pixel-editor/js/_getCursorPosition.js

//get cursor position relative to canvas
function getCursorPosition(e) {
	var x;
	var y;
	if (e.pageX != undefined && e.pageY != undefined) {
		x = e.pageX;
		y = e.pageY;
	}
	else {
		x = e.clientX + document.body.scrollLeft + document.documentElement.scrollLeft;
		y = e.clientY + document.body.scrollTop + document.documentElement.scrollTop;			
	}

	x -= canvas.offsetLeft;
	y -= canvas.offsetTop;

	return [x,y];
}