dolfin team mailing list archive

["Martin Sandve Alnæs" <martinal@xxxxxxxxx>]

2008/10/28 Garth N. Wells <gnw20@xxxxxxxxx>:
>
>
> Anders Logg wrote:
>> On Tue, Oct 28, 2008 at 10:53:01AM +0000, Garth N. Wells wrote:
>>> I started looking at ConstantFunction, and if we keep the old interface
>>> and let ConstantFunction inherit from Function, it will involve quite a
>>> lot of code for something rather simple because on the basis of the cell
>>> type and the rank of the constant, an element needs to be chosen. To
>>> keep ConstantFunction simple, is it reasonable that a ConstantFunction
>>> require a FunctionSpace in its constructor? FFC is already generating
>>> this since it treats constant functions as DG0 elements. This way the
>>> ConstantFunction code will be really simple.
>>>
>>> Garth
>>
>> Locally, DG is always a subspace of whatever element space we have so
>> the interpolation of anything constant will be exact, and thus the
>> interpolation of something constant into whatever element space we
>> have will be exactly equal to the interpolation into DG.
>>
>> So it is no limitation to assume that constant functions are always DG
>> functions. This means we don't need the FunctionSpace argument, but we
>> need to precompile some DG elements.
>>
>
> My point is that there is a bit code to do all this. We have to check
> the cell type (so ConstantFunction will need updating when cell types
> are added), then once the cell type is determined, the right finite
> element for that cell type and the function rank must be created on the
> basis of a string. It isn't very attractive from a long-term maintenance
> point of view. Plus Form already contains the necessary information.
>
> It would be straightforward if the assembler used the
> Function::interpolate(...) which provides the FunctionSpace,
>
>   Function::interpolate(double*, const FunctionSpace&, const ufc::cell&)
>
> Garth
>
>
>> The problem may be if someone wants a DG element which is not
>> precompiled, like some strange vector/tensor-valued constant.
>>
>>
>>
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Anything that depends on precompiled elements is not form compiler independent.

--
Martin