c++ - Parallelizing a for loop using openmp & replacing push_back

Question

I'd like to parallelize the following piece of code but am new to openmp and creating parallel code.

std::vector<DMatch> good_matches;
for (int i = 0; i < descriptors_A.rows; i++) {
   if (matches_RM[i].distance < 3 * min_dist) {
      good_matches.push_back(matches_RM[i]);
   }
}

I have tried

std::vector<DMatch> good_matches;
#pragma omp parallel for
for (int i = 0; i < descriptors_A.rows; i++) {
   if (matches_RM[i].distance < 3 * min_dist) {
      good_matches[i] = matches_RM[i];
   }
}

and

std::vector<DMatch> good_matches;
cv::DMatch temp;
#pragma omp parallel for
for (int i = 0; i < descriptors_A.rows; i++) {
   if (matches_RM[i].distance < 3 * min_dist) {
      temp = matches_RM[i];
      good_matches[i] = temp;
      // AND ALSO good_matches.push_back(temp);
   }

I have also tried

#omp parallel critical 
good_matches.push_back(matches_RM[i]);

This clause works but does not speed anything up. It may be the case that this for loop cannot be sped up but it'd be great if it can be. I'd also like to speed this up as well

std::vector<Point2f> obj, scene;
for (int i = 0; i < good_matches.size(); i++) {
   obj.push_back(keypoints_A[good_matches[i].queryIdx].pt);
   scene.push_back(keypoints_B[good_matches[i].trainIdx].pt);
}

Apologies if this question as been answered and thank you very much to anyone who can help.

Answer 1

I showed how to do this here c-openmp-parallel-for-loop-alternatives-to-stdvector

Make private versions of the std::vector and fill the shared std::vector in a critical section like this:

std::vector<DMatch> good_matches;
#pragma omp parallel
{
    std::vector<DMatch> good_matches_private;
    #pragma omp for nowait
    for (int i = 0; i < descriptors_A.rows; i++) {
       if (matches_RM[i].distance < 3 * min_dist) {
          good_matches_private.push_back(matches_RM[i]);
       }
    }
    #pragma omp critical
    good_matches.insert(good_matches.end(), good_matches_private.begin(), good_matches_private.end());
}