Rename point integral to vertex integral in dolfin/ufl/ffc/ufc

[Martin Sandve Alnæs]

All other integrals have consistent naming across ufl/ufc/ffc/assembler, and this one exception is in the way for generalizing code on the form 'for integral_type in ("cell", "point", ...)'.

I vote for using 'vertex' for the current feature and reserve 'point' to mean arbitrary points within cells in a future implementation.

@johanhake @meg @pefarrell feel free to comment.

[Johan Hake]

The name is not important. If I recall correctly we picked the name such that it could be used for both Vertices and arbitrary points.

If you choose to change the name maybe you can also change PointIntegralSolver to VertexIntegralSolver?

[Martin Sandve Alnæs]

Ok!

[Prof Garth Wells]

Is it really a VertexIntegralSolver, or more a Lagrange NodeIntegralSolver, i.e. solves problems at cell 'nodes'?

[Johan Hake]

What is the difference between solving problems at cell nodes and vertices?

You have a point about Lagrange, as we only accept problems formulated with CG1 test functions.

[Prof Garth Wells]

Vertices are vertices of triangle, etc. 'nodes' are where dofs live, e.g. vertices and side midpoints for P2 elements.

[Martin Sandve Alnæs]

I think it's easily generalisable from vertices to nodes. Maybe "node integral" then?

[Anders Logg (Chalmers)]

Node integral sounds ok, but a comment is needed if this only works for Lagrange nodes.

[Martin Sandve Alnæs]

I think Lagrange only, I think a core property of this functionality is that there is one basis function from the global test space that is 1.0 at each node and the rest is 0.0.

However a different generalisation of vertex integrals I've been thinking about is to treat them as cell integrals over 0d vertex cells embedded in Nd.

Does anyone need a node integral solver?

[Anders Logg (Chalmers)]

Isn't the essential property that we have a nodal basis? It doesn't need to be Lagrange. For all element types we have a basis that satisfies that exactly one dof evaluates to 1 for each basis function so it could well be that the dofs happen to be say tangential component at a point, or some integral moment.

[Lawrence Mitchell]

Note, FWIW, that EnrichedElements are not nodal (since the basis is not reorthogonalised).

[Martin Sandve Alnæs]

I don't know, sounds correct. Still, does anyone need it or is this just a discussion for the sake of it?

However the point/vertex integral is a sum over a set of point coordinates (currently the vertices). From an implementation point of view, the set of dof coordinates can be used without caring about the type of nodal functional. However I don't know if it makes sense. And again, does anyone need it?

In this issue I just proposed a renaming here to enable convenient generalization in some code snippets that does 'if integral type = "point": dosomething("vertex")'.

[Anders Logg (Chalmers)]

I don't need it for anything right now. Just pointing out that NodeIntegral seems like a good name for it since it actually might work for other nodes than only Lagrange point evaluation.

[Marie Elisabeth Rognes]

As far as I remember, it is currently only supported for vertices. The work "point" reflects "point measure" as in point evaluation, "vertex measure" sounds less familiar, but I don't mind that being changed.

We are currently actively using the point measure/integral/solver for CG1. A potentially relevant extension would be evaluation in quadrature points (rather than more general dofs), but this is not needed for our applications in the near future.

[Martin Sandve Alnæs]

Just to clarify one thing here: one of the main reasons we need different names for (i) arbitrary points vs (ii) vertices/nodes, is that the code generation will be different to exploit precomputation of basis functions in (ii).

Also note that (i) is very close to custom integrals, and we have a use case for that which is why I want it.

[Martin Sandve Alnæs]

in next

[Martin Sandve Alnæs]

I've renamed point integral but kept *dP and not renamed PointIntegralSolver.

[Martin Sandve Alnæs]

Removing milestone: 1.6 (automated comment)