YouTube Shorts ads: horizontal video gets blurred bars, make your own

Google Ads Help says 9:16 is strongly recommended for YouTube Shorts ads, and that horizontal assets are shown with a blurred top and bottom. If you want control over what fills that space, render the vertical file yourself with Sume Timeline 1.0, whose video[].fit has cover, contain, stretch and blur modes.

What Google says, and what each fit does

The Google page describes the automatic treatment. The Sume behaviour below comes from the Timeline compiler source in the repo, not from Google.

Render the blur version

Timeline's default output is a 1080x1920 MP4, so only the fit needs setting. The request below keeps a 20-second wide clip whole over a blurred background. A Timeline render needs an audio spine with a declared length, here a Sume-hosted voice file. The plan endpoint is free and checks a request before you pay. A render costs $0.10 per output minute, rounded up.

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: shorts-ad-blur-001" \
  -d '{
    "audio": {"url": "https://media.sume.com/artifacts/artf_demo/voice.wav", "duration_seconds": 20},
    "video": [{
      "source_url": "https://media.sume.com/artifacts/artf_demo/wide.mp4",
      "start": 0, "duration": 20, "fit": "blur"
    }],
    "output": {"width": 1080, "height": 1920}
  }'

When to choose cover instead

A blurred pad shrinks the picture to a band in the middle, so product text gets small. If the subject is centred, cover fills the frame and crops the sides. For an exact crop that keeps an off-centre subject, use video filter ops[] with a fractional crop rectangle first, then place the cropped clip in Timeline.

Keep length in mind too: Google's page says an uploaded Shorts ad video can run up to 3 minutes, but only the first 60 seconds play on the Shorts feed.