convert detected eye region (ROI) to Y channel

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In the following code to detect face and eye

I want to convert the detected eye region only to Y channel of Ycbcr ,

Thanks for any help

Regards

#include "opencv2/objdetect/objdetect.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"

#include <iostream>
#include <stdio.h>

using namespace std;
using namespace cv;


void detectAndDisplay(Mat frame);

String face_cascade_name = "haarcascade_frontalface_alt.xml";
String eyes_cascade_name = "haarcascade_eye_tree_eyeglasses.xml";
 CascadeClassifier face_cascade;
 CascadeClassifier eyes_cascade;

 int main(int argc, const char** argv)
 {
 VideoCapture capture("face.mp4");
 Mat frame;

//-- 1. Load the cascades
if (!face_cascade.load(face_cascade_name)){ printf("--(!)Error loading\n"); return -1; };
if (!eyes_cascade.load(eyes_cascade_name)){ printf("--(!)Error loading\n"); return -1; };

//-- 2. Read the video stream

if (!capture.isOpened())
{
    printf("error to initialize camera");
    return 1;
}
    while (true)
    {
        capture >> frame;

        //-- 3. Apply the classifier to the frame
        if (!frame.empty())
        {
            detectAndDisplay(frame);
        }
        else
        {
            printf(" --(!) No captured frame -- Break!"); break;
        }

        int c = waitKey(10);
        if ((char)c == 'c') { break; }
    }

return 0;
  }

    /** @function detectAndDisplay */
   void detectAndDisplay(Mat frame)
 {
std::vector<Rect> faces;
Mat frame_gray;

cvtColor(frame, frame_gray, CV_BGR2GRAY);
equalizeHist(frame_gray, frame_gray);

//-- Detect faces
face_cascade.detectMultiScale(frame_gray, faces, 1.1, 2, 0 | CV_HAAR_SCALE_IMAGE, Size(30, 30));

for (size_t i = 0; i < faces.size(); i++)
{
    Point center(faces[i].x + faces[i].width*0.5, faces[i].y + faces[i].height*0.5);
    ellipse(frame, center, Size(faces[i].width*0.5, faces[i].height*0.5), 0, 0, 360, Scalar(255, 0, 255), 4, 8, 0);

    Mat faceROI = frame_gray(faces[i]);
    std::vector<Rect> eyes;

    //-- In each face, detect eyes
    eyes_cascade.detectMultiScale(faceROI, eyes, 1.1, 2, 0 | CV_HAAR_SCALE_IMAGE, Size(30, 30));

    for (size_t j = 0; j < eyes.size(); j++)
    {
        Point center(faces[i].x + eyes[j].x + eyes[j].width*0.5, faces[i].y + eyes[j].y + eyes[j].height*0.5);
        int radius = cvRound((eyes[j].width + eyes[j].height)*0.25);
        circle(frame, center, radius, Scalar(255, 0, 0), 4, 8, 0);
    }
    }
   //-- Show what you got
imshow(window_name, frame);
}