I am create a rectangle box in video stream using python. how to apply face detection in rectangular box inside only and don't need out of the rectangular box detect face

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Code:

import cv2

face_cascade = cv2.CascadeClassifier("haarcascade_frontalface_default.xml")
ds_factor = 0.6
font = cv2.FONT_HERSHEY_SIMPLEX

class VideoCamera(object):
    def __init__(self):
        self.video = cv2.VideoCapture(0)

    def __del__(self):
        self.video.release()

    def get_frame(self):
        success, image = self.video.read()
        cv2.rectangle(image, (400, 300), (800, 600), (255, 255, 255), -1)
        cv2.resize(output, None, fx=ds_factor, fy=ds_factor, interpolation=cv2.INTER_AREA)
        gray = cv2.cvtColor(output, cv2.COLOR_BGR2GRAY)
        faces = face_cascade.detectMultiScale(
            gray, scaleFactor=1.1,
            minNeighbors=5,
            minSize=(200, 200),
            flags=cv2.CASCADE_SCALE_IMAGE)

        for (x, y, w, h) in faces:
            cv2.rectangle(output, (x, y), (x + w, y + h), (255, 0, 0), 3)
            cv2.putText(output, 'Number of Faces : ' + str(len(faces)), (40, 40), font, 1, (255, 0, 0), 2)
            break
        ret, jpeg = cv2.imencode('.jpg', output)
        return jpeg.tobytes()