Compose, by Rendobar
Video composition API
Rendobar's compose API renders finished video from a JSON timeline. Tracks hold clips. Clips reference video, image, audio, or text. Transitions sit between them. You POST the timeline and an MP4 comes back. A typical composition renders in about 6 seconds. No template editor, no render farm to run.
$5 in free credits. No credit card required.
What it is
What is a video composition API?
A video composition API renders finished video from a structured description instead of an editing UI. With compose, that description is a JSON timeline: tracks hold clips, clips reference video, image, audio, or text assets, and transitions sit between them. You POST the timeline to the Rendobar API and get an MP4 back, with progress streamed while it renders.
There is no template to build first and no editor to click through. The timeline is plain data, so it can come from your backend, a spreadsheet, a form, or a coding agent. One schema covers a single clip trim and a multi-track composition with nested timelines.
The whole model in five steps
Start with one clip. Layer up.
Each step adds one idea to the JSON. The preview is the real render at that step.
One track, one clip
The smallest timeline. Three seconds of one video, rendered to MP4.
{ "timeline": { "tracks": [{
"clips": [
{ "asset": { "type": "video", "src": ".../sea.mp4" },
"start": 0, "length": 3 }
] }] } }Join two clips with a crossfade
A transition is an item between two clips. Video dissolves and audio crossfades, automatically.
"clips": [
{ "asset": { "src": ".../sea.mp4" }, "length": 3 },
{ "type": "transition",
"transition": "crossfade", "duration": 1 },
{ "asset": { "src": ".../city.mp4" }, "length": 4 }
]Layer a title on top
A second track renders above the first. Text is an asset like any other, with style, timing, and an entrance animation.
"tracks": [
{ "clips": [ ...video clips... ] },
{ "clips": [
{ "asset": { "type": "text", "text": "Summer '26",
"style": { "size": 72, "color": "#FFFFFF",
"animate": { "type": "slideUp" } } },
"start": 0.5, "length": 3 } ] }
]Give a clip an effect
Effects are per-clip fields. Color grades, look presets, chroma key, Ken Burns motion, speed, blur, and opacity keyframes.
{ "asset": { "src": ".../photo.jpg" }, "length": 4,
"color": { "saturation": 1.2 },
"transform": { "scale": 1,
"animateTo": { "scale": 1.35 } } }Or think in scenes
For slideshows and data-driven video, write scenes instead of tracks. Overlays span the whole video. Same engine underneath.
"scenes": [
{ "clips": [{ "asset": { "src": ".../intro.mp4" } }] },
{ "transition": { "transition": "slide", "duration": 0.6 },
"clips": [{ "asset": { "src": ".../product.mp4" } }] }
],
"overlays": [{ "asset": { "type": "image", "src": ".../logo.png" } }]The full vocabulary
Everything in the timeline
Every tile below is a real compose render. Hover to play. Split tiles show the source next to the result.
Transitions
A transition is an item between two clips. Video blends and audio crossfades automatically. Crossfade, fade to black, slide from any edge, zoom, or a hard cut.
{ "type": "transition", "transition": "crossfade", "duration": 1 }Motion
Animate a clip's position, scale, and rotation from a start state to an end state for Ken Burns moves. Change playback speed for slow motion or timelapse. Crop by edge percentages. Flip on either axis.
"transform": { "scale": 1, "animateTo": { "scale": 1.4 } }Color
Grade any clip with brightness, contrast, saturation, gamma, hue, and color temperature. Or pick one of seven named looks. Gaussian blur and unsharp sharpening are one field each.
"color": { "saturation": 1.2, "temperature": 5200 }Compositing
Tracks stack bottom to top. Scale a clip down for picture-in-picture, key out a green screen, blend with seven modes, and animate opacity with keyframes and easing. A clip can even hold a nested timeline.
"opacity": [{ "time": 0, "value": 0 }, { "time": 1, "value": 1, "easing": "smooth" }]Text and titles
Text is an asset like any other. Font, size, weight, color, stroke, background, alignment, and position, plus entrance animations and a drop shadow.
"animate": { "type": "slideUp", "duration": 0.6 }Audio
Every video clip carries its audio with per-clip volume. Add music as an audio asset with fade in and fade out, balance a clip left or right with stereo pan, and let transitions crossfade the mix automatically.
{ "type": "audio", "src": ".../bed.mp3", "volume": 0.4, "fadeIn": 1 }Output
MP4 with H.264, WebM with VP9, animated GIF, MP3 for audio-only exports, and JPG or PNG to capture a single frame at any timestamp. Any resolution up to 4K on every plan, any frame rate, codec overrides when you want them. No resolution tiers.
"output": { "format": "gif", "resolution": { "width": 480, "height": 854 } }The alternative is a filtergraph
The same edit, twice
Two clips, a crossfade, a title. On the left, the FFmpeg filter_complex you would maintain by hand. On the right, the compose timeline. Both run on Rendobar. Pick the one you want to debug at 2am.
ffmpeg -i beach.mp4 -i city.mp4 \ -filter_complex "\ [0:v]trim=0:4,setpts=PTS-STARTPTS,\ scale=1080:1920:force_original_aspect_ratio=increase,\ crop=1080:1920,format=yuv420p[v0]; \ [1:v]trim=3:7,setpts=PTS-STARTPTS,\ scale=1080:1920:force_original_aspect_ratio=increase,\ crop=1080:1920,format=yuv420p[v1]; \ [v0][v1]xfade=transition=fade:duration=1:offset=3[vx]; \ [vx]drawtext=fontfile=Inter.ttf:text='Summer 26':\ fontsize=72:fontcolor=white:x=(w-tw)/2:y=h-320:\ enable='between(t,0.5,3.5)'[vt]; \ [0:a]atrim=0:4,asetpts=PTS-STARTPTS[a0]; \ [1:a]atrim=3:7,asetpts=PTS-STARTPTS[a1]; \ [a0][a1]acrossfade=d=1[ax]" \ -map "[vt]" -map "[ax]" -c:v libx264 -crf 20 out.mp4 # plus the retry logic, the temp files, # the machine it runs on, and the queue in front of it
{
"output": { "format": "mp4",
"resolution": { "width": 1080, "height": 1920 } },
"timeline": {
"tracks": [
{ "clips": [
{ "asset": { "type": "video", "src": ".../beach.mp4" },
"start": 0, "length": 4 },
{ "type": "transition",
"transition": "crossfade", "duration": 1 },
{ "asset": { "type": "video", "src": ".../city.mp4" },
"start": 3, "length": 4 }
] },
{ "clips": [
{ "asset": { "type": "text", "text": "Summer 26",
"style": { "size": 72, "color": "#FFFFFF" } },
"start": 0.5, "length": 3 }
] }
]
}
}When the timeline cannot express something, Rendobar's FFmpeg API takes the raw command. Same platform, same credits, one job type apart.
Built for coding agents
Your agent writes the timeline
A JSON timeline is exactly what coding agents are good at producing. Rendobar ships a native MCP server, so Claude Code, Cursor, and Codex can submit compose jobs, watch progress, and fetch the output without leaving the editor.
Where it sits
compose, template platforms, or raw FFmpeg
| compose | Template platforms | DIY FFmpeg | |
|---|---|---|---|
| You write | a JSON timeline | a template in their editor | filter_complex strings |
| Template editor required | none | yes | none |
| Multi-track layering | yes | varies by plan | yes, by hand |
| Infrastructure to run | none | none | servers, queues, retries |
| Live render progress | percent, fps, speed | rarely | parse stderr yourself |
| Escape hatch to raw FFmpeg | same platform, one job type over | no | you are the escape hatch |
Comparing a specific platform? See the comparison pages.
Three ways in
Submit from anything that can POST
import { createClient, outputUrl } from "@rendobar/sdk";
const client = createClient({ apiKey: process.env.RENDOBAR_API_KEY });
const job = await client.jobs.create({ type: "compose", params: { schemaVersion: 1, output: { format: "mp4", resolution: { width: 1080, height: 1920 }, fps: 30 }, timeline: { tracks: [{ clips: [ { asset: { type: "video", src: "https://cdn.rendobar.com/assets/examples/sea.mp4" }, start: 0, length: 4 }, { type: "transition", transition: "crossfade", duration: 1 }, { asset: { type: "video", src: "https://cdn.rendobar.com/assets/examples/sample.mp4" }, start: 3, length: 4 }, ], }], }, },});
const result = await client.jobs.wait(job.id);console.log(outputUrl(result)); // the rendered MP4import os, requests
resp = requests.post( "https://api.rendobar.com/jobs", headers={"Authorization": f"Bearer {os.environ['RENDOBAR_API_KEY']}"}, json={ "type": "compose", "params": { "schemaVersion": 1, "output": {"format": "mp4", "resolution": {"width": 1080, "height": 1920}, "fps": 30}, "timeline": { "tracks": [{ "clips": [ {"asset": {"type": "video", "src": "https://cdn.rendobar.com/assets/examples/sea.mp4"}, "start": 0, "length": 4}, {"type": "transition", "transition": "crossfade", "duration": 1}, {"asset": {"type": "video", "src": "https://cdn.rendobar.com/assets/examples/sample.mp4"}, "start": 3, "length": 4}, ] }] }, }, },)print(resp.json())curl -X POST https://api.rendobar.com/jobs \ -H "Authorization: Bearer $RENDOBAR_API_KEY" \ -H "Content-Type: application/json" \ -d '{ "type": "compose", "params": { "schemaVersion": 1, "output": { "format": "mp4", "resolution": { "width": 1080, "height": 1920 }, "fps": 30 }, "timeline": { "tracks": [{ "clips": [ { "asset": { "type": "video", "src": "https://cdn.rendobar.com/assets/examples/sea.mp4" }, "start": 0, "length": 4 }, { "type": "transition", "transition": "crossfade", "duration": 1 }, { "asset": { "type": "video", "src": "https://cdn.rendobar.com/assets/examples/sample.mp4" }, "start": 3, "length": 4 } ] }] } } }'FAQ
Questions developers actually search
What is a video composition API?
An API that renders finished video from a structured description instead of an editing UI. With compose the description is a JSON timeline of tracks, clips, transitions, text, and effects. You POST it to the Rendobar API and get an MP4 back.
How do I create a video from JSON?
Write a timeline object with at least one track and one clip, then POST it to /jobs with type "compose". The response carries the job id. When the job completes, the output URL points at the rendered file. The five-step section above walks the whole model.
Which transitions does the compose API support?
Crossfade, fade to black, slide from any of four directions, zoom, and a hard cut. A transition sits between two clips on a track, or between two scenes in scene mode. Audio crossfades automatically.
Can I add text overlays to a video with an API?
Yes. Text is an asset type in the timeline with font, size, weight, color, stroke, background, position, entrance animation, and drop shadow. Put it on an upper track and it renders over the video below.
What output formats does compose support?
MP4 with H.264, WebM with VP9, animated GIF, MP3 for audio-only exports, and JPG or PNG to capture a single frame at any timestamp. Any resolution up to 4K renders on every plan with no resolution tiers, and frame rate is a free choice.
How is compose different from running FFmpeg commands directly?
Both run on Rendobar. compose takes a declarative timeline and handles the filter graphs, sync, and audio mixing for you. The FFmpeg API takes the raw command when you need something the timeline does not express. Same platform, one job type apart.
How much does it cost to render a video?
Pricing is cost-plus per second of compute, and a typical short composition renders in about 6 seconds. Signup includes $5 of free credit with no card required.
Can AI agents like Claude Code use the compose API?
Yes. Rendobar ships a native MCP server, and a JSON timeline is exactly the kind of structured output coding agents are good at producing. Agents can submit compose jobs, watch progress, and fetch the output without leaving the editor.
Render your first composition
$5 of free credit on signup. One POST request to your first MP4.