docs: function onMouseDown( event ) {
  
  		if ( scope.enabled === false ) return;
  
  		event.preventDefault();
  
  		if ( event.button === 0 ) {
  
  			state = STATE.ROTATE;
  
  		} else if ( event.button === 1 ) {
  
  			state = STATE.ZOOM;
  
  		} else if ( event.button === 2 ) {
  
  			state = STATE.PAN;
  
  		}
  
  		domElement.addEventListener( 'mousemove', onMouseMove, false );
  		domElement.addEventListener( 'mouseup', onMouseUp, false );
  		domElement.addEventListener( 'mouseout', onMouseUp, false );
  
  	}