dolfin team mailing list archive

[Neilen Marais <nmarais@xxxxxxxxx>]

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).

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?

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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