Я использовал приведенный ниже код, чтобы добавить наложение изображения поверх видео, а затем экспортировать новое сгенерированное видео в каталог документов. Но как ни странно, видео поворачивается на 90 градусов.

- (void)buildTransitionComposition:(AVMutableComposition *)composition andVideoComposition:(AVMutableVideoComposition *)videoComposition
{
    CMTime nextClipStartTime = kCMTimeZero;
    NSInteger i;

    // Make transitionDuration no greater than half the shortest clip duration.
    CMTime transitionDuration = self.transitionDuration;
    for (i = 0; i < [_clips count]; i++ ) {
        NSValue *clipTimeRange = [_clipTimeRanges objectAtIndex:i];
        if (clipTimeRange) {
            CMTime halfClipDuration = [clipTimeRange CMTimeRangeValue].duration;
            halfClipDuration.timescale *= 2; // You can halve a rational by doubling its denominator.
            transitionDuration = CMTimeMinimum(transitionDuration, halfClipDuration);
        }
    }

    // Add two video tracks and two audio tracks.
    AVMutableCompositionTrack *compositionVideoTracks[2];
    AVMutableCompositionTrack *compositionAudioTracks[2];
    compositionVideoTracks[0] = [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
    compositionVideoTracks[1] = [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
    compositionAudioTracks[0] = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
    compositionAudioTracks[1] = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];

    CMTimeRange *passThroughTimeRanges = alloca(sizeof(CMTimeRange) * [_clips count]);
    CMTimeRange *transitionTimeRanges = alloca(sizeof(CMTimeRange) * [_clips count]);

    // Place clips into alternating video & audio tracks in composition, overlapped by transitionDuration.
    for (i = 0; i < [_clips count]; i++ ) {
        NSInteger alternatingIndex = i % 2; // alternating targets: 0, 1, 0, 1, ...
        AVURLAsset *asset = [_clips objectAtIndex:i];
        NSValue *clipTimeRange = [_clipTimeRanges objectAtIndex:i];
        CMTimeRange timeRangeInAsset;
        if (clipTimeRange)
            timeRangeInAsset = [clipTimeRange CMTimeRangeValue];
        else
            timeRangeInAsset = CMTimeRangeMake(kCMTimeZero, [asset duration]);

        AVAssetTrack *clipVideoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
        [compositionVideoTracks[alternatingIndex] insertTimeRange:timeRangeInAsset ofTrack:clipVideoTrack atTime:nextClipStartTime error:nil];

        /*
        CGAffineTransform t = clipVideoTrack.preferredTransform;
        NSLog(@"Transform1 : %@",t);
        */
        AVAssetTrack *clipAudioTrack = [[asset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
        [compositionAudioTracks[alternatingIndex] insertTimeRange:timeRangeInAsset ofTrack:clipAudioTrack atTime:nextClipStartTime error:nil];

        // Remember the time range in which this clip should pass through.
        // Every clip after the first begins with a transition.
        // Every clip before the last ends with a transition.
        // Exclude those transitions from the pass through time ranges.
        passThroughTimeRanges[i] = CMTimeRangeMake(nextClipStartTime, timeRangeInAsset.duration);
        if (i > 0) {
            passThroughTimeRanges[i].start = CMTimeAdd(passThroughTimeRanges[i].start, transitionDuration);
            passThroughTimeRanges[i].duration = CMTimeSubtract(passThroughTimeRanges[i].duration, transitionDuration);
        }
        if (i+1 < [_clips count]) {
            passThroughTimeRanges[i].duration = CMTimeSubtract(passThroughTimeRanges[i].duration, transitionDuration);
        }

        // The end of this clip will overlap the start of the next by transitionDuration.
        // (Note: this arithmetic falls apart if timeRangeInAsset.duration < 2 * transitionDuration.)
        nextClipStartTime = CMTimeAdd(nextClipStartTime, timeRangeInAsset.duration);
        nextClipStartTime = CMTimeSubtract(nextClipStartTime, transitionDuration);

        // Remember the time range for the transition to the next item.
        transitionTimeRanges[i] = CMTimeRangeMake(nextClipStartTime, transitionDuration);
    }

    // Set up the video composition if we are to perform crossfade or push transitions between clips.
    NSMutableArray *instructions = [NSMutableArray array];

    // Cycle between "pass through A", "transition from A to B", "pass through B", "transition from B to A".
    for (i = 0; i < [_clips count]; i++ ) {
        NSInteger alternatingIndex = i % 2; // alternating targets

        // Pass through clip i.
        AVMutableVideoCompositionInstruction *passThroughInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
        passThroughInstruction.timeRange = passThroughTimeRanges[i];
        AVMutableVideoCompositionLayerInstruction *passThroughLayer = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:compositionVideoTracks[alternatingIndex]];
        /*
        CGAffineTransform rotationTransform = CGAffineTransformMakeRotation(M_PI_2);
        CGAffineTransform rotateTranslate = CGAffineTransformTranslate(rotationTransform,320,0);
        [passThroughLayer setTransform:rotateTranslate atTime:kCMTimeZero];
         */
        passThroughInstruction.layerInstructions = [NSArray arrayWithObject:passThroughLayer];
        [instructions addObject:passThroughInstruction];

        if (i+1 < [_clips count]) {
            // Add transition from clip i to clip i+1.

            AVMutableVideoCompositionInstruction *transitionInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
            transitionInstruction.timeRange = transitionTimeRanges[i];
            AVMutableVideoCompositionLayerInstruction *fromLayer = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:compositionVideoTracks[alternatingIndex]];
            AVMutableVideoCompositionLayerInstruction *toLayer = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:compositionVideoTracks[1-alternatingIndex]];

            if (self.transitionType == SimpleEditorTransitionTypeCrossFade) {
                // Fade out the fromLayer by setting a ramp from 1.0 to 0.0.
                [fromLayer setOpacityRampFromStartOpacity:1.0 toEndOpacity:0.0 timeRange:transitionTimeRanges[i]];
            }
            else if (self.transitionType == SimpleEditorTransitionTypePush) {
                // Set a transform ramp on fromLayer from identity to all the way left of the screen.
                [fromLayer setTransformRampFromStartTransform:CGAffineTransformIdentity toEndTransform:CGAffineTransformMakeTranslation(-composition.naturalSize.width, 0.0) timeRange:transitionTimeRanges[i]];
                // Set a transform ramp on toLayer from all the way right of the screen to identity.
                [toLayer setTransformRampFromStartTransform:CGAffineTransformMakeTranslation(+composition.naturalSize.width, 0.0) toEndTransform:CGAffineTransformIdentity timeRange:transitionTimeRanges[i]];
            }

            transitionInstruction.layerInstructions = [NSArray arrayWithObjects:fromLayer, toLayer, nil];
            [instructions addObject:transitionInstruction];
        }
    }

    videoComposition.instructions = instructions;


}

Пожалуйста, помогите, так как я не могу экспортировать портретное видео в правильном режиме. Любая помощь приветствуется. Спасибо.