Interpolating between different meshes fails

[Harish Narayanan]

When I attempt to interpolate between different meshes, I get completely erroneous results. The following example demonstrates this:

from dolfin import *

mesh1 = UnitSquare(8, 8)
mesh2 = UnitSquare(16, 16)

V1 = FunctionSpace(mesh1, "CG", 1)
V2 = FunctionSpace(mesh2, "CG", 1)

f1 = Function(V1, "500.0 * exp(-(pow(x[0] - 0.5, 2) + pow(x[1] - 0.5, 2)) / 0.02)")
f1.interpolate()

f2 = interpolate(f1, V2)

plot(f1, title="Original function")
plot(f2, title="Interpolated function")
interactive()

Furthermore, when I comment out f1.interpolate(), i.e. keep it defined as an expression, I do not run into this problem. A workaround (as suggested by Anders) for this seems to be to comment out data._ufc_cell = &cell; from dolfin/function/UFCFunction.cpp.

[marc spiegelman (mspieg)]

This is problem exists in the c++ version as well.
It's actually critical for some codes I'm writing where I need to interpolate to a submesh.
Everything worked up to revision 6870, and I suspect it's related to putting ufc_cell into data (as is suggested by the workaround in the original bug report).

[Anders Logg (logg)]

My fault. I've assigned this to milestone 0.93 so it will be fixed before the release.

[Anders Logg (logg)]

Reverted optimization to UFCFunction.cpp (not storing _ufc_cell). The problem was
that the cell might originate from another mesh and so was not valid. We could
consider storing the mesh also and check that as well but the Function class is
up for redesign (simplification) anyway so it can wait until then.

Closing this bug.