rounded_rectangle with radius > 50% and single corner

[rysson]

What did you do?

Use rounded_rectangle() with radius bigger than half of the size with single corner enabled.

What did you expect to happen?

Draw correct corner arc without extra pixels.

What actually happened?

Visible fill color outside of the rounded rectangle.

What are your OS, Python and Pillow versions?

• OS: Linux (Debian Forky)
• Python: 3.13.12
• Pillow: 12.2.0

--------------------------------------------------------------------
Pillow 12.2.0
Python 3.13.12 (main, Feb  4 2026, 15:06:39) [GCC 15.2.0]
--------------------------------------------------------------------
Python executable is /home/user/work/kodi/koframes/.venv/bin/python3
Environment Python files loaded from /home/user/work/kodi/koframes/.venv
System Python files loaded from /usr
--------------------------------------------------------------------
Python Pillow modules loaded from /home/user/work/kodi/koframes/.venv/lib/python3.13/site-packages/PIL
Binary Pillow modules loaded from /home/user/work/kodi/koframes/.venv/lib/python3.13/site-packages/PIL
--------------------------------------------------------------------
--- PIL CORE support ok, compiled for 12.2.0
--- TKINTER support ok, loaded 8.6
--- FREETYPE2 support ok, loaded 2.14.3
--- LITTLECMS2 support ok, loaded 2.18
--- WEBP support ok, loaded 1.6.0
--- AVIF support ok, loaded 1.4.1
--- JPEG support ok, compiled for libjpeg-turbo 3.1.4.1
--- OPENJPEG (JPEG2000) support ok, loaded 2.5.4
--- ZLIB (PNG/ZIP) support ok, loaded 1.3.1, compiled for zlib-ng 2.3.3
--- LIBTIFF support ok, loaded 4.7.1
--- RAQM (Bidirectional Text) support ok, loaded 0.10.3, fribidi 1.0.16, harfbuzz 13.2.1
*** LIBIMAGEQUANT (Quantization method) support not installed
--- XCB (X protocol) support ok
--------------------------------------------------------------------

from PIL import Image, ImageDraw

def run(radius, file):
    mask = Image.new("L", (100, 100), 0)
    draw = ImageDraw.Draw(mask)

    draw.rounded_rectangle((0, 0, *mask.size), radius=radius, fill=128, outline=255,
                           corners=(True, False, False, False))

    mask.save(file)

run(50, "ok.png")
run(53, "looks_ok.png")
run(54, "bug.png")
run(75, "bug75.png")

ok and looks_ok:

Bug:

Look at extra pixel the image top:

Bigger radius makes more extra pixels (radius 75):

Summary

I know radius bigger than 50% has no sense for two nearest corners (neighbors). But it seems have a sense for single (or diagonal) corners.
If you want to disable those radii please update doc and maybe limit radius to half of the size in the code.

CSS for compare

Single corner

div {
width: 100px;
height: 100px;
border-top-left-radius: 75px;
background: red;
}

Diagonal

div {
border-top-left-radius: 75px;
border-botom-right-radius: 75px;
}

Neighbors (limited to 50%)

div {
border-top-left-radius: 75px;
border-top-right-radius: 75px;
}

[radarhere]

I've created #9561 to limit the radius the half of the width or the height.