Function frame_diff 

pub fn frame_diff(a: &CaptureImage, b: &CaptureImage) -> f32

Mean absolute per-channel (RGB) difference between two images, normalized to 0.0..=1.0 (0 = identical, 1 = every channel maximally different). Mismatched dimensions read as fully different (1.0). Alpha is ignored — the capture background is opaque, so alpha carries no signal.