def ndmi(b8, b11):
    """
    Normalized Difference Moisture Index \
    (Gerard et al., 2003).

    .. math:: NDMI = (b8 - b11) / (b8 + b11)

    :param b8: NIR.
    :type b8: numpy.ndarray or float
    :param b11: SWIR 1.
    :type b11: numpy.ndarray or float

    :returns NDMI: Index value

    .. Tip::

        Gerard, F., Plummer, S., Wadsworth, R., Sanfeliu, \
        A. F., Iliffe, L., Balzter, H., & Wyatt, B. 2003. \
        Forest fire scar detection in the boreal forest with \
        multitemporal SPOT-VEGETATION data. IEEE Transactions \
        on Geoscience and Remote Sensing 41(11), 2575-2585. \
        doi:10.1109/tgrs.2003.819190.
    """

    NDMI = (b8 - b11) / (b8 + b11)
    return NDMI