dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Mon, Nov 03, 2008 at 03:55:31PM +0000, Garth N. Wells wrote:
> 
> 
> Anders Logg wrote:
> > On Mon, Nov 03, 2008 at 02:38:36PM +0000, Garth N. Wells wrote:
> >>
> >> Anders Logg wrote:
> >>> On Mon, Nov 03, 2008 at 11:22:26AM +0000, Garth N. Wells wrote:
> >>>> Anders Logg wrote:
> >>>>> On Sun, Nov 02, 2008 at 06:29:25PM +0000, Garth N. Wells wrote:
> >>>>>> Anders Logg wrote:
> >>>>>>> On Sun, Nov 02, 2008 at 05:52:21PM +0000, Garth N. Wells wrote:
> >>>>>>>> Do we want to insist that Dirichlet bc functions that do not appear 
> >>>>>>>> inside a form are constructed with a FunctionSpace? DirichletBC is 
> >>>>>>>> supplied with a FunctionSpace, so if the bc Function does not have a 
> >>>>>>>> FunctionSpace, we could attach one automatically.
> >>>>>>>>
> >>>>>>>> Garth
> >>>>>>>> _______________________________________________
> >>>>>>>> DOLFIN-dev mailing list
> >>>>>>>> DOLFIN-dev@xxxxxxxxxx
> >>>>>>>> http://www.fenics.org/mailman/listinfo/dolfin-dev
> >>>>>>> I think this is already handled. Look in the Poisson demo. It uses a
> >>>>>>> Constant to set the BC and it does not have a FunctionSpace attached
> >>>>>>> to it. The DirichletBC class now uses its own FunctionSpace rather
> >>>>>>> than the one that the Function has (if any). There is a check (in
> >>>>>>> DirichletBC::check()) that checks that the FunctionSpace for the
> >>>>>>> Function is the same as the one in the DirichletBC.
> >>>>>>>
> >>>>>> It works for Constant, but not for Functions. I was getting an error 
> >>>>>> when Function::interpolate is called. Function::interpolate leads to 
> >>>>>> eval being called, in which case there is a test for the FunctionSpace 
> >>>>>> which fails. Constant provides its own eval and therefore doesn't have a 
> >>>>>> problem.
> >>>>>>
> >>>>>> For now, I've added a test in DirichletBC for the FunctionSpace. What we 
> >>>>>> can add is an attach function if there is no FunctionSpace associated.
> >>>>>>
> >>>>>> Garth
> >>>>> In which demo does this show up? Is there a simple way I can comment
> >>>>> something out to reproduce the error so I understand what goes wrong?
> >>>>>
> >>>> Look at /demo/nls/nonlinearpoisson/cpp.
> >>>>
> >>>> If you change
> >>>>
> >>>>    DirichletBoundaryCondition g(V, t);
> >>>>
> >>>> to
> >>>>
> >>>>    DirichletBoundaryCondition g(t);
> >>>>
> >>>> it will break down.
> >>>>
> >>>> Garth
> >>> ok I see the problem now.
> >>>
> >>> The problem is a user may choose to either overload a scalar eval
> >>> function or a tensor eval function and we need to decide which one
> >>> after the callback from ufc::function::evaluate(). If the
> >>> FunctionSpace is not known, we can't decide which one to pick.
> >>>
> >>> If we insist that one should be able to pass a Function without a
> >>> FunctionSpace to a DirichletBC, then we must remove the scalar eval
> >>> function.
> >>>
> >> Fine with me. I think that it makes things simpler because the eval 
> >> interface remains the same for all user-defined functions.
> >>
> >> Garth
> > 
> > ok. It will also look the same as in Python.
> >
> 
> We discussed recently passing an object to eval() which contains some 
> data. It would be useful the object also carried information on the rank 
> and dimension of the function to allow checks and switching between 
> 1D/2D/3D problems.
> 
> Garth

Yes, this would be nice, but it won't work as long as we don't require
that a Function always has a FunctionSpace. We could add some new
classes to handle error checking and data for user-defined Functions:

  void eval(Values& values, Data& data)
  {
    values[0] = sin(data.x[0]);
  }

The class Values could check that data is not assigned to any illegal
indices and it could also check that all values have been assigned
etc, but if the Function does not know its FunctionSpace, this can't
be done.

Another complication related to this but also to the thread-safety of
cell() and facet() is that UFC gets in the middle of the call
sequence:

  assemble()
  |--> Function::interpolate()
       |--> FiniteElement::evaluate_dof()
            |--> ufc::finite_element::evaluate_dof
                 |--> ufc::function::evaluate()
                      |--> Function::eval()

Since eval() is called from the generated UFC code, any arguments like
cell and facet passed from the assembler will be lost on the way.

Should we extend the UFC interface to allow sending a void* to
evaluate_dof which it will propagate to evaluate()?

-- 
Anders

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