js决断控件的位置

function getElementPos(elementId) {
var ua = navigator.userAgent.toLowerCase();
var isOpera = (ua.indexOf('opera') != -1);
var isIE = (ua.indexOf('msie') != -1 && !isOpera); // not opera spoof
var el = document.getElementById(elementId);
if(el.parentNode === null || el.style.display == 'none')
{ return false; }
var parent = null;
var pos = [];
var box;
if(el.getBoundingClientRect) //IE
{ box = el.getBoundingClientRect();
var scrollTop = Math.max(document.documentElement.scrollTop, document.body.scrollTop);
var scrollLeft = Math.max(document.documentElement.scrollLeft, document.body.scrollLeft);
return {x:box.left + scrollLeft, y:box.top + scrollTop}; }
else if(document.getBoxObjectFor) // gecko
{ box = document.getBoxObjectFor(el);
var borderLeft = (el.style.borderLeftWidth)?parseInt(el.style.borderLeftWidth):0;
var borderTop = (el.style.borderTopWidth)?parseInt(el.style.borderTopWidth):0;
pos = [box.x - borderLeft, box.y - borderTop]; } else // safari & opera
{ pos = [el.offsetLeft, el.offsetTop];
parent = el.offsetParent;
if (parent != el)
{ while (parent)
{ pos[0] += parent.offsetLeft;
pos[1] += parent.offsetTop;
parent = parent.offsetParent;
}
}
if (ua.indexOf('opera') != -1 || ( ua.indexOf('safari') != -1 && el.style.position == 'absolute' ))
{ pos[0] -= document.body.offsetLeft;
pos[1] -= document.body.offsetTop;
}
}
if (el.parentNode)
{ parent = el.parentNode; }
else { parent = null; }
while (parent && parent.tagName != 'BODY' && parent.tagName != 'HTML') { // account for any scrolled ancestors
pos[0] -= parent.scrollLeft;
pos[1] -= parent.scrollTop;
if (parent.parentNode)
{ parent = parent.parentNode; }
else { parent = null; } } return {x:pos[0], y:pos[1]}; }

function show()
{
var pos=getElementPos("imga");
var x=pos.x-2;
var y=pos.y-2;
alert(x);
alert(y);
//var mydiv=document.getElementById("autoDiv");
// mydiv.style.position="absolute";
//mydiv.style.left=x;
//mydiv.style.top=y+18;
//mydiv.style.visibility="visible";
}