Basic importing

import numpy as np
import cv2
from matplotlib import pyplot as plt

Loading an image

img = cv2.imread(file_name, 0)

Padding an image

cv2.copyMakeBorder(img, <top>, <bottom>, <left>, <right>, CV2.BORDER_REPLICATE)

Showing an image

plt.imshow(img, cmap='gray', intepolation='bicubic')
plt.show()

Transform a 1-channel array/image to 3-channel

stacked_image = np.stack((a,)*3, -1)

Upsize/rescale an image to larger size

cv2.resize(a, dsize=(224, 224), interpolation=cv2.INTER_CUBIC)

saving and loading numpy data into/from file

numpy.save(file, arr)
back_arr = numpy.load(file)