OpenCV video processing tutorial issue - iphone

Question

I followed the iOS video processing tutorial on the OpenCV site 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
I use a iPhone 3GS for testing.
Here is the full source of my ViewController.h:
#import <UIKit/UIKit.h>
#import <opencv2/highgui/cap_ios.h>
#interface ViewController : UIViewController <CvVideoCameraDelegate>
{
IBOutlet UIImageView* imageView;
IBOutlet UIButton* button;
CvVideoCamera* videoCamera;
}
- (IBAction)actionStart:(id)sender;
#property (nonatomic, strong) CvVideoCamera* videoCamera;
#end
and ViewController.mm:
#implementation ViewController
#synthesize videoCamera;
- (void)viewDidLoad
{
[super viewDidLoad];
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

Answer 1

I tested on an iPhone 4 and it works well. Maybe the iPhone 3GS doesn't have a front facing camera? Then you have to change this line: self.videoCamera.defaultAVCaptureDevicePosition = AVCaptureDevicePositionFront to self.videoCamera.defaultAVCaptureDevicePosition = AVCaptureDevicePositionBack