Figure 7.

Profiles showing the spatial bias for different filters. Estimated background levels of four different methods along a vertical line (pixel column 915) on Slide 1 in the red channel. From bottom: (1) morphological opening γ_{60 × 60}, (2a) quantile opening γ_{60 × 60,{0,1}}, (2b) the latter shifted 17 units upward for visibility (dashed), and at the top (3) the quantile filter γ_{60 × 60,{0.08}} ζ_{3 × 3,7} which almost equals (4) the median background (bold curve). The opening and the corresponding quantile filter do not follow the median background trend well enough.