def gndvi(b3, b8):
    """
    Green Normalized Difference Vegetation Index \
    (Gitelson, Kaufman, and Merzlyak, 1996).

    .. math:: GNDVI = (b8 - b3) / (b8 + b3)

    :param b3: Green.
    :type b3: numpy.ndarray or float
    :param b8: NIR.
    :type b8: numpy.ndarray or float

    :returns GNDVI: Index value

    .. Tip::

        Gitelson, A., Kaufman, Y. J., Merzlyak, M. N. 1996. Use of a green \
        channel in remote sensing of global vegetation from EOS-MODIS. \
        Remote Sensing of Environment 58(3), 289-298. \
        doi:10.1016/s0034-4257(96)00072-7.
    """

    GNDVI = (b8 - b3) / (b8 + b3)
    return GNDVI