Embedding a hex value in a .jpg or .png file that doesn't impact display?

Question

Is there a location inside the .jpg or .png where I can put an arbitrary 32-bit value method for embedding a 32-bit value inside that will not be visible in the image (but would change the hash of the image file.)

Answer 1

Updated Answer

You can do that by adding a comment. If you want a technique that works with both JPEG and PNG and doesn't recompress your image data, I would suggest exiftool.

You can add a comment like this to JPEG or PNG:

exiftool -Comment=32768 a.jpg
exiftool -Comment=32768 a.png

You can retrieve the comment along with all other information using:

exiftool a.jpg

Or, you can get just the comment and its value with:

exiftool -Comment a.jpg   
Comment                         : 32768 

Or, if parsing in a shell script, you can get the unadorned values like this:

exiftool -Comment -s -s -s a.jpg
32768

Original Answer

You can put a number or comment in a file with jhead like this:

jhead -cl "32768" a.jpg
Modified: a.jpg

Then read it back, with jhead:

jhead a.jpg
File name    : a.jpg
File size    : 49732 bytes
...
...
Comment      : 32768   <--- HERE IT IS