# Synthetic test clips

Generated with ffmpeg (see git history / commit for exact commands). Ground truth for
validating marker capture, offset math, and lossless export.

- `sync_test_INSYNC.mp4` — 10s, 640x480 @30fps, black video with a 33ms white flash
  at t=5.000s; near-silent 1kHz tone that jumps to full volume for the same 33ms
  window at t=5.000s. Video and audio events are aligned (offset = 0ms).

- `sync_test_OUTOFSYNC_audio+350ms.mp4` — same content, but the audio track has
  been shifted +350ms later relative to video (via `-itsoffset 0.350` on the audio
  input, stream-copied). The flash event stays at video t=5.000s; the beep event
  now appears at t=5.350s. Exercises the negative-delay_ms / "delay video" branch
  in ffmpeg_export.py.

- `sync_test_OUTOFSYNC_video+350ms.mp4` — same content, but the *video* track has
  been shifted +350ms later instead (via `-itsoffset 0.350` on the video input).
  The beep stays at audio t=5.000s; the flash now appears at t=5.350s. Exercises
  the positive-delay_ms / "delay audio" branch in ffmpeg_export.py.

**Ground truth for the app's marker/offset workflow:**
- Mark Video at the flash → ~5.000s
- Mark Audio at the beep → ~5.350s
- Expected computed `delay_ms` = (video_mark - audio_mark) * 1000 = **-350**
  (audio is already the *late* one here, so the fix is to delay the video by
  350ms to catch up to it -- not to delay the audio further. Negative
  delay_ms → offset the video input, per sync_logic.py's sign convention.)
- Exporting `sync_test_OUTOFSYNC_audio+350ms.mp4` with delay_ms = -350 should
  land both events at t=5.350s in the output (not back at 5.000s -- the
  absolute timeline shifts, only the *relative* sync matters).

Frame at exactly t=5.000s is frame index 150 (30fps) — useful for checking
frame-step precision lands exactly on it.

- `sync_test_MULTITRACK.mp4` — 10s, blue video, two AAC audio tracks: track
  index 1 (default) has a beep at t=3.000s, track index 2 (not default) has
  a beep at t=7.000s. Used to verify clap detection follows whichever audio
  track mpv is actually playing (its `ff-index`), not whichever one
  ffmpeg's own default-stream selection would pick -- see
  scripts/smoke_test_multitrack_clap_detection.py.