iOS Swift - AVCaptureSession - Capture frames respecting frame rate - ios

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)