Crop 21:9 to 9:16: a 608 px strip, then scale to 1080x1920

A 21:9 frame is 2.33 times wider than tall, and a 9:16 frame is 0.5625 times wider than tall, so a vertical cut from ultrawide keeps only about a quarter of the width. On a 2560x1080 source, that is a 608x1080 strip.

In Sume's video filter, the crop op is x: 0.381, y: 0, width: 0.2379, height: 1. The width of 0.2379 x 2560 is 609.02, and the compiler rounds down to even pixels, so you get 608. The value 0.2375 can round to 606 because of floating point, so nudge it up.

Then scale to the delivery size

The crop op keeps geometry. To reach 1080x1920 add a filtergraph. Sume applies ops[] first and then the graph, and scale is on the allowlist (the video filter docs, read 2026-10-05, list geometry filters). A strip of 608x1080 scaled to 1080x1920 is a 1.78x upscale, so expect a softer picture than a native vertical render.

{
  "video_url": "https://media.sume.com/artifacts/artf_demo/wide.mp4",
  "ops": [{"op": "crop", "x": 0.381, "y": 0, "width": 0.2379, "height": 1}],
  "filtergraph": "scale=1080:1920:flags=lanczos"
}

Is a crop the right move?

Only if the subject is centered. The crop is a fixed rectangle, so a subject off the middle gets cut. For a moving subject, or for the full ultrawide picture inside a vertical frame, the Timeline 1.0 fit options are the alternative: cover crops, contain letterboxes, and blur fills the bars with a blurred copy of the clip.

Numbers at a glance

The 1:1 row is 1080 / 2560 = 0.421875. Check any program with the free /v1/video-filter/check before the $0.02 encode.