Function mean_lit_level 

pub fn mean_lit_level(img: &CaptureImage, bg: [u8; 4], eps: u8) -> f32

Mean linear light over the lit set — the level statistic (ADR-0150), in 0.0..=1.0. 0.0 for a frame with no lit pixels, the convention tonal_flatness and boundary_density use.

Every other statistic in this module answers a shape question. This one answers how much light does the picture carry, which is the question a retune asks when it wants to hold level constant across a change, and it is the one question that cannot be asked on the stored bytes: sRGB’s transfer curve is concave, so an encoded mean under-reports a trim by roughly half. linear_diff carries the same reasoning for a two-frame comparison. It is private, so this names it rather than linking it.

The lit predicate is coverage’s, so this is linear light over a CODE-SPACE-selected set. ADR-0150 records why that seam is accepted rather than solved: a lit predicate in linear light would change coverage itself and move blessed baselines across the whole suite, for no gain to the question being asked here. A reader who does not know that will eventually “fix” the wrong half.

Restricted to the lit set, not the frame, which is the substantive half. A preset’s background is a deliberate authored constant; folding it into the level makes the reading mostly a measurement of that constant, and a frame mean read a 30 % source trim as 3 % on the fixture that motivated this. The corollary is a blind spot: a preset that goes wrong by changing its background is invisible here.

Luminance weights are Rec.709 (0.2126/0.7152/0.0722), not the Rec.601 that luma applies to code values — those are the luminance coefficients of sRGB’s own primaries, and they are what every other linear-light reading in this workspace uses.