AVAssetWriter 输出大文件（即使在应用压缩设置时）

Question

我正在开展一个个人 iOS 项目，该项目需要通过 4G 连接将全屏视频（时长 15 秒）上传到后端。虽然我可以很好地拍摄视频，但文件的输出大小达到 30MB，这让我觉得我在压缩方面非常做错了什么。下面是我用来设置 AssetWriter 的代码：

-(void)captureOutput:(AVCaptureFileOutput *)captureOutput didStartRecordingToOutputFileAtURL:(NSURL *)fileURL fromConnections:(NSArray *)connections
{
    NSLog(@"Started Recording! *******************");
    self.movieWriter = [AVAssetWriter assetWriterWithURL:fileURL fileType:AVFileTypeMPEG4 error:nil];
    [self.movieWriter setShouldOptimizeForNetworkUse:YES];

    NSDictionary *videoCleanApertureSettings = @{
                                                 AVVideoCleanApertureWidthKey: [NSNumber numberWithFloat:self.view.frame.size.width],
                                                 AVVideoCleanApertureHeightKey: [NSNumber numberWithFloat:self.view.frame.size.height],
                                                 AVVideoCleanApertureHorizontalOffsetKey: [NSNumber numberWithInt:10],
                                                 AVVideoCleanApertureVerticalOffsetKey: [NSNumber numberWithInt:10],
                                                 };

    NSDictionary *videoCompressionSettings = @{
                                          AVVideoAverageBitRateKey: [NSNumber numberWithFloat:5000000.0],
                                          AVVideoMaxKeyFrameIntervalKey: [NSNumber numberWithInteger:1],
                                          AVVideoProfileLevelKey: AVVideoProfileLevelH264Baseline30,
                                          AVVideoCleanApertureKey: videoCleanApertureSettings,
                                          };

    NSDictionary *videoSettings = @{AVVideoCodecKey: AVVideoCodecH264,
                                    AVVideoWidthKey: [NSNumber numberWithFloat:self.view.frame.size.width],
                                    AVVideoHeightKey: [NSNumber numberWithFloat:self.view.frame.size.height],
                                    AVVideoCompressionPropertiesKey: videoCompressionSettings,
                                    };

    self.movieWriterVideoInput = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeVideo outputSettings:videoSettings];
    self.movieWriterVideoInput.expectsMediaDataInRealTime = YES;
    [self.movieWriter addInput:self.movieWriterVideoInput];

    NSDictionary *audioSettings = @{AVFormatIDKey: [NSNumber numberWithInteger:kAudioFormatMPEG4AAC],
                                    AVSampleRateKey: [NSNumber numberWithFloat:44100.0],
                                    AVNumberOfChannelsKey: [NSNumber numberWithInteger:1],
                                    };

    self.movieWriterAudioInput = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeAudio outputSettings:audioSettings];
    self.movieWriterAudioInput.expectsMediaDataInRealTime = YES;
    [self.movieWriter addInput:self.movieWriterAudioInput];


    [self.movieWriter startWriting];
}

-(void)captureOutput:(AVCaptureFileOutput *)captureOutput didFinishRecordingToOutputFileAtURL:(NSURL *)outputFileURL fromConnections:(NSArray *)connections error:(NSError *)error
{
    NSLog(@"Done Recording!");
    [self.movieWriterVideoInput markAsFinished];
    [self.movieWriterAudioInput markAsFinished];
    [self.movieWriter finishWritingWithCompletionHandler:^{
        AVURLAsset *compressedVideoAsset = [[AVURLAsset alloc] initWithURL:self.movieWriter.outputURL options:nil];
        //Upload video to server

    }];
}

对于实际会话的设置，我使用以下代码：

            //Indicate that some changes will be made to the session
            [self.captureSession beginConfiguration];
            self.captureSession.sessionPreset = AVCaptureSessionPresetHigh;

            AVCaptureInput* currentCameraInput = [self.captureSession.inputs objectAtIndex:0];
            for (AVCaptureInput *captureInput in self.captureSession.inputs) {
                [self.captureSession removeInput:captureInput];
            }


            //Get currently selected camera and use for input
            AVCaptureDevice *videoCamera = nil;
            if(((AVCaptureDeviceInput*)currentCameraInput).device.position == AVCaptureDevicePositionBack)
            {
                videoCamera = [self cameraWithPosition:AVCaptureDevicePositionBack];
            }
            else
            {
                videoCamera = [self cameraWithPosition:AVCaptureDevicePositionFront];
            }

            //Add input to session
            AVCaptureDeviceInput *newVideoInput = [[AVCaptureDeviceInput alloc] initWithDevice:videoCamera error:nil];
            [self.captureSession addInput:newVideoInput];

            //Add mic input to the session
            AVCaptureDevice *audioDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeAudio];
            AVCaptureInput *audioInput = [AVCaptureDeviceInput deviceInputWithDevice:audioDevice error:nil];
            [self.captureSession addInput:audioInput];

            //Add movie output to session
            for (AVCaptureOutput *output in self.captureSession.outputs) {
                [self.captureSession removeOutput:output];
            }

            self.movieOutput = [AVCaptureMovieFileOutput new];
            int32_t preferredTimeScale = 30; //Frames per second
            self.movieOutput.maxRecordedDuration = CMTimeMakeWithSeconds(15, preferredTimeScale); //Setting the max video length
            [self.captureSession addOutput:self.movieOutput];

            //Commit all the configuration changes at once
            [self.captureSession commitConfiguration];

我知道如果我将 AVCaptureSessionPresetHigh 更改为不同的预设，我可以减小最终视频的文件大小，但不幸的是它看起来像 AVCaptureSessionPresetiFrame1280x720 是唯一一个提供我试图捕获的全帧的图像（这使我的输出大小约为 20MB，对于 4G 上传来说仍然太大）。

我花了很多时间在谷歌上搜索和搜索 Stack Overflow 上的其他帖子，但我似乎无法弄清楚我这辈子做错了什么，任何帮助都是非常感谢。

Answer 1

您需要博士学位才能使用 AVAssetWriter - 这很重要：https://developer.apple.com/library/mac/documentation/AudioVideo/Conceptual/AVFoundationPG/Articles/05_Export.html#//apple_ref/doc/uid/TP40010188-CH9-SW1

有一个很棒的库可以做你想做的事情，它只是一个 AVAssetExportSession 的替代品，具有更重要的功能，比如改变比特率：https://github.com/rs/SDAVAssetExportSession

使用方法如下：

-(void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info
{

  SDAVAssetExportSession *encoder = [SDAVAssetExportSession.alloc initWithAsset:[AVAsset assetWithURL:[info objectForKey:UIImagePickerControllerMediaURL]]];
  NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
  NSString *documentsDirectory = [paths objectAtIndex:0];
  self.myPathDocs =  [documentsDirectory stringByAppendingPathComponent:
                      [NSString stringWithFormat:@"lowerBitRate-%d.mov",arc4random() % 1000]];
  NSURL *url = [NSURL fileURLWithPath:self.myPathDocs];
  encoder.outputURL=url;
  encoder.outputFileType = AVFileTypeMPEG4;
  encoder.shouldOptimizeForNetworkUse = YES;

  encoder.videoSettings = @
  {
  AVVideoCodecKey: AVVideoCodecH264,
  AVVideoCompressionPropertiesKey: @
    {
    AVVideoAverageBitRateKey: @2300000, // Lower bit rate here
    AVVideoProfileLevelKey: AVVideoProfileLevelH264High40,
    },
  };
  encoder.audioSettings = @
  {
  AVFormatIDKey: @(kAudioFormatMPEG4AAC),
  AVNumberOfChannelsKey: @2,
  AVSampleRateKey: @44100,
  AVEncoderBitRateKey: @128000,
  };

  [encoder exportAsynchronouslyWithCompletionHandler:^
  {
    int status = encoder.status;

    if (status == AVAssetExportSessionStatusCompleted)
    {
      AVAssetTrack *videoTrack = nil;
      AVURLAsset *asset = [AVAsset assetWithURL:encoder.outputURL];
      NSArray *videoTracks = [asset tracksWithMediaType:AVMediaTypeVideo];
      videoTrack = [videoTracks objectAtIndex:0];
      float frameRate = [videoTrack nominalFrameRate];
      float bps = [videoTrack estimatedDataRate];
      NSLog(@"Frame rate == %f",frameRate);
      NSLog(@"bps rate == %f",bps/(1024.0 * 1024.0));
      NSLog(@"Video export succeeded");
      // encoder.outputURL <- this is what you want!!
    }
    else if (status == AVAssetExportSessionStatusCancelled)
    {
      NSLog(@"Video export cancelled");
    }
    else
    {
      NSLog(@"Video export failed with error: %@ (%d)", encoder.error.localizedDescription, encoder.error.code);
    }
  }];
}

AVAssetWriter 输出大文件（即使在应用压缩设置时）

AVAssetWriter Outputting Large File (even when applying compression settings)

ios

objective-c

video

compression

avassetwriter