Timeline Behavior

Timing model

• Clip timings are [position, end) intervals in seconds
• Transitions overlap clips, compressing the total output duration
• Background music is mixed after visual transitions — crossfades do not affect music volume

Transition compensation

FFmpeg’s xfade transitions overlap two clips, compressing the timeline. A 1s transition between two 10s clips produces 19s of output, not 20s. With multiple transitions, this compounds.

Clips:   [  A (10s)  ][  B (10s)  ]
          ↑ 0         ↑ 10
With 1s fade:
Output:  [  A  ][fade][  B  ]   = 19s total

By default, simple-ffmpegjs automatically adjusts text, subtitle, and standalone audio clip timings to compensate. When you position text at ”15s” or a sound effect at ”15s”, it appears at the correct visual moment regardless of how many transitions preceded it:

await project.load([
  { type: "video", url: "./a.mp4", position: 0, end: 10 },
  {
    type: "video",
    url: "./b.mp4",
    position: 10,
    end: 20,
    transition: { type: "fade", duration: 1 },
  },
  // This appears at the visual 15s mark, even though the output is only 19s long
  { type: "text", text: "Appears at 15s visual", position: 15, end: 18 },
]);