get a blob from a canvas

gistfile1.js

function getBlob(canvas) {
  var data = atob(canvas.toDataURL().replace('data:image/png;base64,', '')),
      bytes = new Uint8Array(data.length);

  for (var i = 0, len = data.length; i < len; i++) {
    bytes[i] = data.charCodeAt(i) & 0xff;
  }

  var bb = new BlobBuilder();
  bb.append(bytes.buffer);
  return bb.getBlob('image/png');
}