Problems with video orientation when exporting using AVMutableVideoComposition

Question

Here is the function I used to export the video:

- (void) videoOutput { //1 - Early exit if there no video file selected if (!self.videoAsset) { UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Error" message:@"Please Load a Video Asset First" delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil]; [alert show]; return; } // 2 - Create AVMutableComposition object. This object will hold your AVMutableCompositionTrack instances. AVMutableComposition *mixComposition = [[AVMutableComposition alloc] init]; // 3 - Video track AVMutableCompositionTrack *videoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid]; [videoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, self.videoAsset.duration) ofTrack:[[self.videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:kCMTimeZero error:nil]; // 3.1 - Create AVMutableVideoCompositionInstruction AVMutableVideoCompositionInstruction *mainInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction]; mainInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, self.videoAsset.duration); // 3.2 - Create an AVMutableVideoCompositionLayerInstruction for the video track and fix the orientation. AVMutableVideoCompositionLayerInstruction *videolayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoTrack]; AVAssetTrack *videoAssetTrack = [[self.videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0]; BOOL isVideoAssetPortrait_ = NO; CGAffineTransform videoTransform = videoAssetTrack.preferredTransform; if (videoTransform.a == 0 && videoTransform.b == 1.0 && videoTransform.c == -1.0 && videoTransform.d == 0) { isVideoAssetPortrait_ = YES; } if (videoTransform.a == 0 && videoTransform.b == -1.0 && videoTransform.c == 1.0 && videoTransform.d == 0) { isVideoAssetPortrait_ = YES; } if (videoTransform.a == 1.0 && videoTransform.b == 0 && videoTransform.c == 0 && videoTransform.d == 1.0) { isVideoAssetPortrait_ = NO; } if (videoTransform.a == -1.0 && videoTransform.b == 0 && videoTransform.c == 0 && videoTransform.d == -1.0) { isVideoAssetPortrait_ = NO; } [videolayerInstruction setTransform:videoAssetTrack.preferredTransform atTime:kCMTimeZero]; [videolayerInstruction setOpacity:0.0 atTime:self.videoAsset.duration]; // 3.3 - Add instructions mainInstruction.layerInstructions = [NSArray arrayWithObjects:videolayerInstruction,nil]; AVMutableVideoComposition *mainCompositionInst = [AVMutableVideoComposition videoComposition]; CGSize naturalSize; if(isVideoAssetPortrait_){ naturalSize = CGSizeMake(videoAssetTrack.naturalSize.height, videoAssetTrack.naturalSize.width); } else { naturalSize = videoAssetTrack.naturalSize; } mainCompositionInst.renderSize = naturalSize; mainCompositionInst.instructions = [NSArray arrayWithObject:mainInstruction]; mainCompositionInst.frameDuration = CMTimeMake(1, 30); // 4 - Get path NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES); NSString *documentsDirectory = [paths objectAtIndex:0]; NSString *myPathDocs = [documentsDirectory stringByAppendingPathComponent: [NSString stringWithFormat:@"FinalVideo-%d.mov",arc4random() % 1000]]; NSURL *url = [NSURL fileURLWithPath:myPathDocs]; // 5 - Create exporter AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPresetHighestQuality]; exporter.outputURL=url; exporter.outputFileType = AVFileTypeQuickTimeMovie; exporter.shouldOptimizeForNetworkUse = YES; exporter.videoComposition = mainCompositionInst; [exporter exportAsynchronouslyWithCompletionHandler:^{ dispatch_async(dispatch_get_main_queue(), ^{ [self exportDidFinish:exporter]; }); }]; } 

The problem is that the first time I use this function to export portrait video, the videoTransform variable (videoAssetTrack.preferredTransform) :

 videoTransform.a == 0 && videoTransform.b == 1.0 && videoTransform.c == -1.0 && videoTransform.d == 0 

And the variable isVideoAssetPortrait _ is equal to YES . All right. However, after the export is completed and saved in Camera Roll, I used this function to reload the video result . This time videoTransform has changed:

 videoTransform.a == 1.0 && videoTransform.b == 0 && videoTransform.c == 0 && videoTransform.d == 1.0 

And isVideoAssetPortrait _ equals NO . This means that after one time export, videoTransform changed its values ​​(portrait orientation → landscape)

I have many questions about the video orientation of AV Foundation, but no solution has been found yet.

Thanks for reading my long explanation. If you have any questions, let me know.

Answer 1

There is no such thing as a portrait / landscape relative to video tracks. It has only dimensions and the transformation used to represent it correctly. By default, a portrait video is encoded because it is created by the camera (say, landscape) and rotated 90 degrees, used to display correctly.

When you export it, the orientation does not change, it is simply transcoded with a physical rotation, so rotation does not need to be displayed correctly. This is why you get the ID matrix a second time (this does not mean that it was changed from portrait to landscape), but this time its natural size is also replaced, and everything should be fine based on your code.

Please indicate what is wrong in a later case.