python - openCV Error in cvtColor function: Assertion failed (scn == 3 || scn == 4) -

i want load video file, convert grayscale , display it. here code:

import numpy np import cv2  cap = cv2.videocapture('cars.mp4')  while(cap.isopened()):     # capture frame-by-frame     ret, frame = cap.read()     #print frame.shape          # our operations on frame come here     gray = cv2.cvtcolor(frame, cv2.color_bgr2gray)      # display resulting frame     cv2.imshow('frame',gray)     if cv2.waitkey(1) & 0xff == ord('q'):         break   # when done, release capture cap.release() cv2.destroyallwindows() 

the video plays in grayscale till end. freezes , window becomes not responding , following error in terminal:

opencv error: assertion failed (scn == 3 || scn == 4) in cvtcolor, file /home/clive/downloads/opencv/opencv-2.4.9/modules/imgproc/src/color.cpp, line 3737 traceback (most recent call last):   file "cap.py", line 13, in <module>     gray = cv2.cvtcolor(frame, cv2.color_bgr2gray) cv2.error: /home/clive/downloads/opencv/opencv-2.4.9/modules/imgproc/src/color.cpp:3737: error: (-215) scn == 3 || scn == 4 in function cvtcolor 

i uncommented statement print frame.shape. keeps printing 720,1028,3. after video plays till end, freezes , after while closes , returns

print frame.shape    attributeerror: 'nonetype' object has no attribute 'shape' 

i understand assertion failure massage means i'm trying convert empty image. added check empty image before starting process using if(ret): statement. (any other way of doing it??)

import numpy np import cv2  cap = cv2.videocapture('cars.mp4')  while(cap.isopened()):     # capture frame-by-frame     ret, frame = cap.read()     #print frame.shape         if(ret): #if cam read successfull         # our operations on frame come here         gray = cv2.cvtcolor(frame, cv2.color_bgr2gray)          # display resulting frame         cv2.imshow('frame',gray)         if cv2.waitkey(1) & 0xff == ord('q'):             break     else:         break  # when done, release capture cap.release() cv2.destroyallwindows() 

this time video plays till end , window still freezes , closes after few seconds. time don't error. why window freeze? how can fix that?

the waitkey() part should not depend on validity of frame, move out of condition:

import numpy np import cv2  cap = cv2.videocapture('cars.mp4')  while(cap.isopened()):     # capture frame-by-frame     ret, frame = cap.read()     #print frame.shape         if(ret): #if cam read successfull         # our operations on frame come here         gray = cv2.cvtcolor(frame, cv2.color_bgr2gray)          # display resulting frame         cv2.imshow('frame',gray)     else:         break      # should called always, frame or not.     if cv2.waitkey(1) & 0xff == ord('q'):         break