diff --git a/frontend/js/labs/stereo.js b/frontend/js/labs/stereo.js
index 0c7f9ba..c401ef9 100644
--- a/frontend/js/labs/stereo.js
+++ b/frontend/js/labs/stereo.js
@@ -3361,6 +3361,19 @@ class StereoSim {
   _drawAngleArc(center, dir1, dir2, angleDeg, radius, color) {
     const n1 = dir1.clone().normalize();
     const n2 = dir2.clone().normalize();
+
+    // Right angle → draw the conventional square corner marker, not an arc.
+    if (Math.abs(angleDeg - 90) < 0.5) {
+      const r = radius * 0.7;
+      const p1 = center.clone().addScaledVector(n1, r);
+      const p2 = center.clone().addScaledVector(n2, r);
+      const p3 = p1.clone().addScaledVector(n2, r);            // outer corner
+      const geo = new THREE.BufferGeometry().setFromPoints([p1, p3, p2]);
+      const mat = new THREE.LineBasicMaterial({ color: new THREE.Color(color), transparent: true, opacity: 0.85 });
+      this._angleGroup.add(new THREE.Line(geo, mat));
+      return;
+    }
+
     const angleRad = angleDeg * Math.PI / 180;
     const steps = Math.max(8, Math.round(angleDeg / 5));
     const pts = [];