img1 = Image.open(img_1_path).convert('RGBA')
img2 = Image.open(img_2_path).convert('RGBA')
arr1 = np.array(img1)
arr2 = np.array(img2)
# record the original shape
shape1 = arr1.shape
shape2 = arr2.shape
# make a 1-dimensional view of arr
flat_arr1 = arr1.ravel()
flat_arr2 = arr2.ravel()