[S II]/Hα (top) and [O I]/Hα (bottom) versus Hα luminosity for SNRs (red dots) compared with the H II regions density distribution (in grayscale). For SNRs that overlap with a H II region, the symbol is an empty circle (in light red). Thresholds of [O I]/Hα=0.1 and [S II]/Hα=0.4 (horizontal dash-dotted lines) are shown. SNRs overlapping with H II regions are selected by other criteria so they can still be identified as SNRs. Emission lines are not corrected for the photoionization.