three.js/src/animation/KeyframeTrack.js

THREE.KeyframeTrack = function ( name, keys ) {

	this.keys = keys || [];	// time in seconds, value as value

	// TODO: sort keys via their times
	this.keys.sort( function( a, b ) { return a.time < b.time; } );

	THREE.Track.call( this, name );

};

THREE.KeyframeTrack.prototype = {

	constructor: THREE.Track,

	getAt: function( time ) {

		if( this.keys.length == 0 ) throw new Error( "no keys in track named " + this.name );
		
		// before the start of the track, return the first key value
		if( this.keys[0].time >= time ) {

			return this.keys[0].value;

		}

		// past the end of the track, return the last key value
		if( this.keys[ this.keys.length - 1 ].time <= time ) {

			return this.keys[ this.keys.length - 1 ].value;

		}

		// interpolate to the required time
		for( var i = 1; i < this.keys.length; i ++ ) {

			if( time <= this.keys[ i ].time ) {

				// linear interpolation to start with
				var alpha = ( time - this.keys[ i - 1 ].time ) / ( this.keys[ i ].time - this.keys[ i - 1 ].time );

				return THREE.AnimationUtils.lerp( this.keys[ i - 1 ].value, this.keys[ i ].value, alpha );

			}
		}

		throw new Error( "should never get here." );

	};

};