iOS: captureOutput:didOutputSampleBuffer:fromConnection is NOT called

Question

I want to pull frames from the live-feed of AVCaptureSession and I am using Apple's AVCam as a test case. Here is the link to AVCam:

https://developer.apple.com/library/ios/samplecode/AVCam/Introduction/Intro.html

I found that that captureOutput:didOutputSampleBuffer:fromConnection is NOT called and I would like to know why or what I am doing wrong.

Here is what I have done:

(1) I make the AVCamViewController a delegate

@interface AVCamViewController () <AVCaptureFileOutputRecordingDelegate, AVCaptureVideoDataOutputSampleBufferDelegate>

(2) I created an AVCaptureVideoDataOutput object and add it to the session

AVCaptureVideoDataOutput *videoDataOutput = [[AVCaptureVideoDataOutput alloc] init];
if ([session canAddOutput:videoDataOutput])
     {
         [session addOutput:videoDataOutput];
     }

(3) I added the delegate method and test by logging a random string to test

- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection
{
    NSLog(@"I am called");

}

The test application works but captureOutput:didOutputSampleBuffer:fromConnection is not called.

(4) I read on SO that the session variable in AVCaptureSession *session = [[AVCaptureSession alloc] init]; being local in viewDidLoad is a possible reason why the delegate is not called and I made it an instance variable of the AVCamViewController class, yet it is not called.

Here is the viewDidLoad method I am testing with (taken from AVCam), I added AVCaptureDataOutput towards the end of the method:

- (void)viewDidLoad
{
    [super viewDidLoad];

    // Create the AVCaptureSession
    session = [[AVCaptureSession alloc] init];
    [self setSession:session];

    // Setup the preview view
    [[self previewView] setSession:session];

    // Check for device authorization
    [self checkDeviceAuthorizationStatus];

    // In general it is not safe to mutate an AVCaptureSession or any of its inputs, outputs, or connections from multiple threads at the same time.
    // Why not do all of this on the main queue?
    // -[AVCaptureSession startRunning] is a blocking call which can take a long time. We dispatch session setup to the sessionQueue so that the main queue isn't blocked (which keeps the UI responsive).

    dispatch_queue_t sessionQueue = dispatch_queue_create("session queue", DISPATCH_QUEUE_SERIAL);
    [self setSessionQueue:sessionQueue];

    dispatch_async(sessionQueue, ^{
        [self setBackgroundRecordingID:UIBackgroundTaskInvalid];

        NSError *error = nil;

        AVCaptureDevice *videoDevice = [AVCamViewController deviceWithMediaType:AVMediaTypeVideo preferringPosition:AVCaptureDevicePositionBack];
        AVCaptureDeviceInput *videoDeviceInput = [AVCaptureDeviceInput deviceInputWithDevice:videoDevice error:&error];

        if (error)
        {
            NSLog(@"%@", error);
        }

        if ([session canAddInput:videoDeviceInput])
        {
            [session addInput:videoDeviceInput];
            [self setVideoDeviceInput:videoDeviceInput];

            dispatch_async(dispatch_get_main_queue(), ^{
                // Why are we dispatching this to the main queue?
                // Because AVCaptureVideoPreviewLayer is the backing layer for AVCamPreviewView and UIView can only be manipulated on main thread.
                // Note: As an exception to the above rule, it is not necessary to serialize video orientation changes on the AVCaptureVideoPreviewLayer’s connection with other session manipulation.

                [[(AVCaptureVideoPreviewLayer *)[[self previewView] layer] connection] setVideoOrientation:(AVCaptureVideoOrientation)[self interfaceOrientation]];
            });
        }

        AVCaptureDevice *audioDevice = [[AVCaptureDevice devicesWithMediaType:AVMediaTypeAudio] firstObject];
        AVCaptureDeviceInput *audioDeviceInput = [AVCaptureDeviceInput deviceInputWithDevice:audioDevice error:&error];

        if (error)
        {
            NSLog(@"%@", error);
        }

        if ([session canAddInput:audioDeviceInput])
        {
            [session addInput:audioDeviceInput];
        }

        AVCaptureMovieFileOutput *movieFileOutput = [[AVCaptureMovieFileOutput alloc] init];
        if ([session canAddOutput:movieFileOutput])
        {
            [session addOutput:movieFileOutput];
            AVCaptureConnection *connection = [movieFileOutput connectionWithMediaType:AVMediaTypeVideo];
            if ([connection isVideoStabilizationSupported])
                [connection setEnablesVideoStabilizationWhenAvailable:YES];
            [self setMovieFileOutput:movieFileOutput];
        }

        AVCaptureStillImageOutput *stillImageOutput = [[AVCaptureStillImageOutput alloc] init];
        if ([session canAddOutput:stillImageOutput])
        {
            [stillImageOutput setOutputSettings:@{AVVideoCodecKey : AVVideoCodecJPEG}];
            [session addOutput:stillImageOutput];
            [self setStillImageOutput:stillImageOutput];
        }

        AVCaptureVideoDataOutput *videoDataOutput = [[AVCaptureVideoDataOutput alloc] init];
        [videoDataOutput setSampleBufferDelegate:self queue:sessionQueue];

        if ([session canAddOutput:videoDataOutput])
        {
            NSLog(@"Yes I can add it");
            [session addOutput:videoDataOutput];
        }

    });
}

- (void)viewWillAppear:(BOOL)animated
{
    dispatch_async([self sessionQueue], ^{
        [self addObserver:self forKeyPath:@"sessionRunningAndDeviceAuthorized" options:(NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew) context:SessionRunningAndDeviceAuthorizedContext];
        [self addObserver:self forKeyPath:@"stillImageOutput.capturingStillImage" options:(NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew) context:CapturingStillImageContext];
        [self addObserver:self forKeyPath:@"movieFileOutput.recording" options:(NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew) context:RecordingContext];
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(subjectAreaDidChange:) name:AVCaptureDeviceSubjectAreaDidChangeNotification object:[[self videoDeviceInput] device]];

        __weak AVCamViewController *weakSelf = self;
        [self setRuntimeErrorHandlingObserver:[[NSNotificationCenter defaultCenter] addObserverForName:AVCaptureSessionRuntimeErrorNotification object:[self session] queue:nil usingBlock:^(NSNotification *note) {
            AVCamViewController *strongSelf = weakSelf;
            dispatch_async([strongSelf sessionQueue], ^{
                // Manually restarting the session since it must have been stopped due to an error.
                [[strongSelf session] startRunning];
                [[strongSelf recordButton] setTitle:NSLocalizedString(@"Record", @"Recording button record title") forState:UIControlStateNormal];
            });
        }]];
        [[self session] startRunning];
    });
}

Can someone please tell me why and suggestions on how to fix it?

Answer 1

I've done a lot of experimenting with this and I think I have the answer probably. I have similar but different code that's written from the ground up rather than being copied from Apple's samples (which are a bit old now).

I think it's the section...

AVCaptureMovieFileOutput *movieFileOutput = [[AVCaptureMovieFileOutput alloc] init];
    if ([session canAddOutput:movieFileOutput])
    {
        [session addOutput:movieFileOutput];
        AVCaptureConnection *connection = [movieFileOutput connectionWithMediaType:AVMediaTypeVideo];
        if ([connection isVideoStabilizationSupported])
            [connection setEnablesVideoStabilizationWhenAvailable:YES];
        [self setMovieFileOutput:movieFileOutput];
    }

From my experiments, this is the thing that causes your problem. In my code, when this is there captureOutput:didOutputSampleBuffer:fromConnection is not called. I think the video system EITHER gives you a series of sample buffers OR records a compressed, optimised movie file to disk, not both. (At least on iOS.) I guess this makes sense/is not surprising but I have not seen it documented anywhere!

Also, at one point, I seemed to be getting errors and/or the buffer callback not occurring when I had the microphone on. Again undocumented, these were error -11800 (unknown error). But I cannot always reproduce that.

Answer 2

Your code looks good to me and I could think of 10 guess-and-check things you could try so I'll take a different approach that will hopefully indirectly fix the issue. Besides the fact that I think AVCam is poorly written, I think it would be better for you to see a an example that only focuses on live video rather than recording video and taking still images. I have provided an example that does just that and no more.

-(void)startSession {
    self.session = [AVCaptureSession new];
    self.session.sessionPreset = AVCaptureSessionPresetMedium;
    AVCaptureDevice *backCamera;
    for (AVCaptureDevice *device in [AVCaptureDevice devices]) {
        if ([device hasMediaType:AVMediaTypeVideo] && device.position == AVCaptureDevicePositionBack) {
            backCamera = device;
            break;
        }
    }
    NSError *error = nil;
    AVCaptureDeviceInput *input = [AVCaptureDeviceInput deviceInputWithDevice:backCamera error:&error];
    if (error) {
        // handle error
    }
    if ([self.session canAddInput:input]) {
        [self.session addInput:input];
    }
    AVCaptureVideoDataOutput *output = [AVCaptureVideoDataOutput new];
    [output setSampleBufferDelegate:self queue:self.queue];
    output.videoSettings = @{(id)kCVPixelBufferPixelFormatTypeKey:@(kCVPixelFormatType_32BGRA)};
    if ([self.session canAddOutput:output]) {
        [self.session addOutput:output];
    }
    dispatch_async(self.queue, ^{
        [self.session startRunning];
    });
}