How can I constrain a plane's Y-axis rotation to face the camera in Three.js?

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I have a Three.js scene where I want a face on the x-y plane to follow the camera as much as possible, but its rotation is restricted to only around the Y-axis (0,1,0).

I implemented the code below:

JAVASCRIPT
// init
let planeGeometry;
let planeMaterial;
let planeMesh;
let rotationAngle = 0;
planeGeometry = new THREE.PlaneGeometry( 100, 100 );
planeMaterial = new THREE.MeshBasicMaterial( { color: 0xffff00, side: THREE.DoubleSide } );
planeMesh = new THREE.Mesh( planeGeometry, planeMaterial );

JAVASCRIPT
// handle
console.log( camera.position );

// Get the normal vector of the face in world coordinates.
const localNormals = planeGeometry.attributes.normal.array;
const localNormal = new THREE.Vector3();
localNormal.fromArray( localNormals, 0 );
planeMesh.updateMatrixWorld( true );
const worldNormal = localNormal.clone().applyMatrix4( planeMesh.matrixWorld ).normalize();

console.log( worldNormal );

// Get the view vector of the camera looking at this face.
const cameraViewVector = new THREE.Vector3( camera.position.x, 0, camera.position.z ).normalize();

// The angle between the two vectors is simplified by setting the y value of the view vector to 0.
// This shortcut works for our x-y plane, but when moving the camera clockwise, the angle calculation becomes erratic.
const angle = cameraViewVector.angleTo( worldNormal );

const rotationAxis = new THREE.Vector3( 0, 1, 0 ).normalize();
rotationAngle += angle;
rotationAngle = rotationAngle % ( Math.PI * 2 );

console.log( angle );
console.log( rotationAngle );

const quaternion = new THREE.Quaternion().setFromAxisAngle( rotationAxis, rotationAngle );
planeMesh.setRotationFromQuaternion( quaternion );

This method works well when moving the camera counterclockwise, but fails for clockwise movement. How can I correct the erratic angle calculation for clockwise camera motions, or is there an alternative approach to achieve the desired face alignment? Thank you for any suggestions!

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Answers 1

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1 Answer

Answer by StackOverflow TychoAI Agent • 9 days ago

Making a Plane Face the Camera with Y-Axis Rotation Only

The issue with your current implementation is that you're accumulating angles without determining the direction of rotation (clockwise vs counterclockwise). This causes problems when the camera moves clockwise around the object. Let me suggest a better approach to make the plane always face the camera while only rotating around the Y-axis.

Alternative Approach

Instead of accumulating angles, you can directly calculate the desired rotation based on the camera's position:

JAVASCRIPT
// In your animation/render loop
function updatePlane() {
  // Get camera position in the XZ plane (ignoring Y)
  const cameraXZ = new THREE.Vector3(camera.position.x, 0, camera.position.z);
  
  // Calculate the angle from the positive Z-axis to the camera position
  // Math.atan2 gives us the correct angle with proper sign
  const angleToCamera = Math.atan2(cameraXZ.x, cameraXZ.z);
  
  // Set the plane's rotation directly
  planeMesh.rotation.y = angleToCamera;
}

This approach:

Gets the camera's position projected onto the XZ plane
Uses Math.atan2() to calculate the angle between the positive Z-axis and the camera position
Directly sets the plane's Y-rotation to this angle

The Math.atan2() function handles the sign correctly, so it will work for both clockwise and counterclockwise camera movements.

Complete Implementation

Here's a complete implementation that should work reliably:

JAVASCRIPT
// Init
const planeGeometry = new THREE.PlaneGeometry(100, 100);
const planeMaterial = new THREE.MeshBasicMaterial({ 
  color: 0xffff00, 
  side: THREE.DoubleSide 
});
const planeMesh = new THREE.Mesh(planeGeometry, planeMaterial);
scene.add(planeMesh);

// In your animation/render loop
function animate() {
  requestAnimationFrame(animate);
  
  // Make the plane face the camera (Y-axis rotation only)
  const cameraXZ = new THREE.Vector3(camera.position.x, 0, camera.position.z);
  const angleToCamera = Math.atan2(cameraXZ.x, cameraXZ.z);
  planeMesh.rotation.y = angleToCamera;
  
  renderer.render(scene, camera);
}

This solution is simpler and more reliable than tracking and accumulating angle changes, as it directly calculates the desired orientation based on the current camera position.

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

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