Toy model to illustrate the problem of self-absorption in the standard Feautrier scheme for a single p-ray (see text). The lower panel shows the opacity distribution for this model. The opacities of the models are chosen such that the same total optical depth over the entire radial interaction zone is reached. The upper panel shows the u function derived from the standard Feautrier solution using an incident intensity I⁺ = 4 from the left side and I⁻ = 0 on the right side. The black lines denote the run of the source function S_line.