def wbi(b2, b4):
    """
    Water Body Index (Domenech and Mallet, 2014).

    .. math:: WBI = (b2 - b4) / (b2 + b4)

    :param b2: Blue.
    :type b2: numpy.ndarray or float
    :param b4: Red.
    :type b4: numpy.ndarray or float

    :returns WBI: Index value

    .. Tip::

        Domenech, E., Mallet, C. 2014. Change Detection in High \
        resolution land use/land cover geodatabases (at object level). \
        EuroSDR official publication, 64.
    """

    WBI = (b2 - b4) / (b2 + b4)
    return WBI