dolfin team mailing list archive

[Johan Hake <hake.dev@xxxxxxxxx>]

On 01/28/2013 11:27 PM, Anders Logg 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.

Yes, but that will only give the mapping from a dolfin::Function to
vertex data. Not the other way around. I guess the mapping would be also
be faster for CG 1 family Functions as it just tabulate dofs.

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

Yes. I have use cases where I need vertex based data into my
dolfin::Functions.

Johan

> --
> Anders
>