Function footprint_diff 

pub fn footprint_diff(
    a: &CaptureImage,
    b: &CaptureImage,
    bg: [u8; 4],
    eps: u8,
    min_lit_frac: f32,
) -> f32

Mean absolute per-channel (RGB) difference measured over the union of lit pixels in the two frames rather than over the whole frame, normalized to 0.0..=1.0 — the footprint statistic of ADR-0091 (Plan 0077 Phase 1).

frame_diff is a mean over every pixel, so a sparse figure’s motion is averaged against the empty frame around it and the statistic scores occupancy — which Plan 0067 Phase 1d measured to be scale-invariant, so no render size recovers the dilution. This is the masked form ADR-0091 offers (chosen over frame_diff / max(occupancy, eps) because the quotient form keeps the whole-frame numerator, so backdrop drift outside the figure still leaks into a statistic that claims to be about the figure): a pixel is in the mask if it differs from bg by more than eps on any RGB channel in either frame, and the mean is taken over the mask only.

The denominator is floored at min_lit_frac of the frame’s pixels — the guard ADR-0091 requires, without which a one-pixel flicker in a nearly-empty frame reads as strong animation (one full-swing pixel over a mask of one is 1.0). Callers state their bound and its derivation; a mask at or under the floor means the statistic is reporting “near-invisible at this size” rather than measuring motion, which is a finding for the coverage gate, not this one.

Mismatched dimensions read as fully different (1.0); an empty mask over a zero floor reads 0.0 (nothing lit in either frame is no motion, not a division). Alpha is ignored, as in frame_diff.