如何保持两个 AVCaptureMovieFileOutput 同步

Question

我有两个摄像头输入进入 OSX 应用程序，我正在尝试使用 AVCaptureMovieFileOutput 保存它们。没过多久，他们的视频就不同步了。经过最小测试后，它们可以关闭 1 到 5 秒。经过一个小时的测试后，他们关闭了 20 秒。我觉得必须有某种简单的解决方案来保持两个输出同步。我们已尝试对会话和输出使用相同的设备，但我们遇到了同样的问题。我们尝试将 fps 强制降低到 15，但仍然没有成功。

设置输出

func assignDeviceToPreview(captureSession: AVCaptureSession, device: AVCaptureDevice, previewView: NSView, index: Int){

    captureSession.stopRunning()

    captureSession.beginConfiguration()

    //clearing out old inputs
    for input in captureSession.inputs {
        let i = input as! AVCaptureInput
        captureSession.removeInput(i)
    }

    let output = self.outputs[index]
    output.outputSettings = [AVVideoCodecKey: AVVideoCodecJPEG]

    //removing old outputs
    for o in captureSession.outputs{
        if let oc = o as? AVCaptureStillImageOutput{
            captureSession.removeOutput(oc)
            print("removed image out")
        }
    }

    //Adding input
    do {

        try captureSession.addInput(AVCaptureDeviceInput(device:device))

        let camViewLayer = previewView.layer!
        camViewLayer.backgroundColor = CGColorGetConstantColor(kCGColorBlack)

        let previewLayer = AVCaptureVideoPreviewLayer(session: captureSession)
        previewLayer.frame = camViewLayer.bounds
        previewLayer.autoresizingMask = [.LayerWidthSizable, .LayerHeightSizable]

        camViewLayer.addSublayer(previewLayer)

        let overlayPreview = overlayPreviews[index]
        overlayPreview.frame.origin = CGPoint.zero

        previewView.addSubview(overlayPreview)

        //adding output
        captureSession.addOutput(output)

        if captureSession == session2{
            let audio = AVCaptureDevice.defaultDeviceWithMediaType(AVMediaTypeAudio)

            do {
                let input = try AVCaptureDeviceInput(device: audio)
                captureSession.addInput(input)
            }
        }

    } catch {
        print("Failed to add webcam as AV input")
    }

    captureSession.commitConfiguration()
    captureSession.startRunning()
}

开始录制

func startRecording(){

    startRecordingTimer()

    let base = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)[0]
    let appFolder = "Sessions"
    let sessionFolder = "session_" + session.UUID

    let path = base+"/"+appFolder+"/"+sessionFolder

    do{
        try NSFileManager.defaultManager().createDirectoryAtPath(path, withIntermediateDirectories: true, attributes: nil)
    }catch{
        print("issue creating folder")
    }

    for fileOutput in fileOutputs{

        let fileName = "cam\(String(fileOutputs.indexOf(fileOutput)!))" + ".mov"

        let fileURL = NSURL.fileURLWithPathComponents([path, fileName])
        fileURLs.append(fileURL!)
        print(fileURL?.absoluteString)

        var captureConnection = fileOutput.connections.first as? AVCaptureConnection
        captureConnection!.videoMinFrameDuration = CMTimeMake(1, 15)
        captureConnection!.videoMaxFrameDuration = CMTimeMake(1, 15)

        if fileOutput == movieFileOutput1{
            fileOutput.setOutputSettings([AVVideoScalingModeKey: AVVideoScalingModeResize, AVVideoCodecKey: AVVideoCodecH264, AVVideoWidthKey: 1280, AVVideoHeightKey: 720], forConnection: captureConnection)
        }else{
            fileOutput.setOutputSettings([AVVideoScalingModeKey: AVVideoScalingModeResizeAspect, AVVideoCodecKey: AVVideoCodecH264, AVVideoWidthKey: 640, AVVideoHeightKey: 360], forConnection: captureConnection)
        }
        captureConnection = fileOutput.connections.first as? AVCaptureConnection
        print(fileOutput.outputSettingsForConnection(captureConnection))

        fileOutput.startRecordingToOutputFileURL(fileURL, recordingDelegate: self)

        print("start recording")
    }

}

Answer 1

为了精确的时间控制，我认为你需要考虑使用较低级别的 AVAssetWriter 框架。这允许您控制单个帧的写入和时间。

使用 AVAssetWriter.startSession(atSourceTime: CMTime) 您可以精确控制每台摄像机何时开始录制。

在写入过程中，使用AVCaptureVideoDataOutputSampleBufferDelegate，可以进一步操作生成的CMSampleBuffer，调整其时序信息，进一步保持两个视频的同步。查看 https://developer.apple.com/reference/coremedia/1669345-cmsamplebuffer 以获取有关调整 CMSampleBuffer 计时部分的参考。

就是说，我从来没有尝试过这个，也不确定这是否会奏效，但我相信如果你沿着这条路走下去，你会接近你想要实现的目标。

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