using  kp_idx_t = size_t;
        using landmark_idx_t = size_t;
        using img_idx_t = size_t;

        std::map<kp_idx_t, std::map<img_idx_t, kp_idx_t>> kp_matches;
        std::map<kp_idx_t, landmark_idx_t> kp_landmark;


        kp_idx_t& kp_match_idx(size_t kp_idx, size_t img_idx) { return kp_matches[kp_idx][img_idx]; };
        bool kp_match_exist(size_t kp_idx, size_t img_idx) { return kp_matches[kp_idx].count(img_idx) > 0; };