Stabilize multi-marker fusion (deterministic order + coarse-bucket weights) + expose inter-marker spread

public/js/ar/fuse.js

@@ -70,11 +70,26 @@ export class WorldFuser {
       const pos = new THREE.Vector3();
       const quat = new THREE.Quaternion();
       camWorld.decompose(pos, quat, _s);
-      cams.push({ pos, quat, w: Math.max(1, e.area) });
+      cams.push({ markerId: e.markerId, pos, quat, w: Math.max(1, e.area) });
     }
     if (!cams.length) return null;
 
-    // weighted position mean + incremental weighted slerp for orientation
+    /* Multi-marker fusion. Two visible markers each give an independent camera-in-world
+     * estimate; if they disagree (pose noise, or real-table spacing not matching the
+     * editor), naive area-weighting oscillates frame to frame because the areas jitter
+     * — this is the "steady on one marker, jittery on two" symptom. Fixes:
+     *  1. Deterministic order (sort by markerId) so the slerp base can't flip.
+     *  2. Robust weights: quantise area into coarse buckets so tiny per-frame area
+     *     wobble doesn't shift the blend; a marker only dominates when genuinely much
+     *     closer. Equal-ish markers then average to a STABLE midpoint, not a moving one. */
+    cams.sort((a, b) => a.markerId - b.markerId);
+    // Quantise confidence into coarse buckets from marker size (sqrt(area) ~ linear
+    // size). Near-equal markers land in the same bucket => equal weight => stable
+    // midpoint. A marker only outweighs another when it's a full bucket closer, so
+    // per-frame area jitter no longer shifts the blend.
+    for (const c of cams) c.w = Math.max(1, Math.round(Math.sqrt(c.w) / 8));
+
+    // weighted position mean + incremental weighted slerp (now order-stable)
     let wSum = cams[0].w;
     const pos = cams[0].pos.clone().multiplyScalar(cams[0].w);
     const quat = cams[0].quat.clone();
@@ -89,6 +104,14 @@ export class WorldFuser {
     }
     pos.multiplyScalar(1 / wSum);
 
+    // spread = how far apart the individual marker estimates are (0 for one marker).
+    // Large spread (>~2cm) means the markers disagree — usually real-table spacing
+    // not matching the editor, which no fusion can fully hide; surfaced in dev HUD.
+    let spread = 0;
+    for (let i = 0; i < cams.length; i++)
+      for (let j = i + 1; j < cams.length; j++)
+        spread = Math.max(spread, cams[i].pos.distanceTo(cams[j].pos));
+
     // ---- 1€ filter (position + orientation) ----
     const now = performance.now();
     const gap = now - this.lastFuseT;
@@ -100,7 +123,7 @@ export class WorldFuser {
     if (!this.smoothPos || gap > 1500) {
       this.smoothPos = pos.clone(); oe.prevPos = pos.clone(); oe.dPos.set(0, 0, 0);
       this.smoothQuat = quat.clone(); oe.prevQuat = quat.clone(); oe.dAngRate = 0;
-      return { position: this.smoothPos.clone(), quaternion: this.smoothQuat.clone(), markerCount: cams.length };
+      return { position: this.smoothPos.clone(), quaternion: this.smoothQuat.clone(), markerCount: cams.length, spread };
     }
 
     // POSITION: derivative -> speed -> dynamic cutoff -> low-pass
@@ -123,6 +146,6 @@ export class WorldFuser {
     this.smoothQuat.slerp(quat, aA);
     oe.prevQuat.copy(quat);
 
-    return { position: this.smoothPos.clone(), quaternion: this.smoothQuat.clone(), markerCount: cams.length };
+    return { position: this.smoothPos.clone(), quaternion: this.smoothQuat.clone(), markerCount: cams.length, spread };
   }
 }