from skimage.io import imread, imsave, imshow
from skimage import img_as_float, img_as_ubyte
import numpy as np

img = imread("img.png")

counts = np.bincount(img.flat, minlength=256)

mn_cdf = -1
cdf = [0] * 256
for i in range(256):
    if i != 0:
        cdf[i] += cdf[i-1]

    cdf[i] += counts[i]
    if mn_cdf == -1 and cdf[i] != 0:
        mn_cdf = cdf[i]

def f(x):
    return round((cdf[x] - mn_cdf) / (img.shape[0] * img.shape[1] - 1) * 255)

for i in range(img.shape[0]):
    for j in range(img.shape[1]):
        img[i, j] = f(img[i,j])

imsave("out_img.png", img)