I want to write a python program that adds a comment to a jpeg file. I've read that a comment in a jpeg file is signaled by the marker 0xfffe. Thus, can I just open the file and append this marker with whatever comment I want following it? My code looks something like this:
file = open("someimage.jpg", "a+b")
file.write("\xff\xfeCOMMENT")
file.close()
Does it matter if my comment is after the end of file marker (0xffd9)? Thanks!
The accepted answer uses EXIF fields. If a standard JPEG comment is needed, you'll need to insert the payload length:
Two bytes giving the-length-of-the-comment-in-bytes-plus-2 as a big-endian (unsigned short) integer must be inserted between the FF FE bytes and the comment.
For example, the sequence of bytes corresponding to a comment Hello is FF FE00 0748 65 6C 6C 6F because the length of the payload (unsigned short + comment) is 7 bytes. If an optional C-style null-terminator is wanted, it's FF FE00 0848 65 6C 6C 6F 00.
The comment segment should strictly be before the end-of-file marker (FF D9) though some image viewers may understand extra data after the marker.
For details, see How do text comments in JPG files work?.
This will work (it will append text beyond the part needed to store the image).
A more sophisticated approach would read the JPG file format and add a comment in EXIF fields. See this StackOverflow discussion: Exif manipulation library for python
See pyexiv for python bindings to exiv2, a tool for reading and writing image metadata.
