ufl team mailing list archive

[Martin Sandve Alnæs <martinal@xxxxxxxxx>]

On Tue, Mar 17, 2009 at 9:14 PM, Kristian Oelgaard
<k.b.oelgaard@xxxxxxxxxx> wrote:
> Quoting Anders Logg <logg@xxxxxxxxx>:
>
>> On Tue, Mar 17, 2009 at 09:05:22PM +0100, Kristian Oelgaard wrote:
>> > Quoting Anders Logg <logg@xxxxxxxxx>:
>> >
>> > > On Tue, Mar 17, 2009 at 10:39:10AM +0100, Kristian Oelgaard wrote:
>> > > > Quoting UFL <ufl@xxxxxxxxxx>:
>> > > >
>> > > > > One or more new changesets pushed to the primary ufl repository.
>> > > > > A short summary of the last three changesets is included below.
>> > > > >
>> > > > > changeset:   739:ace55a2c179c2335fccb620f1c4db1bc5a0f3153
>> > > > > tag:         tip
>> > > > > user:        Anders Logg <logg@xxxxxxxxx>
>> > > > > date:        Sun Mar 15 19:56:10 2009 +0100
>> > > > > files:       scripts/form2ufl
>> > > > > description:
>> > > > > Improvements to form2ufl script
>> > > >
>> > > > There's a bug when translating PoissonDG.form, look at the last *ds
>> term.
>> > > >
>> > > > Kristian
>> > >
>> > > Fixed. I've regenerated all .ufl files now except for
>> TensorWeightedPoisson
>> > > which can't be handled by the script.
>> >
>> > OK.
>> >
>> > > It would be good if we could keep the .ufl files untouched from now on
>> > > until at least the next release (so they are as close as possible to
>> > > the old .form files and generated using form2ufl). If you want, you
>> > > can add some more examples that set options like "quadrature_order".
>> >
>> > The reason that I've changed the *.ufl forms (added the quadrature_order)
>> is
>> > that otherwise the results does not agree, which I think is just as
>> important
>> > if you talk about a foo.form and foo.ufl form being equal.
>>
>> But isn't that a good thing? It indicates that we have not yet figured
>> out the correct quadrature order to use automatically.
>
> That is one way of looking at it, but it makes it difficult to see if the rest
> of the generated code is correct. It's no big deal though, I just add the order
> for the forms that I test without checking the changes in.
>
> Kristian

To really solve this issue, I need to understand what sufficient
quadrature order _is_.

1) Can the discrete operator assembled from a bilinear form be
approximated using quadrature order v.degree() + u.degree()?
This seems to be common, but I don't understand why it is ok.

2) Obviously, using v.degree() is not sufficient for linear forms,
it even fails for f*v*dx. So using v.degree() + f.degree() in this
case seems like the only right thing to do. This can be generalized
to sum the degrees of form arguments in each product.

3) But for forms like "exp(f**2)*v*dx", there's no way we can
decide a sufficient quadrature order, so what do we fall
back to? Should the rhs use the same order as the lhs?
Or will simply 2*v.degree() be fine? The user should probably
be careful in a case like this anyway.

Martin