Keeping a Plane Fixed Relative to Camera with OrbitControls

When using OrbitControls in Three.js, you can keep a plane fixed at the bottom of the camera view by updating the plane's position and rotation in your animation loop to counteract the camera's movement.

Here's how you can achieve this:

Solution Approach

1. Use a separate scene or group for your main object and the plane
2. Update the plane's position and rotation in your render loop based on the camera's position

Implementation Example

JAVASCRIPTCopied
// Setup your scene, camera, and controls as usual
const scene = new THREE.Scene();
const camera = new THREE.PerspectiveCamera(75, window.innerWidth / window.innerHeight, 0.1, 1000);
const renderer = new THREE.WebGLRenderer();
const controls = new THREE.OrbitControls(camera, renderer.domElement);

// Create your main object
const mainObject = new THREE.Mesh(
  new THREE.BoxGeometry(1, 1, 1),
  new THREE.MeshStandardMaterial({ color: 0x00ff00 })
);
scene.add(mainObject);

// Create your plane that will show the shadow
const planeGeometry = new THREE.PlaneGeometry(10, 10);
const planeMaterial = new THREE.MeshStandardMaterial({ 
  color: 0xffffff,
  receiveShadow: true 
});
const plane = new THREE.Mesh(planeGeometry, planeMaterial);
scene.add(plane);

// Setup lighting for shadow
const light = new THREE.DirectionalLight(0xffffff, 1);
light.position.set(5, 5, 5);
light.castShadow = true;
scene.add(light);

// Make sure your object casts shadows
mainObject.castShadow = true;

// Initial camera position
camera.position.z = 5;
camera.position.y = 2;
camera.lookAt(mainObject.position);

// Animation loop
function animate() {
  requestAnimationFrame(animate);
  
  // Update controls
  controls.update();
  
  // Calculate the position for the plane to stay at the bottom of the view
  // This is the key part that keeps the plane fixed relative to camera
  
  // Get the camera's direction vector
  const cameraDirection = new THREE.Vector3();
  camera.getWorldDirection(cameraDirection);
  
  // Calculate a position that's "down" from the camera's perspective
  // You may need to adjust these values based on your scene scale
  const distance = camera.position.length() * 0.5; // Adjust as needed
  
  // Create a vector pointing "down" in camera space
  const downVector = new THREE.Vector3(0, -1, 0);
  
  // Position the plane
  plane.position.copy(camera.position).add(
    downVector.multiplyScalar(distance)
  );
  
  // Make the plane face the camera (optional)
  plane.lookAt(camera.position);
  
  // Keep the plane horizontal by resetting rotation on x and z axes
  plane.rotation.x = -Math.PI / 2; // Keep it flat
  plane.rotation.z = 0;
  
  renderer.render(scene, camera);
}

animate();

Alternative Approach

If you want the plane to be completely fixed in screen space, you could also consider:

1. Using a separate scene and camera for the plane
2. Rendering the plane in a second pass after your main scene
3. Using a shader material to ensure the plane always appears at the bottom of the screen

This approach would be more complex but would guarantee the plane stays fixed regardless of camera movement.

Remember that you'll need to adjust the shadow casting setup to ensure the shadows still project correctly onto your repositioned plane.