dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Tue, Dec 19, 2006 at 04:01:02PM +0100, Johan Hoffman wrote:
> > On Tue, Dec 19, 2006 at 02:59:29PM +0100, Johan Hoffman wrote:
> >> I am now working on mesh refinement/coarsening in Dolfin. There are 2
> >> different approaches; either (1) we have one mesh object that we modify,
> >> or (2) we have a hierarchy of meshes. I think we should support both (1)
> >> and (2). The benefit of (1) is that it is simple and cheap to store,
> >> whereas (2) can make coarsening simpler and is typically needed in
> >> geometric multigrid.
> >>
> >> I plan to add a class MeshHierarchy for the purpose of (2). I want to
> >> keep
> >> it simple, but to minimize storage we need to be somewhat more clever
> >> than
> >> just using a simple array with meshes. I guess something like (i) a
> >> primitive base mesh; mesh0, which is a regular mesh object, together
> >> with
> >> (ii) the number of refinement levels, and (iii) for each level k the
> >> cells
> >> that are part k but not k-1, together with their vertices and edges.
> >
> > Sounds good. The mesh class should support this without difficulty
> > (storing a mesh that is not connected).
> 
> Good.
> 
> > The mapping from one level to the other can use the class MeshFunction
> > in the same way that it is currently used to map entities on a
> > boundary mesh to the original mesh.
> 
> Ok.
> 
> >> Is there any problem in considering (iii) to be some sort of "broken
> >> mesh", and treat mesh_k as a regular mesh object that may not be
> >> connected? If we do this then MeshHierarchy would basically just be a
> >> collection of mesh objects, together with parent-child info.
> >
> > So the parent-child info could be a MeshFunction.
> >
> > I think there should also be some functionality for storing the entire
> > hierarchy so that each mesh in the hierarchy is a complete
> > representation. Maybe there could be a flag to choose how the
> > hierarchy is stored?
> 
> Yes, we could do that. In either case one should be able to ask for mesh_k
> and get a complete mesh object by assembling the "leaf cells" of meshes
> m<k, with the leaf cells being cells without children.
> 
> /Johan

Good idea, then maybe we don't need the alternative full
representation. How about something like this (which is sort of what
we had in the old MeshHierarchy):

  class MeshHierarchy
  public:

    Mesh operator[] (uint i) const;

  }

So one can do

  MeshHierarchy hierarchy;
  Mesh mesh = hierarchy[3];

?

/Anders


> 
> 
> 
> 
> 
> > /Anders
> >
> >
> >> Any other ideas/comments/objections?
> >>
> >> /Johan
> >
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> >
> 
> 
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