ffc team mailing list archive

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

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.

> 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.


> I also don't understand how it can work at all since the quadrature
> points are not known at compile-time. Or do you a fixed set of reference
> polyhedra that you map to?

The "CutCellIntegrator" object is asked for the quadrature points, and 
it returns them. One way to do it is to sub-triangulate polyhedra (I 
think that CGAL can do this). "CutCellIntegrator" would usually be aware 
of intersecting surfaces, etc. It can also return schemes on surfaces.

I would like a quadrature generator on polyhedra to eventually be part 
of DOLFIN.

Garth

> --
> Anders