def gcvi(b3, b8):
    """
    Green Chlorophyll Vegetation Index (Gitelson et al., 2003b).

    .. math:: GCVI = (b8/b3) - 1

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

    :returns GCVI: Index value

    .. Tip::

        Gitelson, A. A., Viña, A., Arkebauer, T. J., Rundquist, D. C., \
        Keydan, G., Leavitt, B. 2003b. Remote estimation of leaf area \
        index and green leaf biomass in maize canopies. Geophysical \
        Research Letters 30(5). doi:10.1029/2002gl016450.
    """

    GCVI = (b8/b3) - 1
    return GCVI