# Replay — Auto-sync video↔tlog via IMU take-off detection (AC-7 / AC-8)

**Task**: AZ-405_replay_auto_sync
**Name**: Auto-sync of video ↔ tlog via IMU take-off detection (AC-7 / AC-8; `--time-offset-ms` remains the manual override)
**Description**: Implement auto-detection of the video↔tlog timestamp offset for the replay CLI, mitigating R-DEMO-1 (recordings are often started independently — camera and FC may be minutes apart). Algorithm: (1) parse the tlog for the IMU take-off pattern — sustained vertical accel > 0.5 g for ≥ 0.5 s + change in attitude rate > 1 rad/s in the same window (typical quadcopter take-off signature); compute `tlog_takeoff_ns`. (2) Analyse the video for motion-onset — pyramidal optical flow magnitude crossing a configurable threshold sustained for ≥ 0.5 s; compute `video_motion_onset_ns`. (3) Offset = `tlog_takeoff_ns - video_motion_onset_ns` (positive offset = video starts before take-off recorded in tlog). Confidence-scoring: confidence is high (≥ 80 %) when both signals are well-defined; low when ambiguous (e.g., fixed-wing hand-launch — no clear vertical-accel-above-0.5g pulse). If confidence < 80 %, log WARN + use the best-guess offset and proceed. `--time-offset-ms` always overrides auto-detect (manual override per AC-7). AC-8 hard-fail (exit code 2): if the resulting offset produces ≤ 95 % of frames matching at least one IMU window within ± 100 ms, the CLI exits with code 2 and prints both the auto-detected offset (if any) and the per-frame match percentage so the operator can debug.
**Complexity**: 5 points
**Dependencies**: AZ-402 (CLI hosts the auto-sync logic at startup); AZ-399 (tlog parser); AZ-398 (VideoFileFrameSource for video-side analysis); AZ-263, AZ-269, AZ-266, AZ-272 (FDR for confidence + decision logging)
**Component**: replay-auto-sync (epic AZ-265 / E-DEMO-REPLAY) — auto-sync helper at `src/gps_denied_onboard/cli/replay_auto_sync.py`
**Tracker**: AZ-405
**Epic**: AZ-265 (E-DEMO-REPLAY)

### Document Dependencies

- `_docs/02_document/contracts/replay/replay_protocol.md` — `time_offset_ms` semantics (Invariant 8).
- `_docs/02_document/architecture.md` — R-DEMO-1 mitigation.
- Epic AZ-265 description in `_docs/02_document/epics.md` — AC-7 / AC-8.

## Problem

Without this task, the replay CLI relies on the operator passing `--time-offset-ms N` manually, which is error-prone (operators often don't have a stopwatch on the moment of take-off; the camera and FC are routinely started at different times). R-DEMO-1 is a recurring real-world concern. AC-7 / AC-8 codify the auto-sync expectation.

## Outcome

- `src/gps_denied_onboard/cli/replay_auto_sync.py`:
  - `detect_tlog_takeoff(tlog_path, target_fc_dialect) -> AutoSyncResult` — returns `(tlog_takeoff_ns, confidence)`.
  - `detect_video_motion_onset(video_path, frame_rate_hz) -> AutoSyncResult` — returns `(video_motion_onset_ns, confidence)`.
  - `compute_offset(tlog_result, video_result) -> AutoSyncOffset` — combines the two; emits final confidence + offset.
  - `validate_offset_or_fail(offset, tlog_path, video_path, ...) -> int` — runs the AC-8 frame-window match-percentage check; returns 0 if ≥ 95 %, 2 otherwise (caller maps to CLI exit code).
- CLI wiring (in `cli/replay.py`): when `--time-offset-ms` is NOT provided, the CLI invokes `detect_*` + `compute_offset` + `validate_offset_or_fail`; if validation returns 2, the CLI exits 2 with the diagnostic message per AC-8.
- INFO log on auto-detect success: `kind="replay.auto_sync.detected"` with `{tlog_takeoff_ns, video_motion_onset_ns, offset_ms, tlog_confidence, video_confidence, combined_confidence}`.
- WARN log on low confidence: `kind="replay.auto_sync.low_confidence"` with the same fields + `proceeding_with_best_guess: true`.
- ERROR log on AC-8 fail: `kind="replay.auto_sync.ac8_validation_failed"` with `{frame_window_match_pct, threshold_pct: 95.0}`.
- FDR records mirror all three log kinds.
- Unit tests: tlog-takeoff detector against synthetic IMU traces (positive case + ambiguous case + hand-launch case); video-motion detector against synthetic video frames; combined offset within tolerance for synchronised inputs; AC-8 validation hard-fails on degenerate offsets.

## Scope

### Included
- Tlog-takeoff detector (sustained vertical accel + attitude rate).
- Video-motion-onset detector (pyramidal optical flow).
- Combined offset computation + confidence.
- AC-8 frame-window match-percentage validator.
- CLI wiring at startup.
- Manual override (`--time-offset-ms`) bypass path.
- Structured logging + FDR.
- Unit tests covering positive / ambiguous / hand-launch / hard-fail cases.

### Excluded
- E2E test against the Derkachi fixture — owned by E2E task (this task ships unit tests; E2E task adds an integration assertion AC-7 / AC-8).
- The CLI argparse + entrypoint — owned by CLI task.
- Modifications to `TlogReplayFcAdapter` — this task consumes the adapter's tlog stream and the FrameSource's video frames; no API changes.

## Acceptance Criteria

**AC-1: Tlog take-off detector positive** — synthetic AP IMU trace with a clear take-off (sustained 1.2 g vertical for 1 s + 1.5 rad/s attitude rate) → `tlog_takeoff_ns` matches the synthetic onset within ± 50 ms; `confidence ≥ 0.85`.

**AC-2: Tlog take-off detector ambiguous** — synthetic IMU with low-amplitude vibration (0.3 g) but no take-off → `confidence < 0.50`.

**AC-3: Tlog take-off detector hand-launch** — synthetic IMU with abrupt 0.8 g impulse but no sustained climb → `confidence < 0.80` (in the WARN-and-proceed regime per AC-7).

**AC-4: Video motion-onset positive** — synthetic 60-frame video with first 10 frames stationary and frames 11+ moving → `video_motion_onset_ns` matches the onset of frame 11 within ± 1 frame.

**AC-5: Combined offset within ± 200 ms (epic AC-7)** — for a fixture with KNOWN ground-truth offset (e.g., constructed test case offset = 5000 ms), `compute_offset` returns within ± 200 ms of ground truth.

**AC-6: Low combined confidence WARN-and-proceed** — when `combined_confidence < 0.80`, `compute_offset` returns the best-guess offset + WARN log; the CLI proceeds (does NOT exit) — verified via the unit test of the CLI wiring.

**AC-7: AC-8 hard-fail exit 2** — wire a `validate_offset_or_fail` against a deliberately-bad offset (e.g., 60 s offset on a 60 s clip — every frame would be off the tlog window); function returns 2; CLI exit code 2; ERROR log + FDR fired.

**AC-8: Manual override bypasses auto-detect** — `--time-offset-ms 5000` passed → auto-detect functions are NOT invoked (verified via call-count assertion); the manual offset flows directly into `TlogReplayFcAdapter`.

**AC-9: Frame-window match-percentage validator** — for a known-good offset, validator computes ≥ 95 % match (returns 0); for a known-bad offset, computes ≤ 95 % (returns 2). Threshold is configurable via `config.replay.auto_sync_match_threshold_pct` (default 95.0).

**AC-10: Confidence-score determinism** — re-run the auto-sync against the same input twice; assert confidence values match within 1e-9 (algorithmic determinism).

## Non-Functional Requirements

- Auto-sync startup overhead p99 ≤ 3 s (within the epic's cold-start ≤ 5 s budget combined with composition).
- Tlog-takeoff detection: full tlog scan ≤ 1 s for tlogs up to 100 MB (typical 1–2 min clip is ~10 MB).
- Video-motion-onset detection: scan the first 10 s of the video; ≤ 1 s on Tier-1 hardware.

## Constraints

- OpenCV (already in deps for video) is the optical flow library.
- pymavlink (already bundled per D-C8-3) is the tlog reader.
- The take-off pattern thresholds (0.5 g, 1 rad/s, 0.5 s sustained) are in `config.replay.auto_sync.takeoff_*` with documented defaults.
- The video-motion threshold is similarly configurable.
- AC-8's 95 % match threshold is configurable per `config.replay.auto_sync_match_threshold_pct`.

## Risks & Mitigation

- **R-DEMO-1 (drift / unsynchronised recordings)** — *Mitigation*: this task IS the mitigation; AC-1..AC-5 cover the positive cases; AC-6 covers the WARN-and-proceed regime; AC-8 covers the hard-fail regime.
- **Risk: optical-flow false-positives on jitter-only video** — *Mitigation*: configurable threshold; sustained-for-0.5 s requirement matches the take-off semantics; AC-2 covers the ambiguous case.
- **Risk: fixed-wing hand-launch hits the WARN regime even on legitimate footage** — *Mitigation*: documented; operator can pass `--time-offset-ms` manually; AC-3 documents the expected confidence drop.
- **Risk: AC-8 95 % threshold too strict for short clips with sparse IMU** — *Mitigation*: threshold is configurable; default 95 % is calibrated for typical tlog rates (50–200 Hz IMU).

## Runtime Completeness

- **Named capability**: video↔tlog auto-sync via IMU take-off detection.
- **Production code**: real OpenCV optical flow, real pymavlink tlog scan, real confidence-scored combined offset, real AC-8 validator.
- **Allowed external stubs**: test fakes only.
- **Unacceptable substitutes**: a hardcoded `time_offset_ms = 0` default (defeats R-DEMO-1 mitigation).

## Contract

Implements epic AZ-265 ACs 7 + 8; mitigates R-DEMO-1.