Function outlier_fraction 

fn outlier_fraction(reference: &[u8], rgba: &[u8], channel_delta: u8) -> f64

Available on crate feature snapshot only.

The fraction of pixels whose largest per-channel difference from the baseline exceeds channel_delta.

A pixel the two implementations resolve differently moves far, so it lands outside channel_delta; one they agree on, or which differs only by the rounding of an interpolated value, stays inside it. Counting them is what separates a difference confined to a few edges — which the perceptual score is too coarse to see past — from one spread over the frame.