Y channel of Ycbcr not dispalyed correctly

edit

I set up ROI for eye region . I converted this region to ycbcr channel , and I splitted Y channel from it.

when I show the ycbcr channel the eye area is showed in normal ycbcr colors,

My problem is that when I show Y channel it doesn't show the ROI Y channel which it should be the same as ycbcr but in grey

this is the output image for Ycbcr and Y channel . any I idea why this happening ?

My code :

for (i = 0; i < faces.size(); i++) // Iterate through all current elements (detected faces)

{

    Point pt1(faces[i].x, faces[i].y); // Display detected faces on main window 
    Point pt2((faces[i].x + faces[i].height), (faces[i].y + faces[i].width));
    rectangle(frame, pt1, pt2, Scalar(0, 255, 0), 2, 8, 0);

    // set ROI for the eyes

    Rect Roi = faces[i];

    Roi.height = Roi.height / 4;

    Roi.y = Roi.y + Roi.height;

    cv::Mat crop = frame(Roi);

    cvtColor(crop, ycbcr, CV_BGR2YCrCb);

    imshow("ROI                   ",ycbcr);

    vector<Mat> channels;

    split(ycbcr, channels);

    medianBlur(channels[0], channels[0], 3);

    Mat eye_region_Ychannel = channels[0].clone();

   imshow("Y", channels[0]);