图片等比例压缩 ---已经测试 好用----图片加载完成以后再进行缩放

//图片等比例压缩

    var scaleImage = function (w, h, o) {
        var img = new Image();
        //img.height=$("#imgBig").height();
        //img.width=$("#imgBig").width();
        img.src = o.src;
        if (img.width > 0 && img.height > 0) {
            if (img.width / img.height >= w / h) {
                if (img.width > w) {
                    o.width = w;
                    o.height = (img.height * w) / img.width;
                } else {
                    o.width = img.width;
                    o.height = img.height;
                }
                o.alt = img.width + "x" + img.height;
            } else {
                if (img.height > h) {
                    o.height = h;
                    o.width = (img.width * h) / img.height;
                } else {
                    o.width = img.width;
                    o.height = img.height;
                }
                o.alt = img.width + "x" + img.height;
            }
        }
    }

 //图片加载完成以后再进行缩放

            document.getElementById("imgBig").onload = function () {

            //调用图片压缩
                scaleImage(734, 450, document.getElementById("imgBig"));
                          }