Synthesis of Ca lines used in this work. In each sub-panel the upper part shows the 1D–LTE synthesis (dashed black line), the 1D–NLTE synthesis from this work (red), and the 3D–LTE synthesis (green) for a T_eff = 4500 K, log g = 2.5, [M/H] = −1.0 model. The lower parts display the corresponding differences in abundances of the same line between the 1D–LTE model and the NLTE or 3D models for various temperatures and metallicities (large symbols T_eff = 4000 K, log g = 1.5; medium symbols T_eff = 4500 K, log g = 2.5; small symbols T_eff = 5000 K, log g = 2.5).