dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Mon, Jan 28, 2013 at 11:19:13PM +0100, Johan Hake wrote:
> On 01/28/2013 11:08 PM, Anders Logg wrote:
> > On Mon, Jan 28, 2013 at 10:37:46PM +0100, Johan Hake wrote:
> >> On 01/28/2013 10:22 PM, Anders Logg wrote:
> >>> On Mon, Jan 28, 2013 at 10:10:40PM +0100, Johan Hake wrote:
> >>>> Hello!
> >>>>
> >>>> I have now added a method to DofMap, which tabulate a map between
> >>>> vertices and dofs. It will only work for DofMaps with dofs on vertices.
> >>>> So basically for any CG 1 FunctionSpaces and will raise error else wise.
> >>>>
> >>>> Usage:
> >>>>
> >>>>   from dolfin import *
> >>>>   import numpy as np
> >>>>   mesh = UnitSquareMesh(10,10)
> >>>>   V = VectorFunctionSpace(mesh, "CG", 1)
> >>>>   u = Function(V)
> >>>>   u.interpolate(Constant((1,2)))
> >>>>   vert_values = np.zeros(mesh.num_vertices()*2)
> >>>>   vert_values[V.dofmap().tabulate_vertex_map(mesh)] = \
> >>>>   u.vector().array()
> >>>>   print vert_values
> >>>>
> >>>> In parallel the map will follow the local dofs. This means that some
> >>>> values in vert_values above will still be 0. The 0 values will then
> >>>> correspond to vertex values which are owned by another process.
> >>>>
> >>>> In C++ the method returns a std::vector<std::size_t>.
> >>>>
> >>>> Questions:
> >>>> Does the name "tabulate_vertex_map" work?
> >>>
> >>> Sounds ok.
> >>>
> >>>> Should a version of this method exist in other classes. FunctionSpace,
> >>>> Function (without the mesh argument of course)?
> >>>
> >>> Yes, would be good to have for convenience.
> >>>
> >>> What is the relation to Function::compute_vertex_values?
> >>
> >> compute_vertex_values works on any GenericFunction. However, for all
> >> Functions from CG 1 families I guess one could use this map to just
> >> tabulate the entries.
> >
> > What does the tabulate_vertex_map method do which
> > compute_vertex_values doesn't? Enable writing to dofs?
>
> RTFC! ;)
>
> tabulate_vertex_map only computes a map based on the prior knowledge of
> the dofs coinciding with the vertices. compute_vertex_values are much
> more general and used
>
>   element.interpolate_vertex_values
>
> to do the heavy lifting.

Yes, so why is tabulate_vertex_map needed, if compute_vertex_values
already computes the vertex values and is much more general?

> I see tabulate_vertex_map as a convenience mapping for users who want
> their data on a vertex based format, or users who want data which
> resides on vertices into dolfin la structures.

What is the use case? If it is for plotting or postprocessing, then
compute_vertex_values should be enough.

Or is the intention to use it for modifying values of CG1 functions
from the outside?

--
Anders