text: import sys
  import cv2
  import numpy as np
  
  img = cv2.imread('../icon/flowers.jpg')
  img = cv2.resize(img, (480, 320))
  
  if img is None:
      print('Image load failed!')
      sys.exit()
  
  cv2.imshow('raw_img', img)
  cv2.moveWindow('raw_img', 100, 100)
  
  val = 0
  mask = np.full((320, 480, 3), (val, val, val)).astype(np.uint8)
  
  while True:
      mask = np.full((320, 480, 3), (val, val, val)).astype(np.uint8)
  
      lightness = cv2.add(img, mask)
      cv2.imshow('light_img', lightness)
      cv2.moveWindow('light_img', 580, 100)
  
      darkness = cv2.subtract(img, mask)
      cv2.imshow('dark_img', darkness)
      cv2.moveWindow('dark_img', 1060, 100)
  
      key = cv2.waitKey()
      if key == 27:
          break
      if key == ord('u'):
          val += 5
      if key == ord('d'):
          val -= 5
  cv2.destroyAllWindows()
  