Slicing of structured curvilinear grids

Is your feature request related to a problem?

Currently NDPointIndex (e.g. with KDTree) only allows advanced indexing.

Can we make it support slicing too (with appropriate semantics)?

Describe the solution you'd like

I just wrote this up

import xarray as xr

ds = xr.tutorial.open_dataset("ROMS_example").set_xindex(("lat_rho", "lon_rho"), xr.indexes.NDPointIndex,)

# intended slicer
slicers = dict(lat_rho=slice(28, 29), lon_rho=slice(-91, -89))

To get this to work, I did this:

import itertools

index = ds.xindexes["lat_rho"]
edges = tuple((slicer.start, slicer.stop) for slicer in slicers.values())
vectorized_sel = {
    name: xr.DataArray(dims=("pts",), data=data)
    for name, data in zip(
        slicers.keys(), map(np.asarray, zip(*itertools.product(*edges)))
    )
}
idxrs = index.sel(vectorized_sel, method="nearest").dim_indexers
new_slicers = {
    name: slice(array.min().item(), array.max().item()) for name, array in idxrs.items()
}
subset =ds.sel(new_slicers)

Output

As you can see: this effectively defines slicing as an operation that "returns a continuous slice of data such that every point within the bounding box defined by the slice objects is returned." Does this make sense?

ds.salt.isel(s_rho=-1, ocean_time=0).plot(x="lon_rho", y="lat_rho")
ds.salt.isel(s_rho=-1, ocean_time=0, **new_slicers).plot(
    x="lon_rho", y="lat_rho", cmap="Blues", add_colorbar=False
)

x0, x1 = slicers["lon_rho"].start, slicers["lon_rho"].stop
y0, y1 = slicers["lat_rho"].start, slicers["lat_rho"].stop
import matplotlib.pyplot as plt

plt.plot([x0, x1, x1, x0, x0], [y0, y0, y1, y1, y0], color="r")

Describe alternatives you've considered

We could mask out data outside the box, but that then becomes a copy instead of a view.

Additional context

No response

Start with xarray.indexes.NDPointIndex and its sel entry point, then trace how the shown slicers become dim_indexers. Clarify and test the intended bounding-box semantics, including whether the result remains a view rather than a masked copy; the work is done when structured curvilinear grids support the requested slice form.