YouTube 9:16 on desktop: white padding, and don't bake bars in

On a computer, YouTube may add padding around a 9:16 vertical video, white by default and dark gray when Dark theme is on. YouTube's advice is to avoid adding padding or black bars directly to your video, because that interferes with its ability to resize the player to your video and the viewer's device. So render the vertical frame at its true 9:16 shape and let YouTube do the padding.

What YouTube does

The standard YouTube aspect ratio on a computer is 16:9, and the player adapts to each video's shape. Padding appears when the video's shape and the device's shape differ, as with vertical video in a desktop browser.

What Sume gives you

Timeline 1.0 defaults to a 1080x1920 output, which is already 9:16, and the Video Router's Auto model offers 16:9 or 9:16 clips. So a vertical clip can enter Timeline with the same shape as the output and need no borders.

Keep the shapes aligned

Timeline lists four fit modes for each slot: cover, contain, stretch and blur, with cover as the default. When the source shape differs from the output, some mode has to decide what to do with the difference. Choose an output size that matches your sources so no fit mode has to invent a border. The unbilled plan route validates a body without creating a job or reserving credits.

curl -X POST https://api.sume.com/v1/timeline-1.0/plan \
  -H "Authorization: Bearer $SUME_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "audio": { "mode": "silence", "duration_seconds": 12 },
    "video": [
      {
        "source_url": "https://media.sume.com/artifacts/artf_demo/vertical.mp4",
        "start": 0,
        "duration": 12,
        "fit": "cover"
      }
    ],
    "output": { "width": 1080, "height": 1920 }
  }'

Checklist

• Final frame is true 9:16 with no baked white or black bars.
• Do not draw a fake background to match the desktop padding color; it changes between themes.
• If a source is 16:9, either keep the output 16:9 or crop with cover on purpose.