code: function(command, stderrLine) {
      var progress = parseProgressLine(stderrLine);
  
      if (progress) {
        // build progress report object
        var ret = {
          frames: parseInt(progress.frame, 10),
          currentFps: parseInt(progress.fps, 10),
          currentKbps: progress.bitrate ? parseFloat(progress.bitrate.replace('kbits/s', '')) : 0,
          targetSize: parseInt(progress.size || progress.Lsize, 10),
          timemark: progress.time
        };
  
        // calculate percent progress using duration
        if (command._ffprobeData && command._ffprobeData.format && command._ffprobeData.format.duration) {
          var duration = Number(command._ffprobeData.format.duration);
          if (!isNaN(duration))
            ret.percent = (utils.timemarkToSeconds(ret.timemark) / duration) * 100;
        }
        command.emit('progress', ret);
      }
    }
docs: Extract progress data from ffmpeg stderr and emit 'progress' event if appropriate
  
  @param {FfmpegCommand} command event emitter
  @param {String} stderrLine ffmpeg stderr data
  @private