Persistent UI in WebXR AR Mode

When entering AR mode with Three.js WebXR, UI elements often disappear because the AR session takes over the rendering context. This is a common issue when developing AR experiences for Android devices.

Solution Approaches

To implement a persistent UI in AR mode, you have several options:

1. Use DOM Overlay

The WebXR DOM Overlay API is specifically designed for this purpose:

JAVASCRIPTCopied
// Set up renderer with xr enabled
const renderer = new THREE.WebGLRenderer();
renderer.xr.enabled = true;

// Configure WebXR session with DOM overlay
function startAR() {
  const sessionInit = {
    optionalFeatures: ['dom-overlay'],
    domOverlay: { root: document.getElementById('ar-ui-container') }
  };
  
  navigator.xr.requestSession('immersive-ar', sessionInit)
    .then((session) => {
      renderer.xr.setSession(session);
      // Session started successfully
    })
    .catch((error) => {
      console.error('Failed to start AR session:', error);
    });
}

Make sure your UI elements are inside the container specified in domOverlay.root.

2. Create Three.js UI Elements

Instead of using DOM elements, create UI elements directly in your Three.js scene:

JAVASCRIPTCopied
// Create a sprite-based UI element
const spriteMap = new THREE.TextureLoader().load('button-texture.png');
const spriteMaterial = new THREE.SpriteMaterial({ map: spriteMap });
const sprite = new THREE.Sprite(spriteMaterial);
sprite.scale.set(1, 0.5, 1);
sprite.position.set(0, 0, -2); // Position in front of the camera
scene.add(sprite);

// Make it interactive
const raycaster = new THREE.Raycaster();
function checkIntersection(controller) {
  raycaster.setFromCamera(new THREE.Vector2(), camera);
  const intersects = raycaster.intersectObject(sprite);
  if (intersects.length > 0) {
    // Handle button press
  }
}

3. Use WebXRManager for Session Management

The WebXRManager can help you manage UI state transitions:

JAVASCRIPTCopied
// Access the WebXRManager
const xrManager = renderer.xr;

// Listen for session start/end to manage UI
xrManager.addEventListener('sessionstart', () => {
  // Adjust UI for AR mode
  document.getElementById('ar-ui-container').classList.add('ar-active');
});

xrManager.addEventListener('sessionend', () => {
  // Restore UI for non-AR mode
  document.getElementById('ar-ui-container').classList.remove('ar-active');
});

Browser Compatibility Note

Not all browsers support DOM overlay. Check compatibility and provide fallbacks where necessary. For Android specifically, Chrome and the WebXR Viewer typically have good support for these features.

Remember that the DOM overlay feature is only available in immersive-ar mode, not in immersive-vr mode.