Axis Correction

public/js/exhibit.js

@@ -69,6 +69,13 @@ function centredCorners(m, w, h) {
 }
 
 // Build the marker->camera matrix (POS-IT), converting js-aruco axes to three.js.
+// POS-IT frame is right-handed (X right, Y up, Z toward viewer). To bring the
+// full RIGID transform into three.js we must CONJUGATE by F = diag(1,-1,-1):
+//   M_three = F * M_pose * F
+// Premultiplying by F alone flips the translation but leaves the rotation's
+// pitch inverted -> "camera up/down moves the ghost the wrong way". The second
+// (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));
@@ -80,7 +87,8 @@ function markerToCameraMatrix(m) {
     R[2][0], R[2][1], R[2][2], t[2],
     0, 0, 0, 1
   );
-  M.premultiply(flipYZ);
+  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]) };
 }