How to fix OpenCV Segmentation Fault? - c++

Question

I am trying to use openCV to detect red round object and draw a circle around that object. However,the segmentation fault occurs when i use circle function to draw circle. I don't know why is it happening and how to fix it? Thanks!!
#include <opencv/cvaux.h>
#include <opencv/highgui.h>
#include <opencv/cxcore.h>
#include <stdlib.h>
#include <cv.hpp>
#include <cxcore.hpp>
#include <highgui.h>
#include <opencv2/core/core.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <iostream>
#include<stdio.h>
#include<math.h>
#include<opencv/cv.h>
#include<opencv/highgui.h>
#include<opencv2/objdetect/objdetect.hpp>
#include<opencv2/highgui/highgui.hpp>
#include<opencv2/imgproc/imgproc.hpp>
#include<vector>
using namespace cv; // if you don want to use scope resolution operator(::) in the code to call the classes or functions from cv namespace, you need this line
using namespace std; // if you don want to use scope resolution operator(::) in the code to call the classes or functions from std namespace, you need this line
int main(int argc, char* argv[]){
VideoCapture capWebcam(0); //use scope resolution operator :: because VideoCapture is a class under namespace of cv
//use VideoCapture class to instantiate an object called capWebcam; here used the constructor of the object immediately to
//grab the only (0) camera
if(capWebcam.isOpened()==false){ //check whether the camera is detected and successfully grabbed
printf("Error: camera not detected!!\n");
cout<<"Error: camera not detected!!\n"<<endl;
return(1);
}
Mat matOriginal; // matrix object used to store image from webcam
Mat matProcessed;
vector<Vec3f> vecCircles; //declare a 3-element vector of type floats, this will be the pass by reference(i.e. a pointer) output of HoughCicles()
vector<Vec3f>::iterator itrCircles; //iterator for circles vector just a counter, but has the same data type from the itrCircles' data member
namedWindow("Original"); //window for original image
namedWindow("Processed"); //window for Processed image
char charCheckForEscKey =0;
while(charCheckForEscKey!=27){ //as long as ESC is not pressed, stays in the while
if(capWebcam.read(matOriginal) == false){ //check to see whether the image read from webcam correctly
cout<<"Error: image frame not read!!\n"<<endl;
break;
} //
inRange(matOriginal, //this time we don't need to pass a pointer; we pass the image as an object instead
Scalar(0,0,175), //specify the lower bound of BGR we want to keep
Scalar(100,100,256), //upper bound of BGR
matProcessed); //return the processed image to another object
GaussianBlur(matProcessed,matProcessed,Size(9,9),1.5,1.5); //take matProcessed image and blur by Gaussian filter(9x9 window with std of 1.5 in both x,y direction) and return to same object
HoughCircles(matProcessed,
vecCircles, //use vector element to receive the x,y,radius of the detected circle
CV_HOUGH_GRADIENT, //algorithms used to detect circles
2, //size of image divided by this value = "accumulator resolution"
matProcessed.rows/4, //min distance between the centers of two detected circles
100, //upper pixel value threshold for canny edge detection to interpret as edge
50, //lower pixel value threshold for canny edge detection to interpret as edge
10, //min radius of a circle can be detected
400); //max radius of a circle can be detected
for(itrCircles = vecCircles.begin();itrCircles != vecCircles.end();itrCircles++) //retrieve the x,y and radius of the detected circles from vecCircles object one by one
cout<< "circle position x = " << (*itrCircles)[0] //because itrCircles is a pointer(pass by reference), to get the value need to use * to dereference
<< ",y = " << (*itrCircles)[1]
<< ",r = " << (*itrCircles)[2] << "\n" << endl;
// draw the center of detected circle in green
circle(matOriginal,
Point((int)(*itrCircles)[0],(int)(*itrCircles)[1]),
3,
Scalar(0,255,0),
CV_FILLED);
// draw the circumference of detected circle
circle(matOriginal,
Point((int)(*itrCircles)[0],(int)(*itrCircles)[1]),
(int)(*itrCircles)[2],
Scalar(0,0,255),
3);
imshow("Original",matOriginal); //show the original mat(image) in Original window
imshow("Processed",matProcessed);// show the processed mat(image) in Processed window
charCheckForEscKey = waitKey(10); // delay 10 ms to allow a time gap to listen to any key pressed
} // end while
return(0);
} // end main

Answer 1

The crash is caused by the missing parenthesis on the for loop, and so the iterators you are using to draw are not correctly initialized. You should do:
for(itrCircles = vecCircles.begin();itrCircles != vecCircles.end();itrCircles++)
{
// your functions
}
May I suggest to drop iterators, and use the foreach loop?
for (const auto& circ : vecCircles)
{
// your functions
}
Here the full example, cleaned from all useless stuff (especially useless headers).
#include <opencv2\opencv.hpp>
#include <iostream>
#include<vector>
using namespace cv;
using namespace std;
int main(){
VideoCapture capWebcam(0);
if (capWebcam.isOpened() == false){
cout << "Error: camera not detected!!\n" << endl;
return -1;
}
Mat matOriginal; // matrix object used to store image from webcam
Mat matProcessed;
vector<Vec3f> vecCircles;
namedWindow("Original"); //window for original image
namedWindow("Processed"); //window for Processed image
char charCheckForEscKey = 0;
while (charCheckForEscKey != 27){ //as long as ESC is not pressed, stays in the while
if (!capWebcam.read(matOriginal)){
cout << "Error: image frame not read!!" << endl;
break;
} //
inRange(matOriginal, //this time we don't need to pass a pointer; we pass the image as an object instead
Scalar(0, 0, 175), //specify the lower bound of BGR we want to keep
Scalar(100, 100, 256), //upper bound of BGR
matProcessed); //return the processed image to another object
GaussianBlur(matProcessed, matProcessed, Size(9, 9), 1.5, 1.5); //take matProcessed image and blur by Gaussian filter(9x9 window with std of 1.5 in both x,y direction) and return to same object
HoughCircles(matProcessed,
vecCircles, //use vector element to receive the x,y,radius of the detected circle
CV_HOUGH_GRADIENT, //algorithms used to detect circles
2, //size of image divided by this value = "accumulator resolution"
matProcessed.rows / 4, //min distance between the centers of two detected circles
100, //upper pixel value threshold for canny edge detection to interpret as edge
50, //lower pixel value threshold for canny edge detection to interpret as edge
10, //min radius of a circle can be detected
400); //max radius of a circle can be detected
for (const auto& circ : vecCircles) //retrieve the x,y and radius of the detected circles from vecCircles object one by one
{
cout << "circle position x = " << circ[0] //because itrCircles is a pointer(pass by reference), to get the value need to use * to dereference
<< ",y = " << circ[1]
<< ",r = " << circ[2] << "\n" << endl;
// draw the center of detected circle in green
circle(matOriginal, Point(circ[0], circ[1]), 3, Scalar(0, 255, 0), CV_FILLED);
// draw the circumference of detected circle
circle(matOriginal, Point(circ[0], circ[1]), circ[2], Scalar(0, 0, 255), 3);
}
imshow("Original", matOriginal); //show the original mat(image) in Original window
imshow("Processed", matProcessed);// show the processed mat(image) in Processed window
charCheckForEscKey = waitKey(10); // delay 10 ms to allow a time gap to listen to any key pressed
} // end while
return(0);
} // end main