segmentation fault: 11, extracting data in vector - c++

Question

I'm trying to write a program with opencv and C++. I have an image and I am trying to get the saturation value of determined pixel which is in the (x, y) point. I use the next sentence to do it:
saturation_level = hsv_chanels[1].at<uchar>(x, y);
The thing is that the program builds OK, but when I try to run, it sometimes works fine and sometimes terminates with this error:
Segmentation fault: 11
Do someone know why this error is appearing? I read that this error appears because of the my computer memory but I don't know why it only appears sometimes.
EDIT:
This is the function I call to find the homography:
Mat ObtenHomografiaSuelo (vector <KeyPoint> keypoints1, vector <KeyPoint> keypoints2, Mat imagen1, Mat imagen2){
//*****************************************************************************
//Find homography mat
//*****************************************************************************
vector < Point2f > image_points[2];
int cont = 0;
vector<Mat> chanels_hsv1; //[0]->H, [1]->S, [2]->V
split( image1, chanels_hsv1 );
vector<Mat> chanels_hsv2;
split( image2, chanels_hsv2 );
for(vector<KeyPoint>::const_iterator it = keypoints1.begin(); it!= keypoints1.end(); ++it){
// Get the position of left keypoints
float x = (it->pt.x);
float y = (it->pt.y);
cout << "1" << endl;
float saturation_level = chanels_hsv1[1].at<uchar>(x, y);
cout << "2" << endl;
double max_level = 70.0;
cout << "3" << endl;
if ((y < camSize.height/4) && (saturatio_level < max_level) ){
cout << "1:" << endl;
waitKey (100);
cout << "y: " << y;
cout << " Saturation_Level: " << nivel_saturacion << endl;
image_points[0].push_back(Point2f(x,y));
cout << "done" << endl;
cont ++;
}
}
cont = 0;
for (vector<KeyPoint>::const_iterator it = keypoints2.begin(); it!=keypoints2.end(); ++it) {
// Get the position of left keypoints
float x = (it->pt.x);
float y = (it->pt.y);
float saturation_level = chanels_hsv2[1].at<uchar>(x, y);
double max_level = 70.0;
if ((y < (camSize.height)/4) && (saturation_level < max_level)){
cout << "2" << endl;
waitKey (100);
cout << "y: " << y;
cout << " Saturation_Level: " << nivel_saturacion << endl;
image_points[1].push_back(Point2f(x,y));
cont ++;
}
}
cout << "We are obtain: " << cont << " points to do the homography" << endl;
waitKey();
Mat H;
H = Mat::zeros(4, 4, CV_64F);
if (cont < 4) {
cout << "Few points to do the homography" << endl;
}
else{
if (image_points[0].size() > image_points[1].size()){
image_points[0].resize(image_points[1].size());
}
else if (image_points[1].size() > image_points[0].size()){
image_points[1].resize(image_points[0].size());
}
H = findHomography (image_points[0], image_points[1], CV_RANSAC, 3);
cout << "done_matrix" << endl;
}
return H;
}
Before to call the function I detect keypoints using Harris or any other detector and the image I passed to the function is a HSV_image converted by cvtColor function.
The error appears in the line that I mentioned before because in the terminal I can se:
1
Segmentation Fault: 11

Answer 1

I've just finished to correct the error, I think it was because I wasn't using efficiently my function. I was using two 'for' staments to cross my two vectors of keypoints detected and the errors sometimes appears at first and others at second one. I don't really know why was the error, I just change my code to do the same think more efficiently and finally it works.
I just changed this lines:
for(vector<KeyPoint>::const_iterator it = keypoints1.begin(); it!= keypoints1.end(); ++it){
// Get the position of left keypoints
float x = (it->pt.x);
float y = (it->pt.y);
cout << "1" << endl;
float saturation_level = chanels_hsv1[1].at<uchar>(x, y);
cout << "2" << endl;
double max_level = 70.0;
cout << "3" << endl;
if ((y < camSize.height/4) && (saturatio_level < max_level) ){
cout << "1:" << endl;
waitKey (100);
cout << "y: " << y;
cout << " Saturation_Level: " << nivel_saturacion << endl;
image_points[0].push_back(Point2f(x,y));
cout << "done" << endl;
cont ++;
}
cont = 0;
for (vector<KeyPoint>::const_iterator it = keypoints2.begin(); it!=keypoints2.end(); ++it) {
// Get the position of left keypoints
float x = (it->pt.x);
float y = (it->pt.y);
float saturation_level = chanels_hsv2[1].at<uchar>(x, y);
double max_level = 70.0;
if ((y < (camSize.height)/4) && (saturation_level < max_level)){
cout << "2" << endl;
waitKey (100);
cout << "y: " << y;
cout << " Saturation_Level: " << nivel_saturacion << endl;
image_points[1].push_back(Point2f(x,y));
cont ++;
}
}
by this ones:
for (int i = 0; i < good_matches.size(); i++) {
int idx1=good_matches[i].queryIdx;
int idx2=good_matches[i].trainIdx;
if (((keypoints[0][idx1].pt.y < (camSize.height/4)) && (canales_hsv1[1].at<uchar>(keypoints[0][idx1].pt.x, keypoints[0][idx1].pt.y) < nivel_maximo)) || ((keypoints[1][idx2].pt.y < (camSize.height/4)) && (canales_hsv2[1].at<uchar>(keypoints[1][idx1].pt.x, keypoints[1][idx2].pt.y) < nivel_maximo)) ) {
cout << "entro" << endl;
matched_points[0].push_back(keypoints[0][idx1].pt);
matched_points[1].push_back(keypoints[1][idx2].pt);
contador ++;
}
}
Currently I only cross the matched keypoints instead of all the keypoints, it requires less computers operations and now it works OK.