dolfin team mailing list archive

[Johan Hake <johan.hake@xxxxxxxxx>]

On Thursday April 8 2010 04:51:24 Marie Rognes wrote:
> There is something suboptimal with regard to assembly over sub_domains
> specified
> by domains (in contrast to markers)
> 
> Say I have some functional
> 
> (*)    M = f*ds
> 
> where f is some function. Let 'domain' be some sub-domain.
> 
> Now, it is not clear to me what
> 
>     v = assemble(M, mesh=mesh, exterior_facet_domains=domain)
> 
> gives. It would be really convenient (for my purposes) if it gave
> the same as
> 
>     v = assemble(M, mesh=mesh)

Considering that ds defaults to ds(0) I think it is a logical behaviour. When 
you want the whole boundary you do not pass any exterior domain. If you want 
the ds form integrated over a certain boundary you pass that boundary.

> It would also make some sense, considering that no special
> tag has been defined in (*), and so the exterior_facet_domains
> argument could be ignored.
> 
> It does not at the moment though. At the moment, it seems
> as though ds -> ds(0) and the sub-domain is converted
> to markers, marking anything inside 'domain' as 0, and
> everything else as 1...?  This is really not obvious to me.

I do not know if changing the default representation of ds, or documenting 
(better) that ds defaults to ds(0) could help. The latter whould be the 
easiest fix.

> Could "we" change the interface to something slightly
> more transparent?
> 
> --
> Marie
> 
> 
> PS: Example attached.
> 
> 
> 
> from dolfin import *
> 
> class Domain(SubDomain):
>     def inside(self, x, on_boundary):
>         return x[0] < 0.5 and on_boundary
> 
> mesh = UnitSquare(8, 8)
> f = Expression("sin(pi*x[0])")
> M = f*ds
> 
> domain = Domain()
> 
> markers = MeshFunction("uint", mesh, 1)
> markers.set_all(0)
> domain.mark(markers, 1)
> 
> v1 = assemble(M, mesh=mesh)
> v2 = assemble(M, mesh=mesh, exterior_facet_domains=domain)
> v3 = assemble(M, mesh=mesh, exterior_facet_domains=markers)
> 
> print "It is not intuitive what these become ..."
> print "v1 = ", v1
> print "v2 = ", v2
> print "v3 = ", v3

At least v2 and v3 add up to v1 :) 

If you changed

  markers.set_all(0)
  domain.mark(markers, 1)

to

  markers.set_all(1)
  domain.mark(markers, 0)

You would get the same.

Johan

> 
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