CV2: "[ WARN:0] terminating async callback" when attempting to take a picture

Question

I am trying to take a picture from the defualt carmera with python, to do this I am using openCV (import cv2 from python shell). However, when I attempt to disable the camera it closes but with the error [ WARN:0] terminating async callback.

This is code I am trying to run:

import cv2

camera_port = 0
camera = cv2.VideoCapture(camera_port)
return_value, image = camera.read()
cv2.imwrite("image.png", image)

camera.release() # Error is here

The code outputs the desired result- it takes a saves an image but I do not understand why the error message occures or how to remove it

Solution 1

I had the same warning. Just modify the line camera = cv2.VideoCapture(camera_port) to camera = cv2.VideoCapture(camera_port, cv2.CAP_DSHOW) and add cv2.destroyAllWindows() as the last line of your code.

Solution 2

camera = cv2.VideoCapture(camera_port,cv2.CAP_DSHOW)

cv2.destroyAllWindows()

Solution 3

It's probably showing a warning because you're not releasing the handle to the webcam.

try adding this to the end of the code

camera.release()
cv2.destroyAllWindows()

I hope this helps!

Solution 4

I did this & I don't see that warning there after.(only for Windows OS)

Open cmd and type:

setx OPENCV_VIDEOIO_PRIORITY_MSMF 0

View more solutions

Comments

[isaac weathers]

This solution worked but defining port instead cv2.VideoCapture(0, cv2.CAP_DSHOW)

[Otto medina]

Using cv2.CAP_DSHOW removes the warning, but slows down my frame rate from 30fps to 7fps, on Windows.

[Otto medina]

Using cv2.CAP_DSHOW removes the warning, but slows down my frame rate from 30fps to 7fps, on Windows

[Otto medina]

On Windows 10 with Python 3.8 and OpenCV 4.2, this does not solve the problem. The warning is also displayed on the OpenCV sample programs, e.g. samples/python/video.py.

[Engr Syed Rowshan Ali]

Correct Answer, don't know why, but mentioning the 2nd parameter solves the warning

[Mrigank Pawagi]

Goodness! Thank you so much!

[Goigle]

It's worth noting that DirectShow is a legacy API and is currently deprecated

[Peter Wood]

@Goigle what would you use instead of CAP_DSHOW?

[Goigle]

@PeterWood The default on windows should be fine (CAP_MSMF), assuming your Win10 is up to date. On Win10 20H2 I experience no issues (that warning just means the stream ended). The only people on my team who have issues are on outdated versions of Windows. This makes sense as DSHOW used to be the default and MSMF is slowly replacing DSHOW

[Christoph Rackwitz]

destroyAllWindows() is absolutely not relevant to this question.