Darken Only and Lighten Only take the per-channel min and max of the two layers

Darken Only builds each output pixel from the smallest (darkest) per-channel value of the foreground and background: result = [min(r1,r2), min(g1,g2), min(b1,b2)]. Lighten Only is its opposite, taking the per-channel maximum. Because the comparison is per channel, the winning colour can differ channel by channel, so the result is not necessarily one of the two input colours. Lighten Only is a workhorse for live visuals: layering bright elements on black backgrounds keeps whichever layer is brighter at each pixel, compositing glows and shapes without washing the scene out the way additive can.

Examples

Lighten Only of a bright shape on black over another shape on black: both shapes show, black stays black. Darken Only of two photos: keeps the darker detail everywhere — useful for combining exposures or removing bright specular blowouts.

Assessment

Given foreground (0.2,0.9,0.4) and background (0.7,0.1,0.5), compute the Darken Only and Lighten Only results. Why is Lighten Only preferred over Add for stacking bright layers on black?