VideoTransform

The viewer's framing controls, on top of VideoScale: force a display aspect the container did not declare (mpv's video-aspect-override), magnify (mpv's video-zoom), and pan the magnified picture (mpv's video-pan-x/-y). The engine owns the value and every renderer is told it, the same delivery law as VideoScale and VideoAdjustments; the renderers fold it into the same one-pass geometry that letterboxes, so it costs nothing per frame.

Order, applied by the geometry: the aspect override reshapes the content, the scale mode fits that shape to the viewport, the zoom scales the fitted rectangle about its centre, and the pan then moves it by a fraction of its own drawn size. Pixel aspect and rotation are folded in before all four, as always.

It also turns and mirrors the picture (#428), on top of what the file asks for, as mpv's video-rotate and VLC's transform do: for a phone clip filmed sideways with no rotation tag, a webcam that recorded mirrored, or a screen mounted on its side. The mirrors apply to the picture as the file shows it, and the turn after them. Every renderer folds the result into the turn it already draws, through orient, so it costs nothing per frame and keeps hardware decoding, and everything that follows the picture's shape follows it.