Output, size and validation

output controls the size, frame rate, format and encoder. Give it an explicit resolution, or a named size preset like "1080p", "tiktok" or "reels".

"output": { "format": "mp4", "resolution": { "width": 1080, "height": 1920 }, "fps": 30 }

A 9:16 canvas with "fit": "cover" or a crop on the clip turns landscape footage into a vertical Reel that fills the frame, with no black bars. "fit": "blur" keeps all of the picture instead, over a blurred copy of itself. background fills whatever the clips do not cover. It takes a hex colour, or "transparent", which png and webm keep as real alpha. mp4 has no transparency, so it refuses that rather than rendering black.

{
  "schemaVersion": 1,
  "output": { "format": "webm", "size": "1080p" },
  "timeline": {
    "background": "transparent",
    "tracks": [{ "clips": [{ "asset": { "type": "image", "src": "https://cdn.rendobar.com/assets/brand/logo-mark.png" } }] }]
  }
}

range renders part of the timeline instead of all of it, and on jpg and png its start is the frame you get.

"output": { "format": "png", "size": "1080p", "range": { "start": 4.5 } }

How a picture fills its box

A wide photograph in a phone-shaped box. contain shows all of it and leaves bars, cover fills the box and cuts what overflows.

before

fit cover

A compose job renders exactly one file. Two sizes of the same edit are two jobs, which is also how you get them rendered at the same time. gif carries no audio, mp3 is audio only and needs no frame size, and jpg and png are a single frame.

Check it before you render

validate parses a timeline exactly as a render would and costs nothing. It returns the params as the engine reads them, anything it had to upgrade from an older shape, and warnings for settings the render will ignore.

const check = await client.compose.validate(params);
if (!check.valid) console.error(check.problems);

curl -X POST https://api.rendobar.com/compose/validate \
  -H "Authorization: Bearer rb_YOUR_KEY" \
  -H "Content-Type: application/json" \
  -d '{ "params": { "schemaVersion": 1, "output": { "format": "mp4", "size": "1080p" }, "timeline": { "tracks": [{ "clips": [{ "asset": { "type": "video", "src": "https://example.com/a.mp4" } }] }] } } }'

Every output field is in the output reference.

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Output, size and validation

How a picture fills its box

Check it before you render

​How a picture fills its box

​Check it before you render

How a picture fills its box

Check it before you render