Deteksi koordimat mouse diatas image

html>
head>
script language="JavaScript">
function point_it(event){
pos_x = event.offsetX?(event.offsetX):event.pageX-document.getElementById("pointer_div").offsetLeft;
pos_y = event.offsetY?(event.offsetY):event.pageY-document.getElementById("pointer_div").offsetTop;
document.getElementById("cross").style.left = (pos_x-1) ;
document.getElementById("cross").style.top = (pos_y-15) ;
document.getElementById("cross").style.visibility = "visible" ;
document.pointform.form_x.value = pos_x;
document.pointform.form_y.value = pos_y;
}
/script>
/head>
body>
form name="pointform" method="post">
div id="pointer_div" onclick="point_it(event)" style = "background-image:url('sun.jpg');width:500px;height:333px;">
img src="point.gif" id="cross" style="position:relative;visibility:hidden;z-index:2;">