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 def make_gray(image_mat, end_index):
  import cv2
  gray_image_array=[]
  for i in np.arange(end_index):
      image = image_mat[i]
      gray_image= cv2.cvtColor(image,cv2.COLOR_BGR2GRAY)
      gray_image_array.append(gray_image)
  return gray_image_array