Trying to solve a problem of preventing duplicate images to be uploaded.
I have two JPGs. Looking at them I can see that they are in fact identical. But for some reason they have different file size (one is pulled from a backup, the other is another upload) and so they have a different md5 checksum.
How can I efficiently and confidently compare two images in the same sense as a human would be able to see that they are clearly identical?
I wrote this script:
import math, operator from PIL import Image def compare(file1, file2): image1 = Image.open(file1) image2 = Image.open(file2) h1 = image1.histogram() h2 = image2.histogram() rms = math.sqrt(reduce(operator.add, map(lambda a,b: (a-b)**2, h1, h2))/len(h1)) return rms if __name__=='__main__': import sys file1, file2 = sys.argv[1:] print compare(file1, file2)
Then I downloaded the two visually identical images and ran the script. Output:
Can anybody tell me what a suitable cutoff should be?
The difference between a.jpg and b.jpg is that the second one has been saved with PIL:
This apparently applies some very very light quality modifications. I've now solved my problem by applying the same PIL save to the file being uploaded without doing anything with it and it now works!
There is a OSS project that uses WebDriver to take screen shots and then compares the images to see if there are any issues (http://code.google.com/p/fighting-layout-bugs/)). It does it by openning the file into a stream and then comparing every bit.
You may be able to do something similar with PIL.
After more research I found
h1 = Image.open("image1").histogram() h2 = Image.open("image2").histogram() rms = math.sqrt(reduce(operator.add, map(lambda a,b: (a-b)**2, h1, h2))/len(h1))
The quickest way to determine if two images have exactly the same contents is to get the difference between the two images, and then calculate the bounding box of the non-zero regions in this image. If the images are identical, all pixels in the difference image are zero, and the bounding box function returns None.
import ImageChops def equal(im1, im2): return ImageChops.difference(im1, im2).getbbox() is None