OpenCV Python Assertion Failed

Question

I am trying to run opencv-python==3.3.0.10 on a macOS 10.12.6 to read from a file and show the video in a window. I have exactly copied the code from here http://opencv-python-tutroals.readthedocs.io/en/latest/py_tutorials/py_gui/py_video_display/py_video_display.html, section 'Playing' video from file.

The code runs correctly and shows the video, however after termination of the video it breaks the program, giving the following error:

Assertion failed: (ec == 0), function unlock, file /BuildRoot/Library/Caches/com.apple.xbs/Sources/libcxx/libcxx-307.5/src/mutex.cpp, line 48.

Does anyone have any idea of what might cause this?

---

Code snippet for your convenience (edited to include some suggestions in the comment)

cap = cv2.VideoCapture('vtest.avi')
while(True):
 ret, frame = cap.read()
 if not ret:
 break
 gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
 cv2.imshow('frame',gray)
 if cv2.waitKey(1) & 0xFF == ord('q'):
 break
 cv2.destroyAllWindows()

• zindarod: After reading frame, add if ret == False: break.
• Campello: Thank you for the comment. I did add that, but still does not fix the problem. I tried to put a flag after the loop while, and the program does not arrive there. It seems that it does not go out of the loop..
• zindarod: comment out the cap.release() function and use while True: .... What's the output?
• Campello: It gives me the same error. Apparently there is a related issue on GitHub for mac: github.com/dthpham/butterflow/issues/12
• Campello: Ok, if I uncomment cv2.destroyAllWindows() it seems to stop crashing. Any ideas why?

Answer 1

It's not clear from your question, but it looks like you're specifically running into a situation where the video completes playing without being interrupted. I think the issue is that the VideoCapture object is already closed by the time you get to cap.release(). I'd recommend putting the call to release inside of the if statement by the break.

I've not had time to experiment, but I normally follow this pattern:

reader = cv2.VideoCapture(<stuff>)
while True:
 success, frame = reader.read()
 if not success:
 break

I'd not had to call release explicitly in those contexts.

• Campello: Thank you for your answer. It did not solve the problem. It does break the loop but if crashes right after (anything after the loop is not executed)
• Campello: Ok, if I uncomment cv2.destroyAllWindows() it seems to stop crashing. Any ideas why?
• cwallenpoole: What about using cv2.namedWindow('frame') at the beginning of the script?