How to record and save at 240 frames per second?

问题

I need to record and save video from an iPhone Xs at the phone's max frame rate (240 fps). The saved file always ends up at 30 fps. I've been through a dozen guides/docs/Stack Overflow posts but have yet to hit on the right solution. I've tested by opening the recorded file in VLC as well as by extracting and counting frames.

What am I doing wrong?

Environment: Xcode 10.1, build target iOS 12.1, tested on an iPhone Xs running iOS 12.1.2

Here I access the device and configure it for the best frame rate it supports:

override func viewDidLoad() {
    super.viewDidLoad()
    let deviceDiscoverSession = AVCaptureDevice.default(.builtInWideAngleCamera, for: AVMediaType.video, position: .back)
    guard let backCameraDevice = deviceDiscoverSession else {
        print("Failed to open back, wide-angle camera")
        return
    }
    self.currentDevice = backCameraDevice
    do {
        let input = try AVCaptureDeviceInput(device: backCameraDevice)
        // configureDevice() // putting this here makes no difference
        self.session.addInput(input)
        configureDevice()
    } catch {
        print(error)
        return
    }
}

func configureDevice() {
    var bestFormat: AVCaptureDevice.Format? = nil
    var bestFrameRateRange: AVFrameRateRange? = nil
    var bestPixelArea: Int32 = 0
    for format in currentDevice!.formats {
        let dims: CMVideoDimensions = CMVideoFormatDescriptionGetDimensions(format.formatDescription)
        let pixelArea: Int32 = dims.width * dims.height
        let ranges = format.videoSupportedFrameRateRanges
        for range in ranges {
            if bestFrameRateRange==nil || range.maxFrameRate > bestFrameRateRange!.maxFrameRate || ((range.maxFrameRate == bestFrameRateRange!.maxFrameRate) && (pixelArea > bestPixelArea)) {
                bestFormat = format as AVCaptureDevice.Format
                bestFrameRateRange = range
                bestPixelArea = pixelArea
            }
        }
    }
    do {
        try currentDevice!.lockForConfiguration()
        if let best_format = bestFormat {
            currentDevice!.activeFormat = best_format
            currentDevice!.activeVideoMinFrameDuration = bestFrameRateRange!.minFrameDuration
            currentDevice!.activeVideoMaxFrameDuration = bestFrameRateRange!.maxFrameDuration
        }
    } catch {
        print(error)
    }

    let movieFileOutput = AVCaptureMovieFileOutput()

    if self.session.canAddOutput(movieFileOutput) {
        self.session.beginConfiguration()
        self.session.addOutput(movieFileOutput)
        self.session.sessionPreset = .high
        if let connection = movieFileOutput.connection(with: .video) {
            if movieFileOutput.availableVideoCodecTypes.contains(.h264) {
                movieFileOutput.setOutputSettings([AVVideoCodecKey:
                    AVVideoCodecType.h264], for: connection)
            }
        }
        self.session.commitConfiguration()
        self.movieFileOutput = movieFileOutput
    }
    currentDevice!.unlockForConfiguration()
}

When the user stops recording, I call a function that in part contains the following code to save the file to the temp directory (later moved to the app's documents directory)

sessionQueue.async {
    if !self.isRecording {
        self.isRecording = true
        let movieFileOutputConnection = self.movieFileOutput!.connection(with: .video)
        movieFileOutputConnection?.videoOrientation = .landscapeRight
        let availableVideoCodecTypes = self.movieFileOutput!.availableVideoCodecTypes
        if availableVideoCodecTypes.contains(.h264) {
           self.movieFileOutput!.setOutputSettings([AVVideoCodecKey: AVVideoCodecType.h264], for: movieFileOutputConnection!)
        }
        let outputFileName = NSUUID().uuidString
        let outputFilePath = (NSTemporaryDirectory() as NSString).appendingPathComponent((outputFileName as NSString).appendingPathExtension("mp4")!)
        self.movieFileOutput?.startRecording(to: URL(fileURLWithPath: outputFilePath), recordingDelegate: self)
    } else {
        self.movieFileOutput?.stopRecording()
        self.isRecording = false
    }
}

The typical answer seems to be to configure the device after adding it to the session. Calling configure before or after adding to the session doesn't seem to make a difference.

I've tried configuring movieFileOutput right before the self.movieFileOutput?.startRecording() call as well as where I show it above. Both give the same results.

回答1:

I figured this out with a deeper read of https://stackoverflow.com/a/41109637/292947

In configureDevice() I was setting self.session.sessionPreset = .high when in fact I need to set self.session.sessionPreset = .inputPriority which is the Swift 4 equivalent to the AVCaptureSessionPresetInputPriority value suggested in the above answer.

来源：https://stackoverflow.com/questions/54351972/how-to-record-and-save-at-240-frames-per-second

标签

ios

swift

avcapturesession