title: Example of three.js KeyframeTrack with smoothing (cubic interpolation)
body: <p>I looking for some clarity on how to implement smoothing (a.k.a. cubic interpolation) on a three.js animation KeyframeTrack. From my testing, parsing the interpolation method in the KeyframeTrack constructor is insufficient.</p>
  
  <pre><code>var position = new THREE.VectorKeyframeTrack(
      '.position',
      [0, 1],
      [
          plane.position.x,
          plane.position.y,
          plane.position.z,
          1.25 * plane.position.x,
          1.25 * plane.position.y,
          1.25 * plane.position.z
      ],
      THREE.InterpolateSmooth
  );
  position.createInterpolant();
  
  var clip = new THREE.AnimationClip('Action', 3, [
      position
  ]);
  
  that.mixer = new THREE.AnimationMixer(plane);
  
  var action = that.mixer.clipAction(clip);
  action.setLoop(THREE.LoopOnce);
  action.startAt(0);                // delay in seconds
  action.clampWhenFinished = true;
  action.play();
  </code></pre>