Rotate an AVAsset with AVAssetExportSession - iphone

Question

I'm trying to rotate a video to its correct orientation using an AVAssetExportSession and I always get the following error:
Error Domain=AVFoundationErrorDomain Code=-11841 "The operation couldn’t be completed. (AVFoundationErrorDomain error -11841.)"
That translates to AVErrorInvalidVideoComposition but I cannot see anything wrong with my video composition. Here's the code:
AVAssetTrack *sourceVideo = [[avAsset tracksWithMediaType:AVMediaTypeVideo] lastObject];
AVAssetTrack *sourceAudio = [[avAsset tracksWithMediaType:AVMediaTypeAudio] lastObject];
CGAffineTransform preferredTransform = [sourceVideo preferredTransform];
AVMutableComposition *composition = [[AVMutableComposition alloc] init];
AVMutableCompositionTrack *compositionVideoTrack = [composition addMutableTrackWithMediaType:AVMediaTypeVideo
preferredTrackID:kCMPersistentTrackID_Invalid];
AVAssetExportSession *exporter = [[[AVAssetExportSession alloc] initWithAsset:composition presetName:AVAssetExportPresetMediumQuality] autorelease];
[compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, avAsset.duration)
ofTrack:sourceVideo
atTime:kCMTimeZero
error:nil];
if( !CGAffineTransformIsIdentity(preferredTransform) ) {
AVMutableVideoComposition *videoComposition = [AVMutableVideoComposition videoComposition];
videoComposition.renderSize = CGSizeMake([avAsset naturalSize].height, [avAsset naturalSize].width);
videoComposition.frameDuration = CMTimeMake(1, compositionVideoTrack.naturalTimeScale);
AVMutableVideoCompositionLayerInstruction *instruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:sourceVideo];
[instruction setTransform:preferredTransform atTime:kCMTimeZero];
AVMutableVideoCompositionInstruction *videoTrackInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
videoTrackInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, avAsset.duration);
videoTrackInstruction.layerInstructions = [NSArray arrayWithObject:instruction];
[videoComposition setInstructions:[NSArray arrayWithObject:videoTrackInstruction]];
exporter.videoComposition = videoComposition;
}
AVMutableCompositionTrack *compositionAudioTrack = [composition addMutableTrackWithMediaType:AVMediaTypeAudio
preferredTrackID:kCMPersistentTrackID_Invalid];
[compositionAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, avAsset.duration)
ofTrack:sourceAudio
atTime:kCMTimeZero
error:nil];
exporter.outputURL = tempPathUrl;
exporter.outputFileType = AVFileTypeQuickTimeMovie;
[exporter exportAsynchronouslyWithCompletionHandler:^{ }];
What could be wrong with the composition? I've been through the documentation and cannot see anything wrong with it so far.

Answer 1

It might have to do with your frame duration. You're using
CMTimeMake(1, naturalTimeScale)
You should check the naturalTimeScale as it does not always equal the fps. See the AVFoundation Programming Guide 'Representations of Time' for more info.

Answer 2

I think it's because the order of width and height arguments is incorrect.
You have:
videoComposition.renderSize = CGSizeMake([avAsset naturalSize].height, [avAsset naturalSize].width);
Shouldn't it be
videoComposition.renderSize = CGSizeMake([avAsset naturalSize].width, [avAsset naturalSize].height);
instead?

Answer 3

The error is from how you set instruction.
... videoCompositionLayerInstructionWithAssetTrack:sourceVideo]
sourceVideo is not a member of the composition. In your case this should be compositionVideoTrack.