Online course trailer: avatar intro plus b-roll in one timeline render

A course creator can build a 30-second trailer by making an avatar intro, generating two or three b-roll clips, and rendering them together with Sume's Timeline 1.0. Timeline takes one audio spine and ordered video slots and returns one MP4. The docs list its public rate as $0.10 per ceil(output minute), with no provider inference, since it only runs ffmpeg.

The generation jobs cost what their own model lines say. The assembly is the cheap part, and it is the part that stops a trailer from being four separate files.

Why assemble instead of generating one long clip

Trend roundups say multimodal workflows that connect text-to-video, image-to-video, voice, scripts and editing are the defining shift, and that 5 to 30 seconds is the practical range for polished clips (AI Video Generation Trends, read 2026-10-07). A trailer is that shape: a spoken hook, then pictures that illustrate it.

Separate jobs also let you redo one piece. If the second b-roll clip is wrong, you regenerate that clip, not the whole trailer.

The parts

Make the intro with the avatar talking-video route: a 9:16 script of about 10 seconds. Make b-roll with POST /v1/videos, where each clip is a visual of the course topic. Every file you place on the timeline must already be a media.sume.com artifact or asset of your workspace, so import anything external with POST /v1/media-imports first.

Use the avatar clip's audio as the spine, or write a separate voiceover. Timeline needs audio.duration_seconds between 1 and 1800 and an audio file unless you declare silence.

curl -X POST https://api.sume.com/v1/timeline-1.0/render \
  -H "Authorization: Bearer $SUME_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: course-trailer-001" \
  -d '{
    "audio": {
      "url": "https://media.sume.com/artifacts/artf_demo/voice.wav",
      "duration_seconds": 30
    },
    "video": [
      { "source_url": "https://media.sume.com/artifacts/artf_demo/intro.mp4", "start": 0, "duration": 10 },
      { "source_url": "https://media.sume.com/artifacts/artf_demo/broll-1.mp4", "start": 10, "duration": 10,
        "transition": { "type": "fade", "duration": 0.25 } },
      { "source_url": "https://media.sume.com/artifacts/artf_demo/broll-2.mp4", "start": 20, "duration": 10,
        "transition": { "type": "fade", "duration": 0.25 } }
    ]
  }'

Plan first, then render

POST /v1/timeline-1.0/plan is an unbilled compile preflight. It checks the schema and the Sume-host URLs and returns the duration, segment count, billable minutes and an estimated cost, without creating a job. Run it on your document before you render.

The default output is 1080 by 1920. Note that a plan cannot predict short-source pad or loop warnings, which show up as soft warnings on the finished job.

What to check before you launch

Watch the whole trailer with sound off, then with sound on. Make sure the b-roll does not promise results the course cannot deliver, and that the avatar is disclosed as AI where your platform or market expects it.