change div based on how far down the page you have scrolled

$(document).ready(function() {
    function isScrolledIntoView(elem) {
        var docViewTop = $(window).scrollTop();
        var docViewBottom = docViewTop + $(window).height();
 
        var elemTop = $(elem).offset().top;
        var elemBottom = elemTop + $(elem).height();
 
        return ((elemBottom >= docViewTop) && (elemTop <= docViewBottom));
    }
 
    var myelement = $('#formcontainer'); // the element to act on if viewable
    $(window).scroll(function() {
        if(isScrolledIntoView(myelement)) {
            // do something when element is scrolled to and viewable
        } else {
            // do something when element is not viewable
        }
    });
});