How to set the Framerate using iOS AVFoundation - ios

Question

The following code successfully converts an array of images into a video. However, the fps parameter has no effect of the framerate so the framerate is alway 1.
The AVVideoMaxKeyFrameIntervalKey and AVVideoMaxKeyFrameIntervalDurationKey settings have no effect on the output.
How do I set a framerate of 30 fps?
func createVideo(images: [UIImage], durationPerImage:Double = 1, fps:Double = 30){
let filePath = URL(fileURLWithPath: NSTemporaryDirectory() + "video.mp4")
let size = images[0].size
let fileManager = FileManager.default
if fileManager.fileExists(atPath: filePath.path) {
try? fileManager.removeItem(at: filePath)
}
let bitRate = size.width * size.height * fps * 4;
let videoWriter = try? AVAssetWriter(outputURL: filePath, fileType: AVFileType.mp4)
let videoSettings: [String : Any] = [
AVVideoCodecKey: AVVideoCodecType.h264,
AVVideoWidthKey: size.width,
AVVideoHeightKey: size.height,
AVVideoCompressionPropertiesKey: [
AVVideoAverageBitRateKey: bitRate,
AVVideoMaxKeyFrameIntervalKey: fps,
AVVideoMaxKeyFrameIntervalDurationKey: (1/fps)
],
]
let bufferAttributes: [String : Any] = [
kCVPixelBufferPixelFormatTypeKey as String: Int(kCVPixelFormatType_32ARGB),
kCVPixelBufferWidthKey as String: size.width,
kCVPixelBufferHeightKey as String: size.height
]
let videoWriterInput = AVAssetWriterInput(mediaType: AVMediaType.video, outputSettings: videoSettings)
videoWriterInput.expectsMediaDataInRealTime = true
videoWriter?.add(videoWriterInput)
let pixelBufferAdaptor = AVAssetWriterInputPixelBufferAdaptor(assetWriterInput: videoWriterInput, sourcePixelBufferAttributes: bufferAttributes)
videoWriter?.startWriting()
videoWriter?.startSession(atSourceTime: CMTime.zero)
var time = CMTime.zero
for image in images {
if videoWriter?.status == .unknown {
videoWriter?.startWriting()
}
if videoWriter?.status == .failed {
print("Error: \(String(describing: videoWriter?.error))")
return
}
var pixelBuffer: CVPixelBuffer?
let status = CVPixelBufferPoolCreatePixelBuffer(kCFAllocatorDefault, pixelBufferAdaptor.pixelBufferPool!, &pixelBuffer)
if status != kCVReturnSuccess { return }
CVPixelBufferLockBaseAddress(pixelBuffer!, [])
let data = CVPixelBufferGetBaseAddress(pixelBuffer!)
let rgbColorSpace = CGColorSpaceCreateDeviceRGB()
let context = CGContext(data: data, width: Int(size.width), height: Int(size.height), bitsPerComponent: 8, bytesPerRow: CVPixelBufferGetBytesPerRow(pixelBuffer!), space: rgbColorSpace, bitmapInfo: CGImageAlphaInfo.noneSkipFirst.rawValue)
context?.translateBy(x: 0, y: size.height)
context?.scaleBy(x: 1.0, y: -1.0)
UIGraphicsPushContext(context!)
image.draw(in: CGRect(x: 0, y: 0, width:size.width, height: size.height))
UIGraphicsPopContext()
CVPixelBufferUnlockBaseAddress(pixelBuffer!, [])
if !pixelBufferAdaptor.append(pixelBuffer!, withPresentationTime: time) { return }
time = CMTimeAdd(time, CMTimeMakeWithSeconds(durationPerImage, preferredTimescale: Int32(fps)))
}
videoWriterInput.markAsFinished()
videoWriter?.finishWriting {
let tracks = AVAsset(url: filePath).tracks(withMediaType: .video)
print("FPS", tracks.first!.nominalFrameRate, tracks.first!.naturalTimeScale)
}
}