Image above video, or left and right: compose image_region axes

With Sume's stack compose, layout.split and layout.image_region have to agree. A horizontal split takes top or bottom; a vertical split takes left or right. Mixing them returns a 400, compose_image_region_wrong_axis.

This is from the Timeline compose docs, read 2026-10-01. Runway's changelog says it now runs in a panel inside Premiere Pro and After Effects; this page covers the API route for the same still-plus-video layout.

Which region goes with which split?

The split names the direction of the cut line. A horizontal split gives you an upper and a lower region; a vertical split gives a left and a right one.

What does ratio do on a vertical split?

ratio is the still's share of the frame, 0.1 to 0.9, and the video takes the exact remainder. On a vertical split the share runs across the width, so on a 720 by 1280 output a ratio of 0.4 is arithmetic for a 288-pixel-wide still and a 432-pixel-wide video. The default horizontal, top, 0.5 layout is covered in stack an image over a video.

How do I put the image on the left?

Set the split to vertical and the region to left.

curl -X POST https://api.sume.com/v1/timeline-1.0/compose \
  -H "Authorization: Bearer $SUME_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: compose-left-001" \
  -d '{
    "operation": "stack",
    "image": { "url": "https://media.sume.com/artifacts/artf_demo/poster.png" },
    "video": { "url": "https://media.sume.com/artifacts/artf_demo/talk.mp4" },
    "layout": { "split": "vertical", "image_region": "left", "ratio": 0.4 }
  }'

What other layout mistakes return 400?

Mixing stack keys with overlay keys: compose_stack_takes_no_overlay_layout or compose_overlay_takes_no_stack_layout. split, image_region and ratio are illegal on an overlay.