使用 AVAssetWriter 捕获音频和视频的损坏视频

Question

我正在使用 AVCaptureSession 来使用视频和音频输入，并使用 AVAssetWriter 对 H.264 视频进行编码。

如果我不编写音频，视频将按预期进行编码。但如果我写音频，我会得到一个损坏的视频。

如果我检查提供给AVAssetWriter 的音频CMSampleBuffer，它会显示以下信息：

invalid = NO
dataReady = YES
makeDataReadyCallback = 0x0
makeDataReadyRefcon = 0x0
formatDescription = <CMAudioFormatDescription 0x17410ba30 [0x1b3a70bb8]> {
mediaType:'soun' 
mediaSubType:'lpcm' 
mediaSpecific: {
    ASBD: {
        mSampleRate: 44100.000000 
        mFormatID: 'lpcm' 
        mFormatFlags: 0xc 
        mBytesPerPacket: 2 
        mFramesPerPacket: 1 
        mBytesPerFrame: 2 
        mChannelsPerFrame: 1 
        mBitsPerChannel: 16     } 
    cookie: {(null)} 
    ACL: {(null)}
    FormatList Array: {(null)} 
} 
extensions: {(null)}

由于它提供 lpcm 音频，我已经为 AVAssetWriterInput 配置了此声音设置（我尝试了一个和两个通道）：

var channelLayout = AudioChannelLayout()
memset(&channelLayout, 0, MemoryLayout<AudioChannelLayout>.size);
channelLayout.mChannelLayoutTag = kAudioChannelLayoutTag_Mono

let audioOutputSettings:[String: Any] = [AVFormatIDKey as String:UInt(kAudioFormatLinearPCM),
                                             AVNumberOfChannelsKey as String:1,
                                             AVSampleRateKey as String:44100.0,
                                            AVLinearPCMIsBigEndianKey as String:false,
                                            AVLinearPCMIsFloatKey as String:false,
                                            AVLinearPCMBitDepthKey as String:16,
                                            AVLinearPCMIsNonInterleaved as String:false,
                                             AVChannelLayoutKey: NSData(bytes:&channelLayout, length:MemoryLayout<AudioChannelLayout>.size)]

self.assetWriterAudioInput = AVAssetWriterInput(mediaType: AVMediaTypeAudio, outputSettings: audioOutputSettings)
self.assetWriter.add(self.assetWriterAudioInput)

当我使用上面的 lpcm 设置时，我无法使用任何应用程序打开视频。我尝试使用kAudioFormatMPEG4AAC 和kAudioFormatAppleLossless，但我仍然收到损坏的视频，但我可以使用 QuickTime Player 8（而不是 QuickTime Player 7）查看视频，但对视频的持续时间感到困惑并且没有声音播放。

录制完成后，我正在打电话：

func endRecording(_ completionHandler: @escaping () -> ()) {
    isRecording = false
    assetWriterVideoInput.markAsFinished()
    assetWriterAudioInput.markAsFinished()
    assetWriter.finishWriting(completionHandler: completionHandler)
}

AVCaptureSession 的配置方式如下：

func setupCapture() {

    captureSession = AVCaptureSession()

    if (captureSession == nil) {
        fatalError("ERROR: Couldnt create a capture session")
    }

    captureSession?.beginConfiguration()
    captureSession?.sessionPreset = AVCaptureSessionPreset1280x720

    let frontDevices = AVCaptureDevice.devices().filter{ ($0 as AnyObject).hasMediaType(AVMediaTypeVideo) && ($0 as AnyObject).position == AVCaptureDevicePosition.front }

    if let captureDevice = frontDevices.first as? AVCaptureDevice  {
        do {
            let videoDeviceInput: AVCaptureDeviceInput
            do {
                videoDeviceInput = try AVCaptureDeviceInput(device: captureDevice)
            }
            catch {
                fatalError("Could not create AVCaptureDeviceInput instance with error: \(error).")
            }
            guard (captureSession?.canAddInput(videoDeviceInput))! else {
                fatalError()
            }
            captureSession?.addInput(videoDeviceInput)
        }
    }

    do {
        let audioDevice = AVCaptureDevice.defaultDevice(withMediaType: AVMediaTypeAudio)
        let audioDeviceInput: AVCaptureDeviceInput
        do {
            audioDeviceInput = try AVCaptureDeviceInput(device: audioDevice)
        }
        catch {
            fatalError("Could not create AVCaptureDeviceInput instance with error: \(error).")
        }
        guard (captureSession?.canAddInput(audioDeviceInput))! else {
            fatalError()
        }
        captureSession?.addInput(audioDeviceInput)
    }

    do {
        let dataOutput = AVCaptureVideoDataOutput()
        dataOutput.videoSettings = [kCVPixelBufferPixelFormatTypeKey as String : kCVPixelFormatType_32BGRA]
        dataOutput.alwaysDiscardsLateVideoFrames = true
        let queue = DispatchQueue(label: "com.3DTOPO.videosamplequeue")
        dataOutput.setSampleBufferDelegate(self, queue: queue)
        guard (captureSession?.canAddOutput(dataOutput))! else {
            fatalError()
        }
        captureSession?.addOutput(dataOutput)

        videoConnection = dataOutput.connection(withMediaType: AVMediaTypeVideo)
    }

    do {
        let audioDataOutput = AVCaptureAudioDataOutput()
        let queue = DispatchQueue(label: "com.3DTOPO.audiosamplequeue")
        audioDataOutput.setSampleBufferDelegate(self, queue: queue)
        guard (captureSession?.canAddOutput(audioDataOutput))! else {
            fatalError()
        }
        captureSession?.addOutput(audioDataOutput)

        audioConnection = audioDataOutput.connection(withMediaType: AVMediaTypeAudio)
    }

    captureSession?.commitConfiguration()

    // this will trigger capture on its own queue
    captureSession?.startRunning()
}

AVCaptureVideoDataOutput 委托方法：

func captureOutput(_ captureOutput: AVCaptureOutput!, didOutputSampleBuffer sampleBuffer: CMSampleBuffer!, from connection: AVCaptureConnection!) {
    //  func captureOutput(captureOutput: AVCaptureOutput, sampleBuffer: CMSampleBuffer, connection:AVCaptureConnection) {

    var error: CVReturn

    if (connection == audioConnection) {
        delegate?.audioSampleUpdated(sampleBuffer: sampleBuffer)
        return
    }

    // ... Write video buffer ...//
}

调用者：

func audioSampleUpdated(sampleBuffer: CMSampleBuffer) {
    if (isRecording) {  
        while !assetWriterAudioInput.isReadyForMoreMediaData {}
        if (!assetWriterAudioInput.append(sampleBuffer)) {
            print("Unable to write to audio input");
        }
    }
}

如果我禁用上面的assetWriterAudioInput.append() 调用，则视频没有损坏，但我当然没有音频编码。如何让视频和音频编码同时工作？

Answer 1

我想通了。我将assetWriter.startSession 源时间设置为0，然后从当前CACurrentMediaTime() 中减去开始时间以写入像素数据。

我把assetWriter.startSession源时间改成CACurrentMediaTime()，写视频帧的时候不要减去当前时间。

旧的开始会话代码：

assetWriter.startWriting()
assetWriter.startSession(atSourceTime: kCMTimeZero)

有效的新代码：

let presentationStartTime = CMTimeMakeWithSeconds(CACurrentMediaTime(), 240)

assetWriter.startWriting()
assetWriter.startSession(atSourceTime: presentationStartTime)