Function tonal_flatness 

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

Share of the lit figure whose luminance falls inside the single most populated narrow luminance band (0.0..=1.0) — “does this picture have any tonal structure”.

coverage and quadrant_spread answer is something there and is it more than a dot, and a fully saturated single-tone mass satisfies both: it is a real shape, of the right size, in every quadrant. This asks the question they cannot — whether the shape has any interior. A figure with falloff, depth or modelling spreads across several buckets; one driven past the tonemap knee collapses into one and reads near 1.0.

Measured over lit pixels only, against the frame’s own sampled background, for the same reason coverage is: a sparse figure on a wide ground would otherwise report the background’s flatness, which is total by construction and says nothing about the scene.

0.0 for a frame with no lit pixels at all — an empty picture makes no claim here, and coverage is the metric that already convicts it.