def gamma(self, x, y, kwargs, diff=diff):
        """
        computes the shear
        :return: gamma1, gamma2
        """
        f_xx, f_xy, f_yx, f_yy = self.hessian(x, y, kwargs, diff=diff)
        gamma1 = 1./2 * (f_xx - f_yy)
        gamma2 = f_xy
        return gamma1, gamma2