$.fn.isOnScreen = function(){

    var win = $(window);

    var viewport = {
        top : win.scrollTop(),
        left : win.scrollLeft()
    };
    viewport.right = viewport.left + win.width();
    viewport.bottom = viewport.top + win.height();

    var bounds = this.offset();
    bounds.right = bounds.left + this.outerWidth();
    bounds.bottom = bounds.top + this.outerHeight();

    return (!(viewport.right < bounds.left || viewport.left > bounds.right || viewport.bottom < bounds.top || viewport.top > bounds.bottom));

};
$.first_time = true;
$(document).ready(function(){
    $(window).scroll(function(){
        if ($('#counter').isOnScreen()) {
            if($.first_time == true){
                $('#bilder').animateNumber({number: 1458},5000);
                $('#durchschn').animateNumber({number: 800},5000);
                $('#trau').animateNumber({number: 48},5000);
                $('#nackt').animateNumber({number: 18},5000);
                $.first_time = false;
            }
        }
    });
});