def _compare_entropy(start_slice, end_slice, slice, difference):
    """
    Calculate the entropy of two slices (from the start and end of an axis),
    returning a tuple containing the amount that should be added to the start
    and removed from the end of the axis.

    """
    start_entropy = utils.image_entropy(start_slice)
    end_entropy = utils.image_entropy(end_slice)
    if end_entropy and abs(start_entropy / end_entropy - 1) < 0.01:
        # Less than 1% difference, remove from both sides.
        if difference >= slice * 2:
            return slice, slice
        half_slice = slice // 2
        return half_slice, slice - half_slice
    if start_entropy > end_entropy:
        return 0, slice
    else:
        return slice, 0