public void TrainRecognizer(List <User> users)
        {
            var imageList = new List <Mat>();
            var indexList = new List <int>();

            var userIndex = 0;

            foreach (var user in users)
            {
                foreach (var userImage in user.UserImages.Where(userImage => File.Exists(userImage.ImageFilePath)))
                {
                    imageList.Add(new Mat(userImage.ImageFilePath).CvtColor(ColorConversionCodes.BGR2GRAY));
                    indexList.Add(userIndex);
                }

                userIndex++;
            }

            Recognizer = EigenFaceRecognizer.Create();
            Recognizer.Train(imageList, indexList);
        }