from skimage.color import rgb2gray
import matplotlib.pyplot as plt

img = plt.imread('cat.jpg')
gray = rgb2gray(img)

gray_r = gray.reshape(gray.shape[0]*gray.shape[1])
global_threshold = gray_r.mean()

for i in range(gray_r.shape[0]):
    if gray_r[i] > global_threshold:
        gray_r[i] = 1
    else:
        gray_r[i] = 0
		
gray = gray_r.reshape(gray.shape[0], gray.shape[1])
plt.imshow(gray, cmap='gray')