def _vel_disp_one(self, kwargs_profile, kwargs_aperture, kwargs_light, kwargs_anisotropy):
        """
        computes one realisation of the velocity dispersion realized in the slit
        :param gamma:
        :param rho0_r0_gamma:
        :param r_eff:
        :param r_ani:
        :param R_slit:
        :param dR_slit:
        :param FWHM:
        :return:
        """

        while True:
            r = self.lightProfile.draw_light(kwargs_light)  # draw r
            R, x, y = util.R_r(r)  # draw projected R
            x_, y_ = util.displace_PSF(x, y, self.FWHM)  # displace via PSF
            bool = self.aperture.aperture_select(x_, y_, kwargs_aperture)
            if bool is True:
                break
        sigma_s2 = self.sigma_s2(r, R, kwargs_profile, kwargs_anisotropy, kwargs_light)
        return sigma_s2