def ppr(b2, b3):
    """
    Plant Pigment Ratio (Metternicht, 2003).

    .. math:: PPR = (b3 - b2)/(b3 + b2)

    :param b2: Blue.
    :type b2: numpy.ndarray or float
    :param b3: Green.
    :type b3: numpy.ndarray or float

    :returns PPR: Index value

    .. Tip::

       Metternicht, G. 2003. Vegetation indices derived from high-resolution \
       airborne videography for precision crop management. International \
       Journal of Remote Sensing 24(14), 2855-2877. \
       doi:10.1080/01431160210163074
    """

    PPR = (b3 - b2)/(b3 + b2)
    return PPR