I’m developing a Flutter app for a caption embeding on a video that needs to export a video file after processing it. I’m using the flutter_ffmpeg_kit package. However, I’m having trouble getting the export to work correctly.

Here’s the code I’m using:

initially this is my stack

                  Expanded(
                    child: Stack(
                      children: [
                        Center(
                          child: _videoPlayerController.value.isInitialized
                              ? AspectRatio(
                                  aspectRatio:
                                      _videoPlayerController.value.aspectRatio,
                                  child: VideoPlayer(_videoPlayerController),
                                )
                              : CircularProgressIndicator(),
                        ),
                        if (_currentCaption.isNotEmpty)
                          Positioned.fill(
                            child: Center(child: _buildCaptionText()),
                          ),
                      ],
                    ),
                  ),

and in export button i executed this function

 Future<void> _exportVideo() async {
    setState(() {
      _isProcessing = true;
    });

    try {
      final directory = await getExternalStorageDirectory();
      final rootPath = directory?.parent.parent.parent.parent.path;
      final mobixPath = path.join(rootPath!, 'Mobix App');
      final appPath = path.join(mobixPath, 'Caption');
      final outputPath = path.join(appPath, 'Output');

      // Create the directories if they don't exist
      await Directory(outputPath).create(recursive: true);

      final timestamp = DateTime.now().millisecondsSinceEpoch;
      final outputFilePath = path.join(outputPath, 'output-$timestamp.mp4');


      // Generate the FFmpeg command
      final ffmpegCommand = _generateFFmpegCommand(
        widget.videoPath,
        outputFilePath,
        widget.words,
        _fontSize,
        _isBold,
        _isItalic,
        _fontColor,
        _backgroundColor,
      );

      // Execute the FFmpeg command
      await FFmpegKit.execute(
        ffmpegCommand,
      ).then(
        (session) async {
          // Update progress if needed
          final returnCode = await session.getReturnCode();
          if (ReturnCode.isSuccess(returnCode)) {
            setState(() {
              _outputFilePath = outputFilePath;
            });
            ScaffoldMessenger.of(context).showSnackBar(
              SnackBar(content: Text('Export successful: $_outputFilePath')),
            );
          } else {
            print('Export failed with rc: $returnCode');

            ScaffoldMessenger.of(context).showSnackBar(
              SnackBar(content: Text('Export failed with rc: $returnCode')),
            );
          }
          setState(() {
            _isProcessing = false;
          });
        },
      );
    } catch (e) {
      print('Export failed: $e');
      ScaffoldMessenger.of(context).showSnackBar(
        SnackBar(content: Text('Export failed: $e')),
      );
      setState(() {
        _isProcessing = false;
      });
    }
  }

  String _generateFFmpegCommand(
    String inputPath,
    String outputPath,
    List<dynamic> words,
    double fontSize,
    bool isBold,
    bool isItalic,
    Color fontColor,
    Color backgroundColor,
  ) {
    final ffmpegCommand = StringBuffer();

    // Add input file
    ffmpegCommand.write('-i $inputPath ');

    // Add subtitles filter
    final subtitleFilter = StringBuffer();
    for (var word in words) {
      final startTime = word['startTime'].toDouble();
      final endTime = word['endTime'].toDouble();
      final caption = word['word'];

      final fontStyle = isBold && isItalic
          ? 'bold italic'
          : isBold
              ? 'bold'
              : isItalic
                  ? 'italic'
                  : 'normal';
      final fontColorHex = fontColor.value.toRadixString(16).substring(2);
      final backgroundColorHex =
          backgroundColor.value.toRadixString(16).substring(2);

      subtitleFilter.write(
          "drawtext=text='$caption':x=(w-tw)/2:y=h-(2*lh):fontcolor=$fontColorHex:fontsize=$fontSize:fontStyle=$fontStyle:box=1:[email protected]:boxborderw=5:enable='between(t,$startTime,$endTime)',");
    }
    ffmpegCommand.write('-vf "${subtitleFilter.toString()}" ');

    // Add output file
    ffmpegCommand.write('$outputPath');

    return ffmpegCommand.toString();
  }

when i run this it returns ReturnCode 1. what am i doing wrong?