def sipi(b3, b4, b8):
    """
    Structure Intensive Pigment Index \
    (Peñuelas, Baret and Filella, 1995).

    .. math:: SIPI = b3/b8 - b4

    :param b3: Green.
    :type b3: numpy.ndarray or float
    :param b4: Red.
    :type b4: numpy.ndarray or float
    :param b8: NIR.
    :type b8: numpy.ndarray or float

    :returns SIPI: Index value

    .. Tip::

       Peñuelas, J., Baret, F., Filella, I. 1995. Semi-empirical \
       indices to assess carotenoids/chlorophyll-a ratio from leaf \
       spectral reflectance. Photosynthetica 31, 221-230.
    """

    SIPI = b3/b8 - b4
    return SIPI