function getElementComputedStyle(elem, prop){
	if (typeof elem!="object") elem = document.getElementById(elem);

	// external stylesheet for Mozilla, Opera 7+ and Safari 1.3+
	if (document.defaultView && document.defaultView.getComputedStyle){
		if (prop.match(/[A-Z]/)) prop = prop.replace(/([A-Z])/g, "-$1").toLowerCase();
			return document.defaultView.getComputedStyle(elem, "").getPropertyValue(prop);
	}

	// external stylesheet for Explorer and Opera 9
	if (elem.currentStyle){
		var i;
		while ((i=prop.indexOf("-"))!=-1) prop = prop.substr(0, i) + prop.substr(i+1,1).toUpperCase() + prop.substr(i+2);
		return elem.currentStyle[prop];
	}
	return "";
}

function ch(id, prop, pos){
	var elem = document.getElementById(id);
	var bg = getElementComputedStyle(id, prop);

	elem.style.backgroundPosition = pos;
}