code: function(cb){
        this.toCanvas(function(canvas){
          var dataUrl = canvas.toDataURL().replace(/^data:image\/(png|jpg);base64,/, "");
          var byteString = atob(escape(dataUrl));
          // write the bytes of the string to an ArrayBuffer
          var ab = new ArrayBuffer(byteString.length);
          var ia = new Uint8Array(ab);
          for (var i = 0; i < byteString.length; i++) {
            ia[i] = byteString.charCodeAt(i);
          }
          var dataView = new DataView(ab);
          var blob = new Blob([dataView], {type: "image/png"});
          cb(blob);
        });
      }
docs: Converts canvas to binary blob.