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

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.)

CodePudding user response：

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