画像が見えたら不透明度を100%にする

/**
 * 要素が見えたら opacity 0 --> 1
 *
<img src="" class="lazy">
<script src="js/lazy.js"></script>
 *
 * @author ao-system
 */
(function(window,document){
	var lazies = [];
	init();
	function init() {
		lazies = document.getElementsByClassName('lazy');
		for (var i = 0; i < lazies.length; i++) {
			(function(i){
				var elmY = offsettop(lazies[i]);
				var scrollY = scrolltop();
				if (elmY <= (scrollY + document.documentElement.clientHeight)) {
					//既に見えているので何もしない
				} else {
					lazies[i].style.opacity = 0;
				}
			})(i);
		}
		document.addEventListener('scroll',function(){onScroll();},false);
	}
	function onScroll() {
		for (var i = 0; i < lazies.length; i++) {
			(function(i){
				if (lazies[i].style.opacity == 0) {
					var elmY = offsettop(lazies[i]);
					var scrollY = scrolltop();
					if (elmY <= (scrollY + document.documentElement.clientHeight)) {
						lazies[i].style.opacity = 1;
					}
				}
			})(i);
		}
	}
	//要素の位置を返す
	function offsettop(elm) {
		var offsettop = elm.offsetTop;
		if (elm.offsetParent) {
			while (elm = elm.offsetParent) {
				offsettop += elm.offsetTop;
			}
		}
		return offsettop;
	}
	//スクロール位置を返す
	function scrolltop() {
		if (document.body && document.body.scrollTop) {
			return document.body.scrollTop;
		}
		if (document.documentElement && document.documentElement.scrollTop) {
			return document.documentElement.scrollTop;
		}
		if (window.pageYOffset) {
			return window.pageYOffset;
		}
		return 0;
	}
})(window,document);