code: def add_bg(img, padding, color=COL_WHITE):
      
      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
docs: 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.