/* 2015-09-28 上传图片*/

    function convertImgToBase64(url, callback, outputFormat){

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

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

        var img = new Image;

        img.crossOrigin = 'Anonymous';

        img.onload = function(){

          var width = img.width;

          var height = img.height;

          // 按比例压缩4倍

          var rate = (width<height ? width/height : height/width)/4;

          canvas.width = width*rate;

          canvas.height = height*rate;

          ctx.drawImage(img,0,0,width,height,0,0,width*rate,height*rate);

          var dataURL = canvas.toDataURL(outputFormat || 'image/png');

          callback.call(this, dataURL);

          canvas = null;

        };

        img.src = url;

      }

       function getObjectURL(file) {

            var url = null ;

            if (window.createObjectURL!=undefined) {  // basic

              url = window.createObjectURL(file) ;

            } else if (window.URL!=undefined) {       // mozilla(firefox)

              url = window.URL.createObjectURL(file) ;

            } else if (window.webkitURL!=undefined) { // web_kit or chrome

              url = window.webkitURL.createObjectURL(file) ;

            }

            return url ;

      }

     // 前端只需要给input file绑定这个change事件即可（上面两个方法不用管）huodong-msg为input class

      $('.huodong-msg').bind('change',function(event){

            var imageUrl = getObjectURL($(this)[0].files[0]);

            convertImgToBase64(imageUrl, function(base64Img){

              // base64Img为转好的base64,放在img src直接前台展示(<img src="data:image/jpg;base64,/9j/4QMZRXh...." />)

              alert(base64Img);

              // base64转图片不需要base64的前缀data:image/jpg;base64

              alert(base64Img.split(",")[1]);

            });

            event.preventDefault();

        });

    /* 2015-09-28 */