iOS rotate every frame of video

问题

I need to rotate a video because of iPhone back camera is recording as if it were always on landscape left. I need to physically rotate video because settings orientation don't work on all browser (ex. Chrome). So I setup some code to read recorded video and saving it after rotation. I used AVAssetExportSession which seems to work quite well apart two problems: - the exported video have some black frames at the beginning - some videos won't be exported.

Is it possible to have some insight about the problem? Thanks!

Code:

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:^{

    //
}];

来源：https://stackoverflow.com/questions/13102646/ios-rotate-every-frame-of-video