Fixed that save_dataset does not propagate fill_value

[TAlonglong]

Fixed that save_dataset does not propagate fill_value into get_enhanced_image. Further does not get_enhanced_image propagate fill_value into add_overlay and add_decorate. Those in turn does not propagate fill_value into pil_image.

[coveralls]

Coverage decreased (-0.02%) to 71.92% when pulling 7da641d on TAlonglong:fix-propagate-fill_value into 078a73a on pytroll:master.

[codecov]

Codecov Report

Merging #423 into master will not change coverage.
The diff coverage is 20%.

@@           Coverage Diff           @@
##           master     #423   +/-   ##
=======================================
  Coverage   71.94%   71.94%           
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  Files         128      127    -1     
  Lines       16884    16884           
=======================================
  Hits        12147    12147           
  Misses       4737     4737

Impacted Files	Coverage Δ
satpy/writers/__init__.py	76.4% <20%> (ø)	⬆️
satpy/version.py
satpy/__init__.py	94.73% <0%> (+0.61%)	⬆️

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[djhoese]

So I've looked at this more and it makes sense that fill_value needs to be passed to the overlay/decorate functions so that pil_image can "finalize" the image properly (adding or not adding an alpha channel as desired). From what I understand the series of events for the bad performing use case is:

1. Provide L or RGB DataArray to get_enhanced_image
2. Add decoration or overlay (using associated kwarg to get_enhanced_image.
3. These call pil_image on the XRImage object which by default will make an 8-bit LA or RGBA DataArray. Which ends up getting returned from get_enhanced_image.
4. Writer tries to save LA/RGBA data to a JPEG/PNG using the PillowWriter (trollimage.xrimage.XRImage.pil_save). This calls XRImage.finalize which sees that fill_value was provided so it fills any invalid (null) pixels in the DataArray with the fill value but it does not remove the Alpha channel. XRImage assumes that since the user provided an Alpha channel that they want that to still exist. Which I'm guessing fails when saved to JPG and produces undesired output with PNG, right?

So the above situation makes sense for why this is happening and producing undesired results. I have no issue with this solution. I do have an issue with the performance of the functions as implemented. IMO the multiple calls to pil_image and / 255.0 to renormalize the data are wasteful. This of course only happens if an image is decorated and "overlayed" in the same call. I don't want this PR to include a change for this but just wanted to mention it as an issue.

[djhoese]

Found a typo

[djhoese]

Typo fill_value versus fille_value.

[TAlonglong]

Thanks

Regarding the pil_image; yes it is a bad solution if both are invoked. But at the time this was implemented I could not see another solution.

I'm happy to discuss this at some point in time.

[djhoese]

Best I could think of is pass the PIL image to these functions. It makes them harder to use outside of the get_enhanced_image function, but not sure that is important. Regardless, it is fine the way it is.