Video starts on a black frame: fix the first Timeline segment

A video opens on black when something delays or fades the first clip. In Timeline 1.0 two rules guard against it: video[0].start must be 0, and the first segment cannot carry a transition. Break either and the request is refused with timeline_must_start_at_zero or transition_on_first_segment. The one remaining way to start dark is the output-level fade_in_seconds, which you set deliberately.

Rules are from Sume's Timeline 1.0 page, read 2026-10-01. Descript's changelog separately notes its smart transitions no longer fade in on the first scene.

What are the two first-segment errors?

Both are stable refusal codes, so they fail at request time instead of rendering a dark first frame.

How do I open on picture with no fade?

Put the clip you want on frame one at video[0] with start: 0 and no transition. Later slots must have increasing starts, and declared starts are authoritative, so you do not shift the first clip to make room for a crossfade. If the first source has leading black of its own, use source_in to skip into the file.

{
  "audio": { "mode": "silence", "duration_seconds": 12 },
  "video": [
    { "source_url": "https://media.sume.com/clip-a.mp4", "start": 0, "duration": 6 },
    {
      "source_url": "https://media.sume.com/clip-b.mp4",
      "start": 6,
      "duration": 6,
      "transition": { "type": "dissolve", "duration": 0.5 }
    }
  ],
  "output": { "width": 1080, "height": 1920 }
}

Does output.fade_in_seconds add black?

Yes, by design. output.fade_in_seconds and fade_out_seconds accept 0 to 5 seconds, and their sum cannot exceed the output length. Leave fade_in_seconds at 0 or omit it if you want the first frame visible. The doc also notes each transition must be at most 1 s, so a dissolve between clips two and three never touches the opening frame.

What if the black frame is inside the source?

Then Timeline is not adding it. Trim past it with source_in on the first slot, or regenerate the clip. For the wider assembly pattern see assembling long-form video with Timeline.