dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Mon, Jan 28, 2013 at 10:33:15PM +0000, Garth N. Wells wrote:
> On 28 January 2013 22:27, Anders Logg <logg@xxxxxxxxx> wrote:
> > 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?
> >
>
> I don't think that it's required, but I guess Johan is adding it by
> popular demand.

I'm not objecting to it. I just came to think about the
compute_vertex_values function and it should be enough to cover most
of the use cases that are the background for the popular demand: users
of Hans Petter's tutorial who want to get the vertex values on a
regular grid for postprocessing in external software like MATLAB.

--
Anders