> ## Documentation Index > Fetch the complete documentation index at: https://rendobar.com/docs/llms.txt > Use this file to discover all available pages before exploring further. > ## Agent Instructions > Keep the API key in an env var (RENDOBAR_API_KEY), never in chat or committed code. > Job types change, so fetch the live catalog from GET https://api.rendobar.com/jobs/types rather than relying on memory. > The full integration prompt for coding agents is at https://rendobar.com/prompts/integrate.md. # How the timeline works > Build a compose timeline from tracks and clips, and learn when to leave start and length out of it entirely. A timeline is tracks, and a track is clips. The first track is the background and every track after it draws over the one before. That is the whole model, and the rest of this page is what falls out of it. 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.** A clip can carry a `start` and a `length` in seconds, and it can leave both out. 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. ## Timing is optional Every clip sits in a **container**: the scene it belongs to, or the whole video. Two sentences cover every case. * **Omit `length`** and the clip runs as long as its **content** does, never past its container. * **Write `length: "end"`** and it runs to the end of its **container**, whatever its content says. Content means the source's duration, or the trim span, or the source duration divided by `speed` (so `speed: 0.5` on a 4 second clip occupies 8 seconds), or a composition's own end. Text and images have no duration of their own, so for them the two sentences say the same thing. A title with no `length` stays up for the whole video, which is what a watermark wants. `start` follows the same shape: omit it and the clip begins after everything already placed on its track, pulled back by any transition immediately before it. Give it a number and that is seconds from the start of its container. Set either field wherever you want something else. Every timeline written with full timing keeps rendering exactly as it did. ## Worked examples A music bed. The source is three minutes long, the video is not, and `"end"` is how you say so without totalling the clips above it. ```json theme={null} "tracks": [ { "clips": [ { "asset": { "type": "video", "src": "https://cdn.rendobar.com/assets/examples/river.mp4" } }, { "asset": { "type": "video", "src": "https://cdn.rendobar.com/assets/examples/forest.mp4" } } ] }, { "clips": [ { "asset": { "type": "audio", "src": "https://cdn.rendobar.com/assets/showcase/music.mp3", "volume": 0.35, "fadeOut": 2 }, "length": "end" } ] } ] ``` A watermark. It has no duration of its own, so it needs nothing at all. ```json theme={null} { "clips": [ { "asset": { "type": "image", "src": "https://cdn.rendobar.com/assets/brand/logo-mark.png" }, "transform": { "scale": 0.15, "position": { "x": "88%", "y": "12%" } }, "opacity": 0.8 } ] } ``` Both stop on the last frame, whatever the clips elsewhere add up to. Add another clip later and they still do. The music bed above, drawn out. Track 1 is the background and the two clips follow each other because neither names a `start`. The music names `length: "end"`, so it stops on the last frame whatever the video adds up to. ```mermaid theme={null} gantt title Two clips and a music bed, in seconds dateFormat X axisFormat %S section Track 2, music audio, length "end" : 0, 12 section Track 1, video river.mp4 : 0, 6 forest.mp4 : 6, 12 ``` ## The rules that fall out of it A clip running to the end of the **video** is measured against every clip that is not, so two of them on different tracks both land on the real end rather than chasing each other. It runs to the last frame, so nothing may follow it on the same track, and a timeline where nothing has a length of its own has no end to measure and is rejected. A clip never runs past its container, so a 6 second video in a 3 second scene plays 3 seconds, and a 2 second video in a 5 second scene plays 2. That applies to a length you wrote as well as one the engine worked out. `"end"` is the opposite request and does not cap: it asks for the container's full extent whatever the source holds. On a source shorter than the container that is impossible, and the job is rejected for outrunning its source. Leave the length off instead, or trim less. Inside a `composition`, `start` and `length` both stay required. A nested timeline's length is set by the clip hosting it, so there is nothing to append to and no container to measure against. ## Add a layer A second track renders over the first. This lays an animated title over the video. ```json theme={null} "tracks": [ { "clips": [ { "asset": { "type": "video", "src": "https://cdn.rendobar.com/assets/examples/sea.mp4" }, "start": 0, "length": 3 } ] }, { "clips": [ { "asset": { "type": "text", "text": "Coastal Escape", "style": { "font": "Inter", "size": 84, "color": "#FFFFFF" } }, "start": 0, "length": 3 } ] } ] ```