Distance to face calculation Windows10 web cam. OpenCv 3.4 Python 3.6

distFace.py

import numpy as np
import imutils
import cv2
from imutils.video import VideoStream
from imutils.video import FPS
import time



def distance_to_camera(knownWidth, focalLength, perWidth):
	# compute and return the distance from the maker to the camera
	return (knownWidth * focalLength) / perWidth

face_cascade = cv2.CascadeClassifier('C:/Users/bbartling/Desktop/Haar/frontalFace10/haarcascade_frontalface_alt2.xml')

#Calculated from a different script
focalLength = 709.0909090909091

#average human head width
knownWidth = 7


# Initialize mutithreading the video stream.
camera = VideoStream(src=0).start()

# Allow the camera to warm up.
time.sleep(2.0)

#start FPS
fps = FPS().start()

roi = None

while True:
        image = camera.read()
        image = imutils.resize(image, width=400)
        
        gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
        faces = face_cascade.detectMultiScale(gray, scaleFactor=1.1, minNeighbors=5);
        for (x, y, w, h) in faces:
                cv2.rectangle(gray,(x,y),(x+w,y+h),(255,255,255),2)
                roi = gray[y:y+h, x:x+w]

        if roi is None:
                pass
        else:
                inches = distance_to_camera(knownWidth, focalLength, roi.shape[1])
                print(inches)
                text = "Inches {}".format(np.int(inches))
                cv2.putText(gray, text, (x - 10, y - 10),
                        cv2.FONT_HERSHEY_SIMPLEX, 0.5, (255, 255, 255), 2)

        
        cv2.imshow("gray", gray)
        key = cv2.waitKey(1) & 0xFF
    
        fps.update()

        # if the `q` key was pressed, break from the loop
        if key == ord("q"):
                break
        
fps.stop()
print("[INFO] elapsed time: {:.2f}".format(fps.elapsed()))
print("[INFO] approx. FPS: {:.2f}".format(fps.fps()))

camera.stop()
cv2.destroyAllWindows()