在视频上叠加图像会降低视频分辨率

当我在视频上叠加图像时，视频质量会大大降低.如果我未设置导出会话的视频组成或将导出质量设置为直通，则视频质量会很好(但是我显然没有覆盖).

When I overlay an image on my video, the video quality is greatly reduced. If I don't set the video composition of the export session or set the export quality to passthrough, the video quality is great (but I get no overlays obviously).

我也传递了本地.mov视频网址来添加叠加层. 我正在使用PHPhotoLibrary将视频保存到相机胶卷中. 使用其他一些功能来转换视频并设置其说明.

I'm passing in a local .mov video url to add the overlays too. I'm using PHPhotoLibrary to save the video to the camera roll. Using some other functions to transform the video and set its instructions.

这一切看起来都非常简单，但是却有损视频质量

It all seems pretty straightforward, but something is killing the video quality

func merge3(url: URL) {

    let firstAsset = AVAsset(url: url)

    // 1 - Create AVMutableComposition object. This object will hold your AVMutableCompositionTrack instances.
    let mixComposition = AVMutableComposition()

    // 2 - Create two video tracks
    guard
      let firstTrack = mixComposition.addMutableTrack(withMediaType: AVMediaType.video,
                                                      preferredTrackID: Int32(kCMPersistentTrackID_Invalid))
      else {
        return
    }
    do {
      try firstTrack.insertTimeRange(CMTimeRangeMake(start: CMTime.zero, duration: firstAsset.duration),
                                     of: firstAsset.tracks(withMediaType: AVMediaType.video)[0],
                                     at: CMTime.zero)
    } catch {
      print("Failed to load first track")
      return
    }

    let s = UIScreen.main.bounds

    let imglogo = UIImage(named: "django")?.scaleImageToSize(newSize: CGSize(width: 250, height: 125))
    let imglayer = CALayer()
    imglayer.contents = imglogo?.cgImage
    imglayer.frame = CGRect(x: s.width / 2 - 125, y: s.height / 2 - 67.5
      , width: 250, height: 125)
    imglayer.opacity = 1.0

    let videolayer = CALayer()
    videolayer.frame = CGRect(x: 0, y: 0, width: s.width, height: s.height)

    let parentlayer = CALayer()
    parentlayer.frame = CGRect(x: 0, y: 0, width: s.width, height: s.height)
    parentlayer.addSublayer(videolayer)
    parentlayer.addSublayer(imglayer)

    // 2.1
    let mainInstruction = AVMutableVideoCompositionInstruction()
    mainInstruction.timeRange = CMTimeRangeMake(start: CMTime.zero,
                                                duration: firstAsset.duration)

    let layercomposition = AVMutableVideoComposition()
    layercomposition.frameDuration = CMTimeMake(value: 1, timescale: 30)
    layercomposition.animationTool = AVVideoCompositionCoreAnimationTool(postProcessingAsVideoLayer: videolayer, in: parentlayer)
    layercomposition.renderSize = CGSize(width: UIScreen.main.bounds.width, height: UIScreen.main.bounds.height)

    // instruction for watermark
    let instruction = AVMutableVideoCompositionInstruction()
    instruction.timeRange = CMTimeRangeMake(start: CMTime.zero, duration: firstAsset.duration)
    _ = mixComposition.tracks(withMediaType: AVMediaType.video)[0] as AVAssetTrack
    let layerinstruction = VideoHelper.videoCompositionInstruction1(firstTrack, asset: firstAsset)
    instruction.layerInstructions = [layerinstruction]
    layercomposition.instructions = [instruction]

    // 4 - Get path
    guard let documentDirectory = FileManager.default.urls(for: .documentDirectory,
                                                           in: .userDomainMask).first else {
                                                            return
    }
    let dateFormatter = DateFormatter()
    dateFormatter.dateStyle = .long
    dateFormatter.timeStyle = .short
    let date = dateFormatter.string(from: Date())
    let url = documentDirectory.appendingPathComponent("mergeVideo-\(date).mov")

    // 5 - Create Exporter
    guard let exporter = AVAssetExportSession(asset: mixComposition,
                                              presetName: AVAssetExportPresetHighestQuality) else {
                                                return
    }
    exporter.outputURL = url
    exporter.outputFileType = AVFileType.mov
    exporter.shouldOptimizeForNetworkUse = true
    exporter.videoComposition = layercomposition

    // 6 - Perform the Export
    exporter.exportAsynchronously() {
      DispatchQueue.main.async {
        self.exportDidFinish(exporter)
      }
    }
  }

func exportDidFinish(_ session: AVAssetExportSession) {

    guard
      session.status == AVAssetExportSession.Status.completed,
      let outputURL = session.outputURL
      else {
        return
    }

    let saveVideoToPhotos = {
      PHPhotoLibrary.shared().performChanges({
        PHAssetChangeRequest.creationRequestForAssetFromVideo(atFileURL: outputURL)
      }) { saved, error in
        let success = saved && (error == nil)
        let title = success ? "Success" : "Error"
        let message = success ? "Video saved" : "Failed to save video"

        let alert = UIAlertController(title: title, message: message, preferredStyle: .alert)
        alert.addAction(UIAlertAction(title: "OK", style: UIAlertAction.Style.cancel, handler: nil))
        self.present(alert, animated: true, completion: nil)
      }
    }

    // Ensure permission to access Photo Library
    if PHPhotoLibrary.authorizationStatus() != .authorized {
      PHPhotoLibrary.requestAuthorization { status in
        if status == .authorized {
          saveVideoToPhotos()
        }
      }
    } else {
      saveVideoToPhotos()
    }
  }

static func videoCompositionInstruction1(_ track: AVCompositionTrack, asset: AVAsset)
    -> AVMutableVideoCompositionLayerInstruction {
      let instruction = AVMutableVideoCompositionLayerInstruction(assetTrack: track)
      let assetTrack = asset.tracks(withMediaType: .video)[0]

      let transform = assetTrack.preferredTransform
      let assetInfo = orientationFromTransform(transform)

      var scaleToFitRatio = UIScreen.main.bounds.width / assetTrack.naturalSize.width
      if assetInfo.isPortrait { // not hit
        scaleToFitRatio = UIScreen.main.bounds.width / assetTrack.naturalSize.height
        let scaleFactor = CGAffineTransform(scaleX: scaleToFitRatio, y: scaleToFitRatio)
        instruction.setTransform(assetTrack.preferredTransform.concatenating(scaleFactor), at: CMTime.zero)
      } else { // hit
        let scaleFactor = CGAffineTransform(scaleX: scaleToFitRatio, y: scaleToFitRatio)
        var concat = assetTrack.preferredTransform.concatenating(scaleFactor)
          .concatenating(CGAffineTransform(translationX: 0, y: UIScreen.main.bounds.width / 4))
        if assetInfo.orientation == .down { // not hit
          let fixUpsideDown = CGAffineTransform(rotationAngle: CGFloat(Double.pi))
          let windowBounds = UIScreen.main.bounds
          let yFix = assetTrack.naturalSize.height + windowBounds.height
          let centerFix = CGAffineTransform(translationX: assetTrack.naturalSize.width, y: yFix)
          concat = fixUpsideDown.concatenating(centerFix).concatenating(scaleFactor)
        }
        instruction.setTransform(concat, at: CMTime.zero)
      }

      return instruction
  }

static 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 = .left
        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
      }
      return (assetOrientation, isPortrait)
  }