实现效果：下拉滚动条时，图片出现在可见区域时，才开始加载

思路：

（1）img标签，把真实的图片地址，放在自己设置的属性里面，如 lazy-src

（2）获取img离页面的高度（在JQ里是offset().top），原生是：

img.getBoundingClientRect().top + document.body.scrollTop||document.documentElement.scrollTop

（3）判断img出现的位置是否在可见区域里:

在浏览器的可见区域，justTop>scrollTop&&offsetTop<(scrollTop+windowHeight)，这里的justTop是图片的offsetTop+图片高度

//保存document在变量里，减少对document的查询
 var doc = document;
 for(var n=0,i = this.oImg.length;n<i;n++){
 //获取图片占位符图片地址
 var hSrc = this.oImg[n].getAttribute(this.sHolder_src);
 if(hSrc){
 var scrollTop = doc.body.scrollTop||doc.documentElement.scrollTop,
 windowHeight = doc.documentElement.clientHeight,
 offsetTop = this.oImg[n].getBoundingClientRect().top + scrollTop,
 imgHeight = this.oImg[n].clientHeight,
 justTop = offsetTop + imgHeight;
 // 判断图片是否在可见区域
 if(justTop>scrollTop&&offsetTop<(scrollTop+windowHeight)){

 this.isLoad(hSrc,n);
 }
 }
 }

以下为详细代码：

function LGY_imgScrollLoad(option){
 this.oImg = document.getElementById(option.wrapID).getElementsByTagName('img');
 this.sHolder_src = option.holder_src;
 this.int();
 }
 LGY_imgScrollLoad.prototype = {
 loadImg:function(){
 //保存document在变量里，减少对document的查询
 var doc = document;
 for(var n=0,i = this.oImg.length;n<i;n++){
 //获取图片占位符图片地址
 var hSrc = this.oImg[n].getAttribute(this.sHolder_src);
 if(hSrc){
 var scrollTop = doc.body.scrollTop||doc.documentElement.scrollTop,
 windowHeight = doc.documentElement.clientHeight,
 offsetTop = this.oImg[n].getBoundingClientRect().top + scrollTop,
 imgHeight = this.oImg[n].clientHeight,
 justTop = offsetTop + imgHeight;
 // 判断图片是否在可见区域
 if(justTop>scrollTop&&offsetTop<(scrollTop+windowHeight)){
 //alert(offsetTop);
 this.isLoad(hSrc,n);
 }
 }

 }
 },
 isLoad:function(src,n){
 var src = src,
 n = n,
 o_img = new Image(),
 _that = this;
 o_img.onload = (function(n){
 _that.oImg[n].setAttribute('src',src);
 _that.oImg[n].removeAttribute(_that.sHolder_src);
 })(n);
 o_img.src = src;
 },
 int:function(){
 this.loadImg();
 var _that = this,
 timer = null;
 // 滚动：添加定时器，防止频繁调用loadImg函数
 window.onscroll = function(){
 clearTimeout(timer);
 timer = setTimeout(function(){
 _that.loadImg();
 },100);
 }
 }
 }

效果图：