private void writeImageDescriptor(OutputStream os) throws IOException
      {
        os.write((int) ',');                // Image Separator
        putShort(thePosition.x, os);
        putShort(thePosition.y, os);
        putShort(theWidth, os);
        putShort(theHeight, os);
        os.write(isInterlaced ? 0x40 : 0);  // packed fields (1 byte)
      }