Corrupted alpha channel after palletization

[bryghtlabs-richard]

What did you do?

I converted an image from "RGBA" to "PA"

What did you expect to happen?

I expected the source RGB data to be palettized in the destination P channel, and the source A data to be copied to the A channel.

What actually happened?

The P channel data was zeroed, and the A data was replaced with 0xFF.

What are your OS, Python and Pillow versions?

• OS: Windows 11
• Python: 3.13.3
• Pillow: 11.2.1

--------------------------------------------------------------------
Pillow 11.2.1
Python 3.13.3 (tags/v3.13.3:6280bb5, Apr  8 2025, 14:47:33) [MSC v.1943 64 bit (AMD64)]
--------------------------------------------------------------------
Python executable is C:\Python313\python.exe
System Python files loaded from C:\Python313
--------------------------------------------------------------------
Python Pillow modules loaded from C:\Users\RichardAllen\AppData\Roaming\Python\Python313\site-packages\PIL
Binary Pillow modules loaded from C:\Users\RichardAllen\AppData\Roaming\Python\Python313\site-packages\PIL
--------------------------------------------------------------------
--- PIL CORE support ok, compiled for 11.2.1
--- TKINTER support ok, loaded 8.6
--- FREETYPE2 support ok, loaded 2.13.3
--- LITTLECMS2 support ok, loaded 2.17
--- WEBP support ok, loaded 1.5.0
*** AVIF support not installed
--- JPEG support ok, compiled for libjpeg-turbo 3.1.0
--- OPENJPEG (JPEG2000) support ok, loaded 2.5.3
--- ZLIB (PNG/ZIP) support ok, loaded 1.3.1.zlib-ng, compiled for zlib-ng 2.2.4
--- LIBTIFF support ok, loaded 4.7.0
*** RAQM (Bidirectional Text) support not installed
*** LIBIMAGEQUANT (Quantization method) support not installed
*** XCB (X protocol) support not installed
--------------------------------------------------------------------

from PIL import Image

#4xRGBA pixels
pixels = bytes([
    0, 1, 2, 3,
    4, 5, 6, 7,
    8, 9, 10, 11,
    12, 13, 14, 15,
    ])

img = Image.frombytes('RGBA', (4, 1), pixels)
print("RGBA data:", list(img.getdata()))
# img = img.convert("PA")#same result as next line
img = img.convert("PA", palette=Image.ADAPTIVE, colors=4)
print("PA data:", list(img.getdata()))
# print("PA pal:", list(img.getpalette(rawmode="RGB")))

Expected

$ python ~/Desktop/rgb2pa.py
RGBA data: [(0, 1, 2, 3), (4, 5, 6, 7), (8, 9, 10, 11), (12, 13, 14, 15)]
PA data: [(0, 3), (1, 7), (2, 11), (3, 15)]

Actual

$ python ~/Desktop/rgb2pa.py
RGBA data: [(0, 1, 2, 3), (4, 5, 6, 7), (8, 9, 10, 11), (12, 13, 14, 15)]
PA data: [(0, 255), (0, 255), (0, 255), (0, 255)]

We know asking good questions takes effort, and we appreciate your time.

Thank you for Pillow and your support too!

[bryghtlabs-richard]

Ohhh, I think the solution to #9153 might fix this too

[bryghtlabs-richard]

I patched my local installation and now get a compatible result(palette order was different than I had guessed).

Thanks!