python - use cv2.VideoCapture to capture a picture

Question

I want to use my laptop webcam to capture a picture using the code below:

import cv2
cap = cv2.VideoCapture(1)
while(True):
    ret, frame = cap.read()
    cv2.imshow('frame', frame)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break
cap.release()


cv2.destroyAllWindows()

but it throws this error:

cv2.imshow('frame', frame) cv2.error: OpenCV(4.0.0) C:projectsopencv-pythonopencvmoduleshighguisrcwindow.cpp:350: error: (-215:Assertion failed) size.width>0 && size.height>0 in function 'cv::imshow'

How can I fix this error?

Answer 1

When OpenCV has problem to get frame from camera or stream then it doesn't raise error but it return False in ret (return status) so you should check it. It also return None in frame and imshow has problem to display None - it has no width and height - so you get error with size.width>0 && size.height>0

As I know mostly laptop webcame has number 0, not 1

This works with my laptop webcam

import cv2

cap = cv2.VideoCapture(0) # zero instead of one

while True:
    ret, frame = cap.read()

    if not ret: # exit loop if there was problem to get frame to display
        break

    cv2.imshow('frame', frame)

    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

cap.release()
cv2.destroyAllWindows()

---

EDIT: as said Dave W. Smith in comment: some laptops may need time to send correct image then here version which doesn't exit loop

while True:
    ret, frame = cap.read()

    if ret: # display only if status (ret) is True and there is frame
        cv2.imshow('frame', frame)

    if cv2.waitKey(1) & 0xFF == ord('q'):
        break