<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_bg(img, padding, color=COL_WHITE):
 """ Adds a padding to the given image as background of specified color :param img: Input image. :param padding: constant padding around the image. :param color: background color that needs to filled for the newly padded region. :return: New image with background. """

    img = gray3(img)
    h, w, d = img.shape
    new_img = np.ones((h + 2*padding, w + 2*padding, d)) * color[:d]
    new_img = new_img.astype(np.uint8)
    set_img_box(new_img, (padding, padding, w, h), img)
    return new_img