CODEWITHSUNDEEP
c++visual-studioopencvvisual-studio-2015
Bernard Allotey
Bernard Allotey

Reputation: 822

Assertion Error when using Contour Area-OpenCV

I get this error whenever I use the contourArea function in openCV Error

Here is my code

while (true)
{
    Mat imgOriginal;

    bool bSuccess = cap.read(imgOriginal); // read a new frame from video



    if (!bSuccess) //if not success, break loop
    {
        cout << "Cannot read a frame from video stream" << endl;
        break;
    }

    Mat imgHSV;

    cvtColor(imgOriginal, imgHSV, COLOR_BGR2HSV); //Convert the captured frame from BGR to HSV

    Mat imgThresholded;

    inRange(imgHSV, Scalar(iLowH, iLowS, iLowV), Scalar(iHighH, iHighS, iHighV), imgThresholded); //Threshold the image

                                                                                                  //morphological opening (removes small objects from the foreground)
    erode(imgThresholded, imgThresholded, getStructuringElement(MORPH_ELLIPSE, Size(5, 5)));
    dilate(imgThresholded, imgThresholded, getStructuringElement(MORPH_ELLIPSE, Size(5, 5)));

    //morphological closing (removes small holes from the foreground)
    dilate(imgThresholded, imgThresholded, getStructuringElement(MORPH_ELLIPSE, Size(5, 5)));
    erode(imgThresholded, imgThresholded, getStructuringElement(MORPH_ELLIPSE, Size(5, 5)));


    int thresh = 100;
    Mat canny_output;
    vector<Vec4i> hierarchy;
    vector<vector<Point> > contours;
    RNG rng(12345);

    Canny(imgThresholded, canny_output, thresh, thresh * 2, 3);
    /// Find contours
    findContours(canny_output, contours, hierarchy, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE, Point(0, 0));

    /// Draw contours
    Mat drawing = Mat::zeros(canny_output.size(), CV_8UC3);
    for (int i = 0; i< contours.size(); i++)
    {
        Scalar color = Scalar(rng.uniform(0, 255), rng.uniform(0, 255), rng.uniform(0, 255));
        drawContours(drawing, contours, i, color, 2, 8, hierarchy, 0, Point());
    }


    double area = contourArea(contours);
    std::cout << area;

    imshow("contours", drawing);

My code was working mostly fine before this, but it is very important that i grab the contour Area, but it keeps giving me this error. Any help is greatly appreciated

Upvotes: 0

Views: 595

Answers (1)

ZdaR
ZdaR

Reputation: 22954

The cv::contourArea() takes a single contour and returns the area of that contour, Since the contours in your case is a list of contours, which may contain 0, or more contours, so you explicitly need to access a contour using subscript as:

if (contours.size() > 0) {
    for (int i=0; i<contours.size(); i++) {
        std::cout << "Contour id : " << i << ", area = " << cv::contourArea(contours[i]) << std::endl;
    }
} else {
    std::cout << "No contours found !" << std::endl;
}

Upvotes: 1

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