OpenCV トラッキングAPIで顔面追尾 ref: http://qiita.com/AkihiroTakai/items/af0670207535b7cd145f

file0.cpp

#include <opencv2/opencv.hpp>

cv::Mat detect_face(const cv::Mat &image, const std::string &cascade_file)
{
    cv::CascadeClassifier cascade;
    std::vector<cv::Rect> faces;

    //検出器読み込み
    cascade.load(cascade_file);

    //顔検出
    cascade.detectMultiScale(image, faces, 1, 3, 0, cv::Size(50, 50));

    //結果描画
    for (const cv::Rect &face_rect : faces) 
    {
        cv::rectangle(image, cv::Point(face_rect.x, face_rect.y), cv::Point(face_rect.x + face_rect.width, face_rect.y + face_rect.height), cv::Scalar(0, 200, 0), 3, CV_AA);
    }

    return image;
}

int main(int argc, char const **argv) 
{
    //画像読み込み
    cv::Mat image = cv::imread(argv[1]);

    //検出器のパス
    std::string filename = argv[2];

    //顔検出
    cv::Mat face_image = detect_face(image, filename);

    //表示
    cv::imshow("detect face", face_image);
    cv::waitKey(0);

    return 0;
}

file1.cpp

#include <opencv2/core.hpp>
#include <opencv2/imgcodecs.hpp>
#include <opencv2/videoio.hpp>
#include <opencv2/highgui.hpp>
#include <opencv2/tracking/tracker.hpp>
#include <opencv2/tracking/tldDataset.hpp>
#include <opencv2/imgproc.hpp>
#include <iostream>

int tracker_init(cv::Ptr<cv::TrackerMedianFlow> &tracker, cv::Rect2d &rect)
{
    cv::VideoCapture cap(0);
    if (!cap.isOpened())
    {
        std::cout << "CAN'T OPEN CAMERA." << std::endl;
        return -1;
    }

    cv::Mat frame;
    cv::namedWindow("Select face");

    while (true)
    {
        cap >> frame;
        if (frame.empty())
        {
            std::cout << "FAILD TO READ A FRAME" << std::endl;
            cv::destroyAllWindows();
            break;
        }

        cv::imshow("Select face", frame);

        switch (cv::waitKey(1))
        {
        case 'q':
            return -1;
        case 't':
            rect = cv::selectROI("tracker", frame);
            tracker->init(frame, rect);
            cv::destroyAllWindows();
            return 1;
        default:
            break;
        }
    }

    return -1;
}

int main(int argc, char **argv)
{
    cv::Ptr<cv::TrackerMedianFlow> tracker = cv::TrackerMedianFlow::create();
    cv::Rect2d roi;

    if (tracker_init(tracker, roi) == -1)
    {
        /*
        * 初期化に失敗
        */
        return -1;
    }

    cv::Scalar color = cv::Scalar(0, 255, 0);
    cv::Mat frame;
    cv::VideoCapture cap(0);
    if (!cap.isOpened())
    {
        std::cout << "CAN'T OPEN CAMERA." << std::endl;
        return -1;
    }

    while (cv::waitKey(1) != 'q')
    {
        cap >> frame;
        if (frame.empty())
        {
            break;
        }

        //更新
        tracker->update(frame, roi);

        //結果表示
        cv::rectangle(frame, roi, color, 1, 1);

        cv::imshow("tracker", frame);
    }

    return 0;
}