Opacity cross-sections for potassium (K) at T = 1500 K. The weak line at 1252.95 nm is interpolated from a pre-computed opacity table. The doublet line at 1252.55 nm (4p²P_3/2 − 5s) is computed ‘on-the-fly’, meaning that the line strength and line width are calculated exactly for each PT point in the modelled atmosphere. This line also receives a PT-dependent shift, resulting in a blueshift at high pressures.