Debug Flipping Fixes

public/js/exhibit.js

@@ -77,10 +77,10 @@ function centredCorners(m, w, h) {
 // (right) multiply by F fixes the rotation handedness while keeping translation
 // correct (F*M*F's translation column is F*t, i.e. flipped exactly once).
 const flipYZ = new THREE.Matrix4().makeScale(1, -1, -1);
-function markerToCameraMatrix(m) {
-  const pose = posit.pose(centredCorners(m, grab.width, grab.height));
-  if (!pose) return null;
-  const R = pose.bestRotation, t = pose.bestTranslation;
+
+// Convert one POS-IT (R,t) solution into a three.js marker->camera matrix via
+// the F*M*F conjugation (see note above).
+function poseToMatrix(R, t) {
   const M = new THREE.Matrix4().set(
     R[0][0], R[0][1], R[0][2], t[0],
     R[1][0], R[1][1], R[1][2], t[1],
@@ -88,8 +88,24 @@ function markerToCameraMatrix(m) {
     0, 0, 0, 1
   );
   M.premultiply(flipYZ);  // F * M
-  M.multiply(flipYZ);     // F * M * F  (conjugation: fixes pitch handedness)
-  return { M, err: pose.bestError, dist: Math.hypot(t[0], t[1], t[2]) };
+  M.multiply(flipYZ);     // F * M * F
+  return M;
+}
+
+// POS-IT returns TWO solutions for a planar marker (the real pose and a mirror
+// twin). Up close they have near-equal error and the solver flip-flops between
+// them -> "rotate 90deg, ghost spins 180deg". We return both; alignWorld picks
+// the one whose resulting world orientation is closest to the previous frame.
+function markerToCameraMatrix(m) {
+  const pose = posit.pose(centredCorners(m, grab.width, grab.height));
+  if (!pose) return null;
+  const best = { M: poseToMatrix(pose.bestRotation, pose.bestTranslation), err: pose.bestError };
+  let alt = null;
+  if (pose.alternativeRotation && pose.alternativeTranslation) {
+    alt = { M: poseToMatrix(pose.alternativeRotation, pose.alternativeTranslation), err: pose.alternativeError };
+  }
+  const t = pose.bestTranslation;
+  return { best, alt, dist: Math.hypot(t[0], t[1], t[2]) };
 }
 
 // anchor world placement -> matrix mapping the MARKER's local frame (as POS-IT
@@ -121,16 +137,49 @@ function anchorToWorldMatrix(anchor) {
 
 // Given a detected marker with a known anchor, compute world->camera and apply
 // it to the `world` group so everything in world coords renders correctly.
+//
+// Planar-ambiguity defeat: POS-IT gives two candidate poses. We compute the
+// resulting world->camera for each, and keep whichever is closest to LAST
+// frame's transform (temporal consistency). This stops the mirror-twin flip
+// that made the ghost spin the wrong way. On first lock we take `best`.
 const tmpAnchorInv = new THREE.Matrix4();
+let lastWorldToCamera = null; // previous frame's chosen transform (for continuity)
+
+function worldToCameraFor(mMatrix, anchorInv) {
+  return new THREE.Matrix4().multiplyMatrices(mMatrix, anchorInv);
+}
+
+// crude distance between two 4x4s: sum of squared element differences of the
+// rotation part (enough to tell the real pose from its mirror twin).
+function matDist(a, b) {
+  const ea = a.elements, eb = b.elements;
+  let s = 0;
+  for (const i of [0, 1, 2, 4, 5, 6, 8, 9, 10]) { const d = ea[i] - eb[i]; s += d * d; }
+  return s;
+}
+
 function alignWorld(m2c, anchor) {
-  // camera = markerToCamera * anchorToWorld^-1   (as a world->camera transform)
-  // world point -> anchor-local -> camera:
-  //   worldToCamera = markerToCamera * (anchorToWorld)^-1
   const aToW = anchorToWorldMatrix(anchor);
   tmpAnchorInv.copy(aToW).invert();
-  const worldToCamera = new THREE.Matrix4().multiplyMatrices(m2c.M, tmpAnchorInv);
-  world.matrix.copy(worldToCamera);
+
+  const candBest = worldToCameraFor(m2c.best.M, tmpAnchorInv);
+  let chosen = candBest;
+
+  if (m2c.alt) {
+    const candAlt = worldToCameraFor(m2c.alt.M, tmpAnchorInv);
+    if (lastWorldToCamera) {
+      // pick the candidate closest to where the world was last frame
+      chosen = matDist(candAlt, lastWorldToCamera) < matDist(candBest, lastWorldToCamera)
+        ? candAlt : candBest;
+    } else {
+      // first lock: trust the lower-error (best) solution
+      chosen = candBest;
+    }
+  }
+
+  world.matrix.copy(chosen);
   world.matrixWorldNeedsUpdate = true;
+  lastWorldToCamera = chosen.clone();
 }
 
 // ---- Build occluders + anchor viz once scene arrives ----------------------