
> > >>>>>>>>> Not sure what we have to do actually. I suppose I want get any
> > >>>>>>>>> clue from the
> > >>>>>>>>>   demo/function/restriction
> > >>>>>>>>>
> > >>>>>>>>> demo?
> > >>>>>>>>> I see that Kent has modified the dolfin.FunctionSpace to
> > >>>>>>>>> reflect the present (broken) implementation of restriction,
> > >>>>>>>>> anything in that line? Should such an implementation has been
> > >>>>>>>>> done in FunctionSpaceBase instead of FunctionSpace? Kent?
> > >>>>>>>>>
> > >>>>>>>>> Johan
> > >>>>>>>>
> > >>>>>>>> This restriction is different in the sense that the restriction
> > >>>>>>>> is made by the standard element on a sub mesh.
> > >>>>>>>> The restriction in focus now is on the complete mesh but for
> > >>>>>>>> only a part of the element.
> > >>>>>>>> Combined, these two features will be very powerful.  I guess the
> > >>>>>>>> sub-mesh restriction stuff is broken due to the parallel mesh.
> > >>>>>>>
> > >>>>>>> I've attached an example solver. To make it work for P^k with k >
> > >>>>>>> 2 we need to restrict some functions to cell facets. Using FFC,
> > >>>>>>> we do
> > >>>>>>>
> > >>>>>>>     scalar  = FiniteElement("Lagrange", "triangle", 3)
> > >>>>>>>     dscalar = FiniteElement("Discontinuous Lagrange", "triangle",
> > >>>>>>> 3)
> > >>>>>>>
> > >>>>>>>     # This syntax restricts the space 'scalar' to cell facets.
> > >>>>>>>     mixed = scalar["facet"] + dscalar
> > >>>>>>>
> > >>>>>>> In PyDOLFIN we could do
> > >>>>>>>
> > >>>>>>>     V_dg = FunctionSpace(mesh, "DG", 3)
> > >>>>>>>     V_cg = FunctionSpace(mesh, "CG", 3, "facet")
> > >>>>>>>
> > >>>>>>>     mixed = V_cg + V_dg
> > >>>>>>
> > >>>>>> I see now that we need to have the above option since the
> > >>>>>> everything (e.g. the dof map) is created and compiled inside
> > >>>>>> functionspace.py.
> > >>>>>
> > >>>>> We could implement it in FunctionSpaceBase, as an operator,
> > >>>>> returning a new FunctionSpaceBase, with the restricted element as
> > >>>>> arguemnt.
> > >>>>>
> > >>>>> However my concerns go to a sub space of a restricted mixed space
> > >>>>> (if such exists) see the other email.
> > >>>>>
> > >>>>> If we go for 1, (as you have already done ;) ), we cannot create a
> > >>>>> restricted mixed space using the addition syntax.
> > >>>>>
> > >>>>>   mixed = V1 + V2
> > >>>>
> > >>>> Do you mean to then restrict mixed? I'm already doing
> > >>>
> > >>> Precisely.
> > >>>
> > >>> After a second thought I think that doing it in the constructor is
> > >>> more clean. Makes life simpler. Is this also possible i UFL, I mean:
> > >>>
> > >>>   e = FiniteElement("CG",cell,0,"facet")
>
> No, and the above element is not even legal, for CG the degree must be >=1
> :)
>
> You have to use the __getitem__ operator of FiniteElementBase
>
> a = element[cell]
>
> or do
>
> e = ElementRestriction(element, domain)
>
> where domain is either a Measure or Cell and only Cell is supported in FFC.
>
> In FFC I might consider adding a default 'domain=None' to the argument list
> of a FiniteElement since it will make some things easier. Maybe we should
> add this in UFL too?

More, harmony between the languages is nice, therefore I asked. Then we should 
probably support both syntaxes in PyDOLFIN, but the __getitem__ syntax might 
be troublesome to implement.

Johan

> Kristian
>
> > >>> Should it?
> > >>
> > >> Is that a PyDOLFIN finite element? If it is, then yes, that's how it
> > >> should work.
> > >
> > > I meant UFL.FiniteElement.
> >
> > OK. I don't know if that is the syntax. Kristian will know.
> >
> > >> Somewhat related, it's a bit annoying in PyDOLFIN that the code is
> > >> recompiled when the mesh is changed. Can we avoid this? Should one
> > >> then create a finite element and a dof map with which to initialise
> > >> the Pythin FunctionSpace?
> > >
> > > Do you mean when one has refined a mesh?
> >
> > Yes, or just changed from one simulation to the next.
> >
> > As far as I understand it is the
> >
> > > reason for this that we need to recompute the dofmap.
> >
> > In C++ we create a dolfin::FunctionSpace from a dolfin::FiniteElement, a
> > dolfin::DofMap and a dolfin::Mesh. The FiniteElement and DofMap are
> > created with a ufc::finite_element and a ufc::dof_map, respectively.
> >
> > In the Python interface, in addition to what we have now, we would need
> > FiniteElement and DofMap objects, with which we could create a
> > FunctionSpace (together with a Mesh). Then, hopefully we could avoid
> > recompilation when just the mesh is changed.
> >
> > Garth
> >
> > > How is this done in the C++ interface.
> > >
> > > Johan
> > >
> > >> Garth
> > >>
> > >>> Johan
> > >>>
> > >>>>      V0 = FunctionSpace(mesh, "CG", 3, "facet")
> > >>>>      V1 = FunctionSpace(mesh, "DG", 3)
> > >>>>
> > >>>>      mixed = V0 + V1
> > >>>>
> > >>>> and it works. This type of restriction (we'll need to sort out our
> > >>>> terminology) impacts the dofmap, which I why I think it needs to be
> > >>>> involved in the creation of the function space.
> > >>>>
> > >>>> Garth
> > >>>>
> > >>>>> If we want a restricted mixed space we need to use the constructor.
> > >>>>>
> > >>>>>   mixed = MixedFunctionSpace((V1,V2),restriction="facet")
> > >>>>>
> > >>>>> Johan
> > >>>>>
> > >>>>>> Garth
> > >>>>>>
> > >>>>>>> or
> > >>>>>>>
> > >>>>>>>     V_dg = FunctionSpace(mesh, "DG", 3)
> > >>>>>>>     V_cg = FunctionSpace(mesh, "CG", 3)
> > >>>>>>>
> > >>>>>>>     mixed = V_cg["scalar"] + V_dg
> > >>>>>>>
> > >>>>>>> Garth
> > >>>>>>>
> > >>>>>>>> Kent
> > >>>>>>>>
> > >>>>>>>> _______________________________________________
> > >>>>>>>> DOLFIN-dev mailing list
> > >>>>>>>> DOLFIN-dev@xxxxxxxxxx
> > >>>>>>>> http://www.fenics.org/mailman/listinfo/dolfin-dev
> > >>>>>>>
> > >>>>>>> -----------------------------------------------------------------
> > >>>>>>>---- -- -
> > >>>>>>>
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