Histograms showing the relative errors in the percentage of the radiation field tensors, , with respect to the reference solution, , at the corrugated surface where the optical depth at the line centre is unity along the disc centre line of sight. The different panels show: (a) GT 3 × 3 and S1 with 8 rays per octant, (b) GT 4 × 4 and S1 with 14 rays per octant, and (c) GT 5 × 5 and S1 with 18 rays per octant.