ffc team mailing list archive

["Garth N. Wells" <gnw20@xxxxxxxxx>]

On 13/04/10 15:45, Anders Logg wrote:
> 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

OK.

> 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)

OK. We have a member function of the object with which we construct 
forms that determines the intersection

> 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.

OK

Garth

> --
> Anders