On Wed, Jun 20, 2012 at 8:40 PM, Blaise Bourdin <bourdin@lsu.edu> wrote:

Hi,

I want to change the way I handle boundary conditions in my finite elements code:
Right now, I skip the rows and columns during the matrices assembly, using a flag (a SectionInt). Using an IS denoting all dof with a BC and MatZeroRowsColumns, MatZeroRowsColumnsIS or MatZeroRowsColumnsLocal would make much more sense.

The difficulty comes up when applying a Dirichlet boundary condition on only one of the dof associated with a vertex (roller boundary conditions in elasticity, for instance). What I can easily get from Sieve is the point number associated with a vertex, for instance, from which I can obtain the _local_ index of the dof associated with a given component of a field at this point. For vectors, I can then set values on the local Vec or the Section. For matrices, though I can't just use MatGetLocalSubMatrix since I need also to modify the out of diagonal block terms.

I guess I need the LocalToGlobalMapping or to renumber the IS describing my BC and convert it to global indices. I am sure that this information is somewhere in Sieve (in the overlap, I guess), but I was thinking that since getting a Local to Global scatter is trivial, there may be a way to obtain the mapping or renumbering the IS from the scatter.

Now that I think about it more, the right way is most likely to get the information I need from Sieve.

Yes, in the new version there is jsut a LocalToGlobalMapping. In C++, you can use the GlobalNumbering,

  numbering->getIndex(point) + dof offset

And for the record, in the old version, where a DMMesh can be associated with several data layout, the following seems to work (adapted from DMCreateLocalToGlobalMapping_Mesh):