Vector subscript out of range error in c++ and opencv

Question

I'm trying to write a program that uses ORB algorithm to detect and compute the keypoints of an image and matches descriptor vectors using FLANN matcher. The issue I am facing is, that every time I run the program on Visual C++, I am getting an error that tells "vector subscript out of range"(I've also attached an image of the error).

The problem seems to be somewhere in the for because when I start the debugger it stops there and I get the error. When I commented the first for to see if the rest is ok, I've got the same error on the second for.

Please help me find the problem.

#include <iostream>
#include <stdio.h>
#include <opencv2/opencv.hpp>
#include <opencv2/features2d.hpp>
#include <opencv2\core\types.hpp>
#include <opencv2\highgui.hpp>
#include <opencv2\core.hpp>
#include <opencv2\opencv_modules.hpp>

using namespace cv;
using namespace std;


int main()
{


Mat img1 = imread("C:\\Users\\patri\\Desktop\\test.bmp");
Mat img2 = imread("C:\\Users\\patri\\Desktop\\test3.bmp");
/*
if (!img1.data || !img2.data)
{
    printf(" --(!) Error reading images \n"); return -1;
}
*/
std::vector<KeyPoint> keypoints_1, keypoints_2;

Mat descriptors_1, descriptors_2;

Ptr<ORB> orb = ORB::create(100, 2, 8, 31, 0, 2, ORB::HARRIS_SCORE, 31, 20);

orb->detectAndCompute(img1, Mat(), keypoints_1, descriptors_1);
orb->detectAndCompute(img2, Mat(), keypoints_2, descriptors_2);

std::cout << "Found " << keypoints_1.size() << " Keypoints " << std::endl;
std::cout << "Found " << keypoints_2.size() << " Keypoints " << std::endl;

Mat out1, out2;
drawKeypoints(img1, keypoints_1, out1, Scalar::all(255));
drawKeypoints(img2, keypoints_2, out2, Scalar::all(255));

imshow("Kpts1", out1);
imshow("Kpts2", out2);
//////////////////////////////////////////////////////////////////////
// Matching descriptor vectors using FLANN matcher

FlannBasedMatcher matcher;
std::vector< DMatch > matches;
//matcher.match(descriptors_1, descriptors_2, matches);


double max_dist = 0; double min_dist = 100;

//calculation of max and min distances between keypoints
for (int i = 0; i < descriptors_1.rows; i++)
{
    double dist = matches[i].distance;
    if (dist < min_dist) min_dist = dist;
    if (dist > max_dist) max_dist = dist;
}

printf("-- Max dist : %f \n", max_dist);
printf("-- Min dist : %f \n", min_dist);


std::vector< DMatch > good_matches;

for (int i = 0; i < descriptors_1.rows; i++)
{
    if (matches[i].distance <= max(2 * min_dist, 0.02))
    {
        good_matches.push_back(matches[i]);
    }
}

//-- Draw only "good" matches
Mat img_matches;
drawMatches(img1, keypoints_1, img2, keypoints_2,
    good_matches, img_matches, Scalar::all(-1), Scalar::all(-1),
    vector<char>(), DrawMatchesFlags::NOT_DRAW_SINGLE_POINTS);

//-- Show detected matches
imshow("Good Matches", img_matches);

for (int i = 0; i < (int)good_matches.size(); i++)
{
    printf("-- Good Match [%d] Keypoint 1: %d  -- Keypoint 2: %d  \n", i, good_matches[i].queryIdx, good_matches[i].trainIdx);
}

waitKey(0);

return 0;
}   

the error I'm getting

Answer 1

std::vector< DMatch > matches; is empty but you are trying to access its elements in the for loop.

std::vector< DMatch > matches;//this creates an empty vector
//you need to push_back some elements in matches before trying to access it in your loops
......

//calculation of max and min distances between keypoints
for (int i = 0; i < descriptors_1.rows; i++)
{
    double dist = matches[i].distance;//this is trying to access the empty vector
    ......
}