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How to increase the accuracy of EigenFaceRecognizer, it recognizes two people as one?

I'm trying to recognize frontal faces using EigenFaceRecognizer in C++.

The problem is:

1) at high threshold, two people are recognized as same person and a "NEW" face is also recognized, instead of stating it as a new face

2) at low thresholds, a face already in the training set is recognized as a new face

3) False Positives are also encountered. Though not a concern but if an easy way is suggested to decrease them it will be appreciated>

Is there any way to improve the recognizer to identify faces accurately?

Following is what I am doing.

#include<opencv2\opencv.hpp>                //For opencv functions
#include<opencv2\highgui\highgui.hpp>       //For window based functions
#include<fstream>                           //For dealing with I/O operations on file

using namespace std;
using namespace cv;

// Function to read the File containing paths and labels of the training images and push them into images and     labels vector
static void read_data(vector <Mat> & images,vector <int>& labels, char separator=' ')
ifstream file("images.txt");   //images.txt contains paths and labels separated by a space
string line;
string a[2];

 while(getline(file,line))  // read images.txt line by line 
     int i=0;
    stringstream iss(line);
    while (iss.good() && i < 2)
    images.push_back(imread(a[0],CV_LOAD_IMAGE_GRAYSCALE)); // a[0] = "path of images"
    labels.push_back(atoi(a[1].c_str()));  //a[1] = "labels"


  // Function to take input from webcam and recognize faces 
   int face_recognition::face_rec(int time_flag, int trigger_flag)
vector<Mat> images;    //stores the paths of all images
vector<int> labels;    //stores the corresponding labels

//function call to function read_data

//take the size of the sample images
int im_width = images[0].cols;           
int im_height = images[0].rows;

//threshold is the minimum value of magnitude of vector of EigenFaces
double threshold=10.0;    

//create instance of EigenFaceRecognizer
Ptr<FaceRecognizer> model = createEigenFaceRecognizer(10,threshold);  
double current_threshold =model->getDouble("threshold");

// set a threshold value, for face prediction

// train the face recognizer using the sample images

// Create face_cascade to detect people
CascadeClassifier face_cascade;
if(!face_cascade.load("haarcascade_frontalface_default.xml"))   // load haarcascade_frontaface_default.xml
    cout<<"ERROR Loading cascade file";
    return 1;

// capture the video input from webcam
VideoCapture capture(CV_CAP_ANY);    

capture.set(CV_CAP_PROP_FRAME_WIDTH, 320);
capture.set(CV_CAP_PROP_FRAME_HEIGHT, 240); 

Size frameSize(static_cast<int>(320), static_cast<int>(240));

//initialize the VideoWriter object
VideoWriter oVideoWriter ("MyVideo.avi", CV_FOURCC('P','I','M','1'), 20, frameSize, true);  // video is save in the  VS  project

    cout<<"Error in camera";
    return 1;

Mat cap_img, gray_img;

//store the detected faces
vector<Rect> faces;   
    //capture frame by frame in cap_img

    // Image conversion: Color to Gray

    //Histogram Equilization to increase contrast by stretching intensity ranges

    // detects faces in the frame
    //CV_HAAR_SCALE_IMAGE to scale the size of the detect face 
    //CV_HAAR_DO_CANNY_PRUNING to increase speed as it skips image regions that are unlikely to contain a face
    face_cascade.detectMultiScale(gray_img,faces,1.1,10,CV_HAAR_SCALE_IMAGE | CV_HAAR_DO_CANNY_PRUNING, Size(20,20),Size(300,300));  

    Mat Normalized;

    //Loop over the detected faces
    for(int i=0;i<faces.size();i++)
        Rect face_i = faces[i];
        Mat face = gray_img(face_i);

        Mat face_resized;

        //resize the detected face to the size of sample images
        resize(face,face_resized, Size(im_width,im_height),1.0,1.0,INTER_CUBIC);  

        // predict the person the face belongs to, returns label
        int predicted_label = -1;

        // Draws a rectangle around the faces
        rectangle(cap_img,face_i, CV_RGB(0,255,0),1);   

        //text to be put with the face, by default "new" for new faces
        string box_text=format("new");  

        // Change the text based on label
                case 0:box_text = format("keanu");
                case 1:box_text = format("selena");
                case 2:box_text = format("shubham");

        // calculate the coordinates to put the text based on the postion of the face 
        int pos_x = max( - 10, 0);
        int pos_y = max( - 10, 0);

        // put text on the output screen
        putText(cap_img, box_text , Point(pos_x,pos_y), FONT_HERSHEY_PLAIN,0.8, CV_RGB(0,255,0), 1,CV_AA);  

        if (box_text=="new")
           oVideoWriter.write(cap_img); //writer the frame into the file

    // show the frame on the result window
    imshow("Press Esc to exit",cap_img); 


return 0;