ufl team mailing list archive

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

On 3 June 2011 22:27, Florian Rathgeber
<florian.rathgeber@xxxxxxxxxxxxxx> wrote:
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> On 03/06/11 21:02, Kristian Ølgaard wrote:
>> On 3 June 2011 21:38, Florian Rathgeber
>> <florian.rathgeber@xxxxxxxxxxxxxx> wrote:
>> 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.
>
> Sorry, I was imprecise. The quadrature_scheme parameter to a
> FiniteElement would usually be a string specifying the name of the
> scheme to be parsed by the form compiler?

Yes, but it could be anything. I don't think UFL does any type checks
on the argument at the moment, but if it did it should probably only
be a check if it is a string or not. What the argument actually means
is left completely to the form compilers.

> I want to use the
> quadrature_scheme argument in UFL to pass the quadrature degree
> (gaussian quadrature) for our own form compiler (which is completely
> separate from FFC) and would hence abuse it to carry an int. Since UFL
> doesn't interpret the parameter in any way I should be fine doing that,
> right?

That's exactly how I think it should be used from UFL.

Kristian

> Florian
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