function onMouseDown( event ) {

		if ( scope.enabled === false ) return;

		if ( event.button === 0 ) {

			state = STATE.ROTATE;

		} else if ( event.button === 1 ) {

			state = STATE.ZOOM;

		} else if ( event.button === 2 ) {

			state = STATE.PAN;

		}

		pointerOld.set( event.clientX, event.clientY );

		domElement.addEventListener( 'mousemove', onMouseMove, false );
		domElement.addEventListener( 'mouseup', onMouseUp, false );
		domElement.addEventListener( 'mouseout', onMouseUp, false );
		domElement.addEventListener( 'dblclick', onMouseUp, false );

	}
@author qiao / https://github.com/qiao
@author mrdoob / http://mrdoob.com
@author alteredq / http://alteredqualia.com/
@author WestLangley / http://github.com/WestLangley