How to create a anAudioSampleBuffer for CMSampleBufferGetFormatDescription in iOS Swift - ios

Question

I have been working on video compression in iOS Swift, and following this SO's answer. It is working fine until I change this piece of code's file format to .mp4
let videoWriter = try! AVAssetWriter(outputURL: outputURL as URL, fileType: AVFileType.mov)
There are reasons that I need the output in .mp4 file format. So when I do that it crashes the app. And gives me this error,
2020-04-27 18:20:52.573614+0500 BrightCaster[7847:1513728] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[AVAssetWriter addInput:] In order to perform passthrough to file type public.mpeg-4, please provide a format hint in the AVAssetWriterInput initializer'
*** First throw call stack:
(0x1b331d5f0 0x1b303fbcc 0x1bd53b2b0 0x102383c0c 0x102382164 0x1021897cc 0x1b6ca73bc 0x1b6caba7c 0x1b6daec94 0x1b7835080 0x1b7834d30 0x1e9d077b4 0x1b786a764 0x1b783eb68 0x1b783f070 0x1e9d468f4 0x1b783f1c0 0x1e9d468f4 0x1b9e21d9c 0x105173730 0x105181710 0x1b329b748 0x1b329661c 0x1b3295c34 0x1bd3df38c 0x1b73c822c 0x10230f8a0 0x1b311d800)
libc++abi.dylib: terminating with uncaught exception of type NSException
So I searched on SO and found this question relevant to my problem.
but now the issue is when I try to add its answer to my function it gives me error anAudioSampleBuffer not defined. As I am totally new to audio/video domain, I am unable to understand why it is giving me this. And how to resolve this.
The piece of code from answer that I am adding with my function is below.
//setup audio writer
//let formatDesc = CMSampleBufferGetFormatDescription(anAudioSampleBuffer)
//let audioWriterInput = AVAssetWriterInput(mediaType: AVMediaType.audio, outputSettings: nil, sourceFormatHint: formatDesc)
let audioWriterInput = AVAssetWriterInput(mediaType: AVMediaType.audio, outputSettings: nil)
audioWriterInput.expectsMediaDataInRealTime = false
videoWriter.add(audioWriterInput)
The commented part is not working. Any help would be appreciated Thanks.
Whole function for conversion is following
func convertVideoToLowQuailtyWithInputURL(inputURL: URL, outputURL: URL, completion: #escaping (Bool , _ url: String) -> Void) {
let videoAsset = AVURLAsset(url: inputURL as URL, options: nil)
let videoTrack = videoAsset.tracks(withMediaType: AVMediaType.video)[0]
let videoSize = videoTrack.naturalSize
let videoWriterCompressionSettings = [
AVVideoAverageBitRateKey : Int(125000)
]
let videoWriterSettings:[String : AnyObject] = [
AVVideoCodecKey : AVVideoCodecH264 as AnyObject,
AVVideoCompressionPropertiesKey : videoWriterCompressionSettings as AnyObject,
AVVideoWidthKey : Int(videoSize.width) as AnyObject,
AVVideoHeightKey : Int(videoSize.height) as AnyObject
]
let videoWriterInput = AVAssetWriterInput(mediaType: AVMediaType.video, outputSettings: videoWriterSettings)
videoWriterInput.expectsMediaDataInRealTime = true
videoWriterInput.transform = videoTrack.preferredTransform
let videoWriter = try! AVAssetWriter(outputURL: outputURL as URL, fileType: AVFileType.mov) // for now its converting in .mov I THINK SO.
videoWriter.add(videoWriterInput)
//setup video reader
let videoReaderSettings:[String : AnyObject] = [
kCVPixelBufferPixelFormatTypeKey as String: Int(kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange) as AnyObject
]
let videoReaderOutput = AVAssetReaderTrackOutput(track: videoTrack, outputSettings: videoReaderSettings)
var videoReader: AVAssetReader!
do{
videoReader = try AVAssetReader(asset: videoAsset)
}
catch {
print("video reader error: \(error)")
completion(false, "")
}
videoReader.add(videoReaderOutput)
//setup audio writer
//let formatDesc = CMSampleBufferGetFormatDescription(anAudioSampleBuffer) // this is giving me error here of un initilize, which I didn't I know.
//let audioWriterInput = AVAssetWriterInput(mediaType: AVMediaType.audio, outputSettings: nil, sourceFormatHint: formatDesc)
let audioWriterInput = AVAssetWriterInput(mediaType: AVMediaType.audio, outputSettings: nil)
audioWriterInput.expectsMediaDataInRealTime = false
videoWriter.add(audioWriterInput)
//setup audio reader
let audioTrack = videoAsset.tracks(withMediaType: AVMediaType.audio)[0]
let audioReaderOutput = AVAssetReaderTrackOutput(track: audioTrack, outputSettings: nil)
let audioReader = try! AVAssetReader(asset: videoAsset)
audioReader.add(audioReaderOutput)
videoWriter.startWriting()
//start writing from video reader
videoReader.startReading()
videoWriter.startSession(atSourceTime: CMTime.zero)
let processingQueue = DispatchQueue(label: "processingQueue1")
videoWriterInput.requestMediaDataWhenReady(on: processingQueue, using: {() -> Void in
while videoWriterInput.isReadyForMoreMediaData {
let sampleBuffer:CMSampleBuffer? = videoReaderOutput.copyNextSampleBuffer();
if videoReader.status == .reading && sampleBuffer != nil {
videoWriterInput.append(sampleBuffer!)
}
else {
videoWriterInput.markAsFinished()
if videoReader.status == .completed {
//start writing from audio reader
audioReader.startReading()
videoWriter.startSession(atSourceTime: CMTime.zero)
let processingQueue = DispatchQueue(label: "processingQueue2")
audioWriterInput.requestMediaDataWhenReady(on: processingQueue, using: {() -> Void in
while audioWriterInput.isReadyForMoreMediaData {
let sampleBuffer:CMSampleBuffer? = audioReaderOutput.copyNextSampleBuffer()
if audioReader.status == .reading && sampleBuffer != nil {
audioWriterInput.append(sampleBuffer!)
}
else {
audioWriterInput.markAsFinished()
if audioReader.status == .completed {
videoWriter.finishWriting(completionHandler: {() -> Void in
completion(true, "\(videoWriter.outputURL)")
})
}
}
}
})
}
}
}
})
}

Answer 1

You can output as mp4, passing audio through (no transcode) by providing that format hint like so:
let audioTrack = videoAsset.tracks(withMediaType: AVMediaType.audio)[0]
let audioWriterInput = AVAssetWriterInput(mediaType: AVMediaType.audio, outputSettings: nil, sourceFormatHint: audioTrack.formatDescriptions[0] as! CMFormatDescription)
Note the new position of audioTrack definition.
I imagine both of Apple's .mov and .mp4 implementations need to know the the compressed audio format to write the file, but I guess .mov is ok with inferring that information after initialisation, where .mp4 is not. Maybe it's another AVFoundation Surprise!.
In your case I saw that it would be tiresome to rework the code to get the audio format from the first sample buffer, but then I remembered that the format is available from the input audio track.