diff --git a/assets/index.html b/assets/index.html
index 5ab8801..5f84c1c 100644
--- a/assets/index.html
+++ b/assets/index.html
@@ -67,6 +67,12 @@
 
         const camera = { fov: 800 };
 
+        function rotateY(p, angle) {
+            const cos = Math.cos(angle);
+            const sin = Math.sin(angle);
+            return { x: p.x * cos - p.z * sin, y: p.y, z: p.x * sin + p.z * cos };
+        }
+
         class Point3D {
             constructor(x, y, z) {
                 this.x = x;