move background position relative to cursor

This has a slight paralaxing effect. Very subtle.

var start_x;
var start_y;
$('.feedback').mouseenter(function(e){
	start_x=e.clientX;
	start_y=e.clientY;
});
	
$('.feedback').mousemove(function(e){
	var window_width=$('.feedback').outerWidth();
	var element_height=$('.feedback').outerHeight();
        $(this).css('background-position-x', '+='+(-(e.clientX-start_x)15)).css('background-position-y', '+='+(-(e.clientY-start_y)7));
		
	start_x=e.clientX;
	start_y=e.clientY;
});