Mismatch alpha channel when converting "P" to "PA" compared to "P" to "RGBA"

[RedShy]

What did you do?

I opened a PNG "P" image and converted it to "PA" and "RGBA".

What did you expect to happen?

The alpha channel for every pixel is the same for the "PA" and "RGBA" conversion

What actually happened?

The alpha channel in the "PA" image is different from the "RGBA" image

What are your OS, Python and Pillow versions?

• OS: Microsoft Windows 10.0.19043 Pro
• Python: 3.10.4
• Pillow: 9.1.1

def output(bytes):
    return ''.join(['\\x%02x' % b for b in bytes])

assert Image.open("tiny.png").mode == "P"

img_PA = Image.open("tiny.png").convert("PA")
img_RGBA = Image.open("tiny.png").convert("RGBA")

print(output(img_PA.tobytes()[slice(1, None, 2)]))   # \x00\x00\xff\xff\x00\x00\xd0\xb7\x00\x00\x38\x00\x00\x00\x15\xc0
print(output(img_RGBA.tobytes()[slice(3, None, 4)])) # \x00\x00\xff\xff\x00\x00\xff\xff\x00\x00\xff\xff\x00\x00\xff\xff

The tiny image used:

I added PR #6337 that should resolve the issue

[Lucas-C]

Thank you for reporting this and submitting a PR @RedShy 👍