ffc team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Tue, Apr 13, 2010 at 07:17:28AM +0800, Garth N. Wells wrote:
>
>
> On 12/04/10 23:35, Anders Logg wrote:
> >On Mon, Apr 12, 2010 at 10:20:13PM +0800, Garth N. Wells wrote:
> >>
> >>
> >>On 12/04/10 21:49, Anders Logg wrote:
> >>>On Mon, Apr 12, 2010 at 09:34:38PM +0800, Garth N. Wells wrote:
> >>>>
> >>>>
> >>>>On 12/04/10 21:29, Anders Logg wrote:
> >>>>>On Mon, Apr 12, 2010 at 09:21:32PM +0800, Garth N. Wells wrote:
> >>>>>>
> >>>>>>
> >>>>>>On 12/04/10 21:19, Garth N. Wells wrote:
> >>>>>>>
> >>>>>>>
> >>>>>>>On 12/04/10 20:47, Anders Logg wrote:
> >>>>>>>>We are doing some work where we need to do run-time quadrature over
> >>>>>>>>arbitrary polyhedra.
> >>>>>>>
> >>>>>>>We (Mehdi and I) do this already (using UFC), so I don't see why a new
> >>>>>>>function is required. Can you explain why evaluate_tensor is not enough?
> >>>>>>>
> >>>>>>
> >>>>>>I meant 'tabulate_tensor'.
> >>>>>
> >>>>>Which function do you call for evaluating the integrand?
> >>>>>
> >>>>
> >>>>We evaluate it inside ufc::tabulate_tensor. We construct our forms
> >>>>with an extra argument, say an object "CutCellIntegrator", which can
> >>>>provide quadrature schemes which depend on the considered cell.
> >>>
> >>>That would require a special purpose code generator.
> >>
> >>What's wrong with that? FFC won't (and shouldn't) be able to do
> >>everything. Just adding a function to UFC won't make FFC do what we
> >>do now. We reuse FFC (import modules) and add special purpose
> >>extensions.
> >
> >Exactly, it won't make FFC do what we need, but we could *use* FFC to
> >do what we need (without adding a special-purpose code generator).
> >
> >>>Having
> >>>evaluate_integrand would allow more flexibility for users to implement
> >>>their own special quadrature scheme.
> >>>
> >>
> >>We make "CutCellIntegrator" an abstract base class, so the user has
> >>*complete* freedom to define the quadrature scheme and the generated
> >>code does not depend on the scheme, since the scheme may depend on
> >>things like how the cell 'cut' is represented.
> >
> >Then it sounds to me like that generated code is not at all special,
> >but instead general purpose and should be added to UFC/FFC.
> >
> >And the most general interface would (I think) be an interface for
> >evaluating the integrand at a given point. We already have the same
> >for basis functions (evaluate_basis_function) so it is a natural
> >extension.
> >
>
> I still don't see the need for 'evaluate_integrand' unless you plan
> to call it directly from the assembler side (i.e. DOLFIN). Is that
> the case? Perhaps you can give me a concrete example of how you plan
> to use it.

Yes, that's the plan. In pseudo-code, this is what we want to do:

  for polyhedron in intersection.cut_cells:

    quadrature_rule = QuadratureRule(polyhedron)
    AK = 0

    for (x, w) in quadrature_rule:

      AK += w * evaluate_integrand(x)

    A += AK

All data representing the geometry, the polyhedra, mappings from those
polyhedra to the original cells etc is in the Intersection class (in
the sandbox):

  intersection = Intersection(mesh0, mesh1)

Eventually, we might want to move part of the functionality into
either DOLFIN or FFC, but having access to and evaluate_integrand
function makes it possible to experiment (from the C++ side) without
the need for building complex abstractions at this point.

--
Anders

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