from PIL import Image, ImageChops

import logging

logging.getLogger('PIL.PngImagePlugin').setLevel(logging.INFO)

def image_diff(diff_path, before_path, after_path, superimpose=False):

""" Returns the percentage of differing pixels or -1 if dimensions differ. """

before = Image.open(before_path)

after = Image.open(after_path)

if before.size != after.size:

return -1

before = before.convert('RGBA')

after = after.convert('RGBA')

mask = ImageChops.difference(before, after)

mask = mask.convert('L')

mask = mask.point(lambda k: 0 if k == 0 else 255)

if mask.getbbox() is None:

return 0

else:

diff = mask.convert('RGB')

if superimpose:

diff.paste(after, mask=mask)

else:

diff.paste((0, 0, 255), mask=mask)

diff.save(diff_path)

w, h = after.size

pixels = 0

for v in mask.getdata():

if v == 255:

pixels += 1

return float(pixels)/(w*h)*100