def rgb_2_gray(image):
    """
    gray = r * 0.3 + g * 0.59 + b * 0.11
    等价于: np.asarray(image.convert('L'))
    等价于: cv2.cvtColor(image,cv2.COLOR_BGR2GRAY)
    """
    gray = (image[:, :, 0] * 0.3 + image[:, :, 1] * 0.59 + image[:, :, 2] * 0.11).astype('int')
    return gray