Render part of a timeline, or one still frame

output.range renders part of a timeline instead of all of it, which is how you get a 6 second preview of a 3 minute edit without rewriting the timeline.

"output": { "format": "png", "size": "1080p", "range": { "start": 4.5 } }

On jpg and png the range’s start is the frame you get. That also fixes a bug: stills ignored the frame you asked for and always returned the timeline’s last frame.

timeline.background fills whatever the clips do not cover. It takes a hex colour, or "transparent", which png and webm keep as real alpha, so an overlay renders with nothing behind it. mp4 has no transparency and refuses it rather than rendering black.

An mp3 renders on a small frame now, over twice as fast, with the same bytes. A codec the container cannot write, like Opus in an mp4 that asks for gif, is refused at submit instead of failing after the work is done.

Every output field is in the compose docs.

Check any of it before you render with validate.