log([O I]/Hα) as a function of log Hα surface brightness for all pixels in all 19 galaxies. The red solid line is the fitted global correlation for all galaxies, with slope and intercept provided in the lower right corner of this plot. The black solid line is representative of the DIG relation, as measured between 0.40 R_max (maximum radial coverage) and 0.60 R_max for each galaxy from Belfiore et al. (2022). The gray area indicates the 3σ range of this relation. The black dashed line indicates the theoretical value of [O I]/Hα = 0.017 to select SNRs (Kopsacheili et al. 2020). The horizontal blue dotted line indicates the empirical value of [O I]/Hα = 0.1 used select SNRs. For NGC 1672, the individual pixels lying above the majority come from an SNe that just happened several days before our observation.