code: public boolean track()
      {
          byte[] imageData = getImageData();
          if (imageData == null)
          {
              return false;
          }
          if (trackFromImage(imageData))
          {
              getGVRContext().getEventManager().sendEvent(this, TrackerEvents.class, "onTrackUpdate", this);
              return updateSkeleton();
          }
          return false;
      }
docs: Compute new body pose from most recent camera image.
  <p>
  If the camera image has not changed since the last call,
  a new body pose is not computed. The previously computed
  body pose is used to update the skeleton.
  @return true if successful (body data is valid) or false on failure