[Don-Yin]

I tried filling a 3×3 white image from (-1, 1) and all nine pixels turned red. A seed beyond the right or bottom edge already leaves the image alone.

from PIL import Image, ImageDraw

im = Image.new("RGB", (3, 3), "white")
ImageDraw.floodfill(im, (-1, 1), (255, 0, 0))
print(im.getpixel((0, 0)))  # (255, 0, 0), despite the seed being outside

Changes proposed in this pull request:

• I checked the seed before pixel access so negative coordinates cannot wrap around.
• I added a regression test for negative x/y seeds and a release note.

I ran the drawing tests, python3 selftest.py, pre-commit run --all-files, and python3 -m tox -e mypy successfully. python3 -m pytest Tests/ had nine Palm/ImageGrab failures that I also reproduced on unchanged main: the local convert executable is broken and screen capture is unavailable.

[radarhere]

I tried filling a 3×3 white image from (-1, 1)

Can I ask - why did you try and do that?

[radarhere]

What you're experiencing is not necessarily a bug.

#3406 suggested

Add a negative indexing to the PixelAccess so we can get the last pixels easily

from PIL import Image

im = Image.open("image.png")
pix = im.load()
print(pix[-1, -1])

When floodfill() goes to get the background colour, it uses the same mechanism.

Pillow/src/PIL/ImageDraw.py

Line 834 in 76a7d26

background = pixel[x, y]

So to the mind of that user, this might also be considered a feature.

Pillow values backwards compatibility, so I'm inclined not to revert this behaviour.

[Don-Yin]

Honestly, I came across it while trying edge cases rather than in a real workflow, so I don't have a strong use case for changing it. Thanks for digging up #3406; I didn't realise the wraparound came from there. I'm happy to close this. I can also send a one-line docstring note instead, saying that negative seed coordinates count from the far edge, if that would help.

[radarhere]

I've created #10297.

So at https://pillow.readthedocs.io/en/stable/reference/ImageDraw.html#PIL.ImageDraw.floodfill, the user would need to click on 'Coordinate system' to see this new note.

See what you think.