Adding text subtitles to video track (in Swift) fails with error code -11841

Question

I have been struggling with adding text subtitles to videos for a while. I have added some links that I referred in detail, but they are not helping.

In below code, I a

Answer 1

You failed to insert trackID when creating layer instruction. Try :

let videolayerInstruction : AVMutableVideoCompositionLayerInstruction = AVMutableVideoCompositionLayerInstruction(vtrack)

Answer 2

I updated it to work with Swift 3 + adding the fix suggested by Deepak.

func mergeVideoWithTheme(outputUrl: NSURL, inputVideoUrl videoUrl: NSURL!, onComplete completionHandler: ((Int) -> ())!) -> Void {
   do {
        // 1. mergeComposition adds all the AVAssets

        var mergeComposition : AVMutableComposition = AVMutableComposition()
        var trackVideo : AVMutableCompositionTrack = mergeComposition.addMutableTrack(withMediaType: AVMediaTypeVideo, preferredTrackID: CMPersistentTrackID())
        //var trackAudio : AVMutableCompositionTrack = mergeComposition.addMutableTrackWithMediaType(AVMediaTypeAudio, preferredTrackID: CMPersistentTrackID())

        // 2. Add a bank for theme insertion later

        //trackVideo.insertTimeRange(range, ofTrack: VideoHelper.Static.blankTrack, atTime: kCMTimeZero, error: nil)

        // 3. Source tracks

        let sourceAsset = AVURLAsset(url: videoUrl as URL, options: nil)
        let sourceDuration = CMTimeRangeMake(kCMTimeZero, sourceAsset.duration)
        let vtrack: AVAssetTrack? = sourceAsset.tracks(withMediaType: AVMediaTypeVideo)[0] as? AVAssetTrack
        let atrack: AVAssetTrack? = sourceAsset.tracks(withMediaType: AVMediaTypeAudio)[0] as? AVAssetTrack

        if (vtrack == nil) {
            return
        }

        let renderWidth = vtrack?.naturalSize.width
        let renderHeight = vtrack?.naturalSize.height
        let insertTime = kCMTimeZero
        let endTime = sourceAsset.duration
        let range = sourceDuration

        // append tracks

        try trackVideo.insertTimeRange(sourceDuration, of: vtrack!, at: insertTime)
        //if(atrack > 0){
        //    trackAudio.insertTimeRange(sourceDuration, ofTrack: atracks[0] as AVAssetTrack, atTime: insertTime, error: nil)
        //}

        // 4. Add subtitles (we call it theme)

        var themeVideoComposition : AVMutableVideoComposition = AVMutableVideoComposition(propertiesOf: sourceAsset)

        // 4.1 - Create AVMutableVideoCompositionInstruction

        let mainInstruction: AVMutableVideoCompositionInstruction = AVMutableVideoCompositionInstruction()
        mainInstruction.timeRange = range

        // 4.2 - Create an AVMutableVideoCompositionLayerInstruction for the video track and fix the orientation.
        let videolayerInstruction : AVMutableVideoCompositionLayerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: vtrack!)
        videolayerInstruction.setTransform(trackVideo.preferredTransform, at: insertTime)
        videolayerInstruction.setOpacity(0.0, at: endTime)

        // 4.3 - Add instructions

        mainInstruction.layerInstructions = NSArray(array: [videolayerInstruction]) as! [AVVideoCompositionLayerInstruction]

        themeVideoComposition.renderScale = 1.0
        themeVideoComposition.renderSize =   CGSize(width: renderWidth!, height:  renderHeight!)
        themeVideoComposition.frameDuration = CMTimeMake(1, 30)
        themeVideoComposition.instructions = NSArray(array: [mainInstruction]) as! [AVVideoCompositionInstructionProtocol]

        // add the theme

        // setup variables

        // add text

        let title = String("Testing this subtitle")

        var titleLayer = CATextLayer()
        titleLayer.string = title
        titleLayer.frame =  CGRect(x: 0, y: 0, width: renderWidth!, height: renderHeight!)
        let fontName: CFString = "Helvetica-Bold" as CFString
        let fontSize = CGFloat(36)
        titleLayer.font = CTFontCreateWithName(fontName, fontSize, nil)
        titleLayer.alignmentMode = kCAAlignmentCenter
        titleLayer.foregroundColor = UIColor.white.cgColor

        var backgroundLayer = CALayer()
        backgroundLayer.frame = CGRect(x: 0, y: 0, width: renderWidth!, height: renderHeight!)
        backgroundLayer.masksToBounds = true
        backgroundLayer.addSublayer(titleLayer)

        // 2. set parent layer and video layer

        var parentLayer = CALayer()
        var videoLayer = CALayer()
        parentLayer.frame =  CGRect(x: 0, y: 0, width: renderWidth!, height: renderHeight!)
        videoLayer.frame =  CGRect(x: 0, y: 0, width: renderWidth!, height: renderHeight!)

        parentLayer.addSublayer(backgroundLayer)
        parentLayer.addSublayer(videoLayer)

        // 3. make animation

        themeVideoComposition.animationTool = AVVideoCompositionCoreAnimationTool(postProcessingAsVideoLayer: videoLayer, in: parentLayer)

        // Remove the file if it already exists (merger does not overwrite)

        let fileManager = FileManager.default
        try fileManager.removeItem(at: outputUrl as URL)

        // export to output url

        var exporter = AVAssetExportSession(asset: mergeComposition, presetName: AVAssetExportPresetHighestQuality)
        exporter?.outputURL = outputUrl as URL
        exporter?.videoComposition = themeVideoComposition
        exporter?.outputFileType = AVFileTypeQuickTimeMovie
        exporter?.shouldOptimizeForNetworkUse = true
        exporter?.exportAsynchronously(completionHandler: {
            if (exporter?.error != nil) {
                print("Error")
                print(exporter?.error)
                print("Description")
                print(exporter?.description)
            }
            completionHandler((exporter?.status.rawValue)!)
        })
    }
        catch{

        }

    }

Answer 3

When I tried this the subtitle did not appear. I fixed it by swapping the order of the sublayers.

i.e

parentLayer.addSublayer(videoLayer)

parentLayer.addSubLayer(backgroundLayer)