删除OpenCV中的匹配项（关键点和描述符）

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

Answer 1

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.

Answer 2

OK, like Micka suggested, I iterate over all keypoints and descriptors and add all of them, except the matches, in a new vector/matrix. There is no possibility to mark them unnecessary.