function canvas2Blob(canvas) {
  var dataURI = canvas.toDataURL();

  // convert base64 to raw binary data held in a string
  // doesn't handle URLEncoded DataURIs
  var byteString = atob(dataURI.split(',')[1]);

  // separate out the mime component
  var mimeString = dataURI.split(',')[0].split(':')[1].split(';')[0];

  // 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);
  }

  // create a blob for writing to a file
  var blob = new Blob([ab], {type: mimeString});
  return blob;
}