def gemi(self):
        n = (2 * (self.nir**2 - self.red**2) + 1.5 * self.nir + 0.5 * self.red) / (
            self.nir + self.red + 0.5
        )
        return n * (1 - 0.25 * n) - (self.red - 0.125) / (1 - self.red)