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我需要旋转视频，因为 iPhone 后置摄像头正在录制，就好像它总是在横向左侧一样。我需要物理旋转视频，因为设置方向不适用于所有浏览器（例如 Chrome）。所以我设置了一些代码来读取录制的视频并在旋转后保存。我使用了 AVAssetExportSession，它似乎工作得很好，除了两个问题： - 导出的视频在开始时有一些黑框 - 一些视频不会被导出。

是否有可能对这个问题有所了解？谢谢！

代码：

AVAsset* asset = [AVURLAsset URLAssetWithURL: videoUrl options:nil];

AVMutableComposition *composition = [AVMutableComposition composition];
AVMutableCompositionTrack *compositionVideoTrack = [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];

AVMutableCompositionTrack *compositionAudioTrack = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];


AVAssetTrack *videoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
AVAssetTrack *audioTrack = [[asset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
[compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, asset.duration) ofTrack:videoTrack atTime:kCMTimeZero error: &error];

[compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, asset.duration) ofTrack:audioTrack atTime:kCMTimeZero error: &error];

AVMutableVideoCompositionInstruction *instruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
instruction.timeRange = CMTimeRangeMake( kCMTimeZero, asset.duration);

AVMutableVideoCompositionLayerInstruction *layerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:compositionVideoTrack];
  [layerInstruction setTransform:transformToApply atTime:kCMTimeZero];

CGAffineTransform transformToApply = ///code to setup transformation 
[layerInstruction setTransform:transformToApply atTime:kCMTimeZero];
[layerInstruction setOpacity:0.0 atTime:asset.duration];

instruction.layerInstructions = [NSArray arrayWithObject: layerInstruction];

AVMutableVideoComposition *videoComposition = [AVMutableVideoComposition videoComposition];
videoComposition.instructions = [NSArray arrayWithObject: instruction];


videoComposition.frameDuration = CMTimeMake( 1, 600);
videoComposition.renderScale = 1.0;
videoComposition.renderSize =  videoOutputSize;

AVAssetExportSession *exportSession = [[AVAssetExportSession alloc] initWithAsset:asset presetName:AVAssetExportPresetMediumQuality] ;
exportSession.outputURL = self.outputUrl;
exportSession.outputFileType = AVFileTypeQuickTimeMovie;
exportSession.videoComposition = videoComposition;
exportSession.shouldOptimizeForNetworkUse = YES;
exportSession.timeRange = CMTimeRangeMake(kCMTimeZero, asset.duration);

__block BOOL finishedExporting = NO;
[exportSession exportAsynchronouslyWithCompletionHandler:^{

    //
}];
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