16:9 insert in a 1080x1920 Short: Timeline fit modes compared

When a 16:9 clip goes into a vertical Timeline, video[].fit decides how it fills the frame. cover, the default, scales it up and crops, keeping about 31.6% of the width. contain fits the whole picture and pads with black. stretch fills the frame and distorts. blur fits the whole picture over a blurred copy of itself.

What each does at 1080x1920

Behaviour is from the Timeline compiler source on main; the numbers are arithmetic for a 1920x1080 source in a 1080x1920 frame.

Picking one

Use cover when the subject is centred and the frame is more important than the edges, such as a face. Use contain for a screen recording or a chart where cropping would cut data, and accept the bars. Use blur when you want the whole picture and bars would look unfinished. Avoid stretch except for abstract footage.

A vertical platform page does not tell you which is right. TikTok's ad page recommends 9:16 and lists a 540x960 minimum, and YouTube's page says Shorts are vertical. Neither says anything about bars. That is a design choice for you.

Mixing fits in one Short

Fit is set per slot, not per render, so one Short can mix them. A generated vertical clip needs no fit decision, because it already matches the frame. A landscape screen recording can use contain or blur, and a landscape b-roll shot can use cover.

Be consistent about the rule you choose. Switching between black bars and blur from shot to shot looks accidental, even if each choice made sense alone. Pick one treatment for non-vertical sources and apply it throughout.

Also keep the first frame in mind. In many apps the first frame is what shows before playback, and a bar-heavy frame looks weaker in a feed than a full-frame one. That is a design view, not a spec; neither page I read says anything about it.

The cost

A blur fit builds extra filter nodes per slot, and the compiler downsizes the blur background to an eighth of the frame to keep it cheap. Timeline is billed by output minute, not by fit mode, at the public rate in the doc ($0.10 per output minute, rounded up), so the choice is about looks and render time. See the 4:5 on a 9:16 canvas comparison for another ratio.

{
  "audio": { "mode": "silence", "duration_seconds": 6 },
  "video": [{ "source_url": "https://media.sume.com/artifacts/artf_demo/wide.mp4", "start": 0, "duration": 6, "fit": "blur" }],
  "output": { "width": 1080, "height": 1920 }
}