Exporting a mirrored video with AVMutableComposition causes resizing issues - ios

Question

Everything works as expected if I turn off the mirroring on the front camera. However, if I turn it on, my final exported video has crucial resizing problems:
This is how I currently manage the mirroring for my videos:
if currentDevice == frontCamera {
if let connection = output.connections.first {
if connection.isVideoMirroringSupported {
connection.automaticallyAdjustsVideoMirroring = false
connection.isVideoMirrored = true //if true, this bug occurs.
}
}
}else {
//disabling photo mirroring on backCamera
if let connection = output.connections.first {
if connection.isVideoMirroringSupported {
connection.automaticallyAdjustsVideoMirroring = false
connection.isVideoMirrored = false
}
}
}
And this is how I export the video:
/// Create AVMutableComposition object. This object will hold the AVMutableCompositionTrack instances.
let mainMutableComposition = AVMutableComposition()
/// Creating an empty video track
let videoTrack = mainMutableComposition.addMutableTrack(withMediaType: AVMediaType.video, preferredTrackID: kCMPersistentTrackID_Invalid)
let videoAssetTrack = videoAsset.tracks(withMediaType: AVMediaType.video)[0]
do {
//Adding the video track
try videoTrack?.insertTimeRange(CMTimeRange(start: kCMTimeZero, duration: videoAsset.duration), of: videoAsset.tracks(withMediaType: AVMediaType.video).first!, at: kCMTimeZero)
} catch {
completion(false, nil)
}
/// Adding audio if user wants to.
if withAudio {
do {
//Adding the video track
let audio = videoAsset.tracks(withMediaType: AVMediaType.audio).first
if audio != nil {
let audioTrack = mainMutableComposition.addMutableTrack(withMediaType: AVMediaType.audio, preferredTrackID: kCMPersistentTrackID_Invalid)
try audioTrack?.insertTimeRange(CMTimeRange(start: kCMTimeZero, duration: videoAsset.duration), of: audio!, at: kCMTimeZero)
}
} catch {
completion(false, nil)
}
}
// * MARK - Composition is ready ----------
// Create AVMutableVideoCompositionInstruction
let compositionInstructions = AVMutableVideoCompositionInstruction()
compositionInstructions.timeRange = CMTimeRange(start: kCMTimeZero, duration: videoAsset.duration)
// Create an AvmutableVideoCompositionLayerInstruction
let videoLayerInstruction = AVMutableVideoCompositionLayerInstruction.init(assetTrack: videoTrack!)
videoLayerInstruction.setTransform(videoAssetTrack.preferredTransform, at: kCMTimeZero)
compositionInstructions.layerInstructions = [videoLayerInstruction]
//Add instructions
let videoComposition = AVMutableVideoComposition()
let naturalSize : CGSize = videoAssetTrack.naturalSize
///Rendering image into video
let renderWidth = naturalSize.width
let renderHeight = naturalSize.height
//Assigning instructions and rendering size
videoComposition.renderSize = CGSize(width: renderWidth, height: renderHeight)
videoComposition.instructions = [compositionInstructions]
videoComposition.frameDuration = CMTime(value: 1, timescale: Int32((videoTrack?.nominalFrameRate)!))
//Applying image to instruction
self.applyVideoImage(to: videoComposition, withSize: naturalSize, image: image)
// Getting the output path
let documentsURL = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first
let outputPath = documentsURL?.appendingPathComponent("lastEditedVideo.mp4")
if FileManager.default.fileExists(atPath: (outputPath?.path)!) {
do {
try FileManager.default.removeItem(atPath: (outputPath?.path)!)
}
catch {
completion(false, nil)
}
}
// Create exporter
let exporter = NextLevelSessionExporter(withAsset: mainMutableComposition)
exporter.outputURL = outputPath
exporter.outputFileType = AVFileType.mp4
exporter.videoComposition = videoComposition
let compressionDict: [String: Any] = [
AVVideoAverageBitRateKey: NSNumber(integerLiteral: 2300000),
AVVideoProfileLevelKey: AVVideoProfileLevelH264BaselineAutoLevel as String
]
exporter.videoOutputConfiguration = [
AVVideoCodecKey: AVVideoCodecType.h264,
AVVideoWidthKey: NSNumber(integerLiteral: Int(naturalSize.width)),
AVVideoHeightKey: NSNumber(integerLiteral: Int(naturalSize.height)),
AVVideoCompressionPropertiesKey: compressionDict
]
exporter.audioOutputConfiguration = [
AVFormatIDKey: kAudioFormatMPEG4AAC,
AVEncoderBitRateKey: NSNumber(integerLiteral: 128000),
AVNumberOfChannelsKey: NSNumber(integerLiteral: 2),
AVSampleRateKey: NSNumber(value: Float(44100))
]
completion(true, exporter)
}
I'm using the NextLevelSessionExporter to export the video. It doesn't matter if I use the default exporter or not, the resizing problems still occur.

Answer 1

There is an active bug that prevents you from exporting mirrored videos correctly. You need a few workarounds:
Turn off the mirroring on the movieOutputFile
Manually flip the video horizontally when needed:
if needsMirroring == true {
var transform:CGAffineTransform = CGAffineTransform(scaleX: -1.0, y: 1.0)
transform = transform.translatedBy(x: -naturalSize.width, y: 0.0)
transform = transform.rotated(by: CGFloat(Double.pi/2))
transform = transform.translatedBy(x: 0.0, y: -naturalSize.width)
videoTransform = transform
}
It took me days to figure this out, hope it helps.