Square YouTube Short (1:1) from a vertical master with Timeline

YouTube accepts a Short that is square or vertical, so a 1:1 file is valid. To turn a 9:16 master into 1080 by 1080 with Sume, call Timeline 1.0 with output.width and output.height both 1080, and choose fit: blur or contain so the whole vertical frame stays visible instead of being cropped.

The YouTube rule is from Upload YouTube Shorts and Understand three-minute YouTube Shorts, read 2026-10-02. Timeline behavior is from the Timeline 1.0 docs.

Is a square video a valid Short?

Yes by YouTube's wording: both pages say square or vertical aspect ratio, up to three minutes. The three-minute page adds that a wider ratio such as 16:9 keeps a video from being a Short. Neither page says one shape performs better, so this post makes no claim about reach.

Why not just crop the vertical master?

A 9:16 frame cropped to 1:1 loses more than 40 percent of its height. Faces and burned-in captions near the top or bottom can be cut. Timeline's fit field gives four choices: cover (the default, which fills and crops), contain (whole frame, with bars), stretch (distorts), and blur (whole frame over a blurred fill).

If your master has captions already burned in, cover will likely cut them. Use contain or blur, or re-burn captions after the recompose by running video captions on the square file.

How do I render it?

Timeline needs a spine: audio.duration_seconds is required (1 to 1800 seconds) plus either an audio file or audio.mode: silence. For a clip you want to keep with its own voice, pass that voice file as audio.url. Each video[] slot has a Sume-hosted source_url, a start that must be 0 for the first slot, and a duration. Output size must be even integers from 256 to 2160, so 1080 by 1080 is valid.

The public rate is $0.10 per ceil(output minute), which makes a 58-second Short one billed minute at the documented rate; confirm in GET /v1/catalog.

curl -X POST https://api.sume.com/v1/timeline-1.0/render \
  -H "Authorization: Bearer $SUME_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: square-short-001" \
  -d '{"audio":{"url":"https://media.sume.com/artifacts/artf_demo/voice.wav","duration_seconds":58},
       "video":[{"source_url":"https://media.sume.com/artifacts/artf_demo/master.mp4",
                 "start":0,"duration":58,"fit":"blur"}],
       "output":{"width":1080,"height":1080}}'

What should I verify afterwards?

Run video inspect with frames: false on the result and confirm width equals height and duration_seconds is under 180. Read warnings[] on the Timeline result: padded or looped sources and an output frame-rate mismatch are reported there rather than failing the job.

Sume does not publish to YouTube and cannot tell you how the square file will look in the Shorts feed. Check it in YouTube's own editor before publishing.