dolfin team mailing list archive

["Garth N. Wells" <gnw20@xxxxxxxxx>]

On 6 September 2011 17:31, Johan Hake <johan.hake@xxxxxxxxx> wrote:
> 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.

Agree.

> '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;
>

The above also permits multiple entries. Perhaps we want

    boost::unordered_map<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.
>

I don't get ' mesh_function.set_all(maxval);' or the code comment.

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

Me too (worried, that is).

Garth

> Johan
>
>
>> --
>> Anders
>
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