segmentation fault in findContours

问题

I am getting segmentation fault error during run time for the below code to find contours. I have referred this post on this form but didn't help me much. I got to know there are some issues with findContours This is another issue of findContours. Please check both the links and help me to resolve this error. I don't know why I am getting segmentation fault error.

 #include "opencv2/objdetect/objdetect.hpp"
 #include "opencv2/highgui/highgui.hpp"
 #include "opencv2/imgproc/imgproc.hpp"

 #include <iostream>
 #include <stdio.h>

 using namespace std;
 using namespace cv;

 string window_name = "Captured rectangle block";
 RNG rng(12345);
 double fps;
 int thresh = 100;

 int main( int argc, const char** argv )
 {
    VideoCapture cap("video.mp4"); // open the video file for reading
    if ( !cap.isOpened() )  // if not success, exit program
    {
         cout << "Cannot open the video file" << endl;
         return -1;
    }
    fps = cap.get(CV_CAP_PROP_FPS); //get the frames per seconds of the video
    cout << "Frame per seconds : " << fps << endl;
    double dWidth = cap.get(CV_CAP_PROP_FRAME_WIDTH);
    double dHeight = cap.get(CV_CAP_PROP_FRAME_HEIGHT);
    Size S(dWidth,dHeight);

    while(1)
    {

        Mat frame;
        Mat threshold_output;
        int skip_frame = 4;
        while(skip_frame)
        {
            printf("inside while loop\n");
            bool bSuccess = cap.read(frame); // read a new frame from video
            skip_frame--;
            if (!bSuccess) //if not success, break loop
            {
                cout << "Cannot read the frame from video file" << endl;
                break;
            }
        }

        //-- 3. Apply the classifier to the frame
        if( frame.empty() )
        { printf(" --(!) No captured frame -- Break!"); break; }

        std::vector<Rect> faces;
        Mat frame_gray;

        cvtColor( frame, frame_gray, CV_BGR2GRAY );
        equalizeHist( frame_gray, frame_gray);

        vector<vector<Point> > contours;
        vector<Vec4i> hierarchy;
        vector<vector<Point> > contours_poly( contours.size() );
        vector<Rect> boundRect( contours.size() );
        vector<Point2f>center( contours.size() );
        vector<float>radius( contours.size() );
        printf("before finding countrs\n");
        threshold( frame_gray, threshold_output, thresh, 255, THRESH_BINARY );
        findContours( threshold_output, contours, hierarchy, RETR_TREE, CHAIN_APPROX_SIMPLE, Point(0, 0));

        contours.resize(contours.size());
        printf("after finding countrs\n");
        for( unsigned int i = 0; i < contours.size(); i++ )
        {   
            printf("inside for loop\n");
            approxPolyDP( Mat(contours[i]), contours_poly[i], 3, true );
            printf("after poly\n");
            boundRect[i] = boundingRect( Mat(contours_poly[i]) );
            printf("after bondrec\n");
            minEnclosingCircle( (Mat)contours_poly[i], center[i], radius[i] );
        }
        Mat drawing = Mat::zeros( threshold_output.size(), CV_8UC3 );
        for( unsigned 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_poly, i, color, 1, 8, vector<Vec4i>(), 0, Point() );
            rectangle( drawing, boundRect[i].tl(), boundRect[i].br(), color, 2, 8, 0 );
            circle( drawing, center[i], (int)radius[i], color, 2, 8, 0 );
        }

        /// Show in a window
        namedWindow( "Contours", CV_WINDOW_AUTOSIZE );
        imshow( "Contours", drawing );
        int c = waitKey(10);
        if( (char)c == 'c' ) { break; }
    }
    return 0;
 }

Error:

Frame per seconds : 15
inside while loop
inside while loop
inside while loop
inside while loop
before finding countrs
after finding countrs
inside for loop
Segmentation fault (core dumped)

回答1:

I don't know openCv, but... I suppose that your contour vector is feeded by findContours().

So, if I'm not wrong, the istruction

vector<vector<Point> > contours;

creates a vector of zero elements; the following istructions

vector<vector<Point> > contours_poly( contours.size() );
vector<Rect> boundRect( contours.size() );
vector<Point2f>center( contours.size() );
vector<float>radius( contours.size() );

create other vectors of zero elements (contours.size() is zero).

Then findContours() is called and contours change size.

The following istruction is unuseful

contours.resize(contours.size());

but the real problem start here

approxPolyDP( Mat(contours[i]), contours_poly[i], 3, true );

when you try to access the first element of a zero size vector (contours_poly)

Suggestion: declare (or resize) the other vectors after the call to findContours()

p.s.: sorry for my bad english

来源：https://stackoverflow.com/questions/36138094/segmentation-fault-in-findcontours