jQuery.fn.center = function () {
    this.css("position","absolute");
    if (!($(window).height() < this.height()) && !($(window).width() < this.width())) {
    	// più alto e più largo
	    this.css("top", ( $(window).height() - this.height() ) / 2+$(window).scrollTop() + "px");
	    this.css("left", ( $(window).width() - this.width() ) / 2+$(window).scrollLeft() + "px");
    } else if (!($(window).height() > this.height()) && !($(window).width() < this.width())) {
    	// meno alto ma più largo
    	this.css("top", "0px");
    	this.css("left", ( $(window).width() - this.width() ) / 2+$(window).scrollLeft() + "px");
    } else if (!($(window).height() < this.height()) && !($(window).width() > this.width())) {
    	// più alto ma meno largo
    	this.css("top", ( $(window).height() - this.height() ) / 2+$(window).scrollTop() + "px");
    	this.css("left", "0px");
	} else {
		// meno alto e meno largo
    	this.css("top", "0px");
    	this.css("left", "0px");
    }
    return this;
}

$(document).ready(function() {
	$(window).resize(function(){
		$('#centerHV_middle').center();
	});
});