Image.convert broken for P to RGB?

[TheRealQuantam]

I'm encountering several severe issues with palettized color in Pillow 9.1.1 on Python 3.7 64-bit. I'm posting this in discussions in case I am doing something wrong, but if I'm not, I'll elevate these to issues. Per request I'm splitting off each issue into a separate thread.

Image.convert from P to RGB behaves like L to RGB and DOES NOT use Image.palette.

Partial code:

img = Image.fromarray(bitmap, "P")
img.palette = img_pal
img.convert("RGB").save("converted as greyscale not palette.png")

bad_pillow.zip

Correct version:

Converted as L to RGB when it should have been converted as P to RGB:

[radarhere]

Testing your code, I think all of your problems can be solved by replacing

img.palette = img_pal

with

img.putpalette(img_pal)

See https://pillow.readthedocs.io/en/stable/reference/Image.html#PIL.Image.Image.putpalette for documentation.

[TheRealQuantam]

Confirmed that this issue is fixed by putpalette. I'll update the other two with the problems remaining.

Thanks for the help.

[TheRealQuantam]

Might it be a good idea to make Image.convert fail if the image is set to P but no palette has been properly set? Silently treating it as L is really confusing.

[radarhere]

If you run

img = Image.fromarray(bitmap, "P")
print(img.palette.palette)

you will see that there is a palette, it's just empty. It doesn't sound right to say that an empty palette is invalid.

Also, the next release of Pillow will change the behaviour of fromarray with "P" mode so that the palette is populated by default. See #6283.

[TheRealQuantam]

I see. Okay. Thanks for the help.