视频中的图像/文本叠加 swift

我正在使用 swift 在视频中使用图像叠加来实现水印效果。我正在使用AVFoundation为此，但不知何故我没有成功。

以下是我的覆盖图像/文本的代码

    let path = NSBundle.mainBundle().pathForResource("sample_movie", ofType:"mp4")
    let fileURL = NSURL(fileURLWithPath: path!)

    let composition = AVMutableComposition()
    var vidAsset = AVURLAsset(URL: fileURL, options: nil)

    // get video track
    let vtrack =  vidAsset.tracksWithMediaType(AVMediaTypeVideo)
    let videoTrack:AVAssetTrack = vtrack[0] as! AVAssetTrack
    let vid_duration = videoTrack.timeRange.duration
    let vid_timerange = CMTimeRangeMake(kCMTimeZero, vidAsset.duration)

    var error: NSError?
    let compositionvideoTrack:AVMutableCompositionTrack = composition.addMutableTrackWithMediaType(AVMediaTypeVideo, preferredTrackID: CMPersistentTrackID())
    compositionvideoTrack.insertTimeRange(vid_timerange, ofTrack: videoTrack, atTime: kCMTimeZero, error: &error)

    compositionvideoTrack.preferredTransform = videoTrack.preferredTransform

    // Watermark Effect
    let size = videoTrack.naturalSize

    let imglogo = UIImage(named: "image.png")
    let imglayer = CALayer()
    imglayer.contents = imglogo?.CGImage
    imglayer.frame = CGRectMake(5, 5, 100, 100)
    imglayer.opacity = 0.6

    // create text Layer
    let titleLayer = CATextLayer()
    titleLayer.backgroundColor = UIColor.whiteColor().CGColor
    titleLayer.string = "Dummy text"
    titleLayer.font = UIFont(name: "Helvetica", size: 28)
    titleLayer.shadowOpacity = 0.5
    titleLayer.alignmentMode = kCAAlignmentCenter
    titleLayer.frame = CGRectMake(0, 50, size.width, size.height / 6)

    let videolayer = CALayer()
    videolayer.frame = CGRectMake(0, 0, size.width, size.height)

    let parentlayer = CALayer()
    parentlayer.frame = CGRectMake(0, 0, size.width, size.height)
    parentlayer.addSublayer(videolayer)
    parentlayer.addSublayer(imglayer)
    parentlayer.addSublayer(titleLayer)

    let layercomposition = AVMutableVideoComposition()
    layercomposition.frameDuration = CMTimeMake(1, 30)
    layercomposition.renderSize = size
    layercomposition.animationTool = AVVideoCompositionCoreAnimationTool(postProcessingAsVideoLayer: videolayer, inLayer: parentlayer)

    // instruction for watermark
    let instruction = AVMutableVideoCompositionInstruction()
    instruction.timeRange = CMTimeRangeMake(kCMTimeZero, composition.duration)
    let videotrack = composition.tracksWithMediaType(AVMediaTypeVideo)[0] as! AVAssetTrack
    let layerinstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: videotrack)
    instruction.layerInstructions = NSArray(object: layerinstruction) as [AnyObject]
    layercomposition.instructions = NSArray(object: instruction) as [AnyObject]

    //  create new file to receive data
    let dirPaths = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)
    let docsDir: AnyObject = dirPaths[0]
    let movieFilePath = docsDir.stringByAppendingPathComponent("result.mov")
    let movieDestinationUrl = NSURL(fileURLWithPath: movieFilePath)       

    // use AVAssetExportSession to export video
    let assetExport = AVAssetExportSession(asset: composition, presetName:AVAssetExportPresetHighestQuality)
    assetExport.outputFileType = AVFileTypeQuickTimeMovie 
    assetExport.outputURL = movieDestinationUrl
    assetExport.exportAsynchronouslyWithCompletionHandler({
        switch assetExport.status{
        case  AVAssetExportSessionStatus.Failed:
            println("failed \(assetExport.error)")
        case AVAssetExportSessionStatus.Cancelled:
            println("cancelled \(assetExport.error)")
        default:
            println("Movie complete")


            // play video
            NSOperationQueue.mainQueue().addOperationWithBlock({ () -> Void in
                self.playVideo(movieDestinationUrl!)
            })
        }
    })    

通过这段代码，我没有实现覆盖......我不知道我做错了什么......

问题：

• 这段代码有什么遗漏吗？或者这段代码有什么问题吗？
• 此代码是否仅适用于录制的视频或所有视频（包括图库中的视频）？

@El Captain 提供的代码可以工作。只是缺少了：

    assetExport.videoComposition = layercomposition

您可以在实例化后立即添加它AVAssetExportSession

NOTE:最初提供的代码只会导出视频轨道，但不会导出音频轨道。如果您需要音轨，您可以在配置后添加类似的内容作曲视频轨道:

let compositionAudioTrack:AVMutableCompositionTrack = composition.addMutableTrackWithMediaType(AVMediaTypeAudio, preferredTrackID: CMPersistentTrackID())
    for audioTrack in audioTracks {
        try! compositionAudioTrack.insertTimeRange(audioTrack.timeRange, ofTrack: audioTrack, atTime: kCMTimeZero)
    }