dolfin team mailing list archive

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

On 5 May 2011 20:04, Johan Hake <johan.hake@xxxxxxxxx> wrote:
> On Thursday May 5 2011 10:16:11 Neilen Marais wrote:
>> Hi,
>>
>> On Thu, Apr 21, 2011 at 6:18 PM, Johan Hake <johan.hake@xxxxxxxxx> wrote:
>> >> > We should probably ignore all the ufc::cell versions, and add unit
>> >> > test for the one we do expose...
>> >>
>> >> Please excuse my naivete, but what is the essential difficulty in
>> >> exposing the functions to the python side?
>> >
>> > We have not paid too much attention making methods taking generated code,
>> > ufc objects, available from Python. This is too low level, and some of
>> > the ufc objects gets reordered when the dolfin objects gets
>> > instantitated. This is mostly the case when run in parrallel.
>> >
>> >> I ask since from a perspective of both testability of my own code and
>> >> of learning what FEniCS does it is quite usefull to be able to call
>> >> the lower level functions from python. For instance, I also want to
>> >> use finite_element.map_to_reference_cell(), but it seems to give the
>> >> same error that evaluate_basis_all() did before you fixed it :)
>> >
>> > Yes, this would be the same case here. We also need to consider the
>> > interface. It would clutter the interface to duplicate too many
>> > functions. But if Garth or Anders is not opposed to it we could expose
>> > that one too.
>>
>> For me it would make sense to allow these things from python, since
>> that is usually where I'd want to experiment with a new technique
>> before trying to implement it fast in C++. I'm a little out of my
>> depth here, but to me seems that "dolfin" is an easier to use wrapper
>> around the lower level UFL stuff.
>>
>> I would potentially be happy to code to a slightly lower level, but
>> let me sketch out what I'm looking for in more detail. I don't want to
>> have to implement mesh interconnectivity handling and DOF mapping from
>> scratch. At the same time I want the ability to formulate things at
>> the level of basis function evaluations as needed. In part this is
>> simply because the "standard" dolfin doesn't cater quite that well for
>> stuff I need for electromagnetic simulation (yet?), and in part since
>> I may want to try new and/or hare-brained things :)
>>
>> I'm quite happy to work with FEniCS developers to get the needed
>> functionality into dolfin, and that is really first prize for me. In
>> the meantime, however, my slave-driving overlords want a working EM
>> FEM solver with academic outputs, so I can't neccesarily wait for that
>> to happen first :)
>>
>> >> If it is not to difficult you could teach me how to fix such problems.
>> >> Then I could go fishing rather than asking for fish periodically :)
>> >
>> > We need more fishermen! Just look at the last commit, where i added the
>> > dolfin version. It should be pretty straight forward to implement a
>> > similar method for the other methods which takes a ufc::cell.
>>
>> OK, as far as I can tell the trick is just to cast the dolfin cell to
>> a UFC cell. I tried the patch below to get mapt_to_reference_cell()
>> working, but receive:
>>
>> RuntimeError: map_to_reference_cell not yet implemented (introduced in UFC
>> 2.0).
>
> This you need to address to the FFC list, as FFC has not generated code for
> that ufc method.
>
>> Am I right in assuming that this is a limitation in the current dolfin
>> C++ codebase rather than an issue with the code I added?
>>
>> On another note, if it simply down to casting a dolfin cell to a UFC
>> cell, could one not define a python casting function that returns a
>> "UFC compatible" python object, and then tell the user that if they
>> want to access the lower level UFC functionality they need to do that
>> first?
>
> I think this should be OK for UFCCell. But we should typically not expose
> ufc::dof_map, as this is might be reordered.
>
> One way to handle this is to add a typemap at the Python side, so you can send
> a dolfin::Cell to any method accepting a ufc::cell. This will fix the Python
> side of this request.
>
> But I wonder why we need a dolfin version of any of these methods as one in
> C++ can just create a UFCCell and pass that one instead.

Because it's easier when you iterate over cells in the mesh to just use that -->
less code to be written by me.

> So a question to all C++ users. Would you mind if I removed all dolfin::Cell
> versions of the dolfin::FiniteElement::foo and force you to call these methods
> using a create UFCCell instead of dolfin::Cell.

Yes.

If not I think we need to add
> dolfin::Cell version of all ufc::cell methods in dolfin::FiniteElement,
> otherwise we will get new requests in the future.

Why isn't it possible to create a UFCCell in Python and pass that as argument?
Everything is so easy in Python, I've heard.

Kristian

> Johan
>
>
>>
>> Cheers
>> Neilen
>>
>>
>> --- dolfin/fem/FiniteElement.h        2011-04-20 23:21:30 +0000
>> +++ dolfin/fem/FiniteElement.h        2011-05-05 16:49:15 +0000
>> @@ -192,6 +192,16 @@
>>        _ufc_element->evaluate_basis_all(values, coordinates, ufc_cell);
>>      }
>>
>> +    /// Map from coordinate x in cell to coordinate xhat in reference cell
>> +    void map_to_reference_cell(double* xhat,
>> +                               const double* x,
>> +                            const Cell& cell) const
>> +    {
>> +      assert(_ufc_element);
>> +      UFCCell ufc_cell(cell);
>> +      _ufc_element->map_to_reference_cell(xhat, x, ufc_cell);
>> +    }
>> +
>>
>> > Johan
>> >
>> >> Cheers
>> >> Neilen
>> >>
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