RGB通道转为灰度图

import cv2
import os
import numpy as np
img_dir =''   #文件路径
img_names = sorted(os.listdir(img_dir))
for i in range(len(img_names)):
    img = cv2.imread(img_dir +img_names[i],0)
    new_img = np.array([img,img,img])
    new_img = new_img.transpose((1,2,0))
    cv2.imwrite(''+img_names[i],new_img)