OpenCV : Face Detection

Why do you want to detect face? In general, face detection is used in many areas, such as auto focusing, game, etc. Today, we will make a program to synthesize images on the face by using face detection.

First, we need a "haarcascade_frontalface_alt2.xml" file. This is used for face detect algorithm. We can take this file in following url. 
https://github.com/opencv/opencv/blob/master/data/haarcascades/haarcascade_frontalface_alt2.xml

 And also we need images to synthesize on the face. I made a simple animal head for you.
  

Now, let's start make face detecting program. The full code is as follows.


preprogress.h

#ifndef DETACTFACE_PREPROGRASS_H#define DETACTFACE_PREPROGRASS_H
#include <opencv2/opencv.hpp>
using namespace cv;
using namespace std;

//load specific cascade file

void load_cascade(CascadeClassifier& cascade, string frame){
    string path = "./res/haarcascades/";
    string full_path = path + frame;
    CV_Assert(cascade.load(full_path)); //CV_Assert is like error exception
}
// convert image coloar to grayMat 

convertToGray(Mat img){
    Mat gray;
    cvtColor(img, gray, CV_BGR2GRAY);
    equalizeHist(gray, gray);
    return gray;
}

// synthesize image and detect face roi

Mat getResultImg(Mat originMat, Mat maskMat, Rect roiRect){
    Mat grayMask;
    Mat mask_inv;
    cvtColor(maskMat, grayMask, CV_BGR2GRAY);
    threshold(grayMask, grayMask, 10, 255, 0);
    bitwise_not(grayMask, mask_inv);

    Mat imgRoi;
    imgRoi = originMat(roiRect);

    Mat back;
    Mat mask_for;

    bitwise_and(maskMat, maskMat, mask_for, grayMask);
    bitwise_and(imgRoi, imgRoi, back, mask_inv);

    Mat dst;
    add(back, mask_for, dst);
    dst.copyTo(imgRoi);
    return originMat;
}
#endif //DETACTFACE_PREPROGRASS_H

main.cpp

#include <iostream>
#include "preprograss.h"
using namespace std;
using namespace cv;

int main() {
    CascadeClassifier face_cascade, eyes_cascade;
    load_cascade(face_cascade, "haarcascade_frontalface_alt2.xml");
    Mat animalImg = imread("./res/pic/animal_head.png");
    Mat frame;
    VideoCapture stream(0);
    if(!stream.isOpened()) cout << "Cannot open camera";
    while (1){
        if (!(stream.read(frame))) break;
        flip(frame, frame, 1); // flip image left and right        

        Mat gray = convertToGray(frame);
        vector<Rect> faces, eyes;
        face_cascade.detectMultiScale(gray, faces, 1.1, 2, 0, Size(100, 100)); // detect face        

        if(faces.size() > 0){
//            rectangle(frame, faces[0], Scalar(0,0,255), 3);            

            Mat newAni;
            float ratio = faces[0].width/animalImg.size().width; // get ratio detected face's width and animal image's width            

            int ani_height = (int)(animalImg.size().height * ratio); // get animal image's height for resizing            

            resize(animalImg, newAni, Size(faces[0].width, ani_height));
            int pointY = (faces[0].y - ani_height) > 0 ? (faces[0].y - ani_height) : 0; // if y coordinate is negative, it is zero            

            frame = getResultImg(frame, newAni, Rect(faces[0].x, pointY, newAni.size().width, newAni.size().height));
        }
        imshow("detact face", frame);

        if (waitKey(5) >= 0) break;
    }
    return 0;
}

• Detect face

faces.size() is count of detected faces 

face_cascade.detectMultiScale(gray, faces, 1.1, 2, 0, Size(100, 100));

void detectMultiScale( InputArray image,
                      CV_OUT std::vector<Rect>& objects,
                      double scaleFactor = 1.1,
                      int minNeighbors = 3, int flags = 0,
                      Size minSize = Size(),
                      Size maxSize = Size() );

• Masking Images

Synthesize two images. This is explained in detail at the link below.
https://canysway.blogspot.kr/2018/05/opencv-tutorial-image-synthesis-roi.html

// synthesize image and detect face roiMat getResultImg(Mat originMat, Mat maskMat, Rect roiRect){
    Mat grayMask;
    Mat mask_inv;

  ....