For a given projected distance the wing emission comes from the thick part of the line of sight, for radii less than r_i. The emission from the thin part of the line of sight is hidden to the observer.For the blue wing, the observer can observe the emission coming from z = 0 (with v_z = 0) to z = z_m (with v_z = −v_m). For the red wing, the observer can observe the emission coming from z = −z_a (with r = r_i, v_z = v_a) to z = −z_m (with v_z = +v_m), and, since the corresponding part of the equal-LOS-velocity surface facing the observer is missing (it would be outside the infall radius), from z = 0 (with v_z = 0) to z = −z_b (with v_z = v_a). The velocity v_a is the LOS velocity of the equal-LOS-velocity surface that intersects the sphere r = r_i at a projected distance p, i.e. v_a = v_z(p, −z_a) = v_z(p, −z_b).