python - Error printing image with Raspberry Pi and OpenCV face detection (imshow function) -

i'm trying build simple surveillance program raspberry pi camera, using opencv face detection. code:

# -*- coding: utf-8 -*-  picamera.array import pirgbarray picamera import picamera import time import cv2  #setting camera  camera = picamera() camera.resolution = (320, 240) camera.framerate = 30 rawcapture = pirgbarray(camera, size=(320, 240))  #basic cascade classifier face_cascade = cv2.cascadeclassifier('haarcascade_frontalface_default.xml') #0.1 sec warmup  time.sleep(0.1)  # capture frames camera frame in camera.capture_continuous(rawcapture, format="bgr", use_video_port=true):       image = frame.array     gray = cv2.cvtcolor(image, cv2.color_bgr2gray)     faces = face_cascade.detectmultiscale(gray, 1.3, 5)      #rectangle  loop     for(x, y, w, h) in faces:         image = cv2.rectangle(image,(x, y), (x+w, y+h), (255, 0, 0), 2)      # show frame [here comes problem]     cv2.imshow("frame", image)     #   cv2.imshow("gray", gray     key = cv2.waitkey(100) & 0xff  # clear stream in preparation next frame rawcapture.truncate(0) # if `q` key pressed, break loop if key == ord("q"):     break 

the point that, when draw rectangle in image, comes error:

opencv error: assertion failed (size.width>0 && size.height>0) in imshow, file /home/pi/opencv-2.4.10/modules/highgui/src/window.cpp, line 261 traceback (most recent call last): file "camera2.py", line 36, in cv2.imshow("frame", image) cv2.error: /home/pi/opencv-2.4.10/modules/highgui/src/window.cpp:261: error: (-215) size.width>0 && size.height>0 in function imshow

if put cv2.imshow right before rectangle loop, works perfectly. also, printing faces object shows detectmultiscale recognizing faces. should do?

thanks.