json format not correct

[oym1994]

Hi, I use nlohmann /json to create a json file. I want to watch it in vscode using json viewer but I get the error " json format not correct" and can't see the content. What should I do next?
Thank you!

[nlohmann]

How does the JSON file look like? What is the exact error message?

[oym1994]

Hi, thanks for ur response.

Actually ur project is adopte in OpenVSLAM. In the last function of this file map_database.cc , the reconstructed map is converted
to json file
`
void map_database::to_json(nlohmann::json& json_keyfrms, nlohmann::json& json_landmarks) {
std::lock_guardstd::mutex lock(mtx_map_access_);

// Save each keyframe as json
spdlog::info("encoding {} keyframes to store", keyframes_.size());
std::map<std::string, nlohmann::json> keyfrms;
for (const auto id_keyfrm : keyframes_) {
    const auto id = id_keyfrm.first;
    const auto keyfrm = id_keyfrm.second;
    assert(keyfrm);
    assert(id == keyfrm->id_);
    assert(!keyfrm->will_be_erased());
    keyfrm->graph_node_->update_connections();
    assert(!keyfrms.count(std::to_string(id)));
    keyfrms[std::to_string(id)] = keyfrm->to_json();
}
json_keyfrms = keyfrms;

// Save each 3D point as json
spdlog::info("encoding {} landmarks to store", landmarks_.size());
std::map<std::string, nlohmann::json> landmarks;
for (const auto id_lm : landmarks_) {
    const auto id = id_lm.first;
    const auto lm = id_lm.second;
    assert(lm);
    assert(id == lm->id_);
    assert(!lm->will_be_erased());
    lm->update_normal_and_depth();
    assert(!landmarks.count(std::to_string(id)));
    landmarks[std::to_string(id)] = lm->to_json();
}
json_landmarks = landmarks;

}
`

Whie opeing the generated json file by using vscode with json viewer, it gives the following error:
"The file is not displayed in the edit because it is either binary or uses an unsupported text encoding."

Thank you!

[nlohmann]

How do you write json_landmarks to a file?

[oym1994]

Here is the code for converting landmarks and keyframes to json file
`
nlohmann::json keyframe::to_json() const {
// extract landmark IDs
std::vector landmark_ids(landmarks_.size(), -1);
for (unsigned int i = 0; i < landmark_ids.size(); ++i) {
if (landmarks_.at(i) && !landmarks_.at(i)->will_be_erased()) {
landmark_ids.at(i) = landmarks_.at(i)->id_;
}
}

// extract spanning tree parent
auto spanning_parent = graph_node_->get_spanning_parent();

// extract spanning tree children
const auto spanning_children = graph_node_->get_spanning_children();
std::vector<int> spanning_child_ids;
spanning_child_ids.reserve(spanning_children.size());
for (const auto spanning_child : spanning_children) {
    spanning_child_ids.push_back(spanning_child->id_);
}

// extract loop edges
const auto loop_edges = graph_node_->get_loop_edges();
std::vector<int> loop_edge_ids;
for (const auto loop_edge : loop_edges) {
    loop_edge_ids.push_back(loop_edge->id_);
}

return {{"src_frm_id", src_frm_id_},
        {"ts", timestamp_},
        {"cam", camera_->name_},
        {"depth_thr", depth_thr_},
        // camera pose
        {"rot_cw", convert_rotation_to_json(cam_pose_cw_.block<3, 3>(0, 0))},
        {"trans_cw", convert_translation_to_json(cam_pose_cw_.block<3, 1>(0, 3))},
        // features and observations
        {"n_keypts", num_keypts_},
        {"keypts", convert_keypoints_to_json(keypts_)},
        {"undists", convert_undistorted_to_json(undist_keypts_)},
        {"x_rights", stereo_x_right_},
        {"depths", depths_},
        {"descs", convert_descriptors_to_json(descriptors_)},
        {"lm_ids", landmark_ids},
        // orb scale information
        {"n_scale_levels", num_scale_levels_},
        {"scale_factor", scale_factor_},
        // graph information
        {"span_parent", spanning_parent ? spanning_parent->id_ : -1},
        {"span_children", spanning_child_ids},
        {"loop_edges", loop_edge_ids}};

}

nlohmann::json landmark::to_json() const {
return {{"1st_keyfrm", first_keyfrm_id_},
{"pos_w", {pos_w_(0), pos_w_(1), pos_w_(2)}},
{"ref_keyfrm", ref_keyfrm_->id_},
{"n_vis", num_observable_},
{"n_fnd", num_observed_}};

}
`

The final wrting code is:
`
nlohmann::json keyfrms;
nlohmann::json landmarks;
map_db_->to_json(keyfrms, landmarks);

nlohmann::json json{{"cameras", cameras},
                    {"keyframes", keyfrms},
                    {"landmarks", landmarks},
                    {"frame_next_id", static_cast<unsigned int>(data::frame::next_id_)},
                    {"keyframe_next_id", static_cast<unsigned int>(data::keyframe::next_id_)},
                    {"landmark_next_id", static_cast<unsigned int>(data::landmark::next_id_)}};

std::ofstream ofs(path, std::ios::out | std::ios::binary);

if (ofs.is_open()) {
    spdlog::info("save the MessagePack file of database to {}", path);
    const auto msgpack = nlohmann::json::to_msgpack(json);
    ofs.write(reinterpret_cast<const char*>(msgpack.data()), msgpack.size() * sizeof(uint8_t));
    ofs.close();
}
else {
    spdlog::critical("cannot create a file at {}", path);
}

`

The saved json file can be read by the json program, but it can't be openned in vscode.
Thank you!

[nlohmann]

You seem to write a binary file in MessagePack format, not a JSON file. This could explain the error message.

[oym1994]

Got it! Thank you for your kind response!