dispatcher — trim auto prefix hors-eau avant reconstruct

Les sessions record commencent systematiquement avec l AUV sur le pont
ou en surface. Les frames hors-eau polluent le model et bloquent l alignment
ICP du stitch.

trim_above_water_prefix detecte underwater par absorption du canal rouge
(mean_R < mean_G-5 ET mean_R < mean_B-5) et exige un streak consecutif
de 10 frames underwater pour lock le start. Tout ce qui precede est
supprime avant demo.py.

Opencv charge les frames en REDUCED_COLOR_4 pour acceleration.
Execute dans le venv lingbot-map cote worker (cv2 dispo).

scripts/dispatcher.py

@@ -142,6 +142,68 @@ def count_frames(worker: dict, frames_dir: str) -> int:
         return 0
 
 
+# Record sessions always start with the AUV on deck or at the surface — these frames pollute
+# reconstruction. Detect the first sustained underwater run (red channel absorbed by water, so
+# mean_R < mean_G and mean_R < mean_B) and delete the hors-eau prefix before demo.py runs.
+_AUTO_TRIM_SCRIPT = r"""
+import cv2, glob, os, sys
+frames_dir = sys.argv[1]
+need_streak = 10            # consecutive underwater frames required to lock start
+paths = sorted(glob.glob(os.path.join(frames_dir, 'frame_*.jpg')))
+if not paths:
+    print('TRIM_RESULT 0 0'); sys.exit(0)
+start = 0
+streak = 0
+for i, p in enumerate(paths):
+    img = cv2.imread(p, cv2.IMREAD_REDUCED_COLOR_4)
+    if img is None:
+        continue
+    mean_b, mean_g, mean_r = [float(c) for c in cv2.mean(img)[:3]]
+    # Underwater = red is absorbed → R noticeably lower than both G and B
+    underwater = mean_r < mean_g - 5 and mean_r < mean_b - 5
+    if underwater:
+        streak += 1
+        if streak >= need_streak:
+            start = i - need_streak + 1
+            break
+    else:
+        streak = 0
+if start <= 0:
+    print(f'TRIM_RESULT 0 {len(paths)}'); sys.exit(0)
+for p in paths[:start]:
+    try: os.remove(p)
+    except OSError: pass
+print(f'TRIM_RESULT {start} {len(paths) - start}')
+"""
+
+
+def trim_above_water_prefix(worker: dict, frames_dir: str) -> tuple[int, int]:
+    """Delete leading out-of-water frames. Returns (removed, remaining)."""
+    script_remote = f"/tmp/cosma-trim-{os.getpid()}.py"
+    # Write script on worker and run it inside the lingbot-map venv (has cv2)
+    rc, _, err = ssh(
+        worker["ssh_alias"],
+        f"cat > {shlex.quote(script_remote)} << 'PYEOF'\n{_AUTO_TRIM_SCRIPT}\nPYEOF",
+        timeout=15,
+    )
+    if rc != 0:
+        print(f"  ↳ trim script upload failed: {err[:150]}")
+        return (0, 0)
+    rc, out, err = ssh(
+        worker["ssh_alias"],
+        f"source {shlex.quote(worker['lingbot_path'])}/.venv/bin/activate && "
+        f"python3 {shlex.quote(script_remote)} {shlex.quote(frames_dir)}; rm -f {shlex.quote(script_remote)}",
+        timeout=600,
+    )
+    for line in out.splitlines():
+        if line.startswith("TRIM_RESULT"):
+            parts = line.split()
+            removed, remaining = int(parts[1]), int(parts[2])
+            return (removed, remaining)
+    print(f"  ↳ trim unexpected output: {out[:200]} / {err[:200]}")
+    return (0, 0)
+
+
 def scp_to_worker(local_path: str, worker: dict, remote_path: str):
     """Copy a file to the worker.
 
@@ -235,6 +297,11 @@ def do_extract(job: sqlite3.Row, worker: dict) -> str:
         # are 1-11 GB each. Frames are already extracted so worker_src is no longer needed.
         ssh(worker["ssh_alias"], f"rm -f {shlex.quote(worker_src)}")
         set_status(job["id"], frame_count=idx, progress=min(99, idx * 100 // total_frames_est))
+    # Drop the hors-eau prefix before reconstruction — always present at session start.
+    removed, remaining = trim_above_water_prefix(worker, frames_dir)
+    if removed:
+        print(f"  ↳ job #{job['id']}: trimmed {removed} out-of-water frames, {remaining} kept")
+        set_status(job["id"], frame_count=remaining)
     # Trim once per job so LVM thin pool on the host actually reclaims the freed blocks.
     ssh(worker["ssh_alias"], "sudo fstrim / 2>/dev/null || fstrim / 2>/dev/null", timeout=60)
     return frames_dir