js canvas图片压缩

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);
  }
}