ufl team mailing list archive

[Florian Rathgeber <florian.rathgeber@xxxxxxxxxxxxxx>]

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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? 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?

Florian
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