What is FFMPEG?

FFMPEG is a collection of powerful and complicated video and audio processing tools. These tools are written in C++. Due to the complexity of building and coding for these tools, FFMPEG is primarily utilized through command-line (CLI) scripts.

FFMPEG scripts are cryptic!

FFMPEG CLI scripts are notoriously difficult to read and write. It’s very easy to accidentally break a script.

ffmpeg \
   -f lavfi -i testsrc \
   -f lavfi -i testsrc \
   -f lavfi -i testsrc \
   -f lavfi -i testsrc \
   -filter_complex \
"[0:v]  pad=iw*2:ih*2 [a]; \
 [1:v]  negate        [b]; \
 [2:v]  hflip         [c]; \
 [3:v]  edgedetect    [d]; \
 [a] [b] overlay=w   [x]; \
 [x] [c] overlay=0:h [y]; \
 [y] [d] overlay=w:h [out]" \
  -map "[out]" \
  -c:v ffv1   -t 5   multiple_input_grid.avi

FFMPEG filter graphs are inherently complex - there’s no getting around that. However, that complexity doesn’t need to be compounded by trying to work with shell scripts. The ffmpeg_cli package makes it possible to compose these commands in Dart.

FFMPEG commands in Dart

FFMPEG supports two styles of commands. FFMPEG calls them “simple” and “complex”. The difference is that a “simple” command has a single pipeline of steps from input to out. A “complex” command contains an entire graph of steps. The ffmpeg_cli packages supports both.

Compose a simple command

final command = FfmpegCommand.simple(
  inputs: [
    // The input video is "raw-video.mov".
    FfmpegInput.asset(“raw-video.mov”),
  ],
  args: [
    // Cut the first 2 minutes and 30 seconds off the video.
    CliArg("ss", "2:30")
  ],
  // Transcode and output to a file called "my-video.mp4".
  outputFilepath: “my-video.mp4”,
);

Compose a complex command

final command = FfmpegCommand.complex(
  // Take three inputs: an intro, some primary content, and an outro.
  inputs: [
    FfmpegInput.asset("assets/intro.mp4"),
    FfmpegInput.asset("assets/content.mp4"),
    FfmpegInput.asset("assets/outro.mov"),
  ],
  args: [
    // Send typical mapping args to send the result of the filter graph to
    // the output file.
    CliArg(name: 'map', value: outputStream.videoId!),
    CliArg(name: 'map', value: outputStream.audioId!),
    // A couple additional settings that seem to be necessary in some cases
    // to get the expected output. Play with these as needed.
    const CliArg(name: 'y'),
    const CliArg(name: 'vsync', value: '2'),
  ],
  // Configure the whole FFMPEG filter graph.
  filterGraph: FilterGraph(
    chains: [
      // Add a filter to the graph.
      FilterChain(
        // Send all three of our input videos into this filter.
        inputs: [
          const FfmpegStream(videoId: "[0:v]", audioId: "[0:a]"),
          const FfmpegStream(videoId: "[1:v]", audioId: "[1:a]"),
          const FfmpegStream(videoId: "[2:v]", audioId: "[2:a]"),
        ],
        // Apply a concatenation filter, which plays each input back-to-back.
        filters: [
          ConcatFilter(segmentCount: 3, outputVideoStreamCount: 1, outputAudioStreamCount: 1),
        ],
        // Send the result to the final output file.
        outputs: [
          outputStream,
        ],
      ),
    ],
  ),
  outputFilepath: "/my/output/file.mp4",
);

Run the command

Once you've assembled an FfmpegCommand, you can execute it from Dart. Behind the scenes, the ffmpeg_cli package takes your Dart object, converts it to a standard FFMPEG CLI command, and then runs that command in an invisible shell.

// Create the command.
final command = createMyCommand();

// Execute command.
final process = await Ffmpeg().run(command: command);

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