Converting F images to I turns large values, inf and NaN into -2147483648

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@usmanmateen is already working on this.

Since Oct 31, 2026.

  • #10373 by @usmanmateen — open

Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
72/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
c, python
Domain
backend

Research direction

The bug is the unchecked cast i = f; in the f2i function in src/libImaging/Convert.c; clip32() in src/libImaging/Filter.c shows the existing range guard to follow. Reproduce with the Python snippet in the issue, then confirm the F to I results. Done when values above the INT32 range clip to 2147483647, -inf clips to -2147483648, NaN becomes 0, and the round trip returns 2147483647.

Written by the indexing model from the issue text.

Description

Conversion
What did you do?

Converted a float (F) image with large or non-finite values to I, for example a float TIFF or a NumPy result with inf or NaN pixels, or an I image that went through F for processing:

import numpy as np
from PIL import Image

im = Image.fromarray(np.array([[1.5, 70000, 3e9, np.inf, -np.inf, np.nan]], np.float32), "F")
print("I", np.asarray(im.convert("I")).tolist())
print("L", np.asarray(im.convert("L")).tolist())

print(Image.new("I", (1, 1), 2**31 - 1).convert("F").convert("I").getpixel((0, 0)))
What did you expect to happen?

Values outside the I range clipped to it, the way F to L and F to I;16 clip to theirs, and the round trip to keep 2147483647:

I [[1, 70000, 2147483647, 2147483647, -2147483648, 0]]
L [[1, 255, 255, 255, 0, 0]]
2147483647
What actually happened?

Every value above the INT32 range, inf and NaN become -2147483648, the most negative value:

I [[1, 70000, -2147483648, -2147483648, -2147483648, -2147483648]]
L [[1, 255, 255, 255, 0, 0]]
-2147483648

The round trip fails because 2147483647 isn't representable as a float32 and rounds up to 2147483648.0, which is just past the INT32 range.

f2i in Convert.c casts with i = f; and no range check. Converting an out-of-range float or NaN to an integer is undefined behavior in C (-fsanitize=float-cast-overflow reports it), and the result depends on the CPU: x86 gives INT_MIN, while ARM saturates and gives INT32_MAX for large values and 0 for NaN. clip32() in Filter.c already guards the same overflow to INT_MIN.

I can open a PR that clips to the INT32 range in f2i and maps NaN to 0.

What are your OS, Python and Pillow versions?
  • OS: Ubuntu 26.04 (WSL2), x86-64
  • Python: 3.12.13
  • Pillow: 13.0.0
Dominant language
Python
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