[python] Real time face detection OpenCV, Python

I am trying to make a basic program for a real time face detection. Here's my code (I am a newbie in OpenCV) :

import numpy as np import cv2 cam = cv2.VideoCapture(0) name = 'detect' face_cascade = cv2.CascadeClassifier('haarcascade_frontalface_default.xml') cv2.namedWindow(name, cv2.WINDOW_AUTOSIZE) while True:     s, img = cam.read()     gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)     faces = face_cascade.detectMultiScale(gray, 1.3, 5)     #print s     for (x,y,w,h) in faces:         img = cv2.rectangle(img,(x,y),(x+w,y+h),(255,0,0),2)     cv2.imshow(name, img)         k = cv2.waitKey(0)     if k == 27:         cv2.destroyWindow("Detect")         break 

But when I run this code I get this error :

OpenCV Error: Bad flag (parameter or structure field) (Unrecognized or unsupported array type) in cvGetMat, file /build/buildd/opencv-2.4.2+dfsg/modules/core/src/array.cpp, line   2482 Traceback (most recent call last): File "mytry.py", line 27, in <module> cv2.imshow(name, img)     cv2.error: /build/buildd/opencv-2.4.2+dfsg/modules/core/src/array.cpp:2482: error: (-206) Unrecognized or unsupported array type in function cvGetMat 

I am a newbie in OpenCV ! Please tell me what is wrong with the code, why does this error arises what changes should I make ?

This question is related to python opencv

The answer is

Your line:

img = cv2.rectangle(img,(x,y),(x+w,y+h),(255,0,0),2) 

will draw a rectangle in the image, but the return value will be None, so img changes to None and cannot be drawn.