Panel a) mean filament FWHM as a function of central intrinsic (crest, background subtracted) filament A_V. The black solid curve shows the model for isothermal pressure-confined filaments from Fischera & Martin (2012) for an external pressure p_ext/k = 2 × 10⁴ K cm^-3, i.e. environment A_V≈ 2.8 mag for μ = 2.33. Blue and red symbols represent the average change of filament width and crest A_V for the transition from a core-dominated subcritical state (SB) to a wing-dominated supercritical state (SP). The average background for each filament group (A_V (Env.); right axis), derived from the background maps provided by getsources (Sect. 3), is indicated with a black symbol at the characteristic crest A_V of the group. The clear increase in environment column density with regime (crest A_V) is highlighted with a green arrow. Vertical dashed lines mark the approximate boundaries of regimes 2 and 3 for a filament with FWHM ≈ 0.13 pc. Boundaries would shift to lower A_V for larger FWHM. Error bars are the standard error on the mean for each type of filament. Panel b) same as panel a), but highlighting the theoretical curves corresponding to the average background of each group. Models were derived using the equations included in Fischera & Martin (2012).