unxnut's profile - activity

I have an application where I need to look at multiple [almost] concentric circles. I have been having issues using HoughCircles and so, I decided to create a test image with a single circle. My image was created with the following command:

cv::Mat img = cv::Mat::zeros ( 480, 640, CV_8UC1 );
cv::circle ( img, cv::Point(200,200), 175, cv::Scalar(0XFF), 1);

I tried to run HoughCircles on this image as follows:

std::vector<cv::Vec3f> circles;
cv::HoughCircles(img, circles, cv::HOUGH_GRADIENT, 1, 10, 239, 20 );

I get a plethora of circles when there is clearly only one circle in the input image. I have been struggling with this for a while now and will appreciate any help. I should mention that these circles are off-center from where I created the center (200,200), and their radius is all over the place as well.

I seem to be having a problem trying to compute the mean and standard deviation of pixels in an image. Can someone look at my code and point out what I am doing wrong? I am unable to get correct results.

cv::Mat             img;
cv::Mat     mean;
cv::Mat     stddev;

cv::meanStdDev ( img, mean, stddev );
uchar       mean_pxl = mean.data[0];
uchar       stddev_pxl = stddev.data[0];

I have also tried to get the data using mean.at<uchar>(0) without success.

Thanks for your help.

I am trying to find circles in the attached image.

I have used the function cv::HoughCircles(img, circles, cv::HOUGH_GRADIENT, 1, 1);. However, it is unable to detect the circle in this image or other similar images. Can someone point what I may be doing wrong?

You may want to check getAffineTransform.

Your input image is of lesser width than the window title. Change the window title to a smaller string and the problem will go away. The utility is trying to show the complete window name string.

If it is periodic and you know the period, you may be able to look for the specified intensity and pattern and use a mask to clear it.

Try reading Wikipedia entry on OpenCV

You are trying to catch cv::Exception. Right?

I believe that if the write fails, it will issue an exception that can be caught.

Did you change project properties?

Can you replace ${OpenCV_LIBS} with opencv_core, opencv_highgui, and opencv_imgcodecs, and see if it fixes the linking? If yes, then, your variable is not set.

I had an issue with "incorrect hash" that seemed to persist. I fixed it by adding -DWITH_IPP=OFF. I thought I'll add it here for completeness.

Yes, it should detect the object when it moves elsewhere within the frame. However, it won't help you with scaling.

No, I gave up on it. I have been working on Windows for my OpenCV work.

Seems to me that cap >> mySrc captures two frames and buffers the second one. So, when you try to display the second image, it picks it up from the camera buffer. There is a lag of one frame as long as you keep hitting n.

Your code looks fine. So, the thing to check will be if you are linking with the right libraries. Also, are the two versions of OpenCV compiled with the same version of Visual Studio as that makes a difference in some cases (as I discovered painfully).

You should build with cmake. It should have automatically selected Visual Studio 2010 for you. On my system, it shows the current generator as Visual Studio 10 2010.

Looks like you should apply dilate, erode, erode, dilate to clean up the image. Then, you can use findcontours and eliminate contours with "small" area as well as any contours that touch the image borders. You may have to experiment a bit with the first part.

You are opening a .jpg file that I assume is a still image. VideoCapture opens this file but then, there is no way for it to go to a specified frame position because there is none.

I feel obligated to answer my question as I figured out the problem after a lot of wasted time. I had originally compiled OpenCV using Visual Studio 2010 and had been working fine. About a month ago, I needed to develop something using C++-11 features due to a collaboration and so, I installed Visual 2013 but did not recompile OpenCV to work with 2013. Things had been working fine till I encountered this issue with findContours even though I did not use any C++-11 features in this code. I compiled my code using 2010 and things worked as expected. Lesson learned: do not mix code from different versions of Visual Studio, even on the same machine. Sorry to waste your time.

approxPoly didn't help either :-(

I have the image here. I am using OpenCV 3.0 on Visual Studio 2013.

/= is not an error. However, in my 640x480 image, I get 71 contours. As I go through the contours, the first one has 4 points and an area of 56 (reasonable), the second one has 516 points and an area of 5.69e+19 (not right) and the third one has 120195045 points and issues an exception when computing area. I am perplexed.

I am writing a function that takes a binary 8-bit image and trying to find the average area of blobs in there. My function is:

void rm_noise(cv::Mat& img, cv::Mat& clean_img)
{
    std::vector<std::vector<cv::Point>> contours;
    std::vector<cv::Vec4i> hierarchy;
    cv::findContours ( img, contours, hierarchy, cv::RETR_EXTERNAL, cv::CHAIN_APPROX_SIMPLE);
    if ( contours.empty() )
        return;

    clean_img = cv::Mat::zeros(img.size(), CV_8UC1);
    for (size_t i = 0; i < hierarchy.size(); i++)
    {
        cv::drawContours(clean_img, contours, i, cv::Scalar(0XFF), cv::FILLED, cv::LINE_AA, hierarchy);
    }
    cv::imshow("edges", clean_img);
    cv::waitKey();

    double area = 0;
    //for (std::vector<std::vector<cv::Point>>::const_iterator c = contours.begin(); c != contours.end(); c++)
    for (size_t i = 0; i >= 0; i = hierarchy[i][0])
    {
        const std::vector<cv::Point>& c = contours[i];
        double a = std::fabs(cv::contourArea(cv::Mat(c)));
        area += a;
    }
    double avg_area = area /= contours.size();
}

findContours seems to work correctly as I can see the contours drawn properly using a loop with drawContours. However, when I try to get the information on contours such as contourArea or even bounding box, I get strange results. The results seem to give me contour area as very large (something of the order of 10^19 pixels) which is obviously wrong. And the code seg faults. Can someone help me with this as I have already wasted about five days trying different combinations to no avail.

And you can easily do that by specifying that area. For example, for your frame, you can do the operation on frame.operator()(rect) where rect is a rectangle defining your region of interest.

Will meanStdDev solve your problem?

You can do that in Makefile if you are using one.

I think you are using forward slash on the command line argument when you should be using backward slash to separate directories. That is how you have showed your command line parameters above the code.

And then, how did you populate it? Did you use findcontours? If so, and the size is 0, that means no contours were found as pointed by @berak.