Eased punch-in on a screen recording

Give it a screen recording, the piece's time span and two or three crop rectangles on the screen (in source pixels). You get the piece rendered at the output size with a smooth push or pan between those rectangles. Use it for zooms on real footage; a compositor camera over layers you built is hn4a-compositor-in-scene-titles, and an animated crop on a compositor video layer doesn't work (hn4a-compositor-motion-graphics).

Inputs

• The recording and the piece's start and end.
• A fixed region around the screen window, with the output's aspect (1.125 for a 1080×960 half of a 9:16 split screen).
• Keyframes: fractions of the piece (0, 0.6, 1) → a crop rectangle inside the region. Read the rectangles off frames at the piece's start and end, cropped to the screen window with a 100 px grid and source-pixel labels.

Run it

1. Crop the region and upscale it 2× with lanczos. The upscale is what keeps the move smooth: zoompan on the source pixels steps visibly.
2. zoompan with d=1 (one output frame per input frame) and s= the output size. Zoom = region width ÷ crop width; x and y are the crop's position inside the region, doubled for the upscale.
3. Ease between keyframes with a smoothstep on the output frame number on: p = (on/N − f0)/(f1 − f0), e = p·p·(3 − 2p), then interpolate the rectangle with e. Write the expressions from a small Python script per piece rather than by hand.
4. Hold the last key when a graphic must land on a still frame.

Tips and traps

• One move a piece, calm: one drift and one push, never a chase.
• Keep text legible: no crop narrower than about 300 source pixels for UI text.
• Pages scroll mid-piece. Frame loosely enough to survive it, or end the move before the scroll.
• A static punch-in on a whole UI window is unreadable on a phone. Push onto one element.

Check it

ffprobe the frame count against the piece's duration × fps, and a contact sheet at the start, middle and end of each move to confirm it lands on the element.