Impact of a velocity component perpendicular to the LOS. Top: rotational velocity v_rot (in units of c) as a function of the disc radius (in units of R_S). Middle, both as a function of the radius; left: angle θ between the LOS and the total velocity for v_rad = 0.1, 0.3 c (colour coding; see legend); right: relativistic factors ψ_rot, ψ_rot³ for the same total velocity (dark and light colours, respectively). For comparison, dashed lines report ψ, ψ³, i.e. the same factors computed neglecting v_rot. Bottom: ratio between the relativistic factors: dark ad light colours correspond to ψ_rot/ψ and ψ_rot³/ψ³, respectively. Colour coding as above.