Line core formation properties within a cut through x = 0 Mm of the filament synthesis. The source function S_ν and integrated opacity τ_ν, and associated contribution function C_I are shown for four of the six line core transitions shown in Figs. 5 and 7. The dashed lines overlaid on the τ_ν and C_I quantities are the location of τ_ν = 1, and the average formation height z_a quantity calculated for both an MPI-AMRVAC-only and FAL-C-inclusive stratification, respectively, and overlap in the case of Mg II h&k. S_ν and C_I quantities in units of log₁₀(ergs s⁻¹ cm⁻² sr⁻¹ Hz⁻¹).