dolfin team mailing list archive

[mspieg <mspieg@xxxxxxxxxxxxxxxxx>]

On Oct 16, 2009, at 6:04 PM, Anders Logg wrote:

> On Fri, Oct 16, 2009 at 03:34:06PM -0400, mspieg wrote:
> > Hi All,
> >   I'm trying to update a working 0.9.3 code to  0.9.4, and I need  
> > to understand
> > the new interfaces for Function::eval a bit better.
> >
> > I had things working fine with the old interface
> >
> > Function::eval (double *values, const double *x, const ufc::cell  
> > &ufc_cell,
> > uint cell_index)
> >
> > and am trying to understand how to efficiently use
> >
> > Function::eval (double *values, const double *x, const Cell  
> > &dolfin_cell, const
> > ufc::cell &ufc_cell)
> >
> > (My codes need to do a large number of evals at arbitrary points  
> > and cells
> > efficiently)
> >
> > among other things, is there an easy way to update the index of a  
> > cell, without
> > constructing a new Cell every time? (or shouldn't I be worrying  
> > about this).
>
> Constructing a cell should not cost that much. It just stores the
> index, the dimension and a reference to the mesh.
>
> If you are using iterators over a mesh, then you already have the cell
> by dereferencing the pointer. The typical usage would be
>
> UFCCell ufc_cell(mesh);
> for (CellIterator cell(mesh); !mesh.end(); ++cell)
> {
>   ufc_cell.update(*cell);
>
>   f.eval(values, x, *cell, ufc_cell);
> }
>
> But perhaps you don't use iterators?

Thanks Anders,
  I haven't been using iterators because I've been overloading eval 
(double* values, Data& data) and getting the cells passed through  
data during assembly

the issue is that I'm not necessarily evaluating at the position and  
cell in data, but at another point and cell derived from the current  
location and cell (it's basically a particle tracking problem).

Ideally, given data I would set another Data structure (data_prime)  
with the new point, cell and ufc_cell, then call eval as needed.  As  
long as the cost to construct a cell is small, it will probably be  
easy enough to do with the current code (the problem is I need to do  
this at every quadrature point, and I found previously that  
construction of UFCCell at every point was exceptionally expensive).

Anyway, I'll play around a bit more and if I have specific issues or  
design questions, I'll let you know.

cheers
marc




> > Anyway, I need to play with things a bit more, but looking at the  
> > source code
> > for eval,  I was also wondering
> >  if lines 320 and 321 are required as line 322 doesn't seem to  
> > require cell.
> >
> >  typo? bug? or general ignorance on my part (probably the latter).
>
> Yes, lines 320-321 should be removed. Thanks for noticing. I've fixed
> this now.
>
> --
> Anders
>
>
> > cheers
> > marc
> >
> > void Function::eval(double* values, const Data& data) const
> >
> > 00311 {
> > 00312   assert(values);
> > 00313   assert(_function_space);
> > 00314
> > 00315   // Check if UFC cell if available and cell matches
> > 00316   if (data._dolfin_cell && _function_space->has_cell 
> > (*data._dolfin_cell))
> > 00317   {
> > 00318     // Efficient evaluation on given cell
> > 00319     assert(data._ufc_cell);
> > 00320     const uint cell_index = data._ufc_cell->entity_indices 
> > [data._ufc_cell->topological_dimension][0];
> > 00321     Cell cell(_function_space->mesh(), cell_index);
> > 00322     eval(values, data.x, *data._dolfin_cell, *data._ufc_cell);
> > 00323   }
> > 00324   else
> > 00325   {
> > 00326     // Redirect to point-based evaluation
> > 00327     eval(values, data.x);
> > 00328   }
> > 00329 }
> >
> > ----------------------------------------------------
> > Marc Spiegelman
> > Lamont-Doherty Earth Observatory
> > Dept. of Applied Physics/Applied Math
> > Columbia University
> > http://www.ldeo.columbia.edu/~mspieg
> > tel: 845 704 2323 (SkypeIn)
> > ----------------------------------------------------
>
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> > DOLFIN-dev@xxxxxxxxxx
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>
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----------------------------------------------------
Marc Spiegelman
Lamont-Doherty Earth Observatory
Dept. of Applied Physics/Applied Math
Columbia University
http://www.ldeo.columbia.edu/~mspieg
tel: 845 704 2323 (SkypeIn)
----------------------------------------------------