Sample of eye segmenting

//Convert to binary image by thresholding it
Mat thresholded_binary;
threshold(gray, thresholded_binary, 220, 255, THRESH_BINARY);

//Find all contours
vector< vector<Point> > contours;
findContours(gray.clone(), contours, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_NONE);

//Define some variables for postprocessing
vector<Point> pupil;

for (int i = 0; i < contours.size(); i++)
{
    double area = contourArea(contours[i]);
    Rect rect = boundingRect(contours[i]);
    int radius = rect.width/2;

    // If contour is big enough and has round shape
    // Then it is the pupil --> assign it to the pupil vector
    if (area >= 30 && abs(1 - ((double)rect.width / (double)rect.height)) <= 0.2)
    {
        pupil = contours[i];
    }
}

//Look for the founding box of the pupil contour
Rect region_of_interest = boundingRect( pupil );

// Create a small image region and a small mask
Mat small_region = gray( region_of_interest);
Mat small_mask = thresholded_binary( region_of_interest);

//Now make everything except pupil area black
Mat combination = small_region & (small_mask * 255);

//Show the result
imshow("segmented eye", combination);

//Now you can loop over image pixels and look for white ones