Python OpenCV error: (-215) size.width>0 && size.height>0 in function imshow -

i run program , returns me error message:

error: (-215) size.width>0 && size.height>0 in function imshow

how can solve? source code:

import numpy np import cv2  cap = cv2.videocapture(0)  ret,frame = cap.read()   r,h,c,w = 250,90,400,125 track_window = (c,r,w,h)  roi = frame[r:r+h, c:c+w] hsv_roi =  cv2.cvtcolor(frame, cv2.color_bgr2hsv) mask = cv2.inrange(hsv_roi, np.array((0., 60.,32.)), np.array((180.,255.,255.))) roi_hist = cv2.calchist([hsv_roi],[0],mask,[180],[0,180]) cv2.normalize(roi_hist,roi_hist,0,255,cv2.norm_minmax)  term_crit = ( cv2.term_criteria_eps | cv2.term_criteria_count, 10, 1 )  while(1):     ret ,frame = cap.read()     if ret == true:         hsv = cv2.cvtcolor(frame, cv2.color_bgr2hsv)         dst = cv2.calcbackproject([hsv],[0],roi_hist,[0,180],1)          ret, track_window = cv2.meanshift(dst, track_window, term_crit)          x,y,w,h = track_window         img2 = cv2.rectangle(frame, (x,y), (x+w,y+h), 255,2)           cv2.imshow('rilevamento',img2)          k = cv2.waitkey(60) & 0xff         if k == 27:             break         else:             cv2.imwrite(chr(k)+".jpg",img2)      else:         break  cv2.destroyallwindows() cap.release() 

according opencv documentation cv2.rectangle draws rectangle on image gave in parameter , doesn't return anything.

then need change line :

cv2.imshow('rilevamento',img2)

to

cv2.imshow('rilevamento', frame)

hope helps.