jQuery function to determine if an element is positioned within the viewport.

isOnScreen.js

function isOnScreen(elem) {
	if( elem.length == 0 ) {
		return;
	}
	var $window = jQuery(window)
	var viewport_top = $window.scrollTop()
	var viewport_height = $window.height()
	var viewport_bottom = viewport_top + viewport_height
	var $elem = jQuery(elem)
	var top = $elem.offset().top
	var height = $elem.height()
	var bottom = top + height
	return (top >= viewport_top && top < viewport_bottom) ||
	(bottom > viewport_top && bottom <= viewport_bottom) ||
	(height > viewport_height && top <= viewport_top && bottom >= viewport_bottom)
}