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
