Define "image group"

[dstansby]

The term "image group" is used a few times within the specification, but it doesn't seem to be defined anywhere. It would be useful to define this term somewhere.

[joshmoore]

👍 Going through https://ngff.openmicroscopy.org/0.5/

• "series" MUST be a list of string objects, each of which is a path to an image group.
• The "labels" group is nested within an image group,
• This object MAY include a key image, whose value MUST be a string specifying the relative path to a Zarr image group.
• The default value is ../../ since most labeled images are stored in a "labels" group that is nested within the original image group.

Each of these equals "Zarr group with a multiscales" (and "labels" group is Zarr group which is both a multiscales and a label")