test_8bit_with_16bit_depth fails on s390x (64 bit/big endian machine) about imageio

[FirefoxMetzger]

I can reproduce the exception on an amd64 architecture using the following snippet:

>>> import imageio.v3 as iio
>>> import numpy as np
>>> img = iio.imread("imageio:chelsea.png", mode="L")
>>> img.dtype
dtype('uint8')
>>> iio.imwrite("foo.png", img.astype("<u2"))
>>> iio.imwrite("foo.png", img.astype(">u2"))
Traceback (most recent call last):
  File "/Users/<user>/projects/imageio/.venv/lib/python3.10/site-packages/PIL/PngImagePlugin.py", line 1277, in _save
    rawmode, mode = _OUTMODES[mode]
KeyError: 'I;16B'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/<user>/projects/imageio/imageio/v3.py", line 139, in imwrite
    with imopen(
  File "/Users/<user>/projects/imageio/imageio/core/v3_plugin_api.py", line 367, in __exit__
    self.close()
  File "/Users/<user>/projects/imageio/imageio/plugins/pillow.py", line 123, in close
    self._flush_writer()
  File "/Users/<user>/projects/imageio/imageio/plugins/pillow.py", line 457, in _flush_writer
    primary_image.save(self._request.get_file(), **self.save_args)
  File "/Users/<user>/projects/imageio/.venv/lib/python3.10/site-packages/PIL/Image.py", line 2413, in save
    save_handler(self, fp, filename)
  File "/Users/<user>/projects/imageio/.venv/lib/python3.10/site-packages/PIL/PngImagePlugin.py", line 1280, in _save
    raise OSError(msg) from e
OSError: cannot write mode I;16B as PNG

This originates from Pillow. Internally their PNG writer maintains a list of valid input buffer dtypes and I;16B (big-endian 16-bit) is not listed there. However, I;16 (little-endian 16-bit) is. If I add I;16B in their list of allowed dtypes everything runs as it should. --- I will create an issue over at pillow and ask if they can add it, since this looks like a bug/oversight to me :)

---

The reason this error shows up on s390x is because it is a big-endian architecture. For PNGs (though this will likely extend in the future) ImageIO will unpack pixels into native endianness. As such you get a big-endian buffer on a big-endian machine, which is more performant but, for now, not allowed as input when writing with pillow.

[FirefoxMetzger]

xref: python-pillow/Pillow#7301