static void error_callback( const char* msg, void* ){
  stringstream ss;
  ss << "OpenJPEG error :: " << msg;
  throw file_error( ss.str() );
}
static void error_callback( const char* msg, void* ){
  stringstream ss;
  ss << "OpenJPEG error :: " << msg;
  throw file_error( ss.str() );
}