Allow to keep units in composite

[gerritholl]

Feature Request

Is your feature request related to a problem? Please describe.

We have single-band "composites" that we use for our IR channels, so that we can apply a crude stretch enhancement. For reference, our enhancement is:

  dwd_IR_120:
    standard_name: dwd_IR_120
    operations:
    - name: stretch
      method: !!python/name:satpy.enhancements.stretch
      kwargs: {stretch: crude, min_stretch: 313.5, max_stretch: 186}

and the composite

  dwd_IR_120:
    compositor: !!python/name:satpy.composites.GenericCompositor
    prerequisites:
      - wavelength: 12.0
    standard_name: dwd_IR_120

Using this composite, we write files that contain temperature units in the header. There are presently wrong; see #2018. We cannot presently use the solution in #2021 (which sets an offset prior to applying any other enhancements), because Satpy helpfully deletes units metadata when constructing the composite:

satpy/satpy/composites/__init__.py

Lines 418 to 422 in 82112a6

	# remove metadata that shouldn't make sense in a composite
	new_attrs["wavelength"] = None
	new_attrs.pop("units", None)
	new_attrs.pop('calibration', None)
	new_attrs.pop('modifiers', None)

With the unit metadata gone by the time we are saving our data file, we cannot safely convert the units between K and °C.

Describe the solution you'd like

I would like that either:

1. The GenericCompositor retains units (and other) metadata if the compositor has exactly one input, or
2. the GenericCompositor retains units metadata if all inputs have the same units, or
3. the GenericCompositor retains units metadata if explicitly asked for by the user, or
4. a different solution I'm not thinking of right now.

Describe any changes to existing user workflow

Alternatives 1 and 2 may result in composites retaining unit metadata where this is not currently the case. Depending on operations applied, those metadata may not be meaningful. Alternative 3 should not lead to any changes to existing user workflow.

Additional context

We could pass a flag to the writer forcing the temperature conversion without checking. This is a risky alternative, but would not require any changes to the widely used GenericCompositor.

I am open to other ideas.

[gerritholl]

• The GenericCompositor retains units (and other) metadata if the compositor has exactly one input, or

• the GenericCompositor retains units metadata if all inputs have the same units, or

• the GenericCompositor retains units metadata if explicitly asked for by the user, or

• a different solution I'm not thinking of right now.

Another option could be to define an entirely different compositor for this use case.

[gerritholl]

Oh, maybe we should be using SingleBandCompositor…

[djhoese]

Yes, SingleBandCompositor is designed for this use case. If you use GenericCompositor it assumes you are creating an image and makes some rigid decisions based on that.