Populate space_unit attribute upon dataset loading/creation?

[niksirbi]

Proposal

PR #384 introduces an attribute—provisionally named space_unit—that specifies the spatial unit (e.g. "mm", "pixels", etc.). This attribute is optionally added by transformation functions such as scale.

I wonder whether it might be beneficial to create this attribute right from the start when loading a dataset via from_file() or from_numpy(), for both "poses" and "bboxes" datasets.

This would mirror our handling of the time_unit attribute:

• Set to "frames" if fps=None.
• Set to "seconds" if a valid fps value is supplied.

Making this change would likely require updates to load_poses.py, load_bboxes.py, and possibly validators.datasets.py. For all of our supported formats—except, potentially, "Anipose"—the spatial unit would be "pixels".

Caveats

• time_unit is currently a Dataset-level attribute, because both the "position" and "confidence" arrays have a time dimension.
• space_unit in the transforms module is added at the DataArray level, which makes sense since those transforms operate on individual data arrays. As a result, transforms only modify the attribute on a per-array basis.
• It is not entirely clear where space_unit should be added when creating a "poses" or "bboxes" dataset. Logically, it might belong at the DataArray level (because confidence does not have a space dimension). For a "poses" dataset, space_unit would then be an attribute of position, whereas for a "bboxes" dataset it would be an attribute of both position and shape.
• If we retain time_unit at the Dataset level but introduce space_unit at the DataArray level, it becomes more difficult to unify them into a common units={dim_name: dim_unit} dictionary. Perhaps all dimension units should be defined at the array level.

Alternative

We could leave things as they are: space_unit would not exist by default and would only be added by transformations, at which point the user must specify or decide upon the spatial unit.

See Also

• PR Add transforms module with scale function #384
• Related to issue Make xarray objects unit-aware via Pint-xarray #141
• Somewhat related to issue Provide coordinates for both time and frame #117