Visualization of the ray tracing parametrization. The orange solid line represents the trajectory of photons emitted at (R, φ, ϑ) in the direction k^★ with the emission angle α with respect to the radial vector n. R represents the radius of emission. The photon reaches the observer plane (at infinity) at the impact point (A = b cos ρ, B = b sin ρ) at a distance b. The direction k toward the observer is defined to lie in the x, z-plane, inclined by the inclination i with respect to the z-axis, which is also the axis of rotation. The angle between n and k is the apparent emission angle Ψ. The magenta dashed line represents the second possible trajectory, Ψ^∗, to the observer of a photon emitted at (R, φ, ϑ). In this case the photon is emitted toward the neutron star, that is, α > 90° and exhibits a periastron at (R_p,Ψ_p).