Schematics showing how we extract a pair of EW and D-index from a single spaxel, given a choice of the free parameters (highlighted in red) of our modeling. An ensemble of n clouds produces the absorption. The spaxel is only partially covered by gas, so that the flux is either equal to F_los = F_bgexp( − τ) if the clouds are intercepted or to F_bg = 1 if they are not. The total flux of the spaxel F_spaxel depends on the value of the covering fraction C_frac. For more details, see Sect. 4.1.1.