docs: def tweak_diff(diff, opacity):
      """Adjust a difference map into an opacity mask for a given lowest opacity.
  
      Performs a linear map from [0; 255] to [opacity; 255].
  
      The result is that similar areas will have a given opacity, while
      dissimilar areas will be opaque.
      """
      mask = diff.point(lambda i: opacity + i * (255 - opacity) // 255)
      return mask