appearing-filter.js

jQuery.expr[':'].appearing = function(elem) {

  var $window = $(window),
      windowViewTop = $window.scrollTop(),
      windowViewBottom = windowViewTop + $window.height(),
      elemTop = $(elem).offset().top,
      elemBottom = elemTop + $(elem).height(),

      isAppearingFully = ((elemTop >= windowViewTop) && (elemBottom <= windowViewBottom)),
      isAppearingBottom = ((windowViewTop > elemTop) && (windowViewTop < elemBottom)),
      isAppearingTop = ((windowViewBottom > elemTop) && (windowViewTop < elemBottom));

  return isAppearingFully || isAppearingBottom || isAppearingTop;

};