jQuery: is visible on screen.

functions.js

(function ($) {

	"use strict";

	function isSceenVisible( element ) {

		var $el = $( element );

		// Stop here if the element isn't visible.
		if ( !$el.is( ':visible' ) || '0' == $el.css( 'opacity' ) )
			return false;

		var viewportTop = $(window).scrollTop(),
			viewportHeight = $(window).height(),
			viewportBottom = viewportTop + viewportHeight,
			top = $el.offset().top,
			height = $el.height(),
			bottom = top + height;

		switch ( true ) {
			case top >= viewportTop && top < viewportBottom:
				return true;
				break;
			case bottom > viewportTop && bottom <= viewportBottom:
				return true;
				break;
			case height > viewportHeight && top <= viewportTop && bottom >= viewportBottom:
				return true;
				break;
		}

		return false;
	}

})(jQuery);