Delete matches in OpenCV (Keypoints and descriptors)

Question

I want to check a scene image against two train images. For that, I detect features and compute descriptors of both training images. Before detecting, computing and matching the scene image, I will delete all matches of train1 and train2. Because these matches won't facilitate the matching of the scene image with train1 and train2.

So, I match train1 with train2 and get a vector of the matches with the trainIdx and queryIdx. But how can I delete these matches in the keypoints-vector and the descriptor matrix of train1 and train2?

Best regards, dwi

keypoints are a vector so create a new vector named unmatched keypoints abd add all indices tthat arent matched yet. descriptors are a matrix so create a new mat with all cols (?) indices that werent matched yet. to cheat maybe you can instead just use all keyp. and descriptors but set the descrptot entries of matched features to FLT_MAX — Micka
ok, create a new vector and add all others is possible. But I have problems with the matrix. If I use SIFT or SURF, it is easy, but with BRISK or other "string"-descriptors, I do not understand the matrix usage. I do not have as many rows as I got keypoints. :-/ Is there a way to mark keypoints/descriptors as redundant? Can I set their values NULL or something? — dwi

Satya Satya · Accepted Answer · 2016-11-22T09:00:00

I would have done like below:

std::vector<cv::KeyPoint> keypoints[2];
cv::Mat descriptor[2];
std::vector< cv::DMatch > matches;

/* 
      Write code to generate the keypoints, descriptors and matches here...
      keypoint[0] -> Train Image 1 keypoints
      keypoint[1] -> Train Image 2 keypoints
      descriptor[0] -> Train Image 1 descriptors
      descriptor[1] -> Train Image 2 descriptors
      matches -> matched between train image 1 and 2

*/

// Logic to keep unmatched keypoints and corresponding descriptors
for (int idx = 0; idx < 2; idx++) {
    std::vector<bool> isMatched(keypoints[idx].size(), false);

    // Mark all matched keypoint as true
    for (int i = 0; i < matches.size(); i++) {
        if (idx == 0) {
            isMatched[matches[i].queryIdx] = true;
        }
        else {
            isMatched[matches[i].trainIdx] = true;
        }
    }

    std::vector<cv::KeyPoint>::const_iterator itr = keypoints[idx].begin();

    // New descriptor length will be old descriptor length minus matched keypoints size
    int descriptor_length = keypoints[idx].size() - matches.size();

    // Create temporary descriptor of new descriptor length
    cv::Mat tempDescriptor(descriptor_length, descriptor[idx].cols, descriptor[idx].depth());
    int count = 0;
    for (int i = 0; i < isMatched.size(); i++) {
        // Remove matched keypoints
        if (isMatched[i] == true) {
            itr = keypoints[idx].erase(itr);
        }
        else {
            descriptor[idx].row(i).copyTo(tempDescriptor.row(count));
            itr++;
            count++;
        }
    }

    descriptor[idx].release();
    descriptor[idx] = tempDescriptor.clone();
}

I hope this will help.

Delete matches in OpenCV (Keypoints and descriptors)

2 Answers