Issue with AVAssetWriter Input: MPEG4 - ios

Question

I am compressing videos with AVAssetWriter. If i set the video compression file to Quicktime movie it works fine, however i would like to export it to MPEG4, but it gives me this error while running:
In order to perform passthrough to file type public.mpeg-4, please provide a format hint in the AVAssetWriterInput initializer'
Here is the specific code where i declare the file type:
let videoInputQueue = DispatchQueue(label: "videoQueue")
let audioInputQueue = DispatchQueue(label: "audioQueue")
let formatter = DateFormatter()
formatter.dateFormat = "yyyy'-'MM'-'dd'T'HH':'mm':'ss'Z'"
let date = Date()
let documentsPath = NSTemporaryDirectory()
let outputPath = "\(documentsPath)/\(formatter.string(from: date)).mp4"
let newOutputUrl = URL(fileURLWithPath: outputPath)
do{
assetWriter = try AVAssetWriter(outputURL: newOutputUrl, fileType: AVFileTypeMPEG4)
}catch{
assetWriter = nil
}
guard let writer = assetWriter else{
fatalError("assetWriter was nil")
}
writer.shouldOptimizeForNetworkUse = true
writer.add(videoInput)
writer.add(audioInput)
Here is the full code for my compression:
func compressFile(urlToCompress: URL, completion:#escaping (URL)->Void){
//video file to make the asset
var audioFinished = false
var videoFinished = false
let asset = AVAsset(url: urlToCompress)
//create asset reader
do{
assetReader = try AVAssetReader(asset: asset)
} catch{
assetReader = nil
}
guard let reader = assetReader else{
fatalError("Could not initalize asset reader probably failed its try catch")
}
let videoTrack = asset.tracks(withMediaType: AVMediaTypeVideo).first!
let audioTrack = asset.tracks(withMediaType: AVMediaTypeAudio).first!
let videoReaderSettings: [String:Any] = [kCVPixelBufferPixelFormatTypeKey as String!:kCVPixelFormatType_32ARGB ]
// ADJUST BIT RATE OF VIDEO HERE
let videoSettings:[String:Any] = [
AVVideoCompressionPropertiesKey: [AVVideoAverageBitRateKey:self.bitrate],
AVVideoCodecKey: AVVideoCodecH264,
AVVideoHeightKey: videoTrack.naturalSize.height,
AVVideoWidthKey: videoTrack.naturalSize.width
]
let assetReaderVideoOutput = AVAssetReaderTrackOutput(track: videoTrack, outputSettings: videoReaderSettings)
let assetReaderAudioOutput = AVAssetReaderTrackOutput(track: audioTrack, outputSettings: nil)
if reader.canAdd(assetReaderVideoOutput){
reader.add(assetReaderVideoOutput)
}else{
fatalError("Couldn't add video output reader")
}
if reader.canAdd(assetReaderAudioOutput){
reader.add(assetReaderAudioOutput)
}else{
fatalError("Couldn't add audio output reader")
}
let audioInput = AVAssetWriterInput(mediaType: AVMediaTypeAudio, outputSettings: nil)
let videoInput = AVAssetWriterInput(mediaType: AVMediaTypeVideo, outputSettings: videoSettings)
videoInput.transform = videoTrack.preferredTransform
//we need to add samples to the video input
let videoInputQueue = DispatchQueue(label: "videoQueue")
let audioInputQueue = DispatchQueue(label: "audioQueue")
let formatter = DateFormatter()
formatter.dateFormat = "yyyy'-'MM'-'dd'T'HH':'mm':'ss'Z'"
let date = Date()
let documentsPath = NSTemporaryDirectory()
let outputPath = "\(documentsPath)/\(formatter.string(from: date)).mp4"
let newOutputUrl = URL(fileURLWithPath: outputPath)
do{
assetWriter = try AVAssetWriter(outputURL: newOutputUrl, fileType: AVFileTypeMPEG4)
}catch{
assetWriter = nil
}
guard let writer = assetWriter else{
fatalError("assetWriter was nil")
}
writer.shouldOptimizeForNetworkUse = true
writer.add(videoInput)
writer.add(audioInput)
writer.startWriting()
reader.startReading()
writer.startSession(atSourceTime: kCMTimeZero)
let closeWriter:()->Void = {
if (audioFinished && videoFinished){
self.assetWriter?.finishWriting(completionHandler: {
self.checkFileSize(sizeUrl: (self.assetWriter?.outputURL)!, message: "The file size of the compressed file is: ")
completion((self.assetWriter?.outputURL)!)
print("Completed 1")
})
self.assetReader?.cancelReading()
}
}
audioInput.requestMediaDataWhenReady(on: audioInputQueue) {
while(audioInput.isReadyForMoreMediaData){
let sample = assetReaderAudioOutput.copyNextSampleBuffer()
if (sample != nil){
audioInput.append(sample!)
}else{
audioInput.markAsFinished()
DispatchQueue.main.async {
audioFinished = true
closeWriter()
print("Completed 2")
}
break;
}
}
}
videoInput.requestMediaDataWhenReady(on: videoInputQueue) {
//request data here
while(videoInput.isReadyForMoreMediaData){
let sample = assetReaderVideoOutput.copyNextSampleBuffer()
if (sample != nil){
videoInput.append(sample!)
}else{
videoInput.markAsFinished()
DispatchQueue.main.async {
videoFinished = true
print("Completed 3")
closeWriter()
}
break;
}
}
}
}

Answer 1

By creating your audio AVAssetWriterInput with nil outputSettings you indicate that you want to pass through your audio data. The assetWriterInputWithMediaType:outputSettings: header file comment says:
AVAssetWriter only supports passing through a restricted set of media types and subtypes. In order to pass through media data to files other than AVFileTypeQuickTimeMovie, a non-NULL format hint must be provided using +assetWriterInputWithMediaType:outputSettings:sourceFormatHint: instead of this method.
A format description is needed and luckily you can get one from the sample buffers you encounter:
let formatDesc = CMSampleBufferGetFormatDescription(anAudioSampleBuffer)!
let audioInput = AVAssetWriterInput(mediaType: AVMediaTypeAudio, outputSettings: nil, sourceFormatHint: formatDesc)
So if it's that easy to do, why doesn't AVAssetWriter do it for us? I guess because it would awkwardly push AVAssetWriter's usual initialization until some point after you've appended a few CMSampleBuffers, or (maybe?) because not all CMSampleBuffers have a format description.

Answer 2

I ran into this problem when trying to pass a .mp4 into an AVAssetWriter.
Specifically:
AVAssetReaderTrackOutput(track:,outputSettings:) outputSettings parameter to nil
and
AVAssetWriterInput(mediaType:,outputSettings:) outputSettings parameter to nil
Using this code from samkirkiles or for more a detailed look.
The problems are on lines 38 and 53:
// outputSettings: nil is the problem
let assetReaderAudioOutput = AVAssetReaderTrackOutput(track: audioTrack, outputSettings: nil)
let audioInput = AVAssetWriterInput(mediaType: AVMediaType.audio, outputSettings: nil)
Using a .mov that line works fine but using a .mp4 the nil value in the outputSettings parameter causes a crash: outputSettings: nil
To fix it add a dictionary with some audioSettings into the outputSettings parameter:
This is for line 38:
let audioOutputSettingsDict: [String : Any] = [
AVFormatIDKey: kAudioFormatLinearPCM,
AVSampleRateKey: 44100
]
let assetReaderAudioOutput = AVAssetReaderTrackOutput(track: audioTrack, outputSettings: audioOutputSettingsDict)
This is for line 53:
let audioInputSettingsDict: [String:Any] = [AVFormatIDKey : kAudioFormatMPEG4AAC,
AVNumberOfChannelsKey : 2,
AVSampleRateKey : 44100.0,
AVEncoderBitRateKey: 128000 // he uses 250000 in his code via self.bitRate
]
let audioInput = AVAssetWriterInput(mediaType: AVMediaType.audio, outputSettings: audioInputSettingsDict)