addNonemptyTrack = function(
				trackType, trackName, animationKeys, propertyName, destTracks ) {

			// only return track if there are actually keys.
			if ( animationKeys.length !== 0 ) {

				var times = [];
				var values = [];

				THREE.AnimationUtils.flattenJSON(
						animationKeys, times, values, propertyName );

				// empty keys are filtered out, so check again
				if ( times.length !== 0 ) {

					destTracks.push( new trackType( trackName, times, values ) );

				}

			}

		}
Reusable set of Tracks that represent an animation.

@author Ben Houston / http://clara.io/
@author David Sarno / http://lighthaus.us/