def rgbToGray(r, g, b):
    """
    Converts RGB to GrayScale using luminosity method
    :param r: red value (from 0.0 to 1.0)
    :param g: green value (from 0.0 to 1.0)
    :param b: blue value (from 0.0 to 1.0)
    :return GreyScale value (from 0.0 to 1.0)
    """
    g = 0.21*r + 0.72*g + 0.07*b
    return g