function preview_picture(pic){

  var r=new FileReader();

  r.readAsDataURL(pic);

  r.onload=function(e){

    dealImage(this.result,{width:600},function(data){

                picture=data

          $('#pic-box').attr("src",data);

        });

  }

}

function dealImage(path, obj, callback){

  var img = new Image();

  img.src = path;

  img.onload = function(){

  var that = this;

  // 默认按比例压缩

  var w = that.width,

  h = that.height,

  scale = w / h;

  w = obj.width || w;

  h = obj.height || (w / scale);

  var quality = 0.7; // 默认图片质量为0.7

  //生成canvas

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

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

  // 创建属性节点

  var anw = document.createAttribute("width");

  anw.nodeValue = w;

  var anh = document.createAttribute("height");

  anh.nodeValue = h;

  canvas.setAttributeNode(anw);

  canvas.setAttributeNode(anh);

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

  // // 图像质量

  if(obj.quality && obj.quality <= 1 && obj.quality > 0){

  quality = obj.quality;

  }

  // // quality值越小，所绘制出的图像越模糊

  var base64 = canvas.toDataURL('image/jpeg', quality );

  // // 回调函数返回base64的值

  callback(base64);

  }

}