dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Fri, Sep 02, 2011 at 02:35:57PM -0700, Johan Hake wrote:
> What is the different between a MeshMarker and a MeshFunction? Is MeshMarker a
> MeshFunction but instead of storing the values in line with its global entity
> index it stores it wrt the global cell entity index together with its local
> entity index?

Yes, that and values don't need to be stored on the entire mesh, only
for a subset, so you can mark just 3 facets without needing to store
markers for a million facets.

Copy paste from the MeshMarker docstring:

  /// The MeshMarkers class can be used to store data associated with
  /// a subset of the entities of a mesh of a given topological
  /// dimension. It differs from the MeshFunction class in two ways.
  /// First, data does not need to be associated with all entities
  /// (only a subset). Second, data is associated with entities
  /// through the corresponding cell index and local entity number
  /// (relative to the cell), not by global entity index, which means
  /// that data may be stored robustly to file.


> Also, will this take over for the way we use MeshFunctions in the assembler,
> or will a MeshFunction be generated by a MeshMarker before assemble gets
> called?

I think we will do that as a first step (convert from MeshMarker to
MeshFunction) since then we don't need to touch the assembler. Then
later we can think about using MeshMarkers directly.

> I think I also get confused with the naming here. If my explaination of what
> MeshMarker is doing is correct, a MeshMarker and a MeshFunction are
> essentially doing the same thing. What differs is the way the data is stored.
> This is not reflected in the naming of the classes

It was the best I could come up with. Feel free to suggest something
else. SubsetMeshFunction would also be confusing since it's not really
a MeshFunction.

Either way, I expect the MeshMarkers class to be used mostly
internally by the MeshDomains class.

--
Anders



> Johan
>
> On Friday September 2 2011 13:58:13 Anders Logg wrote:
> > Typically through the Mesh class and then they are not visible:
> >
> >   subdomain0 = MySubDomain0()
> >   subdomain1 = MySubDomain1()
> >
> >   subdomain0.mark_facets(mesh, 0)
> >   subdomain1.mark_facets(mesh, 1)
> >
> > But one should also be able to use it directly, like so:
> >
> >   markers = MeshMarkers("uint", mesh, mesh.topology().dim() - 1)
> >   markers.set_marker(cell_index, local_entity, marker_value)
> >
> >   marker = markers.get_marker(i)
> >
> > where marker = (cell_index, local_entity, marker_value) or perhaps
> > marker = ((cell_index, local_entity), marker_value). In C++, I
> > currently use the latter so I can use std::pair and it somehow makes
> > sense.
> >
> > We should probably also add FacetMarkers, CellMarkers etc in the same
> > was as for MeshFunction.
> >