Create-a-simple-web-AR-application.md

@@ -139,4 +139,52 @@ The entrypoint to WebXR is through **XRSystem.requestSession()**. Use the **imme
 
 An **XRReferenceSpace** describes the coordinate system used for objects within the virtual world. The 'local' mode is best suited for an AR experience, with a reference space that has an origin near the viewer and stable tracking.
 
-To create an **XRSession** and **XRReferenceSpace**, add this code to the bottom of the **activateXR()** function:
\ No newline at end of file
+To create an **XRSession** and **XRReferenceSpace**, add this code to the bottom of the **activateXR()** function:
+```
+// 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');
+```
+## Render the scene
+Now you can render the scene. **XRSession.requestAnimationFrame()** schedules a callback which is executed when the browser is ready to draw a frame.
+
+During the animation frame callback, call **XRFrame.getViewerPose()** to obtain the viewer's pose relative to the local coordinate space. This is used to update the in-scene camera, changing how the user views the virtual world before the renderer draws the scene using the updated camera.
+
+Add this code to the bottom of the **activateXR()** function:
+```
+// 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);
+```
+