js利用html5的canvas实现图像等比例压缩

function getBase64Image(img) {
      var readImg = new FileReader();
      readImg.readAsDataURL(img);
      readImg.onload = function (readEvent){
       var base64="";
       if(img.size>524288){
       var tmpimg = new Image;
       var quality = 0.9;
       var cv = document.createElement("canvas");
       var drawer = cv.getContext("2d");
       tmpimg.src = this.result;
       cv.width=tmpimg.width/4;
       cv.height=tmpimg.height/4;
       drawer.drawImage(tmpimg, 0, 0,cv.width,cv.height);
             var tmpsrc = cv.toDataURL("image/jpeg", quality);
             //console.log(tmpsrc);
             base64 = tmpsrc;
             //alert(1);
      }else{
       base64 = readEvent.target.result;
       //alert(2);
   }
      //.substr(23);  // base64字符串   
      //var base64 = readEvent.target.result;
      $("#image").attr("src", base64);
      $.ajax({
      type:"post",
      url:"../sendBase.ilf",
      data:{
       "bases":base64,
       "name":img.name,
       "uuid":uuid
     },
      dataType:"json",
      async:false,
      success:function(data){
       if(data == "1"){
        console.log("Upload Success!");
        success=success+1;
        //alert(success+"--"+length);
      if(success==length){
       length=0;
       success=0;
       alert("Upload Success!");
      }
      allFiles=null;
      imgFile=null;
      $("#image").empty();
       }else{
        console.log("Upload failure!");
        alert("Upload failure!");
        allFiles=null;
      imgFile=null;
      $("#image").empty();
       }
      }
     }); 
      };
 }