code: def main():
      img_channels = 3
      img_dir = "./DRIVE/training/images"
      roi_dir = "./DRIVE/training/mask"
      assert os.path.exists(
          img_dir
      ), f"image dir: '{img_dir}' does not exist."
      assert os.path.exists(
          roi_dir
      ), f"roi dir: '{roi_dir}' does not exist."
  
      img_name_list = [
          i for i in os.listdir(img_dir) if i.endswith(".tif")
      ]
      cumulative_mean = np.zeros(img_channels)
      cumulative_std = np.zeros(img_channels)
      for img_name in img_name_list:
          img_path = os.path.join(img_dir, img_name)
          ori_path = os.path.join(
              roi_dir, img_name.replace(".tif", "_mask.gif")
          )
          img = np.array(Image.open(img_path)) / 255.0
          roi_img = np.array(Image.open(ori_path).convert("L"))
  
          img = img[roi_img == 255]
          cumulative_mean += img.mean(axis=0)
          cumulative_std += img.std(axis=0)
  
      mean = cumulative_mean / len(img_name_list)
      std = cumulative_std / len(img_name_list)
      print(f"mean: {mean}")
      print(f"std: {std}")
  
label_desc: Use image node_id 1 for calling a global function with example usage: main() without return types