tmp = cv2.cvtColor(crop_image, cv2.COLOR_BGR2GRAY)
 _, alpha = cv2.threshold(tmp, 0, 255, cv2.THRESH_BINARY)
 b, g, r = cv2.split(crop_image)
 rgba = [b, g, r, alpha]
 dst = cv2.merge(rgba, 4)
 # 注意保存成png格式！！！jpg的话还是黑色背景(255)
 cv2.imwrite("test.png", dst)