Relative Cursor position in an Element (using MooTools)

cursor.js

$( 'foo' ).addEvent( 
	'mousemove',
	function( event ) {
		var pos = this.getPosition( );
		var relPos = {};
		relPos.x = event.page.x - pos.x;
		relPos.y = event.page.y - pos.y;
		console.log( relPos );
	}
);