CODEWITHSUNDEEP
pythonpython-imaging-libraryjpeg
VY_CMa
VY_CMa

Reputation: 315

Save JPEG comment using Pillow

I need to save an Image in Python (created as a Numpy array) as a JPEG file, while including a "comment" in the file with some specific metadata. This metadata will be used by another (third-party) application and is a simple ASCII string. I have a sample image including such a "comment", which I can read out using Pillow (PIL), via the image.info['comment'] or the image.app['COM'] property. However, when I try a simple round-trip, i.e. loading my sample image and save it again using a different file name, the comment is no longer preserved. Equally, I found no way to include a comment in a newly created image.

I am aware that EXIF tags are the preferred way to save metadata in JPEG images, but as mentioned, the third-party application only accepts this data as a "comment", not as EXIF, which I cannot change. After reading this question, I looked into the binary structure of my sample file and found the comment at the start of the file, after a few bytes of some other (meta)data. I do however not know a lot about binary file manipulation, and also I was wondering if there is a more elegant way, other than messing with the binary...

EDIT: minimum example:

from PIL import Image
img = Image.open(path)  # where path is the path to the sample image

# this prints the desired metadata if it is correctly saved in loaded image
print(img.info["comment"])

img.save(new_path)  # save with different file name
img.close()

# now open to see if it has been saved correctly
new_img = Image.open(new_path)
print(new_img.info['comment'])  # now results in KeyError

I also tried img.save(new_path, info=img.info), but this does not seem to have an effect. Since img.info['comment'] appears identical to img.app['COM'], I tried img.save(new_path, app=img.app), again does not work.

Upvotes: 4

Views: 1119

Answers (1)

Sam Mason
Sam Mason

Reputation: 16194

Just been having a play with this and I couldn't see anything directly in Pillow to support this. I've found that the save() method supports a parameter called extra that can be used to pass arbitrary bytes to the output file.

We then just need a simple method to turn a comment into a valid JPEG segment, for example:

import struct
from PIL import Image

def make_jpeg_variable_segment(marker: int, payload: bytes) -> bytes:
    "make a JPEG segment from the given payload"
    return struct.pack('>HH', marker, 2 + len(payload)) + payload

def make_jpeg_comment_segment(comment: bytes) -> bytes:
    "make a JPEG comment/COM segment"
    return make_jpeg_variable_segment(0xFFFE, comment)

# open source image
with Image.open("foo.jpeg") as im:
    # save out with new JPEG comment
    im.save('bar.jpeg', extra=make_jpeg_comment_segment("hello world".encode()))

# read file back in to ensure comment round-trips
with Image.open('bar.jpeg') as im:
    print(im.app['COM'])
    print(im.info['comment'])

Note that in my initial attempts I tried appending the comment segment at the end of the file, but Pillow wouldn't load this comment even after calling the .load() method to force it to load the entire JPEG file.

Update: The upcoming version Pillow version 9.4.0 will support this by passing a comment parameter while saving, e.g.:

with Image.open("foo.jpeg") as im:
  im.save('bar.jpeg', comment="hello world")

hopefully that makes things easier!

Upvotes: 4

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