>通过AVAssetTrack设置自然大小.
>在AVMutableVideoComposition renderFrame中将视频约束为纵向大小.
>将新视频边界锁定到录制的视频宽度和高度.

下面的代码与我需要帮助的问题不同.我正在尝试添加的图像覆盖了整个纵向视图,并且边缘周围都有边框.该应用程序也只允许肖像.

func processVideoWithWatermark(video: AVURLAsset,watermark: UIImage,completion: @escaping (Bool) -> Void) {

    let composition = AVMutableComposition()
    let asset = AVURLAsset(url: video.url,options: nil)

    let track =  asset.tracks(withMediaType: AVMediaTypeVideo)
    let videoTrack:AVAssetTrack = track[0] as AVAssetTrack
    let timerange = CMTimeRangeMake(kCMTimeZero,asset.duration)

    let compositionVideoTrack:AVMutableCompositionTrack = composition.addMutableTrack(withMediaType: AVMediaTypeVideo,preferredTrackID: CMPersistentTrackID())

    do {
        try compositionVideoTrack.insertTimeRange(timerange,of: videoTrack,at: kCMTimeZero)
        compositionVideoTrack.preferredTransform = videoTrack.preferredTransform
    } catch {
        print(error)
    }

//      let compositionAudioTrack:AVMutableCompositionTrack = composition.addMutableTrack(withMediaType: AVMediaTypeAudio,preferredTrackID: CMPersistentTrackID())
//      
//      for audioTrack in asset.tracks(withMediaType: AVMediaTypeAudio) {
//          do {
//              try compositionAudioTrack.insertTimeRange(audioTrack.timeRange,of: audioTrack,at: kCMTimeZero)
//          } catch {
//              print(error)
//          }
//          
//      }
//      
    let size = videoTrack.naturalSize

    let watermark = watermark.cgImage
    let watermarklayer = CALayer()
    watermarklayer.contents = watermark
    watermarklayer.frame = CGRect(x: 0,y: 0,width: screenWidth,height: screenHeight)
    watermarklayer.opacity = 1

    let videolayer = CALayer()
    videolayer.frame = CGRect(x: 0,height: screenHeight)

    let parentlayer = CALayer()
    parentlayer.frame = CGRect(x: 0,width: size.width,height: size.height)
    parentlayer.addSublayer(videolayer)
    parentlayer.addSublayer(watermarklayer)

    let layercomposition = AVMutableVideoComposition()
    layercomposition.frameDuration = CMTimeMake(1,30)
    layercomposition.renderSize = CGSize(width: screenWidth,height: screenHeight)
    layercomposition.renderScale = 1.0
    layercomposition.animationTool = AVVideoCompositionCoreAnimationTool(postProcessingAsVideoLayer: videolayer,in: parentlayer)

    let instruction = AVMutableVideoCompositionInstruction()
    instruction.timeRange = CMTimeRangeMake(kCMTimeZero,composition.duration)

    let videotrack = composition.tracks(withMediaType: AVMediaTypeVideo)[0] as AVAssetTrack
    let layerinstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: videotrack)

    layerinstruction.setTransform(videoTrack.preferredTransform,at: kCMTimeZero)

    instruction.layerInstructions = [layerinstruction]
    layercomposition.instructions = [instruction]

    let filePath = NSTemporaryDirectory() + self.fileName()
    let movieUrl = URL(fileURLWithPath: filePath)

    guard let assetExport = AVAssetExportSession(asset: composition,presetName:AVAssetExportPresetHighestQuality) else {return}
    assetExport.videoComposition = layercomposition
    assetExport.outputFileType = AVFileTypeMPEG4
    assetExport.outputURL = movieUrl

    assetExport.exportAsynchronously(completionHandler: {

        switch assetExport.status {
        case .completed:
            print("success")
            print(video.url)
            self.saveVideoToUserLibrary(fileURL: movieUrl,completion: { (success,error) in
                if success {
                    completion(true)
                } else {
                    completion(false)

                }
            })

            break
        case .cancelled:
            print("cancelled")
            break
        case .exporting:
            print("exporting")
            break
        case .Failed:
            print(video.url)
            print("Failed: \(assetExport.error!)")
            break
        case .unknown:
            print("unknown")
            break
        case .waiting:
            print("waiting")
            break
        }
    })

}