Qr Recognition

  • This tutorial assumes that RAPP API and OpenCV are installed and built
  • This tutorial uses the 0.7.0 version of the C++ API

We are going to do a project using OpenCV, RAPP and CMake.

Using CMake requires you to configure your CMakeLists.txt in the root of your RAPP project. The structure is the following:


Source code

We create a project called qr_recognition with a folder source where we have our example qr_recognition.cpp. You can see the complete example here.

In this example we are taking images from a camera and then we call the RAPP platform and look for qr_codes. We use a usb camera and OpenCV for image acquisition:

cv::VideoCapture camera(0); 
if(!camera.isOpened()) { 
    std::cout << "Failed to connect to the camera" << std::endl;
    return -1;

We created the parameter camera which is our dev0. You will have to check if your camera is in the correct device and change the number if it's necessary. In the case that the program don't recognise any devices, the program will close.

In other examples, we only see the result with a stdout. Now we would like to see the position of the qr_codes in the image that our camera is recording. So, we are going to create an OpenCV window:

cv::namedWindow("QR recognition", cv::WINDOW_AUTOSIZE);

The size of the window depends of your camera resolution. At the same time, we can initialize our OpenCV matrix where we are going to save the image data from the camera:

cv::Mat frame;

At this point, we can continue our program like RAPP API examples. We are going to initialize the platform information and the service controller, which is in charge of make the cloud calls to the RAPP platform:

rapp::cloud::platform info = {"rapp.ee.auth.gr", "9001", "rapp_token"}; 
rapp::cloud::service_controller ctrl(info);

One the parts that is needed to make the call is a callback:

auto callback = [&](std::vector<rapp::object::qr_code> qr_codes) { 
    std::cout << "Found: " << qr_codes.size() << " qr_codes" << std::endl; 
    for(auto each_qr_code : qr_codes) {
                      cv::Point(each_qr_code.get_left_x(), each_qr_code.get_left_y()),
                      cv::Point(each_qr_code.get_right_x(), each_qr_code.get_right_x()),
                      1, 8, 0);

This callback shows how many qr_codes have been found and, if there are any, and it draws a blue rectangle around the qr code. In this example, we repeat the call every 500 ms.

auto before = std::chrono::system_clock::now();
for (;;) {
    auto now = std::chrono::system_clock::now();
    auto elapsed = std::chrono::duration_cast<std::chrono::milliseconds>(now - before).count(); 


We have updated helloworld/CMakeLists.txt to use opencv:

find_package(OpenCV REQUIRED)
target_link_libraries(qr_recognition ${RAPP_LIBRARIES}

Building your code

The next step is about create our build folder and run our code.

Now we are going to work in the terminal.

  1. Go to your project path (in our case qr_recognition/)
  2. Build your project

    mkdir build
    cd build 
    cmake ..
  3. If everything is ok, you will have created your executable qr_recognition in the folder build.
  4. Run your executable