code: def _generate_ffmpeg_cmd(
          self,
          cmd: List[str],
          input_source: Optional[str],
          output: Optional[str],
          extra_cmd: Optional[str] = None,
      ) -> None:
          
          self._argv = [self._ffmpeg]
  
          # start command init
          if input_source is not None:
              self._put_input(input_source)
          self._argv.extend(cmd)
  
          # exists a extra cmd from customer
          if extra_cmd is not None:
              self._argv.extend(shlex.split(extra_cmd))
  
          self._merge_filters()
          self._put_output(output)
docs: Generate ffmpeg command line.