Question

With the following code the image showed as above is converted as below image… Their it’s showing black background with gray lines…..i want white background with gray lines ..
Please guide me .. i am new to iPhone
Thanks alot in Advance

- (void)viewDidLoad
{
    [super viewDidLoad];

    // Initialise video capture - only supported on iOS device NOT simulator
#if TARGET_IPHONE_SIMULATOR
    NSLog(@"Video capture is not supported in the simulator");
#else
    _videoCapture = new cv::VideoCapture;
    if (!_videoCapture->open(CV_CAP_AVFOUNDATION))
    {
        NSLog(@"Failed to open video camera");
    }
#endif

    // Load a test image and demonstrate conversion between UIImage and cv::Mat
    UIImage *testImage = [UIImage imageNamed:@"testimage.jpg"];

    double t;
    int times = 10;

    //--------------------------------
    // Convert from UIImage to cv::Mat
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    t = (double)cv::getTickCount();

    for (int i = 0; i < times; i++)
    {
        cv::Mat tempMat = [testImage CVMat];
    }

    t = 1000 * ((double)cv::getTickCount() - t) / cv::getTickFrequency() / times;

    [pool release];

    NSLog(@"UIImage to cv::Mat: %gms", t);

    //------------------------------------------
    // Convert from UIImage to grayscale cv::Mat
    pool = [[NSAutoreleasePool alloc] init];

    t = (double)cv::getTickCount();

    for (int i = 0; i < times; i++)
    {
        cv::Mat tempMat = [testImage CVGrayscaleMat];
    }

    t = 1000 * ((double)cv::getTickCount() - t) / cv::getTickFrequency() / times;

    [pool release];

    NSLog(@"UIImage to grayscale cv::Mat: %gms", t);

    //--------------------------------
    // Convert from cv::Mat to UIImage
    cv::Mat testMat = [testImage CVMat];

    t = (double)cv::getTickCount();

    for (int i = 0; i < times; i++)
    {
        UIImage *tempImage = [[UIImage alloc] initWithCVMat:testMat];
        [tempImage release];
    }

    t = 1000 * ((double)cv::getTickCount() - t) / cv::getTickFrequency() / times;

    NSLog(@"cv::Mat to UIImage: %gms", t);

    // Process test image and force update of UI 
    _lastFrame = testMat;
    [self sliderChanged:nil];
}
- (IBAction)capture:(id)sender
{
    if (_videoCapture && _videoCapture->grab())
    {
        (*_videoCapture) >> _lastFrame;
        [self processFrame];
    }
    else
    {
        NSLog(@"Failed to grab frame");        
    }
}
- (void)processFrame
{
    double t = (double)cv::getTickCount();

    cv::Mat grayFrame, output;

    // Convert captured frame to grayscale
    cv::cvtColor(_lastFrame, grayFrame, cv::COLOR_RGB2GRAY);

    // Perform Canny edge detection using slide values for thresholds
    cv::Canny(grayFrame, output,
              _lowSlider.value * kCannyAperture * kCannyAperture,
              _highSlider.value * kCannyAperture * kCannyAperture,
              kCannyAperture);

    t = 1000 * ((double)cv::getTickCount() - t) / cv::getTickFrequency();

    // Display result 
    self.imageView.image = [UIImage imageWithCVMat:output];
    self.elapsedTimeLabel.text = [NSString stringWithFormat:@"%.1fms", t];
}

Answer 1

Inverting a grayscale 1-channel Mat is:

Mat invertedOutput = 255 - output;