PIL doesn't write JPEG comment

[smason]

Following up on an answer I gave to the following stackoverflow question: https://stackoverflow.com/a/74658310/1358308

OP asked how to write a JPEG comment into the file, i.e. not an EXIF comment, but as a COM segment. The closest issue I could find, #4450, was about reading JPEG comments.

While looking for a solution I cloned the repo (hash 5bdf63e) to see what was going on, but couldn't see anything relevant.

Would be happy to turn this into a pull-request if you think this is a reasonable idea. I think I'd fix by modifying _save() to pull in the comment (from the applist) by appending to extra in a similar way as icc_profile:

Pillow/src/PIL/JpegImagePlugin.py

Lines 714 to 736 in 9486786

	extra = info.get("extra", b"")
	icc_profile = info.get("icc_profile")
	if icc_profile:
	ICC_OVERHEAD_LEN = 14
	MAX_BYTES_IN_MARKER = 65533
	MAX_DATA_BYTES_IN_MARKER = MAX_BYTES_IN_MARKER - ICC_OVERHEAD_LEN
	markers = []
	while icc_profile:
	markers.append(icc_profile[:MAX_DATA_BYTES_IN_MARKER])
	icc_profile = icc_profile[MAX_DATA_BYTES_IN_MARKER:]
	i = 1
	for marker in markers:
	size = struct.pack(">H", 2 + ICC_OVERHEAD_LEN + len(marker))
	extra += (
	b"\xFF\xE2"
	+ size
	+ b"ICC_PROFILE\0"
	+ o8(i)
	+ o8(len(markers))
	+ marker
	)
	i += 1

Comments appreciated!

[nicbou]

Thanks for this patch Sam. It's a significant improvement. Could you add a mention in the documentation?

[radarhere]

It should already be mentioned in the documentation, under https://pillow.readthedocs.io/en/stable/handbook/image-file-formats.html#jpeg-saving, thanks to #6784

Saving
The save() method supports the following options:
...
comment
A comment about the image.

New in version 9.4.0.