Reference: pmToBitmap

pmToBitmap(pm, x) returns the pixmap pm with only two colours, black and white, where the black pixels come from "dark" pixels in pm, and white pixels come from "bright" pixels in pm. The threshold for a pixel to be considered "bright" is given by x ∈ [0, 1]. Thus, if x = 0, all pixels will be considered bright, and if x = 1, no pixels will be considered bright.

The "brightness" property is defined as the HSL lightness value.