CvVideoCamera processImage doesn't change the image - c++

Question

Simple example of CvVideoCamera usage doesn't work as expected. The image modified in delegate's processImage is displayed with no changes at all.
ViewController.h:
#import <UIKit/UIKit.h>
#import <opencv2/videoio/cap_ios.h>
#interface ViewController : UIViewController
#property (weak, nonatomic) IBOutlet UIButton *btnStart;
#property (weak, nonatomic) IBOutlet UIImageView *myimg;
#end
ViewController.mm:
#import "ViewController.h"
#import <opencv2/videoio/cap_ios.h>
#interface ViewController ()<CvVideoCameraDelegate>
#property (strong, nonatomic) CvVideoCamera *videoCamera;
#end
#implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
self.videoCamera = [[CvVideoCamera alloc] initWithParentView:self.myimg];
[self.videoCamera setDelegate:self];
self.videoCamera.defaultAVCaptureDevicePosition = AVCaptureDevicePositionBack;
self.videoCamera.defaultAVCaptureSessionPreset = AVCaptureSessionPresetHigh;
self.videoCamera.defaultAVCaptureVideoOrientation = AVCaptureVideoOrientationPortrait;
self.videoCamera.defaultFPS = 30;
self.videoCamera.useAVCaptureVideoPreviewLayer = YES;
}
- (IBAction)actionStart:(id)sender {
[self.videoCamera start];
}
- (void)processImage:(cv::Mat &)image;
{
cv::Mat image_copy;
cvtColor(image, image_copy, CV_BGRA2BGR);
bitwise_not(image_copy, image_copy);
cvtColor(image_copy, image, CV_BGR2BGRA);
}
Once I press the button the normal camera stream is rendered with no traces of any interventions. I also tried to draw shapes and even calling image.release() inside of processImage with no effect.
I assume I'm missing something obvious here.
There's the example repository
This is what I'm getting on the screen

Answer 1

This line was the root of the issue:
self.videoCamera.useAVCaptureVideoPreviewLayer = YES;
once removed, everything works as expected.