A 1.5-second clip allows a 0.75-second fade: Timeline limits

In a Timeline 1.0 render, a fade next to a 1.5-second clip can be at most 0.75 seconds. The docs set two caps on video[].transition: a duration of 1 second or less, and 50% or less of the shorter neighbouring slot. The smaller number wins, so a short clip limits the fade before the 1-second cap does.

The rules

Transitions apply only to slots after the first. The type must be one of fade, wipeleft, wiperight, slideup, slidedown or dissolve. The duration must also be at least one output frame. A slot must last at least 0.2 seconds.

The shorter neighbour sets the cap

Take two clips next to each other. The transition cannot be longer than half of the shorter one. The table gives the maximum for a few lengths of that shorter clip.

Where the 50% rule bites, and what to do in a fast edit

The 1-second cap only decides for neighbours of 2 seconds or longer. Below that, the 50% rule decides, so a sequence of very short clips needs very short fades. A 0.5-second clip allows only a 0.25-second fade, which at 24 fps is 6 frames. A request for a longer fade is refused with transition_too_long before any money is reserved, so a bad value costs you a retry, not a charge.

A montage of 1.5-second clips can use a 0.75-second fade, but not every pair needs one. The docs refuse more than eight adjacent fades with too_many_chained_transitions and tell you to insert a hard cut. A pattern that works: fade into every third clip and hard-cut between the rest.

The docs also say that declared starts are authoritative. The compiler compensates for the overlap of a cross-fade and never pre-shifts your start values. You should write start as the time on the audio spine where the clip begins, not as the time where its fade begins.

Seven 1.5-second clips in a row

Take seven clips of 1.5 seconds with a fade between each pair. That is six transitions, under the limit of eight, and each fade can be 0.75 seconds at most. The overlap shortens the visible time of every clip, and the compiler adjusts for it. With a 0.5-second fade instead, each clip keeps more of its own picture. Pick the length by how it looks, then confirm with the plan call that nothing was snapped.

Check before you pay

The unbilled POST /v1/timeline-1.0/plan runs the pure compiler, so use it to look at a transition setup before the render. Soft problems such as snapped transitions appear as warnings in the render result. They are not failures. If the output frame rate matters for alignment, set output.fps to 24, 25, 30 or 60 yourself instead of leaving it to follow the sources.