//压缩图片方法

function compressImg(file,callback){

    var src;

    var fileSize = parseFloat(parseInt(file['size'])/1024/1024).toFixed(2);

    var read = new FileReader();

    read.readAsDataURL(file);

    read.onload = function (e) {

        var img = new Image();

        img.src = e.target.result;

        img.onload = function(){

            //默认按比例压缩

            var w = this.width,

                h = this.height;

            //生成canvas

            var canvas = document.createElement('canvas');

            var ctx = canvas.getContext('2d');

            var base64;

            // 创建属性节点

            canvas.setAttribute("width", w);

            canvas.setAttribute("height", h);

            ctx.drawImage(this, 0, 0, w, h);

            if(fileSize<1){

                //如果图片小于一兆 那么不执行压缩操作

                base64 = canvas.toDataURL(file['type'], 1);

            }else if(fileSize>1&&fileSize<2){

                //如果图片大于1M并且小于2M 那么压缩0.5

                base64 = canvas.toDataURL(file['type'], 0.5);

            }else{

                //如果图片超过2m 那么压缩0.2

                base64 = canvas.toDataURL(file['type'], 0.2);

            }

            // 回调函数返回file的值

            callback(base64);

        };

    };

};