Neutral fraction of hydrogen with respect to time (bottom axis) and redshift (top axis) for ζ30 (left panel) and ζ55 (right panel). The crosses stand for the average non-zero signal fraction obtained from 21 cm images at different values of z (as if taken along the line of sight). The dashed areas stand for the true Q_HI computed from true maps of t_reion(r) (mean and std). The dotted lines stand for the average Q_HI obtained from maps of t_reion(r) predicted by the CNN.