to_image changes image mode

[gerritholl]

Describe the bug

Calling to_image changes a dataset with mode P to one with mode L.

To Reproduce

import xarray as xr
from trollimage.xrimage import XRImage
from satpy.writers import to_image
da = xr.DataArray(
        data=[[[1, 2, 3], [4, 5, 6]]],
        dims=("bands", "y", "x"),
        coords={"bands": ["P"]},
        attrs={"mode": "P"})
im = to_image(da)
print(im.mode)
# direct does retain it
im = XRImage(da)
print(im.mode)

Expected behavior

Expected output:

P
P

Actual results

Actual output:

L
P

Environment Info:

• OS: openSUSE Leap 13.0
• Satpy Version: v0.38.0-65-g6dfb9b93

Additional context

This happens due to the .squeeze() call in satpy.writers.to_image:

satpy/satpy/writers/__init__.py

Line 483 in 6dfb9b9

dataset = dataset.squeeze()

This deletes the bands dimension for mode P or L. Then XRImage.__init__ adds a bands dimension again with mode L.

The conversion presents an obstacle for handling P-mode images.

[djhoese]

Wow nice catch! This looks like it was added in #83 in one of the first PRs adding dask/xarray support to Satpy.

I don't think we need the squeeze. Maybe try removing it and if tests pass then 👍

CC @mraspaud who originally added the code.