Flutter视频压缩技术：如何在应用中优化视频文件的质量和大小？

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为什么Flutter应用需要视频压缩功能？

移动应用程序中，视频占用了大量的存储空间和带宽。这在一定程度上影响了应用程序的性能和用户体验。因此，在许多应用程序中，我们需要对视频文件进行压缩以优化其大小和质量。通过压缩视频，可以有效地减小视频文件的体积，并且可在保证画质的前提下，大幅降低视频传输和播放时所需的带宽。对于 Flutter 应用程序而言，视频压缩同样也是非常重要的。

常见的视频压缩算法和格式

视频压缩算法可以分为有损压缩和无损压缩两种方式。有损压缩是一种在保证视觉质量的情况下，通过舍弃冗余数据来压缩视频的方式。相反，无损压缩则可保留所有视频数据，但通常需要更大的存储空间。

在实际应用中，我们常用以下几种视频压缩格式：H.264，HEVC，AV1 等等，其中 H.264 是目前移动应用程序中最主流的压缩格式之一，支持广泛，文件体积和清晰度较为平衡。以 Flutter 应用程序为例，我们可以使用 FFmpeg 库进行视频压缩，以支持各种流行的视频压缩格式。接下来，我们将介绍如何使用 FFmpeg 压缩 Flutter 应用程序的视频。

使用FFmpeg库压缩Flutter应用中的视频

如何在Flutter应用中集成FFmpeg库？

若需使用FFmpeg进行视频压缩，我们首先需要将 FFmpeg 库集成到 Flutter 应用程序中。下面是一些基本操作步骤：

从FFmpeg官网下载 FFmpeg 库并解压文件。
在 Flutter 应用程序的 Android 端目录中添加 FFmpeg 的 gradle 依赖项。
在 Flutter 应用程序的 iOS 端目录中添加 FFmpeg 的pod依赖项。
在 Flutter 应用程序使用 FFmpeg 库时初始化所需配置和参数。

以下是一些关键代码步骤和教程供您参考：

1.下载和解压 FFmpeg 库

$ curl -LO http://ffmpeg.org/releases/ffmpeg-<version>.tar.bz2

$ tar jxvf ffmpeg-<version>.tar.bz2

这里需要您自行选择您需要下载的对应版本。

2.添加 FFmpeg 的 gradle 依赖项

在应用程序的 android/app/build.gradle 文件中，添加以下依赖项：

repositories {

    jcenter()

}

dependencies {

    implementation 'com.arthenica:mobile-ffmpeg-full:4.4.LTS'

}

使用上面的依赖项后，我们就可以在应用程序中使用 FFmpeg 库了。

3.在 iOS 端目录中添加 FFmpeg 的pod依赖项

在应用程序的 ios/Podfile 文件中，添加以下依赖项：

target 'Runner' do

  use_frameworks!

  # Pods for Runner

  # Add the following line:

  pod 'MobileFFmpeg', '4.4.LTS'

end

添加依赖项后，我们可以使用 CocoaPods 更新我们的依赖：

cd ios

pod install

4.初始化FFmpeg库配置和参数

在使用 FFmpeg 进行任何操作之前，我们需要通过一些设置来初始化 FFmpeg 库的基本配置和参数。这一步需要在启动应用程序时进行，根据以下代码执行即可：

import 'package:flutter_video_compress/flutter_video_compress.dart';

FlutterVideoCompress flutterVideoCompress = FlutterVideoCompress();

await flutterVideoCompress.getFFmpegVersion();

flutterVideoCompress.setLogLevel(LogLevel.AV_LOG_ERROR);

await flutterVideoCompress.loadFFmpeg();

以上是Flutter中集成 FFmpeg 库的基本代码，这为后续的视频压缩操作打下了必要的基础。接下来，我们将介绍 FFmpeg 库的基本视频压缩操作。

使用FFmpeg库进行视频压缩的基本步骤

在对视频进行压缩之前，我们需要对视频进行解码，并根据要求对其进行重新编码。这个过程需要借助于 FFmpeg 库，并完成以下几个步骤：

打开输入文件；
解码输入文件；
进行需要的操作（如压缩、转换等）；
将操作后的输出写入到输出文件中；
关闭输入文件和输出文件。

await flutterVideoCompress.executeWithArguments([

    '-y',

    '-i',

    'input.mp4',

    '-c:v',

    'libx264',

    '-crf',

    '18',

    '-preset',

    'superfast',

    '-c:a',

    'aac',

    '-b:a',

    '128k',

    '-strict',

    '-2',

    'output.mp4',

]);

该示例中，input.mp4 是我们需要压缩的文件名，通过指定 -c:v libx264 -crf 18 -preset superfast 对视频进行了压缩处理，并且 -c:a aac -b:a 128k 对音频进行了编码，保证音频的质量，最终生成 output.mp4 文件。

这就是使用 FFmpeg 进行视频压缩的基本流程，接下来，我们将详细讲解如何使用 Dart 语言封装 FFmpeg 命令。

使用Dart语言封装FFmpeg命令

在 Flutter 应用程序中使用 FFmpeg 库进行视频压缩时，我们通常需要输入大量的参数才能开始命令执行。为了方便操作，我们常常会使用 Dart 语言的特性来封装 FFmpeg 命令。

通常，我们使用 Process.run 方法执行命令：

import 'dart:convert';

import 'dart:io';

await Process

    .run('ffmpeg', ['-i', 'input.mp4', '-vf', 'scale=-1:360', '-c:v', 'libx264', '-preset', 'fast', '-crf', '23', '-ac', '1', '-ar', '44100', '-acodec', 'aac', 'output.mp4'])

    .then((ProcessResult result) {

      print('standard out:\n${result.stdout}\n');

      print('standard in:\n${result.stderr}\n');

      String message = result.stderr.toString();

      if (message.contains('Cannot find ffmpeg')) {

        throw ('${message.toString()}');

      }

    });

以上示例中，我们使用 Process.run 方法执行 FFmpeg 命令， -i input.mp4指定需要压缩的文件名，-vf scale=-1:360 指定视频框的大小为 360，-c:v libx264 -preset fast -crf 23 是视频压缩参数，-ac 1 -ar 44100 -acodec aac 是音频编码参数，最终我们通过 output.mp4 输出压缩后的视频文件。

然而，对于多次执行相似操作的情况，我们并不希望每次都输入一大堆长串的命令参数。这时候，我们可以使用 Dart 语言中的类来对 FFmpeg 命令进行封装，方便测试和重用。

在以下示例中，我们将基本的 FFmpeg 命令封装在 FFmpegCommands 类中，并通过 toCommand 方法将参数转换为一条命令行命令：

class FFmpegCommands {

  String _inputPath;

  String _outputPath;

  List<String> _videoFilters;

  List<String> _audioFilters;

  String _crf;

  String _bitrate;

  FFmpegCommands({

    @required String inputPath,

    @required String outputPath,

    List<String> videoFilters,

    List<String> audioFilters,

    String crf = '23',

    String bitrate = '1024k',

  })  : assert(inputPath != null),

        assert(outputPath != null) {

    this._inputPath = inputPath;

    this._outputPath = outputPath;

    this._videoFilters = videoFilters;

    this._audioFilters = audioFilters;

    this._crf = crf;

    this._bitrate = bitrate;

  }

  String toCommand() {

    List<String> commands = [];

    commands.addAll([

      'ffmpeg',

      '-i',

      this._inputPath,

    ]);

    if (this._videoFilters != null) {

      commands.addAll(['-vf', this._videoFilters.join(',')]);

    }

    if (this._crf != null) {

      commands.addAll(['-crf', this._crf]);

    }

    if (this._bitrate != null) {

      commands.addAll(['-b:a', this._bitrate]);

    }

    commands.addAll(['-y', this._outputPath]);

    return commands.join(' ');

  }

}

使用 FFmpeg 命令封装类虽然不会改变最终的操作，但能提高重用率和可维护性。下面，我们以例子，具体展示如何使用它进行视频压缩。

void main() async {

  FFmpegCommands ffmpegCommands = FFmpegCommands(

    inputPath: 'input.mp4',

    outputPath: 'output.mp4',

    videoFilters: ['scale=-1:360'],

    crf: '23',

    bitrate: '1024k',

  );

  await Process

      .run(ffmpegCommands.toCommand(), [])

      .then((ProcessResult result) {

        print(result.stdout);

        print(result.stderr);

      });

}

在上面这个例子中，我们首先构造了一个 FFmpegCommands 对象，其中包含了全部参数，然后通过 ffmpegCommands.toCommand() 方法生成可执行的命令行命令，最终通过 Process.run 方法执行压缩操作。

以上就是使用 Dart 语言封装 FFmpeg 命令的方法，接下来，我们将结合实例，讲解如何在 Flutter 应用程序中使用 FFmpeg 库进行视频压缩。

Flutter应用中视频压缩的最佳实践

我们将通过实例，介绍在 Flutter 应用程序中如何使用 FFmpeg 库进行视频压缩的最佳实践。

选择最合适的视频压缩算法和格式

在实际应用中，我们根据视频压缩的需求，选择最适合的压缩算法和格式。

尺寸压缩：对于需要压缩视频大小的需求，可以使用 FFmpeg 库中的 scale 滤镜来改变视频的分辨率，从而减小视频文件大小，例如：

String command = 'ffmpeg -i input.mp4 -vf scale=-1:360 output.mp4';

视频编码压缩：在保证视频质量的前提下，可以选择压缩视频的编码格式，例如使用 H.264 或者 HEVC 等。
音频编码压缩：选择合适的音频编码格式也可以减小视频文件大小。

综合考虑需求，我们需要结合实际进行选择。

配置FFmpeg工具进行压缩

在选择完合适的压缩算法和格式后，便可以使用 FFmpeg 工具进行压缩。

String command = 'ffmpeg -i input.mp4 -c:v libx264 -crf 23 -preset fast -c:a aac -b:a 128k output.mp4';

await Process.run(command, []);

在执行命令前，我们需要确保我们已经按照前面所述集成了 FFmpeg 库。除此之外，根据具体需要，我们可以传递不同的参数来实现不同效果的压缩。

例如，如果我们在压缩过程中想要保留视频的宽高比例，我们可以将压缩命令改为：

String command = 'ffmpeg -i input.mp4 -c:v libx264 -filter:v "scale=-1:360:force_original_aspect_ratio=decrease,pad=360:360:(ow-iw)/2:(oh-ih)/2" -crf 23 -preset fast -c:a aac -b:a 128k output.mp4';

这里使用了 pad 滤镜来保持宽高比例，同时使用 force_original_aspect_ratio=decrease 保证视频宽高比不变。

使用Dart语言封装FFmpeg命令

使用 Dart 语言封装 FFmpeg 命令可以让我们更好地进行参数的组合和维护，以下是使用 FFmpeg 命令封装类进行视频压缩的示例：

class FFmpegCommands {

  String _inputPath;

  String _outputPath;

  List<String> _videoFilters;

  List<String> _audioFilters;

  String _crf;

  String _bitrate;

  FFmpegCommands({

    @required String inputPath,

    @required String outputPath,

    List<String> videoFilters,

    List<String> audioFilters,

    String crf = '23',

    String bitrate = '1024k',

  })  : assert(inputPath != null),

        assert(outputPath != null) {

    this._inputPath = inputPath;

    this._outputPath = outputPath;

    this._videoFilters = videoFilters;

    this._audioFilters = audioFilters;

    this._crf = crf;

    this._bitrate = bitrate;

  }

  String toCommand() {

    List<String> commands = [];

    commands.addAll([

      'ffmpeg',

      '-i',

      this._inputPath,

    ]);

    if (this._videoFilters != null) {

      commands.addAll(['-vf', this._videoFilters.join(',')]);

    }

    if (this._crf != null) {

      commands.addAll(['-crf', this._crf]);

    }

    if (this._bitrate != null) {

      commands.addAll(['-b:a', this._bitrate]);

    }

    commands.addAll(['-y', this._outputPath]);

    return commands.join(' ');

  }

}

void main() async {

  FFmpegCommands ffmpegCommands = FFmpegCommands(

    inputPath: 'input.mp4',

    outputPath: 'output.mp4',

    videoFilters: ['scale=-1:360', 'pad=360:360:(ow-iw)/2:(oh-ih)/2:color=black'],

    crf: '23',

    bitrate: '1024k',

  );

  await Process

      .run(ffmpegCommands.toCommand(), [])

      .then((ProcessResult result) {

        print(result.stdout);

        print(result.stderr);

      });

}

在上述示例中，我们首先定义了 FFmpegCommands 类，然后构造了一个对象来指定 FFmpeg 命令的各个参数，最后通过 toCommand 方法生成可执行的命令行命令，并通过 Process.run 方法执行视频压缩。整个过程中，我们可以自由设置 FFmpeg 命令中的各个参数来实现不同的视频压缩效果。

实现视频压缩进度的更新与回调

在视频压缩过程中，我们通常希望能够实时显示压缩的进度，并提供进度条供用户观察操作进度。为了实现这一目标，我们可以通过监听 FFmpeg 工具的输出流来更新压缩进度。

以下是实现视频压缩进度更新和回调的示例代码：

class VideoCompressUtil {

  static final FlutterVideoCompress _flutterVideoCompress = FlutterVideoCompress();

  static StreamSubscription _subscription;

  static int _prevProgress;

  static Future<void> compressVideo({

    @required String inputPath,

    @required String outputPath,

    List<String> videoFilters,

    List<String> audioFilters,

    String crf = '23',

    String bitrate = '1024k',

    Function(int) onProgress,

  }) async {

    if (_subscription != null) {

      throw 'Another FFmpeg compression is already in progress.';

    }

    int totalDuration = await _flutterVideoCompress.getMediaInformation(inputPath: inputPath).then((info) => info

        .duration

        .inMilliseconds);

    int previousPercent = 0;

    final Completer completer = Completer();

    FFmpegCommands cmd = FFmpegCommands(

      inputPath: inputPath,

      outputPath: outputPath,

      videoFilters: videoFilters,

      audioFilters: audioFilters,

      crf: crf,

      bitrate: bitrate,

    );

    String command = cmd.toCommand();

    print(command);

    _prevProgress = 0;

    _subscription = _flutterVideoCompress.pipe(command).listen((RetrieveData stdout) async {

      String data = utf8.decode(stdout.data);

      if (data.contains('frame=')) {

        String progressString = data.split('frame=')[1].split('fps=')[0].trim();

        int progress = int.parse(progressString);

        if (previousPercent != ((progress * 100) ~/ totalDuration)) {

          previousPercent = ((progress * 100) ~/ totalDuration);

          onProgress(previousPercent);

        }

      }

      if (data.contains('Stream mapping')) {

        _prevProgress = null;

      }

      if (data.contains('size=')) {

        String durString = data.split("Duration:")[1].split(",")[0].trim();

        Duration duration = Duration(

            hours: int.parse(durString.split(":")[0]),

            minutes: int.parse(durString.split(":")[1]),

            seconds: int.parse(durString.split(":")[2].split(".")[0]),

            milliseconds: int.parse(durString.split(":")[2].split(".")[1]));

        int totalDuration = duration.inSeconds;

        double _progress = 0;

        RegExp timeRegExp = new RegExp(r"(?:(\d+):)?(\d{2}):(\d{2})\.(\d{1,3})");

        String lastMatch;

        data.split('\n').forEach((line) {

          lastMatch = line;

          if (line.contains('time=')) {

            final match = timeRegExp.allMatches(line).elementAt(0);

            final hours = match.group(1) != null ? int.parse(match.group(1)) : 0;

            final minutes = int.parse(match.group(2));

            final seconds = int.parse(match.group(3));

            final videoDurationInSeconds = (hours * 3600) + (minutes * 60) + seconds;

            _progress = (videoDurationInSeconds / totalDuration) * 100;

            if ((_progress - _prevProgress).abs()>= 1.0) {

              _prevProgress = _progress.toInt();

              onProgress(_prevProgress);

            }

          }

        });

        completer.complete();

      }

      // Output FFmpeg log to console.

      print(data);

    });

    await completer.future;

    await _subscription.cancel();

    _subscription = null;

    _prevProgress = 0;

  }

}

在上述代码中，我们定义了 VideoCompressUtil 类，通过内部的 FFmpegCommands 类封装了 FFmpeg 命令，并监听 FFmpeg 工具输出流来实现视频压缩进度的更新和回调。在进行压缩过程中，我们通过传递 onProgress 参数来实现压缩进度的实时更新。

总结

本文介绍了使用 Flutter 开发视频压缩功能的方法，包括集成 FFmpeg 库、使用 FFmpeg 命令进行视频压缩、封装 FFmpeg 命令并实现压缩进度更新和回调等。

在实际开发中，视频压缩是一项经常用到的技术，通过掌握本文所述的方法和技巧，我们可以轻易地实现视频压缩功能，并让用户更好地体验应用的功能和服务，兄弟们赶紧去敲一遍试试。