ufl team mailing list archive

[Kristian Ølgaard <k.b.oelgaard@xxxxxxxxx>]

On 3 June 2011 21:38, Florian Rathgeber
<florian.rathgeber@xxxxxxxxxxxxxx> wrote:
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> On 24/05/11 14:31, Martin Sandve Alnæs wrote:
>> On 24 May 2011 11:39, Kristian Ølgaard <k.b.oelgaard@xxxxxxxxx> wrote:
>>> On 24 May 2011 11:23, Martin Sandve Alnæs <martinal@xxxxxxxxx> wrote:
>>>> On 23 May 2011 12:05, Kristian Ølgaard <k.b.oelgaard@xxxxxxxxx> wrote:
>>>>> On 23 May 2011 11:20, Martin Sandve Alnæs <martinal@xxxxxxxxx> wrote:
>>>>>> Is this documented somewhere? Since it is a UFL argument, it should be
>>>>>> documented in UFL.
>>>>>
>>>>> It's not documented anywhere I think. True, it is a UFL argument, but
>>>>> UFL doesn't know anything about quadrature schemes and I don't think
>>>>> it needs to. The argument just has to be a string, then it's up to the
>>>>> form compilers how to deal with the argument.
>>>>> FFC currently supports two different schemes, the 'default' scheme
>>>>> which is the hardcoded quadrature schemes which Garth implemented and
>>>>> the 'canonical' scheme which is a Gauss--Legendre--Jacobi quadrature
>>>>> scheme mapped onto simplices.
>>>>
>>>> Ok. And why does a FiniteElement need to have an associated a quad scheme?
>>>
>>> Because the dofs of quadrature elements are located at the quadrature
>>> points with a value of 1 at those points and zero elsewhere.
>>> Derivatives are not defined. This enables this type of element to be
>>> handled like any other finite element by the form compilers.
>>> However, it also means that FiniteElement("Quadrature", triangle, 2)
>>> will have 4 dofs if the 'canonical' scheme is used and only 3 dofs if
>>> the 'default' scheme is used.
>>> Therefore the scheme has to be included when identifying unique elements.
>>
>> Thanks, that makes perfect sense.
>
> Kristian, could you provide an example how this is used? I have a use
> case where I want to specify a quadrature scheme and this would come in
> really handy. However I don't currently see how to access this
> information in the form compiler.

Currently, only the FFC compiler supports different quadrature schemes.
This is selected from the command line using the '-q scheme' options
where 'scheme'
can be one of 'default' or 'canonical'. 'default' is, well, the default.
To use a different scheme you can add one to
ffc/ffc/quadrature_schemes.py and perhaps submit a patch if you think
it might be useful for others.

The 'quadrature_scheme' argument to FiniteElements in UFL is not used
by the form compilers at the moment so it can be ignored for now.

Kristian

> Florian
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