index.html

<!doctype html>
<html>
<head>
  <meta charset="UTF-8">
  <meta name="viewport"
        content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
  <title>Tetrahedron Demo</title>

  <!-- three.js -->
  <script src="https://unpkg.com/three@0.126.0/build/three.js"></script>
</head>
<body>

<!-- Starting an immersive WebXR session requires user interaction.
    We start this one with a simple button. -->
<button onclick="activateXR()">Start Tetrahedron</button>
<script>
async function activateXR() {
  // Add a canvas element and initialize a WebGL context that is compatible with WebXR.
  const canvas = document.createElement("canvas");
  document.body.appendChild(canvas);
  const gl = canvas.getContext("webgl", {xrCompatible: true});

  const scene = new THREE.Scene();

  // Add lighting to the scene
  const light = new THREE.DirectionalLight(0xffffff, 1);
  light.position.set(1, 1, 1).normalize();
  scene.add(light);

  const ambientLight = new THREE.AmbientLight(0x404040); // soft white light
  scene.add(ambientLight);

  // Color of the tetrahedron geometry.
  const material = new THREE.MeshPhysicalMaterial({ color: 0xC70039, roughness: 0.5, metalness: 0.5, clearcoat: 1.0, clearcoatRoughness: 0.1 });

  // Create the tetrahedron and add it to the demo scene.
  const tetrahedron = new THREE.Mesh(new THREE.TetrahedronGeometry(0.5), material);
  
  // Position the tetrahedron
  tetrahedron.position.set(0, 0, -1);

  // Add the mesh to the scene
  scene.add(tetrahedron);

  // Set up the WebGLRenderer, which handles rendering to the session's base layer.
  const renderer = new THREE.WebGLRenderer({
    alpha: true,
    preserveDrawingBuffer: true,
    canvas: canvas,
    context: gl
  });
  renderer.autoClear = false;

  // The API directly updates the camera matrices.
  // Disable matrix auto updates so three.js doesn't attempt
  // to handle the matrices independently.
  const camera = new THREE.PerspectiveCamera();
  camera.matrixAutoUpdate = false;

  // Initialize a WebXR session using "immersive-ar".
  const session = await navigator.xr.requestSession("immersive-ar");
  session.updateRenderState({
    baseLayer: new XRWebGLLayer(session, gl)
  });

  // A 'local' reference space has a native origin that is located
  // near the viewer's position at the time the session was created.
  const referenceSpace = await session.requestReferenceSpace('local');

  // Create a render loop that allows us to draw on the AR view.
  const onXRFrame = (time, frame) => {
    // Queue up the next draw request.
    session.requestAnimationFrame(onXRFrame);

    // Bind the graphics framebuffer to the baseLayer's framebuffer
    gl.bindFramebuffer(gl.FRAMEBUFFER, session.renderState.baseLayer.framebuffer);

    // Retrieve the pose of the device.
    // XRFrame.getViewerPose can return null while the session attempts to establish tracking.
    const pose = frame.getViewerPose(referenceSpace);
    if (pose) {
      // In mobile AR, we only have one view.
      const view = pose.views[0];

      const viewport = session.renderState.baseLayer.getViewport(view);
      renderer.setSize(viewport.width, viewport.height);

      // Use the view's transform matrix and projection matrix to configure the THREE.camera.
      camera.matrix.fromArray(view.transform.matrix);
      camera.projectionMatrix.fromArray(view.projectionMatrix);
      camera.updateMatrixWorld(true);

      // Render the scene with THREE.WebGLRenderer.
      renderer.render(scene, camera);
    }
  };
  session.requestAnimationFrame(onXRFrame);
}
</script>
</body>
</html>