Selection procedure of gas cells (gray squares) along the line of sight of a stellar particle (yellow circle). The stellar particle, with comoving coordinates (x_*, y_*, z_*), is a dimensionless point, while gas cells have a linear extension L_g in (x, y, z), assuming cubic cells with volume V_g. The z-axis grows from right to left, meaning that darker gray gas cells are in front of the star particle and along its line of sight (LoS), compared to the observer located at the origin n of the z-axis. Lighter gray gas cells are not included in the selection, because they are behind or out of the LoS of the star particle.