fix: reject the mirrored coplanar-ambiguity pose — camera-below-sheet and teleport gates, reproj gate 8px -> 4px, rejection reason reported

public/js/ar/cv-worker.js

@@ -8,8 +8,8 @@
  *
  * Protocol (main <-> worker):
  *   -> { type:'init' }                          <- { type:'ready' } | { type:'failed', msg }
- *   -> { type:'solve', id, obj, img, n, W,H,f }  <- { type:'pose', id, ok, C, m3, reproj, n }
- *   -> { type:'reset' }                          (drop temporal warm start)
+ *   -> { type:'solve', id, obj, img, n, W,H,f }  <- { type:'pose', id, ok, C, m3, reproj, n, why }
+ *   -> { type:'reset' }                          (drop temporal warm start + gate state)
  * obj/img are plain Float64Array payloads (transferred, so no copy).
  * The worker never touches three.js: it returns the camera position and the
  * 3x3 world-from-camera rotation, and the client builds the quaternion.
@@ -17,8 +17,28 @@
 
 let cv = null;
 let prev = null;              // { r:[3], t:[3], at } temporal warm start
+let lastGood = null;          // { C:[3], at } last ACCEPTED camera position
+let rejectStreak = 0;
 const PREV_TTL = 1500;
-const MAX_REPROJ_PX = 8;
+
+/* Sanity gates. Every crest on a printed sheet is coplanar, and a coplanar point
+ * set has TWO poses that reproject almost equally well — the true one and a
+ * mirrored twin. With only one or two crests in frame, solvePnP intermittently
+ * returns the twin, which on device looks like ghosts snapping to the wrong side
+ * and vanishing. Reprojection error alone can't separate them (the twin fits the
+ * pixels), so we reject on physics instead:
+ *   - the camera is always ABOVE the sheet (world +Y); the twin usually isn't
+ *   - a phone can't teleport 20cm between frames 50ms apart
+ * The streak escape hatch stops a bad lastGood from locking tracking out.
+ *
+ * These gates SUPPRESS the symptom. The cure is non-coplanar geometry: one crest
+ * standing vertical removes the ambiguity outright, because no mirrored pose can
+ * fit points that don't share a plane. */
+const MAX_REPROJ_PX = 4;      // was 8 — that accepted visibly wrong poses
+const MIN_CAM_Y_CM = 1;       // camera below the sheet is never real
+const MAX_JUMP_CM = 20;       // per accepted-pose gap, scaled by elapsed time
+const JUMP_GRACE_MS = 500;
+const MAX_REJECT_STREAK = 12;
 
 // reusable Mats — allocating 7 Mats per frame was a measurable cost
 let bufN = 0;
@@ -57,7 +77,7 @@ function solve(msg) {
     let ok = false;
     try { ok = cv.solvePnP(objM, imgM, K, dist, rvec, tvec, false, cv.SOLVEPNP_SQPNP); } catch { ok = false; }
     if (!ok) { try { ok = cv.solvePnP(objM, imgM, K, dist, rvec, tvec, false, cv.SOLVEPNP_IPPE); } catch { ok = false; } }
-    if (!ok) return { ok: false };
+    if (!ok) return reject('nosolve', 0);
     cv.solvePnP(objM, imgM, K, dist, rvec, tvec, true, cv.SOLVEPNP_ITERATIVE);
   }
 
@@ -68,9 +88,7 @@ function solve(msg) {
     err += Math.hypot(proj.data64F[2 * i] - img[2 * i], proj.data64F[2 * i + 1] - img[2 * i + 1]);
   }
   err /= n;
-  if (err > MAX_REPROJ_PX) { prev = null; return { ok: false, reproj: err }; }
-
-  prev = { r: [...rvec.data64F], t: [...tvec.data64F], at: Date.now() };
+  if (err > MAX_REPROJ_PX) { prev = null; return reject('reproj', err); }
 
   cv.Rodrigues(rvec, R);
   const d = R.data64F;          // row-major world->cvCam
@@ -81,9 +99,32 @@ function solve(msg) {
     -(d[1] * t[0] + d[4] * t[1] + d[7] * t[2]),
     -(d[2] * t[0] + d[5] * t[1] + d[8] * t[2]),
   ];
+
+  // ---- gates ----
+  const now = Date.now();
+  const forced = rejectStreak >= MAX_REJECT_STREAK;   // escape hatch
+  if (!forced) {
+    if (C[1] < MIN_CAM_Y_CM) { prev = null; return reject('below', err); }
+    if (lastGood && (now - lastGood.at) < JUMP_GRACE_MS) {
+      const dt = Math.max(1, now - lastGood.at) / 1000;
+      const jump = Math.hypot(C[0] - lastGood.C[0], C[1] - lastGood.C[1], C[2] - lastGood.C[2]);
+      // allowance grows with elapsed time so real movement is never blocked
+      if (jump > MAX_JUMP_CM * Math.max(1, dt * 4)) { prev = null; return reject('jump', err); }
+    }
+  }
+
+  rejectStreak = 0;
+  prev = { r: [...rvec.data64F], t: [...tvec.data64F], at: now };
+  lastGood = { C, at: now };
+
   // world-from-threeCam = R^T * diag(1,-1,-1), row-major for Matrix4.set
   const m3 = [d[0], -d[3], -d[6], d[1], -d[4], -d[7], d[2], -d[5], -d[8]];
-  return { ok: true, C, m3, reproj: err };
+  return { ok: true, C, m3, reproj: err, forced };
+}
+
+function reject(why, reproj) {
+  rejectStreak++;
+  return { ok: false, why, reproj };
 }
 
 self.onmessage = (ev) => {
@@ -119,13 +160,13 @@ self.onmessage = (ev) => {
     return;
   }
 
-  if (msg.type === 'reset') { prev = null; return; }
+  if (msg.type === 'reset') { prev = null; lastGood = null; rejectStreak = 0; return; }
 
   if (msg.type === 'solve') {
     if (!cv) return self.postMessage({ type: 'pose', id: msg.id, ok: false });
     let out;
     try { out = solve(msg); }
-    catch (e) { prev = null; out = { ok: false }; }
+    catch (e) { prev = null; out = { ok: false, why: 'throw' }; }
     self.postMessage({ type: 'pose', id: msg.id, n: msg.n, ...out });
   }
 };