dolfin team mailing list archive

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

Anders Logg wrote:
> On Thu, Nov 26, 2009 at 11:07:20PM +0000, Garth N. Wells wrote:
>>
>> Anders Logg wrote:
>>> On Thu, Nov 26, 2009 at 10:55:14PM +0000, Garth N. Wells wrote:
>>>> noreply@xxxxxxxxxxxxx wrote:
>>>>> ------------------------------------------------------------
>>>>> revno: 4323
>>>>> committer: Anders Logg <logg@xxxxxxxxx>
>>>>> branch nick: dolfin-dev
>>>>> timestamp: Thu 2009-11-26 11:15:01 +0100
>>>>> message:
>>>>>   Use Lagrange as default element instead of quadrature for expressions.
>>>> I thought that we agreed that this is a bad idea.
>>>>
>>>> Garth
>>> I don't see why it is such a bad idea. Either we approximate a
>>> coefficient by interpolating it into a finite element space or we
>>> evaluate it at quadrature points. Just two different approaches to
>>> approximation.
>>>
>> We discussed this a few days ago and agreed that it's not good.
> 
> I don't remember why I agreed really. I keep getting back to thinking
> it's not that bad. :-)
> 
>> How should the order be chosen?
> 
> The order can be chosen to preserve the order of approximation, for
> which it is enough to take the degree to be the same as that for the
> test and trial functions.
> 
> This is what we do in bench/fem/convergence which shows the expected
> convergence rates for degree 1-5 (but I haven't checked in a while).
> 
>> All the finite element codes that
>> I'm aware of would evaluate an Expression at quadrature points, so
>> that's what most users would expect.
> 
> DOLFIN has been interpolating to a FEM space for the last 5 years and
> that has worked fine... :-)
> 

Why did we add QuadratureElements then?

It's not just a question of what works, it's what would one expect.

Garth

>>> Besides, it's the only thing that currently works because of the bug
>>> in the quadrature code generation that I wasn't able to fix.
>>>
>> That's a different motivation ;).
> 
> Yes, this is why I changed.
> 
>>> The choice of element is controlled by the variable named "Family" in
>>> the function _auto_select_element_from_shape in expression.py.
>>>
>>> We can make this a global parameter and set the default tor
>>> "Quadrature" once it works,
>> I'm happy is "Quadrature" is the default, but if it's not the default
>> will we ever get around to fixing it?
> 
> I think Kristian is looking into it. In the meantime, we can go ahead
> and update all the demos and prepare for a release. This is a bug in
> FFC, not DOLFIN, and it doesn't change the interface.
> 
> We might also want to make some cleanups in expression.py. There are
> still a bunch of references to function spaces which should not be
> there anymore.
> 
> --
> Anders

-- 
Dr Garth N Wells
Department of Engineering
University of Cambridge
Trumpington Street
Cambridge CB2 1PZ
United Kingdom

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