Video API transitions: six types, one second max, eight in a row

The Sume timeline supports six transition types: fade, wipeleft, wiperight, slideup, slidedown and dissolve. A transition goes on any slot after the first, lasts at most 1 second, may not exceed half of the shorter neighboring clip, and must span at least one output frame. More than eight adjacent fades is refused with too_many_chained_transitions, and the fix is a hard cut.

The rules in one table

A slot with a transition

This is the slot from the documented render example. Starts are on the audio spine, and the docs say the compiler compensates for the crossfade, so you do not pre-shift later starts yourself.

{
  "source_url": "https://media.sume.com/artifacts/artf_demo/talk.mp4",
  "start": 8,
  "duration": 16,
  "transition": { "type": "fade", "duration": 0.25 }
}

Practical guidance

Transitions need frame alignment to the output rate, so changing output.fps between 24, 25, 30 and 60 can turn a valid duration into one that snaps. Soft warnings such as snapped transitions do not fail the job, but read them before you rely on the render.

• Use 0.25 to 0.5 seconds for fades between talking clips so the cut still reads as a cut.
• Short neighbors cap the transition: next to a 0.6-second clip, the longest transition is 0.3 seconds.
• In a fast montage, alternate transitions with hard cuts to stay under the eight-fade limit.
• Plan first with the free plan endpoint, which reports transition errors before you spend.

Choosing a type

The docs list the types but not a style guide, so the uses above are suggestions, not product claims. Whatever type you choose, keep one type per video for consistency, and test a render at 0.25 seconds first, because a transition that is too long hides the end of a shot. If a render warns that a transition was snapped, the requested duration did not land on a frame boundary at your output frame rate, so the compiler adjusted it.