Thursday 4 October 2018

Person detection(Yolo v3) with the helps of mxnet, able to run on gpu/cpu

    In this post I will show you how to do object detection with the helps of the cpp-package of mxnet. Why do I introduce mxnet? Because following advantages make it a decent library for standalone project development

1. It is open source and royalty free
2. Got decent support for GPU/CPU
3. Scaling efficiently to multiple GPU and machines
4. Support cpp api, which means you do not need to ask the users to install python environment , shipped the source codes in order to run your apps
5. mxnet support many platforms, including windows, linux, mac, aws, android, ios
6. It got a lot of pre-trained models
7. MMDNN support mxnet, which mean we can convert the models trained by different libraries to mxnet(although not all of the models could be converted).

Step 1 : Download model and convert it to the format can load by cpp package

1. Install anaconda(the version come with python3)
2. Install mxnet from the terminal of anaconda
3. Install gluon-cv from the terminal of anaconda
4. Download model and convert it by following scripts

import gluoncv as gcv
from gluoncv.utils import export_block

net = gcv.model_zoo.get_model('yolo3_darknet53_coco', pretrained=True)
export_block('yolo3_darknet53_coco', net)

 Step 2 : Load the models after convert

void load_check_point(std::string const &model_params,
                      std::string const &model_symbol,
                      Symbol *symbol,
                      std::map<std::string, NDArray> *arg_params,
                      std::map<std::string, NDArray> *aux_params,
                      Context const &ctx)
    Symbol new_symbol = Symbol::Load(model_symbol);
    std::map<std::string, NDArray> params = NDArray::LoadToMap(model_params);
    std::map<std::string, NDArray> args;
    std::map<std::string, NDArray> auxs;
    for (auto iter : params) {
        std::string type = iter.first.substr(0, 4);
        std::string name = iter.first.substr(4);
        if (type == "arg:")
            args[name] = iter.second.Copy(ctx);
        else if (type == "aux:")
            auxs[name] = iter.second.Copy(ctx);

    *symbol = new_symbol;
    *arg_params = args;
    *aux_params = auxs;

    You could use the load_check_point function as following

    Symbol net;
    std::map<std::string, NDArray> args, auxs;
    load_check_point(model_params, model_symbols, &net, &args, &auxs, context);

    //The shape of the input data must be the same, if you need different size,
    //you could rebind the Executor or create a pool of Executor.
    //In order to create input layer of the Executor, I make a dummy NDArray.
    //The value of the "data" could be change later
    args["data"] = NDArray(Shape(1, static_cast<unsigned>(input_size.height),
                                 static_cast<unsigned>(input_size.width), 3), context);
    executor_.reset(net.SimpleBind(context, args, std::map<std::string, NDArray>(),
                                   std::map<std::string, OpReqType>(), auxs));

    model_params is the location of the weights(ex : yolo3_darknet53_coco.params), model_symbols(ex : yolo3_darknet53_coco.json) is the location of the symbols saved as json.

Step 3: Convert image format

    Before we feed the image into the executor of mxnet, we need to convert them.

NDArray cvmat_to_ndarray(cv::Mat const &bgr_image, Context const &ctx)
    cv::Mat rgb_image;
    cv::cvtColor(bgr_image, rgb_image, cv::COLOR_BGR2RGB); 
    rgb_image.convertTo(rgb_image, CV_32FC3);
    //This api copy the data of rgb_image into NDArray. As far as I know,
    //opencv guarantee continuous of cv::Mat unless it is sub matrix of cv::Mat
    return NDArray(rgb_image.ptr<float>(),
                   Shape(1, static_cast<unsigned>(rgb_image.rows), static_cast<unsigned>(rgb_image.cols), 3),

Step 4 : Perform object detection on video

void object_detector::forward(const cv::Mat &input)
    //By default, input_size_.height equal to 256 input_size_.width equal to 320.
    //Yolo v3 has a limitation, width and height of the image must be divided by 32.
    if(input.rows != input_size_.height || input.cols != input_size_.width){
        cv::resize(input, resize_img_, input_size_);
        resize_img_ = input;

    auto data = cvmat_to_ndarray(resize_img_, *context_);
    //Copy the data of the image to the "data"
    //Forward is an async api.

Step 5 : Draw bounding boxes on image

void plot_object_detector_bboxes::plot(cv::Mat &inout,
                                       std::vector<mxnet::cpp::NDArray> const &predict_results,
                                       bool normalize)
    using namespace mxnet::cpp;

    //1. predict_results get from the output of Executor(executor_->outputs)
    //2. Must Set Context as cpu because we need process data by cpu later
    auto labels = predict_results[0].Copy(Context(kCPU, 0));
    auto scores = predict_results[1].Copy(Context(kCPU, 0));
    auto bboxes = predict_results[2].Copy(Context(kCPU, 0));
    //1. Should call wait because Forward api of Executor is async
    //2. scores and labels could treat as one dimension array
    //3. BBoxes can treat as 2 dimensions array

    size_t const num = bboxes.GetShape()[1];
    for(size_t i = 0; i < num; ++i) {
        float const score = scores.At(0, 0, i);
        if (score < thresh_) break;

        size_t const cls_id = static_cast<size_t>(labels.At(0, 0, i));
        auto const color = colors_[cls_id];
        //pt1 : top left; pt2 : bottom right
        cv::Point pt1, pt2;
        //get_points perform normalization
        std::tie(pt1, pt2) = normalize_points(bboxes.At(0, i, 0), bboxes.At(0, i, 1),
                                              bboxes.At(0, i, 2), bboxes.At(0, i, 3),
                                              normalize, cv::Size(inout.cols, inout.rows));
        cv::rectangle(inout, pt1, pt2, color, 2);

        std::string txt;
        if (labels_.size() > cls_id) {
            txt += labels_[cls_id];
        std::stringstream ss;
        ss << std::fixed << std::setprecision(3) << score;
        txt += " " + ss.str();
        put_label(inout, txt, pt1, color);

     I only mentioned the key points in this post, if you want to study the details, please check it on github.

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