How a background idle is made to loop with no visible seam: grade the still → animate with a strict subtle-motion prompt (breathing only, no actions) → boomerang loop → moving film-grain. Because motion is tiny and the scene is drift-free, the reversal is invisible. Sample: Sci-Fi Noir, 480p.
Final loop graded · subtle · boomerang · grain
Watch the patrons — breathing only; the loop point just flows.