How to work with PID controller / OpenCV for A.R Drone 2.0?

edit

I have been looking at codes for autonomous drones and encountered this one on this repository: https://github.com/puku0x/cvdrone . Im trying to understand the code, I am new to controller algorithms and OpenCV. I tried going on the OpenCV website and understand functions but it didn't help very much. Any help would be appreciated.

    if (contour_index >= 0) {
        // Moments
        cv::Moments moments = cv::moments(contours[contour_index], true);
        double marker_y = (int)(moments.m01 / moments.m00);
        double marker_x = (int)(moments.m10 / moments.m00);

        // Show result
        cv::Rect rect = cv::boundingRect(contours[contour_index]);
        cv::rectangle(image, rect, cv::Scalar(0, 255, 0));


       // Tracking
        if (track) {
            const double kp = 0.005;
            vx = kp * (binalized.rows / 2 - marker_y);;
            vy = 0.0;
            vz = kp; 
            vr = kp * (binalized.cols / 2 - marker_x);
            std::cout << "(vx, vy, vz, vr)" << "(" << vx << "," << vy << "," << vz << "," << vr << ")" << std::endl;
            std::cout << "Altitude = " << ardrone.getAltitude() << "%" << std::endl;
        }
              // Marker tracking
    if (track) {
        // PID gains
        const double kp = 0.001;
        const double ki = 0.000;
        const double kd = 0.000;

        // Errors
        double error_x = (binalized.rows / 2 - marker.y);   // Error front/back
        double error_y = (binalized.cols / 2 - marker.x);   // Error left/right

        // Time [s]
        static int64 last_t = 0.0;
        double dt = (cv::getTickCount() - last_t) / cv::getTickFrequency();
        last_t = cv::getTickCount();

        // Integral terms
        static double integral_x = 0.0, integral_y = 0.0;
        if (dt > 0.1) {
            // Reset
            integral_x = 0.0;
            integral_y = 0.0;
        }
        integral_x += error_x * dt;
        integral_y += error_y * dt;

        // Derivative terms
        static double previous_error_x = 0.0, previous_error_y = 0.0;
        if (dt > 0.1) {
            // Reset
            previous_error_x = 0.0;
            previous_error_y = 0.0;
        }
        double derivative_x = (error_x - previous_error_x) / dt;
        double derivative_y = (error_y - previous_error_y) / dt;
        previous_error_x = error_x;
        previous_error_y = error_y;

        // Command velocities
        vx = kp * error_x + ki * integral_x + kd * derivative_x;
        vy = 0.0;//kp * error_y + ki * integral_y + kd * derivative_y;
        vz = 0.0;
        vr = 0.0;

    }
    }

    // Display the image
    cv::putText(image, (track) ? "track on" : "track off", cv::Point(10, 20), cv::FONT_HERSHEY_SIMPLEX, 0.5, (track) ? cv::Scalar(0, 0, 255) : cv::Scalar(0, 255, 0), 1, cv::LINE_AA);
    cv::imshow("camera", image);
    ardrone.move3D(vx, vy, vz, vr);
}