python - Compute hash of only the core image data (excluding metadata) for an image

Question

I'm writing a script to calculate the MD5 sum of an image excluding the EXIF tag.

In order to do this accurately, I need to know where the EXIF tag is located in the file (beginning, middle, end) so that I can exclude it.

How can I determine where in the file the tag is located?

The images that I am scanning are in the format TIFF, JPG, PNG, BMP, DNG, CR2, NEF, and some videos MOV, AVI, and MPG.

Answer 1

It is much easier to use the Python Imaging Library to extract the picture data (example in iPython):

In [1]: import Image

In [2]: import hashlib

In [3]: im = Image.open('foo.jpg')

In [4]: hashlib.md5(im.tobytes()).hexdigest()
Out[4]: '171e2774b2549bbe0e18ed6dcafd04d5'

This works on any type of image that PIL can handle. The tobytes method returns the a string containing the pixel data.

BTW, the MD5 hash is now seen as pretty weak. Better to use SHA512:

In [6]: hashlib.sha512(im.tobytes()).hexdigest()
Out[6]: '6361f4a2722f221b277f81af508c9c1d0385d293a12958e2c56a57edf03da16f4e5b715582feef3db31200db67146a4b52ec3a8c445decfc2759975a98969c34'

On my machine, calculating the MD5 checksum for a 2500x1600 JPEG takes around 0.07 seconds. Using SHA512, it takes 0,10 seconds. Complete example:

#!/usr/bin/env python3

from PIL import Image
import hashlib
import sys

im = Image.open(sys.argv[1])
print(hashlib.sha512(im.tobytes()).hexdigest(), end="")

For movies, you can extract frames from them with e.g. ffmpeg, and then process them as shown above.