How to combine video clips with different orientation using AVFoundation

This is what I do. I then use an AVAssetExportSession to create the actual file. but I warn you, the CGAffineTransforms are sometimes applied late, so you'll see a or two of the original before the video transforms. I have no clue why this happens, a different combination of videos will yield the expected result, but sometimes its off.

AVMutableComposition *composition = [AVMutableComposition composition];    AVMutableCompositionTrack *compositionVideoTrack = [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];AVMutableVideoComposition *videoComposition = [AVMutableVideoComposition videoComposition]; videoComposition.frameDuration = CMTimeMake(1,30); videoComposition.renderScale = 1.0;AVMutableVideoCompositionInstruction *instruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];AVMutableVideoCompositionLayerInstruction *layerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:compositionVideoTrack];// Get only paths the user selected NSMutableArray *array = [NSMutableArray array]; for(NSString* string in videoPathArray){if(![string isEqualToString:@""]){    [array addObject:string];} self.videoPathArray = array;float time = 0;for (int i = 0; i<self.videoPathArray.count; i++) {    AVURLAsset *sourceAsset = [AVURLAsset URLAssetWithURL:[NSURL fileURLWithPath:[videoPathArray objectAtIndex:i]] options:[NSDictionary dictionaryWithObject:[NSNumber numberWithBool:YES] forKey:AVURLAssetPreferPreciseDurationAndTimingKey]];    NSError *error = nil;    BOOL ok = NO;    AVAssetTrack *sourceVideoTrack = [[sourceAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];    CGSize temp = CGSizeApplyAffineTransform(sourceVideoTrack.naturalSize, sourceVideoTrack.preferredTransform);    CGSize size = CGSizeMake(fabsf(temp.width), fabsf(temp.height));    CGAffineTransform transform = sourceVideoTrack.preferredTransform;    videoComposition.renderSize = sourceVideoTrack.naturalSize;    if (size.width > size.height) {        [layerInstruction setTransform:transform atTime:CMTimeMakeWithSeconds(time, 30)];    } else {        float s = size.width/size.height;        CGAffineTransform new = CGAffineTransformConcat(transform, CGAffineTransformMakeScale(s,s));        float x = (size.height - size.width*s)/2;        CGAffineTransform newer = CGAffineTransformConcat(new, CGAffineTransformMakeTranslation(x, 0));        [layerInstruction setTransform:newer atTime:CMTimeMakeWithSeconds(time, 30)];    }    ok = [compositionVideoTrack insertTimeRange:sourceVideoTrack.timeRange ofTrack:sourceVideoTrack atTime:[composition duration] error:&error];    if (!ok) {        // Deal with the error.        NSLog(@"something went wrong");    }    NSLog(@"\n source asset duration is %f \n source vid track timerange is %f %f \n composition duration is %f \n composition vid track time range is %f %f",CMTimeGetSeconds([sourceAsset duration]), CMTimeGetSeconds(sourceVideoTrack.timeRange.start),CMTimeGetSeconds(sourceVideoTrack.timeRange.duration),CMTimeGetSeconds([composition duration]), CMTimeGetSeconds(compositionVideoTrack.timeRange.start),CMTimeGetSeconds(compositionVideoTrack.timeRange.duration));    time += CMTimeGetSeconds(sourceVideoTrack.timeRange.duration);}instruction.layerInstructions = [NSArray arrayWithObject:layerInstruction];instruction.timeRange = compositionVideoTrack.timeRange; videoComposition.instructions = [NSArray arrayWithObject:instruction];

This is what I do. I then use an AVAssetExportSession to create the actual file. but I warn you, the CGAffineTransforms are sometimes applied late, so you'll see a or two of the original before the video transforms. I have no clue why this happens, a different combination of videos will yield the expected result, but sometimes its off.

Here is @bogardon's answer in swift 4+

 import ARKitclass ARKitSampleViewController: UIViewController {    var label: UILabel?    var planeFound = false    func plane(from anchor: ARPlaneAnchor?) -> SCNNode? {        let plane = SCNPlane(width: CGFloat(anchor?.extent.x ?? 0.0), height: CGFloat(anchor?.extent.z ?? 0.0))        plane.firstMaterial?.diffuse.contents = UIColor.clear        let planeNode = SCNNode(geometry: plane)        planeNode.position = SCNVector3Make(anchor?.center.x ?? 0.0, 0, anchor?.center.z ?? 0.0)        // SCNPlanes are vertically oriented in their local coordinate space.        // Rotate it to match the horizontal orientation of the ARPlaneAnchor.        planeNode.transform = SCNMatrix4MakeRotation(-.pi * 0.5, 1, 0, 0)        return planeNode    }// MARK: -  ARSCNViewDelegate    func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) {        if planeFound == false {            if (anchor is ARPlaneAnchor) {                DispatchQueue.main.async(execute: {                    self.planeFound = true                    self.label?.text = "DANCEFLOOR FOUND. LET'S BOOGIE"                    let overlay = UIView(frame: self.view.frame)                    overlay.backgroundColor = UIColor.black                    overlay.alpha = 0                    if let label = self.label {                        self.view.insertSubview(overlay, belowSubview: label)                    }                    UIView.animate(withDuration: 1.5, delay: 2, options: .curveEaseIn, animations: {                        self.label?.alpha = 0                        overlay.alpha = 0.5                    }) { finished in                        let planeAnchor = anchor as? ARPlaneAnchor                        // Show the disco ball here                    }                })            }        }    }}