);
        window.requestAnimationFrame(onAnimationFrame);
      }

      function onVRRequestPresent () {
        vrDisplay.requestPresent([{ source: webglCanvas }]).then(function () {
        }, function () {
          VRSamplesUtil.addError("requestPresent failed.", 2000);
        });
      }

      function onVRExitPresent () {
        if (!vrDisplay.isPresenting)
          return;

        vrDisplay.exitPresent().then(function () {
        }, function () {
          VRSamplesUtil.addError("exitPresent 