Watermark (overlap) a video with an image on iOS - ios

Question

I try in vain to watermark an existing video with the AVFoundation library. I followed the instructions and tried to rewrite the code in Swift from the third answer in this question: iPhone Watermark on recorded Video. – but this doesn't work for me.
Every time when run my code I only see a black video rendered with the length of my source video which needs to be watermarked. My goal is to fade in the watermark after 5 seconds.
Here is my code:
let composition = AVMutableComposition()
let vidAsset = AVURLAsset(URL: NSURL(fileURLWithPath: moviePath), options: nil)
// GET THE VIDEO TRACK
let vtrack = vidAsset.tracksWithMediaType(AVMediaTypeVideo)
let videoTrack:AVAssetTrack = vtrack[0]
let vid_timerange = CMTimeRangeMake(kCMTimeZero, vidAsset.duration)
do {
let compositionvideoTrack:AVMutableCompositionTrack = composition.addMutableTrackWithMediaType(AVMediaTypeVideo, preferredTrackID: CMPersistentTrackID())
try compositionvideoTrack.insertTimeRange(vid_timerange, ofTrack: videoTrack, atTime: kCMTimeZero)
compositionvideoTrack.preferredTransform = videoTrack.preferredTransform
} catch {
print(error)
}
let animationImage: UIImage = self.artworkImage
let artWorkOverlayLayer: CALayer = CALayer()
artWorkOverlayLayer.contents = (animationImage.CGImage as! AnyObject)
artWorkOverlayLayer.frame = CGRectMake(0, 0, 512, 512)
artWorkOverlayLayer.opacity = 0
artWorkOverlayLayer.masksToBounds = true
let animation: CABasicAnimation = CABasicAnimation(keyPath: "opacity")
animation.duration = 10
animation.repeatCount = 0
animation.autoreverses = false
animation.fromValue = Int(0.0)
animation.toValue = Int(1.0)
animation.beginTime = 5.0
artWorkOverlayLayer.addAnimation(animation, forKey: "animateOpacity")
let videolayer = CALayer()
videolayer.frame = CGRectMake(0, 0, videoTrack.naturalSize.width, videoTrack.naturalSize.height)
let parentlayer = CALayer()
parentlayer.frame = CGRectMake(0, 0, videoTrack.naturalSize.width, videoTrack.naturalSize.height)
parentlayer.addSublayer(artWorkOverlayLayer)
let layercomposition = AVMutableVideoComposition()
layercomposition.frameDuration = CMTimeMake(1, 30)
layercomposition.renderSize = videoTrack.naturalSize
layercomposition.animationTool = AVVideoCompositionCoreAnimationTool(postProcessingAsVideoLayer: videolayer, inLayer: parentlayer)
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! [AVVideoCompositionLayerInstruction]
layercomposition.instructions = NSArray(object: instruction) as! [AVVideoCompositionInstructionProtocol]
// EXPORT
let filePath: NSURL = NSURL.fileURLWithPath(NSTemporaryDirectory().stringByAppendingString("output-tmp.mp4"))
let assetExportSession: AVAssetExportSession! = AVAssetExportSession(asset: composition, presetName: AVAssetExportPresetHighestQuality)
assetExportSession.outputFileType = AVFileTypeMPEG4
assetExportSession.outputURL = filePath
assetExportSession.videoComposition = layercomposition
assetExportSession.exportAsynchronouslyWithCompletionHandler({() -> Void in
print(filePath)
})