dolfin team mailing list archive

[Johan Hake <johan.hake@xxxxxxxxx>]

On Monday September 5 2011 00:09:58 Anders Logg wrote:
> On Sun, Sep 04, 2011 at 11:23:04PM -0700, Johan Hake wrote:
> > On Friday September 2 2011 23:19:22 Anders Logg wrote:
> > > 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.
> > 
> > ok, I will see what I can do.
> 
> Thanks!
> 
> > > 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.
> > 
> > Ok.
> > 
> > > > 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.
> > 
> > Ok.
> > 
> > Not sure these are better, but they might reflect the difference between
> > this guy and a MeshFunction in a slightly more intuitive way.
> > 
> >   MeshEntityFunction, LocalMeshEntityFunction, LocalMeshFunction,
> >   SubMeshFunction
> 
> I'm not sure those are much better, and I don't think it would be
> correct to call them something containing "Function" since they are
> not really functions. With a MeshFunction, one can input x (a mesh
> entity) and get y = f(x) (the value of the MeshFunction at that
> entity). That's not possible with MeshMarkers; they are just a
> collection of markers, not really a function since the value is only
> defined on a subset and one would need to loop through the list of
> values to get the value at any entity where the value is actually
> defined.

What with MeshValueCollection? As it is a templated class I do not think 
Marker is an appropriated name. 'Collection' says that the class is not 
defined over the whole Mesh.

Two questions:

How can the following code work:

      // Get marker data
      const std::vector<uint>& marker = _markers[i];
      const uint cell_index   = marker[0];
      const uint local_entity = marker[1];
      const T marker_value    = marker[2];

when _markers is declared as:

    // The markers
    std::vector<std::pair<std::pair<uint, uint>, T> > _markers;

What is the logic behind:

    // Set all value of mesh function to maximum value (not all will
    // be set) by markers below
    mesh_function.set_all(maxval);

Isn't it more natural to initiate the values to zero? Also it makes no sense 
in conjunction with boundary markers. Then all boundary faces gets marked with 
the largest marker value. I cannot see how that could be correct.

> So MeshMarkers may not be that bad. I'm starting to get used to
> it... :-)

That's what worries me :)

Johan

 
> --
> Anders