iOS video processing tutorial problem

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I followed the iOS video processing tutorial and managed to compile and install it on the device. However when I click on the "Start" button, nothing visible happens. Apparently neither my - (void)processImage:(Mat&)image callback is never called. Could you give me some hints what could be the problem?

Some things I checked:

• actionStart is called, and both log lines prints YES
• the self object is valid when assigned to self.videoCamera.delegate
• imageView is valid and visible on the screen
• videoCamera is initialized when its start method is called

Here is the full source of my ViewController.mm:

@implementation ViewController

@synthesize videoCamera;

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

    self.videoCamera = [[CvVideoCamera alloc] initWithParentView:imageView];
    self.videoCamera.delegate = self;

    self.videoCamera.defaultAVCaptureDevicePosition = AVCaptureDevicePositionFront;
    self.videoCamera.defaultAVCaptureSessionPreset = AVCaptureSessionPreset352x288;
    self.videoCamera.defaultAVCaptureVideoOrientation = AVCaptureVideoOrientationPortrait;
    self.videoCamera.defaultFPS = 30;
    self.videoCamera.grayscaleMode = NO;

}

#pragma mark - UI Actions

- (IBAction)actionStart:(id)sender;
{
    [self.videoCamera start];
    NSLog(@"video camera running: %d", [self.videoCamera running]);
    NSLog(@"capture session loaded: %d", [self.videoCamera captureSessionLoaded]);
}

#pragma mark - Protocol CvVideoCameraDelegate

- (void)processImage:(cv::Mat&)image
{
    // Do some OpenCV stuff with the image
    cv::Mat image_copy;
    cvtColor(image, image_copy, CV_BGRA2BGR);

    // invert image
    bitwise_not(image_copy, image_copy);
    cvtColor(image_copy, image, CV_BGR2BGRA);
}

@end