compose renders a video from a JSON timeline. Start with one clip, then layer up: more clips, transitions, overlays, and effects. This page builds from the smallest render to the full model. Each step adds one idea.
Your simplest render
The smallest timeline is one track with one clip. This trims three seconds of a video and renders it to MP4. The whole API call:
import { createClient, outputUrl } from "@rendobar/sdk";
const client = createClient({ apiKey: "rb_YOUR_KEY" });
const job = await client.jobs.create({
type: "compose",
params: {
schemaVersion: 1,
output: { format: "mp4", resolution: { width: 1280, height: 720 }, fps: 30 },
timeline: {
tracks: [
{ clips: [
{ asset: { type: "video", src: "https://cdn.rendobar.com/assets/examples/a.mp4", trim: { from: 0, to: 3 } }, start: 0, length: 3 },
] },
],
},
},
});
const result = await client.jobs.wait(job.id);
console.log(outputUrl(result));
Assets: video, image, text, audio, composition · Output: mp4 / webm / gif / mp3 (audio only) / jpg / png (still frame)
Join two clips
Put a second clip on the same track and a transition between them. The clips overlap by the transition’s duration, so the second clip starts a little before the first ends.
"clips": [
{ "asset": { "type": "video", "src": "a.mp4" }, "start": 0, "length": 3 },
{ "type": "transition", "transition": "crossfade", "duration": 1 },
{ "asset": { "type": "video", "src": "b.mp4" }, "start": 2, "length": 3 }
]
slide also accepts direction: "left" | "right" | "up" | "down".
| Transition | Effect |
|---|
crossfade | A dissolves directly into B |
fade | A dips through black, then B fades up |
slide | B slides in over A from an edge (directional) |
zoom | B grows in from the center |
How the timeline works
The model is two ideas:
- Tracks stack. The first track is the background. Each later track composites on top of the ones below it: overlays, titles, picture-in-picture.
- Clips sit in time. Each clip has a
start and a length in seconds. Two clips on the same track are joined by a transition. A gap between them plays nothing.
So a multi-track timeline is layers (tracks) of sequences (clips). That is the whole structure.
Add a layer
A second track renders over the first. This lays an animated title over the video.
"tracks": [
{ "clips": [
{ "asset": { "type": "video", "src": "a.mp4" }, "start": 0, "length": 5 }
] },
{ "clips": [
{ "asset": { "type": "text", "text": "Coastal Escape", "style": { "size": 84 } },
"start": 0, "length": 5 }
] }
]
Effects on a clip
Any clip takes effects as fields. Add one field at a time, on the clip it applies to.
Color grade
color grades a clip. Set contrast, saturation, temperature, brightness, and gamma to set a cinematic mood, or push them to lift flat, hazy footage.
"color": { "contrast": 1.12, "saturation": 1.2, "temperature": 6800, "gamma": 0.95 }
Filters
filter applies a one-shot look: greyscale, negative, boost, muted, lighten, darken, or contrast.
transform positions and scales a clip. Add animateTo and it animates from the start values to the end values over the clip, the Ken Burns move that brings a still photo to life.
"transform": {
"scale": 1.1,
"position": { "x": "38%", "y": "48%" },
"animateTo": { "scale": 1.32, "position": { "x": "62%", "y": "52%" } }
}
Picture-in-picture
Scale a clip down and position it on an overlay track to put a webcam over your content, for reactions and screen-shares.
"transform": { "scale": 0.3, "position": { "x": "80%", "y": "75%" } }
Speed
speed is a multiplier. Below 1 slows a clip down for a dramatic action beat, above 1 speeds a slow process into a timelapse.
Chroma key
chromaKey removes a solid background color so you can drop a presenter onto any scene. Put the keyed clip on an overlay track above the new background.
"chromaKey": { "color": "#66DD09", "similarity": 0.28 }
Blend modes
blendMode controls how an overlay composites with the track below: screen, multiply, overlay, add, and more. A light-leak clip over your footage with screen adds a cinematic glow.
"blendMode": "screen",
"opacity": 0.7
Opacity and blur
opacity takes a number or keyframes. Keyframe it from 0 to 1 and back for a clean fade in and out. blur softens a clip, useful as a backdrop that keeps overlaid text readable.
"opacity": [
{ "time": 0, "value": 0 },
{ "time": 0.6, "value": 1 },
{ "time": 2.4, "value": 1 },
{ "time": 3, "value": 0 }
]
Text and titles
A text asset renders a styled title. Set the font, size, weight, color, position, and an animate entrance (fade, slideUp, slideDown, slideLeft, slideRight). Put one text clip on its own track for a title card, another low and left for a lower-third name tag.
"asset": {
"type": "text",
"text": "Coastal Escape",
"style": {
"font": "Inter",
"size": 84,
"weight": 800,
"color": "#FFFFFF",
"position": { "x": "50%", "y": "38%" },
"animate": { "type": "slideUp", "duration": 0.7 }
}
}
Set the canvas
output controls the resolution, frame rate, and format. A 9:16 canvas with a crop on the clip turns landscape footage into a vertical Reel that fills the frame, no black bars.
"output": { "format": "mp4", "resolution": { "width": 1080, "height": 1920 }, "fps": 30 }
Scene-first authoring
Tracks give you precise multi-track control. For a sequential edit, you can instead describe a list of scenes. Each scene is a self-contained segment with its own clips and an optional transition into the next. overlays is a flat list of clips that sit over the whole composition, like a persistent logo or watermark.
{
"scenes": [
{ "duration": 4, "transition": { "transition": "crossfade", "duration": 0.6 },
"clips": [{ "asset": { "type": "video", "src": "a.mp4" } }] },
{ "duration": 4, "clips": [{ "asset": { "type": "video", "src": "b.mp4" } }] }
],
"overlays": [
{ "asset": { "type": "image", "src": "logo.png" }, "start": 0, "length": 8,
"transform": { "scale": 0.2, "position": { "x": "88%", "y": "12%" } } }
]
}
start and length to span the whole scene. Every clip field works the same as in tracks mode. scenes and tracks are mutually exclusive: a timeline uses one or the other.
Reference
Output
The output object controls the render target.
| Field | Values |
|---|
format | mp4 (default), webm, gif (no audio), mp3 (audio only), jpg, png (still frame) |
resolution | { width, height } (integer pixels) |
fps | number |
videoCodec | h264, vp9 (optional; per-format default otherwise) |
audioCodec | aac, opus, mp3 (optional) |
frameTime | second of the timeline to capture for jpg / png (default 0) |
Assets
A clip’s asset is one of:
type | Fields |
|---|
video | src, trim { from, to }, volume (default 1) |
image | src |
audio | src, trim, volume, fadeIn, fadeOut (seconds) |
text | text, style |
composition | timeline, a nested { tracks } rendered and composited as a single clip |
Clip fields
Every clip has asset, start, length, plus any of:
| Field | What it does |
|---|
opacity | 0..1, or keyframes [{ time, value, easing: step | linear | smooth }] |
transform | position {x,y}, scale, rotate, anchor (center | topLeft), fit (contain | fill), animateTo (Ken Burns end state) |
crop | top / right / bottom / left, each a percent string like "10%" |
speed | playback rate (0.5 = slow motion, 2 = fast, and pitch shifts with speed) |
color | brightness (-1..1), contrast (0..4), saturation (0..3), gamma (0.1..10), hue (-180..180), temperature (1000..40000 K) |
filter | greyscale, negative, boost, muted, lighten, darken, contrast (mutually exclusive with color) |
blur / sharpen | gaussian blur strength / unsharp amount |
chromaKey | color (hex), similarity (0..1) |
blendMode | normal, multiply, screen, overlay, darken, lighten, add, difference |
pan | stereo balance, -1 (left) to 1 (right) |
flip | { horizontal, vertical } |
Transitions
A track item { "type": "transition", "transition": <kind>, "duration": <seconds>, "direction"?: <dir> }:
crossfade, fade, slide (direction: left / right / up / down), zoom, none (hard cut).
Text style
Fields on a text asset’s style:
font, size, weight (100..1000), color, align (left / center / right), position {x,y}, stroke {color,width}, background, shadow {color,opacity,offsetX,offsetY}, animate {type: fade \| slideUp \| slideDown \| slideLeft \| slideRight, duration}. 
