OpenCV vector access error

Question

I am using this code from the project FingertipTuio3D

std::vector<cv::Point2i> detectFingertips(cv::Mat1f z, float zMin = 0.0f, float zMax = 0.75f, cv::Mat1f& debugFrame = cv::Mat1f()) {
    using namespace cv;
    using namespace std;    
    bool debug = !debugFrame.empty();

    vector<Point2i> fingerTips;

    Mat handMask = z < zMax & z > zMin;

    std::vector<std::vector<cv::Point>> contours;
    findContours(handMask.clone(), contours, CV_RETR_LIST, CV_CHAIN_APPROX_SIMPLE); // we are cloning here since method will destruct the image

    if (contours.size()) {
        for (int i=0; i<contours.size(); i++) {
            vector<Point> contour = contours[i];
            Mat contourMat = Mat(contour);
            double area = cv::contourArea(contourMat);

            if (area > 3000)  { // possible hand
                Scalar center = mean(contourMat);
                Point centerPoint = Point(center.val[0], center.val[1]);

                vector<Point> approxCurve;
                cv::approxPolyDP(contourMat, approxCurve, 20, true);

                vector<int> hull;
                cv::convexHull(Mat(approxCurve), hull);

                // find upper and lower bounds of the hand and define cutoff threshold (don't consider lower vertices as fingers)
                int upper = 640, lower = 0;
                for (int j=0; j<hull.size(); j++) {
                    int idx = hull[j]; // corner index
                    if (approxCurve[idx].y < upper) upper = approxCurve[idx].y;
                    if (approxCurve[idx].y > lower) lower = approxCurve[idx].y;
                }
                float cutoff = lower - (lower - upper) * 0.1f;

                // find interior angles of hull corners
                for (int j=0; j<hull.size(); j++) {
                    int idx = hull[j]; // corner index
                    int pdx = idx == 0 ? approxCurve.size() - 1 : idx - 1; //  predecessor of idx
                    int sdx = idx == approxCurve.size() - 1 ? 0 : idx + 1; // successor of idx

                    Point v1 = approxCurve[sdx] - approxCurve[idx];
                    Point v2 = approxCurve[pdx] - approxCurve[idx];

                    float angle = acos( (v1.x*v2.x + v1.y*v2.y) / (norm(v1) * norm(v2)) );

                    // low interior angle + within upper 90% of region -> we got a finger
                    if (angle < 1 && approxCurve[idx].y < cutoff) {
                        int u = approxCurve[idx].x;
                        int v = approxCurve[idx].y;

                        fingerTips.push_back(Point2i(u,v));

                        if (debug) {
                            cv::circle(debugFrame, approxCurve[idx], 10, Scalar(1), -1);
                        }
                    }
                }

                if (debug) {
                    // draw cutoff threshold
                    cv::line(debugFrame, Point(center.val[0]-100, cutoff), Point(center.val[0]+100, cutoff), Scalar(1.0f));

                    // draw approxCurve
                    for (int j=0; j<approxCurve.size(); j++) {
                        cv::circle(debugFrame, approxCurve[j], 10, Scalar(1.0f));
                        if (j != 0) {
                            cv::line(debugFrame, approxCurve[j], approxCurve[j-1], Scalar(1.0f));
                        } else {
                            cv::line(debugFrame, approxCurve[0], approxCurve[approxCurve.size()-1], Scalar(1.0f));
                        }
                    }

                    // draw approxCurve hull
                    for (int j=0; j<hull.size(); j++) {
                        cv::circle(debugFrame, approxCurve[hull[j]], 10, Scalar(1.0f), 3);
                        if(j == 0) {
                            cv::line(debugFrame, approxCurve[hull[j]], approxCurve[hull[hull.size()-1]], Scalar(1.0f));
                        } else {
                            cv::line(debugFrame, approxCurve[hull[j]], approxCurve[hull[j-1]], Scalar(1.0f));
                        }
                    }
                }
            }
        }
    }

    return fingerTips;
}

When the code reaches this point vector<Point> contour = contours[i]; it fails with an AccessViolation:

Unhandled exception at 0x00b85039 in FingertipTuio3d.exe: 0xC0000005: Access violation reading location 0x00000008.

That location is in the size_type size() const function of std::vector.

Any idea what is causing the problem, and how it might be fixed?

Answer 1

Does your findContours actually put anything into each contour?

You are checking that the array exists but not that the individual elements are valid

Answer 2

Can you access the elements via an iterator? IOW, if you add the following code immediately after the call to loadContours, what output do you get?

int i = 0;
for (std::vector<std::vector<cv::Point> >::iterator it = contours.begin();
     it != contours.end();
     ++it)
{
  std::cout << "contours[" << i << "].size() == " << it->size() << std::endl;
}

Answer 3

because contours is a vector of vector (std::vector> contours;)

and contour is a vector of points(vector contour)

so it cannot do vector contour = contours[i];

if you want to copy contours[i] you need to take another vector of vector.

Answer 4

This is build linked error. look at this:

--#ifdef _DEBUG 
--#pragma comment(lib,"opencv_core249d.lib")
--#else
--#pragma comment(lib,"opencv_core249.lib")    <<---- your checked!!
--#endif  

:-)