def mndwi(b3, b11):
    """
    Modified Normalized Difference Water Index (Xu, 2006).

    .. math:: MNDWI = (b3 - b11) / (b3 + b11)

    :param b3: Green.
    :type b3: numpy.ndarray or float
    :param b11: SWIR 1.
    :type b11: numpy.ndarray or float

    :returns MNDWI: Index value

    .. Tip::

        Xu, H. (2006). Modification of normalised difference water index \
        (NDWI) to enhance open water features in remotely sensed imagery. \
        International Journal of Remote Sensing 27(14), 3025-3033. \
        doi:10.1080/01431160600589179.
    """

    MNDWI = (b3 - b11) / (b3 + b11)
    return MNDWI