Swarm proxy refresh is broken whenever reference quantities are set (dimensional coords into a non-dimensional kd-tree)

[lmoresi]

Found while adding the linear-exact local RBF interpolator (feature/linear-rbf).

The defect

SwarmVariable._rbf_to_meshVar gathers proxy node values from the particles:

new_coords = meshVar.coords          # swarm.py, _rbf_to_meshVar
...
values = kdt.rbf_interpolator_local(new_coords, D, nnn, ...)

MeshVariable.coords dimensionalises when the model has reference quantities active — it returns a UnitAwareArray in metres. The kd-tree it is handed comes from Swarm._get_kdtree(), built from self._particle_coordinates.data, which is non-dimensional, so KDTree.coord_units is None.

ckdtree.KDTree._convert_coords_to_tree_units then raises:

ValueError: KD-tree was built with dimensionless coordinates, but query
coordinates have units 'meter'. Convert to dimensionless first.

So under an active units model the proxy refresh does not work at all. It fails loudly rather than silently corrupting, which is the one piece of good news.

Two more sites have the identical pattern, in IndexSwarmVariable._update_proxy_variables:

kd_swarm.query(self._meshLevelSetVars[0].coords, k=1, sqr_dists=False)
kd.query(self._meshLevelSetVars[0].coords, k=self.nnn, sqr_dists=False)

grep coords_nd src/underworld3/swarm.py returns nothing — the swarm module is the outlier. Code that gets this right: MeshVariable._get_kdtree uses coords_nd; SemiLagrangian uses psi_star[i].coords_nd; _apply_monotone_limit non-dimensionalises explicitly.

Fix

.coords -> .coords_nd at the three sites.

Related

SwarmVariable._create_proxy_variable never forwards units= to the proxy MeshVariable constructor, so a variable created as units="kg/m^3" has var.units == kg/m**3 while var._meshVar.units is None and var.sym is a dimensionless symbol. Probably the same root cause — the proxy path predates the units work.

Test gap

No test exercises a proxied swarm variable under model.set_reference_quantities(...). A regression test should.

A # TODO(BUG): marker pointing here has been added at the _rbf_to_meshVar site on feature/linear-rbf.

[lmoresi]

Fixed by PR #434 (proxy refresh converts to the kd-tree's non-dimensional frame under an active units model), merged to development on 2026-07-27. (Manual close — PRs target development, so auto-close doesn't fire.)

[lmoresi]

Closed by #434, which fixed the coordinate half (.coords → .coords_nd at the three live sites).

The second defect reported here — _create_proxy_variable not forwarding units=, so var.units is set while var._meshVar.units is None — was not fixed and is now tracked separately as #439. It needs a units-boundary decision rather than a coordinate fix, since attaching units to the proxy makes var.sym unit-aware everywhere it is composed into an expression.