Question

I would like to create a program that compares two images. It must take in 2 folders, each full of pictures and each picture in one folder has a pair in the other folder (with the same filename) ex: folder1 has 3 pictures: [a.png, b.png, c.png], and folder2 has 3 pictures: [a.png, b.png, c.png]. I would like to take folder1/a and compare it to folder2/a. I do not know where to start.

Answer 1

You mentioned images instead of file so I assume you don’t want to compare raw data but the image pixels and you may also want to have threshold for difference between images

Easiet way to do that is compare two images using PIL. PIL has a histogram function, you can use that to get histogram for both images and then getting a RMS(root mean square) e.g.

import ImageChops
import math, operator

def rmsdiff(im1, im2):
    "Calculate the root-mean-square difference between two images"

    h = ImageChops.difference(im1, im2).histogram()

    # calculate rms
    return math.sqrt(reduce(operator.add,
        map(lambda h, i: h*(i**2), h, range(256))
    ) / (float(im1.size[0]) * im1.size[1]))

copied from http://effbot.org/zone/pil-comparing-images.htm

once you have a function which returns rmsdiff, you can use that in another function which iterates thru all files e.g.

def diff_folder(folder1, folder2):
    for path1 in glob.glob(folder1+"/*.png"):
        filename = os.path.basename(path1)
        path2 = os.path.join(folder2, filename)
        if not os.path.exists(path2):
            continue
        im1 = Image.open(path1)
        im2 = Image.open(path2)
        diff = rmsdiff(im1, im2)
        if diff > threshold:
           print "different"