An example of the N-PDF fitting. Left: the N-PDF of Ophiuchus (gray histogram). The error bars at the lower-density end show the uncertainties due to the map area. The orange line shows the fitted curve and the black vertical line shows the fitted threshold column density. Right: corner plot of the fitted parameters. σ_η represents the dimensionless dispersion, and μ is the mean of the lognormal component. N_threshold is the transitional column density, related to η_t via the relation η_t = ln(N_threshold/⟨N_H2⟩). α denotes the slope of the power-law tail.