Detectar cara open cv (usando tkinter)

from tkinter import *
from PIL import Image
from PIL import ImageTk
import cv2
import imutils
def iniciar():
    global cap
    cap = cv2.VideoCapture(0, cv2.CAP_DSHOW)
    visualizar()
def visualizar():
    global cap
    if cap is not None:
        ret, frame = cap.read()
        face_cascade = cv2.CascadeClassifier('haarcascade_frontalface_default.xml')
        eye_cascade = cv2.CascadeClassifier('haarcascade_eye.xml')
        if ret == True:
            gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
            faces = face_cascade.detectMultiScale(gray, 1.3, 5)
            gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
            faces = face_cascade.detectMultiScale(gray, 1.3, 5)
            for (x, y, w, h) in faces:
                cv2.rectangle(frame, (x, y), (x + w, y + h), (255, 0, 0), 5)
                roi_gray = gray[y:y + w, x:x + w]
                roi_color = frame[y:y + h, x:x + w]
                eyes = eye_cascade.detectMultiScale(roi_gray, 1.3, 5)
                for (ex, ey, ew, eh) in eyes:
                    cv2.rectangle(roi_color, (ex, ey), (ex + ew, ey + eh), (0, 255, 0), 5)
            frame = imutils.resize(frame, width=640)
            frame = cv2.cvtColor(frame, cv2.COLOR_BGR2RGB)
            im = Image.fromarray(frame)
            img = ImageTk.PhotoImage(image=im)
            lblVideo.configure(image=img)
            lblVideo.image = img
            lblVideo.after(10, visualizar)
        else:
            lblVideo.image = ""
            cap.release()
def finalizar():
    global cap
    cap.release()
cap = None
root = Tk()
btnIniciar = Button(root, text="Iniciar", width=45, command=iniciar)
btnIniciar.grid(column=0, row=0, padx=5, pady=5)
btnFinalizar = Button(root, text="Finalizar", width=45, command=finalizar)
btnFinalizar.grid(column=1, row=0, padx=5, pady=5)
lblVideo = Label(root)
lblVideo.grid(column=0, row=1, columnspan=2)
root.mainloop()

Este código foi em  grande parte baseado num código do Blog da Gabriela Solano.