On Mon, Sep 08, 2008 at 09:04:22AM -0400, Shawn Walker wrote: > > > On Mon, 8 Sep 2008, Johan Hoffman wrote: > > >> 2008/9/8 Dag Lindbo <[EMAIL PROTECTED]>: > >>> Anders Logg wrote: > >>>> There seems to be a problem (among many) with the current design of > >>>> the Function classes (see thread "evaluating higher order mesh > >>>> function"). > >>>> > >>>> In particular, the finite element is missing in DiscreteFunction. My > >>>> suggestion would be to just add it and let a DiscreteFunction consist > >>>> of the following four items which are always available: > >>>> > >>>> mesh, x, dof_map, finite_element > >>>> > >>>> Is this enough, and what other issues to we need to fix? > >>>> > >>> > >>> I'm not sure I agree that the dof map and finite element should be owned > >>> by the discrete function. There was a great suggestion from Martin, in a > >>> thread "Abstraction idea" from 06/05/2008, to create a class > >>> FunctionSpace where the mesh, element and dof_map(s) are aggregated. > >>> Citing Martin: > >>> U = FunctionSpace(mesh, dofmapset, form, 0) # or something similar > >>> u = Function(U) > >>> v = Function(U) > >>> > >>> This seems a solid approach to me since it would provide a way of > >>> encapsulating the mathematical formulation of the problem, which is more > >>> or less const and likely to be reused by many discrete functions in a > >>> solver. > >>> > >>> It seems to me that there is an obvious risk that a lot of redundant > >>> initialization would occur if all discrete functions should own their > >>> own elements and dof maps. There seems to be consensus that the mesh > >>> should be "global" for efficiency reasons, so why not treat the function > >>> space the same way? > >>> > >>> Is there a problem with an approach where the funciton _always_ owns the > >>> vector and _never_ owns the function space (and mesh)? A very strict > >>> design would avoid shared/smart pointers, provide a comprehensible user > >>> interface and probably help the parallellization effort. > >>> > >>> /Dag > >> > >> If the Function always owns the vector, there are cases you'll have to > >> make unneccessary copies of a vector, in particular such scenarios > >> may occur when trying to combine dolfin with something else. > >> > >> If the Function never owns the function space, it must always be > >> constructed explicitly by the user. This may not be a bad thing. > >> However, if the Function is loaded from a file, nobody owns the > >> FunctionSpace. > >> > > > > Conceptually, I agree with Dag (and Martin?) that it is natural to have > > global function spaces. And if the explicit construction of such spaces > > can be made simple, it may not be a bad thing but a natural part in > > setting up the mathematical problem. And I do not really like that > > functions should be initialized from a form, which defines an equation. > > I agree very much with this last sentence. > > > I think one idea was to not force less mathematically oriented users to > > worry about function spaces. I guess there are (at least) 2 types of > > functions: (i) functions part of the form, and (ii) functions not part of > > the form, but used in pre/postprocessing etc. > > Do you really think it is a huge leap for users to know what a function > space is? They don't need to know functional analysis. > > > For (i) it may be natural to construct the function space from the form, > > and for (ii) it may be convenient in some cases, but it is not really > > obvious that this is the best solution. > > > > Maybe an explicit construction of a function space can come with a > > default, such as a nodal basis of piecewise linears? > > Note: it must be possible to update the function space if the mesh > changes. I guess this would be automatic since the function space > references the mesh. Assuming the mesh does not change topology; then I > guess there would be some interpolations happening, which could be done by > a higher level module or the user. > > This thread started from me trying to add a Function to the MeshGeometry > class for use in higher order shaped elements. Maybe it would be easier > to change that, than the whole Function setup? Unless the Function class > needs to change anyway.
It needs to change anyway. -- Anders
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