Compare pngs in python

gistfile1.txt

import os
from scipy.misc import imread
from scipy.linalg import norm
from scipy import sum, average


def compare_images(img1, img2):
    # normalize to compensate for exposure difference, this may be unnecessary
    # consider disabling it
    img1 = normalize(img1)
    img2 = normalize(img2)
    # calculate the difference and its norms
    diff = img1 - img2  # elementwise for scipy arrays
    m_norm = sum(abs(diff))  # Manhattan norm
    z_norm = norm(diff.ravel(), 0)  # Zero norm
    return (m_norm, z_norm)


def to_grayscale(arr):
    "If arr is a color image (3D array), convert it to grayscale (2D array)."
    if len(arr.shape) == 3:
        return average(arr, -1)  # average over the last axis (color channels)
    else:
        return arr


def normalize(arr):
    rng = arr.max()-arr.min()
    amin = arr.min()
    return (arr-amin)*255/rng


def main():
    path = "D:/PyCharmProjects/EvoArt/Temp"
    os.chdir(path)

    img1 = to_grayscale(imread("Pic1.png").astype(float))
    img2 = to_grayscale(imread("Pic.png").astype(float))

    n_m, n_0 = compare_images(img1, img2)
    print("Manhattan norm:", n_m, "/ per pixel:", n_m / img1.size)
    print("Zero norm:", n_0, "/ per pixel:", n_0 * 1.0 / img1.size)


if __name__ == "__main__":
    main()