AVAssetWriterInput - add image with a specific duration - swift

Question

I am using these piece of code to create a timelapse movie with a collection of images.
https://github.com/acj/TimeLapseBuilder-Swift/blob/master/TimeLapseBuilder/TimeLapseBuilder.swift
So now I want to add a specific duration between the images. For example... every image should be showed for 3 seconds but the fps of the video should be still 30 fps.
Can somebody help me with this?
videoWriterInput.requestMediaDataWhenReady(on: media_queue) {
let fps: Int32 = 30
let frameDuration = CMTimeMake(1, fps)
let currentProgress = Progress(totalUnitCount: Int64(self.photoURLs.count))
var frameCount: Int64 = 0
var remainingPhotoURLs = [String](self.photoURLs)
while videoWriterInput.isReadyForMoreMediaData && !remainingPhotoURLs.isEmpty {
let nextPhotoURL = remainingPhotoURLs.remove(at: 0)
let lastFrameTime = CMTimeMake(frameCount, fps)
let presentationTime = frameCount == 0 ? lastFrameTime : CMTimeAdd(lastFrameTime, frameDuration)
if !self.appendPixelBufferForImageAtURL(nextPhotoURL, pixelBufferAdaptor: pixelBufferAdaptor, presentationTime: presentationTime) {
error = NSError(
domain: kErrorDomain,
code: kFailedToAppendPixelBufferError,
userInfo: ["description": "AVAssetWriterInputPixelBufferAdapter failed to append pixel buffer"]
)
break
}
frameCount += 1
currentProgress.completedUnitCount = frameCount
progress(currentProgress)
}
videoWriterInput.markAsFinished()
videoWriter.finishWriting {
if let error = error {
failure(error)
} else {
success(videoOutputURL)
}
self.videoWriter = nil
}
}
} else {
error = NSError(
domain: kErrorDomain,
code: kFailedToStartAssetWriterError,
userInfo: ["description": "AVAssetWriter failed to start writing"]
)
}
Many thanks for your help!