iOS Swift - AVCaptureSession - Capture frames respecting frame rate

Question

I'm trying to build an app which will capture frames from the camera and process them with OpenCV before saving those files to the device, but at a specific frame rate.

What I'm stuck on at the moment is the fact that AVCaptureVideoDataOutputSampleBufferDelegate doesn't appear to respect the AVCaptureDevice.activeVideoMinFrameDuration, or AVCaptureDevice.activeVideoMaxFrameDuration settings.

captureOutput runs far quicker than 2 frames per second as the above settings would indicate.

Do you happen to know how one could achieve this, with or without the delegate?

ViewController:

override func viewDidLoad() {
    super.viewDidLoad()

}

override func viewDidAppear(animated: Bool) {
    setupCaptureSession()
}

func setupCaptureSession() {

    let session : AVCaptureSession = AVCaptureSession()
    session.sessionPreset = AVCaptureSessionPreset1280x720

    let videoDevices : [AVCaptureDevice] = AVCaptureDevice.devices() as! [AVCaptureDevice]

    for device in videoDevices {
        if device.position == AVCaptureDevicePosition.Back {
            let captureDevice : AVCaptureDevice = device

            do {
                try captureDevice.lockForConfiguration()
                captureDevice.activeVideoMinFrameDuration = CMTimeMake(1, 2)
                captureDevice.activeVideoMaxFrameDuration = CMTimeMake(1, 2)
                captureDevice.unlockForConfiguration()

                let input : AVCaptureDeviceInput = try AVCaptureDeviceInput(device: captureDevice)

                if session.canAddInput(input) {
                    try session.addInput(input)
                }

                let output : AVCaptureVideoDataOutput = AVCaptureVideoDataOutput()

                let dispatch_queue : dispatch_queue_t = dispatch_queue_create("streamoutput", nil)
                output.setSampleBufferDelegate(self, queue: dispatch_queue)

                session.addOutput(output)

                session.startRunning()

                let previewLayer = AVCaptureVideoPreviewLayer(session: session)
                previewLayer.connection.videoOrientation = .LandscapeRight

                let previewBounds : CGRect = CGRectMake(0,0,self.view.frame.width/2,self.view.frame.height+20)
                previewLayer.backgroundColor = UIColor.blackColor().CGColor
                previewLayer.frame = previewBounds
                previewLayer.videoGravity = AVLayerVideoGravityResizeAspectFill
                self.imageView.layer.addSublayer(previewLayer)

                self.previewMat.frame = CGRectMake(previewBounds.width, 0, previewBounds.width, previewBounds.height)

            } catch _ {

            }
            break
        }
    }

}

func captureOutput(captureOutput: AVCaptureOutput!, didOutputSampleBuffer sampleBuffer: CMSampleBuffer!, fromConnection connection: AVCaptureConnection!) {
    self.wrapper.processBuffer(self.getUiImageFromBuffer(sampleBuffer), self.previewMat)
}

Answer 1

So I've figured out the problem.

In the comments section for AVCaptureDevice.h above the activeVideoMinFrameDuration property it states:

On iOS, the receiver's activeVideoMinFrameDuration resets to its default value under the following conditions:

• The receiver's activeFormat changes
• The receiver's AVCaptureDeviceInput's session's sessionPreset changes
• The receiver's AVCaptureDeviceInput is added to a session

The last bullet point was causing my problem, so doing the following solved the problem for me:

        do {

            let input : AVCaptureDeviceInput = try AVCaptureDeviceInput(device: captureDevice)

            if session.canAddInput(input) {
                try session.addInput(input)
            }

            try captureDevice.lockForConfiguration()
            captureDevice.activeVideoMinFrameDuration = CMTimeMake(value: 1, timescale: 2)
            captureDevice.activeVideoMaxFrameDuration = CMTimeMake(value: 1, timescale: 2)
            captureDevice.unlockForConfiguration()

            let output : AVCaptureVideoDataOutput = AVCaptureVideoDataOutput()

            let dispatch_queue : dispatch_queue_t = dispatch_queue_create("streamoutput", nil)
            output.setSampleBufferDelegate(self, queue: dispatch_queue)

            session.addOutput(output)