[dune-pdelab] DiscreteGridFunction interface

[Felix Heimann]

Hallo Everybody,

I agree, the PDELab GridFunction interface should allow access to the 
derivatives (preferably of any order) and those implementations that can 
provide them should do so.

As those people using finite elements with sophisticated local->global 
transformations had to write their own grid functions so far, this issue 
should not prevent us from improving what we have (not saying that it 
should not be addressed in near future).

And kudos to him who implements...

Best regards,
Felix


On 08.08.2012 16:37, Jö Fahlke wrote:
> Am Wed,  8. Aug 2012, 15:57:34 +0200 schrieb Christian Engwer:
>> On Wed, Aug 08, 2012 at 03:15:29PM +0200, Jö Fahlke wrote:
>>> Am Wed,  8. Aug 2012, 10:10:53 +0200 schrieb Christian Engwer:
>>>> I'd like to get a new topic on the never ending
>>>> interface-discussion-list.
>>>>
>>>> Why doesn't the DiscreteGridFunction allow the evaluation of
>>>> derivatives?
>>>>
>>>> DiscreteGridFunction is based on the LocalFunctions which allow to
>>>> evaluate derivatives and tell till which order. If nobody comes up
>>>> with a good reason not to do it, I'd like to include such
>>>> functionality in the DiscreteGridFunction. I currently have the
>>>> problem, that I'd like to implement different kinds of error
>>>> functionals, some of them work require the gradient, some the function
>>>> itself and I would prefer not to expose this part to the user.
>>>
>>> One reason is that the GridFunction interface does not include evaluating the
>>> derivatives.
>>
>> Well, this is not really a reason. The interface in some sense is also
>> a description of what is there.
>>
>>> Another reason is that the local-valued LocalFiniteElements only provide
>>> local-valued derivatives.  You still need to know which transformation to
>>> apply, which (in general) depends on the kind of element.  This is of course a
>>> non-issue with global-valued finite elements.
>>
>> Again it is not _really_ a reason. It is just that it is somewhat
>> harder to do it, but doesn't really give a reason why not to do it now.
>
> Rereading your question again I see I gave answers to a slightly different
> question.  All the reasons mentioned were reasons why nobody had done this up
> to now.  They're definitely not hard reasons why this shouldn't be done.
>
> As I said, I think it's a good idea to do let the GridFunction provide the
> derivates, where available.
>
> Regards,
> Jö.
>
>
>
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-- 
Felix Heimann
Universität Heidelberg
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