Display a OpenCV Mat using a UIImageView.

showOpenCVMat.mm

void showOpenCVMat(cv::Mat img, UIImageView* imageView) {
  cv::Mat converted;
  img.convertTo(converted, CV_8U);
  // convert the cv::Mat to UIImage
  NSData *data = [NSData dataWithBytes:converted.data length:converted.elemSize() * converted.total()];
  CGColorSpaceRef colorSpace =
      CGColorSpaceCreateDeviceGray();  // CGColorSpaceCreateDeviceRGB();
  CGDataProviderRef provider =
      CGDataProviderCreateWithCFData((__bridge CFDataRef)data);
  CGImageRef imageRef = CGImageCreate(
      converted.cols, converted.rows, 8, 8, converted.cols, colorSpace,
      kCGBitmapByteOrderDefault | kCGImageAlphaNone, provider, NULL, false,
      kCGRenderingIntentDefault);
  UIImage *image = [UIImage imageWithCGImage:imageRef];
  CGImageRelease(imageRef);
  CGDataProviderRelease(provider);
  CGColorSpaceRelease(colorSpace);

  // show the UIImage on the UIImageView
  dispatch_async(dispatch_get_main_queue(), ^{
    imageView.image = image;
  });
}

In your ViewController's viewDidLoad method:

- (void)viewDidLoad {
  [super viewDidLoad];

  UIImageView *imageView = [UIImageView new];
  imageView.frame        = self.view.bounds;
  [self.view addSubview:imageView];

  // ...
  showOpenCVMat(img, imageView);
}