AVMutableVideoCompositionLayerInstruction -Misalignment when Merging Videos

I got it working for both portrait and landscape.

I tested this answer with videos recorded in portrait, landscape left/right, upside down, front camera, and the back camera. I haven't had any issues. I'm far from a CGAffineTransform expert, so if anyone has a better answer please post it.

Ray Wenderlich's merging code works, but it doesn't work for videos with different orientations. I used this answer to check the properties of the preferredTransform for the orientation check.

One thing to point out is the comments from DonMag told me about the benefit of using 720x1280. The code below will merge all of the videos together with a renderSize of 720x1280 which will keep them the same size.

code:

// class property
let renderSize = CGSize(width: 720, height: 1280) // for higher quality use CGSize(width: 1080, height: 1920)

func mergVideos() {

    let mixComposition = AVMutableComposition()
            
    let videoCompositionTrack = mixComposition.addMutableTrack(withMediaType: .video, preferredTrackID: Int32(kCMPersistentTrackID_Invalid))
    let audioCompositionTrack = mixComposition.addMutableTrack(withMediaType: .audio, preferredTrackID: Int32(kCMPersistentTrackID_Invalid))
    
    var count = 0
    var insertTime = CMTime.zero
    var instructions = [AVMutableVideoCompositionInstruction]()
    
    for videoAsset in arrOfAssets {

        guard let firstTrack = videoAsset.tracks.first, let _ = videoAsset.tracks(withMediaType: .video).first else { continue }

        do {
    
            try videoCompositionTrack?.insertTimeRange(CMTimeRangeMake(start: .zero, duration: videoAsset.duration), of: videoAsset.tracks(withMediaType: .video)[0], at: insertTime)

            if let audioTrack = videoAsset.tracks(withMediaType: .audio).first {
                try audioCompositionTrack?.insertTimeRange(CMTimeRangeMake(start: .zero, duration: videoAsset.duration), of: audioTrack, at: insertTime)
            }

            let layerInstruction = videoCompositionInstruction(firstTrack, asset: videoAsset, count: count)
    
            let videoCompositionInstruction = AVMutableVideoCompositionInstruction()
            videoCompositionInstruction.timeRange = CMTimeRangeMake(start: insertTime, duration: videoAsset.duration)
            videoCompositionInstruction.layerInstructions = [layerInstruction]

            instructions.append(videoCompositionInstruction)
    
            insertTime = CMTimeAdd(insertTime, videoAsset.duration)

            count += 1

        } catch { }
    }
    
    let videoComposition = AVMutableVideoComposition()
    videoComposition.instructions = instructions
    videoComposition.frameDuration = CMTimeMake(value: 1, timescale: 30)
    videoComposition.renderSize = self.renderSize // <--- **** IMPORTANT ****

    // ...
    exporter.videoComposition = videoComposition
}

Most important part of this answer that replaces the RW code:

func videoCompositionInstruction(_ firstTrack: AVAssetTrack, asset: AVAsset, count: Int) -> AVMutableVideoCompositionLayerInstruction {

    let instruction = AVMutableVideoCompositionLayerInstruction(assetTrack: firstTrack)

    let assetTrack = asset.tracks(withMediaType: .video)[0]            
    let t = assetTrack.fixedPreferredTransform // new transform fix 
    let assetInfo = orientationFromTransform(t)

    if assetInfo.isPortrait {

        let scaleToFitRatio = self.renderSize.width / assetTrack.naturalSize.height
        let scaleFactor = CGAffineTransform(scaleX: scaleToFitRatio, y: scaleToFitRatio)
        var finalTransform = assetTrack.fixedPreferredTransform.concatenating(scaleFactor)

       // From the OP that I used for the portrait part. I haven't tested this, his words: "if video not taking entire screen and leaving some parts black - don't know when actually needed so test"
        if assetInfo.orientation == .rightMirrored || assetInfo.orientation == .leftMirrored {
            finalTransform = finalTransform.translatedBy(x: -transform.ty, y: 0)
        }
        instruction.setTransform(finalTransform, at: CMTime.zero)

    } else {

        let renderRect = CGRect(x: 0, y: 0, width: self.renderSize.width, height: self.renderSize.height)
        let videoRect = CGRect(origin: .zero, size: assetTrack.naturalSize).applying(assetTrack.fixedPreferredTransform)

        let scale = renderRect.width / videoRect.width
        let transform = CGAffineTransform(scaleX: renderRect.width / videoRect.width, y: (videoRect.height * scale) / assetTrack.naturalSize.height)
        let translate = CGAffineTransform(translationX: .zero, y: ((self.renderSize.height - (videoRect.height * scale))) / 2)

        instruction.setTransform(assetTrack.fixedPreferredTransform.concatenating(transform).concatenating(translate), at: .zero)
    }

    if count == 0 {
        instruction.setOpacity(0.0, at: asset.duration)
    }
    
    return instruction
}

New orientation check:

func orientationFromTransform(_ transform: CGAffineTransform) -> (orientation: UIImage.Orientation, isPortrait: Bool) {
    var assetOrientation = UIImage.Orientation.up
    var isPortrait = false
    
    if transform.a == 0 && transform.b == 1.0 && transform.c == -1.0 && transform.d == 0 {
        assetOrientation = .right
        isPortrait = true
    } else if transform.a == 0 && transform.b == 1.0 && transform.c == 1.0 && transform.d == 0 {
        assetOrientation = .rightMirrored
        isPortrait = true
    } else if transform.a == 0 && transform.b == -1.0 && transform.c == 1.0 && transform.d == 0 {
        assetOrientation = .left
        isPortrait = true
    } else if transform.a == 0 && transform.b == -1.0 && transform.c == -1.0 && transform.d == 0 {
        assetOrientation = .leftMirrored
        isPortrait = true
    } else if transform.a == 1.0 && transform.b == 0 && transform.c == 0 && transform.d == 1.0 {
        assetOrientation = .up
    } else if transform.a == -1.0 && transform.b == 0 && transform.c == 0 && transform.d == -1.0 {
        assetOrientation = .down
    }
}

preferredTransform fix:

extension AVAssetTrack {
    
    var fixedPreferredTransform: CGAffineTransform {
        var t = preferredTransform
        switch(t.a, t.b, t.c, t.d) {
        case (1, 0, 0, 1):
            t.tx = 0
            t.ty = 0
        case (1, 0, 0, -1):
            t.tx = 0
            t.ty = naturalSize.height
        case (-1, 0, 0, 1):
            t.tx = naturalSize.width
            t.ty = 0
        case (-1, 0, 0, -1):
            t.tx = naturalSize.width
            t.ty = naturalSize.height
        case (0, -1, 1, 0):
            t.tx = 0
            t.ty = naturalSize.width
        case (0, 1, -1, 0):
            t.tx = naturalSize.height
            t.ty = 0
        case (0, 1, 1, 0):
            t.tx = 0
            t.ty = 0
        case (0, -1, -1, 0):
            t.tx = naturalSize.height
            t.ty = naturalSize.width
        default:
            break
        }
        return t
    }
}

Solution without a large switch/case: https://developer.apple.com/forums/thread/705269?answerId=741570022#741570022