var scrollPos, sectionHeight, imgWrapStop,
            scrollSection = $('.imac-scrolling'),
            imac          = scrollSection.find('.retina-imac'),
            imacWrap      = $('.imac-container'),
            imacOffset    = imac.offset().top,
            $window       = $(window); 


    setVars = function(){
      scrollPos     = $window.scrollTop();
      sectionHeight = scrollSection.outerHeight();
      imgWrapStop   = ((imacOffset + sectionHeight) - (imac.outerHeight()));
    }

    toggleFixed = function(img, imgWrap, imgTop){
      var isFixed = scrollSection.hasClass('fixed');

      if (scrollPos >= imgTop && !isFixed && scrollPos <= imgWrapStop) {  // When we're in the zone
         imgWrap.addClass('fixed');
         img.css({
           top:0,
           bottom: 'auto'
         });
      } else if (scrollPos >= imgWrapStop) { // when we're out of the zone
         img.css({
           bottom:0,
           top: 'auto'
         });
        imgWrap.removeClass('fixed');
      } else {
        imgWrap.removeClass('fixed');
      }
    }  

    translateScroll = function(selector, value, startPos, endPos) {
      if (scrollPos > startPos && scrollPos < endPos) { // Only run if we're in the zone
          var disToScroll = (startPos - endPos),
              disScrolled = (scrollPos - startPos),  // Distance already scrolled
              transY = ((value / disToScroll) * disScrolled ),
              transYround = +transY.toFixed(2),
              transCss = 'translate3d( 0, ' + transYround +'px, ' + '0)';


          function prefixTransform(selector, translate) {// prefixer function
             selector.css('transform', translate);
             selector.css('-moz-transform', translate);
             selector.css('-webkit-transform', translate);
             selector.css('-o-transform', translate);
             selector.css('-ms-transform', translate);
          }
          prefixTransform(selector, transCss);
         
      }
    }


    scrollFix = function(){
      window.requestAnimationFrame(function(){
        setVars();
        toggleFixed(imac, imacWrap, imacOffset);
        translateScroll(imac, 50, imacOffset, imgWrapStop);
      });
    }
    init = function(){ scrollIntervalID = setInterval(scrollFix, 10);}
    init();