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LED light sync

An alternative to hand claps: an LED that flashes once at the start of every recording. Go2Kin finds the flash in each camera's video and aligns the videos on it. Hands-free, works at any distance the cameras can see the LED, and unaffected by room noise.

🚧 TODO: photo of the LED unit and its placement in the lab.

What you need

  • A white LED visible to all cameras, off by default, switched on for 1 second by an external trigger within the first 6 seconds of each recording. It must be clearly brighter than its background (no PWM dimming).
  • Place it at a similar height in every camera view where nobody will walk in front of it during the first seconds (e.g. on the floor near the centre of the volume).

Workflow

  1. In the bottom bar, set Sync to Light. The choice is remembered between sessions.

  2. In the Calibration tab, section LED Sync ROI, click Record LED clip & set ROI. All connected cameras record a short clip (it stops by itself after ~7 s — trigger the LED as you would for a trial). A window then shows one frame per camera:

    • drag the time slider until the LED is on,
    • click the LED in each view — a red square (the region Go2Kin will watch) and a magnified crop appear; adjust ROI size if the LED is large or small,
    • OK when every camera has a square.

    The section shows ROI set: 4 cameras. If a calibration is loaded, the ROIs are saved into it straight away; otherwise they are saved when you Apply Calibration.

  3. Run the extrinsic calibration as usual — its recording is synced with the LED too, so the Calibrate button stays disabled until the ROI is set for every connected camera.

  4. Record trials in the Recording tab as usual. No clapping needed.

Set ROI from folder… lets you define the ROI from any existing recording folder (for example a trial's video/ folder) instead of recording a new clip.

Redo the ROI whenever a camera or the LED is moved. Loading a calibration restores the ROIs saved with it; Load Intrinsics Only clears them, like the extrinsics.

Sync outputs

Same as audio sync — trimmed MP4s in synced/ and the 2×2 stitched_videos.mp4 (the LED should light up in the same frame in all four cells) — plus sync_led_signal.png, the ROI brightness over time for each camera with the detected on/off instants. The trial's trial.json records "sync_method": "light".

If it fails

The red SYNC ISSUE popup lists the reason; the trial is discarded (or the extrinsic calibration aborted) exactly as for a failed audio sync:

  • LED not detected in ROI — the LED did not flash, is outside the square (camera or LED moved → redo the ROI), or is too dim / too small: move it closer, increase its brightness or the ROI size.
  • LED already on at start / never switches off within the search window — the trigger fired too early or too late; the flash must start after the recording begins and finish within the first 6 s.
  • Brightness pulse of N frames does not match the LED — something else changed brightness inside the square (someone walked through, a screen changed). Keep the square on the LED only.
  • LED on/off edges give offsets differing by N frames — the two edges disagree; usually a partially hidden LED in one camera. Check the view and re-record.
  • No LED ROI for GPn (before recording) — set the ROI in the Calibration tab, or switch Sync back to Manual.