Find if drawn rectangle is visible on screen in opencv python

Question

I have some code which will draw a rectangle using cv2.rectangle function based on other parameters. This is part of a face detection program. The rectangle is shown on screen within an image using imshow . However, due to the variable nature of the rectangle's dimensions, there are situations when the rectangle is not visible. After the line of code which draws the rectangle in the image, how do I detect if the rectangle is visible or not. I want this so that I can find when a face is not detected. This is my code:

import numpy as np
import cv2

# multiple cascades: https://github.com/Itseez/opencv/tree/master/data/haarcascades

# https://github.com/Itseez/opencv/blob/master/data/haarcascades/haarcascade_frontalface_default.xml
face_cascade = cv2.CascadeClassifier('haarcascade_frontalface_default.xml')
# https://github.com/Itseez/opencv/blob/master/data/haarcascades/haarcascade_eye.xml
eye_cascade = cv2.CascadeClassifier('haarcascade_eye.xml')

cap = cv2.VideoCapture(0)

while 1:
    ret, img = cap.read()
    gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
    faces = face_cascade.detectMultiScale(gray, 1.3, 5)

    for (x,y,w,h) in faces:
        cv2.rectangle(img,(x + (w / 4), y + (h / 4)),(x+(3 * w / 4),y + (3 * h / 4)),(255,0,0),2)

    cv2.imshow('img',img)
    k = cv2.waitKey(30) & 0xff
    if k == 27:
        break

cap.release()
cv2.destroyAllWindows()

Thanks!

Answer 1

Add print len(faces) in the while loop. If its zero, then no face detected. No rectangle.