Ah, my intuition for the local order of nodes was wrong.

For example, the following code
---
from firedrake import *

mesh = UnitIntervalMesh(4)
V = FunctionSpace(mesh, "CG", 4)
u = Function(V).interpolate(Expression("x[0]"))

print_kernel ="""
printf("%f %f %f %f %f\\n", u[0][0], u[1][0], u[2][0], u[3][0], u[4][0]);
"""

par_loop(print_kernel, dx, {"u": (u, RW)})
print u.dat.data
---
produces
0.250000 0.000000 0.187500 0.125000 0.062500
0.500000 0.250000 0.437500 0.375000 0.312500
0.750000 0.500000 0.687500 0.625000 0.562500
1.000000 0.750000 0.937500 0.875000 0.812500

(our IntervalMesh cells are 'backwards', just to make things more fun, but this shouldn't be the case in extruded meshes)

In other words, it looks like the CG 'ends' come before the middle, so try 0, 1, 3, 4.

Thanks for spotting this, because it means some of my own visualisation will be wrong!

On 14 September 2015 at 19:57, Cotter, Colin J <colin.cotter@imperial.ac.uk> wrote:
Dear friends,
  I have been doing some stuff with a DG1 x CG2 field, as we use it for temperature in the compatible FEM framework. I'm getting some funny
output when trying to interpolate. In the following code:

https://gist.github.com/colinjcotter/96e23745d40a73303b05

I interpolate a function to a DG1 x CG2 field. The function represented should
be continuous.

Then, I remap it using a Firedrake kernel to a DG1 x DG1 field. The function
should still be continuous.

When I visualise the field in Paraview, I see that it has jumps in the vertical
direction.

What went wrong?

cheers
--cjc