Why cv2.imwrite lags 1 step?

Question

My code:

import cv2

cap = cv2.VideoCapture(0)
cv2.namedWindow('frame', cv2.WINDOW_NORMAL)
while True:
    key = cv2.waitKey(0) & 0xFF
    ret, frame = cap.read()
    cv2.imshow('frame', frame)
    if key == ord('q'):
        cap.release()
        cv2.destroyAllWindows()
        break
    if key == ord('c'):
        cv2.imwrite('capture.jpg', frame)
cap.release()
cv2.destroyAllWindows()

1. I run this code.
2. It shows the gray display.
3. I point the camera at an object and press the 'c' key.
4. It shows not the object image but the image of what the camera pointed at when I run the code, and saves it.
5. I point the camera at somewhere else and press 'c' key agein.
6. It shows the image of the object which it saw at 3. and save it.

The camera lags 1 step. Why?

Answer 1

This could be to do with a lack of cv::waitKey(0) and the window is not getting updated, although this is odd.

Try adding a cv::waitKey command after imshow like this

import cv2

cap = cv2.VideoCapture(0)
cv2.namedWindow('frame', cv2.WINDOW_NORMAL)
while True:
    key = cv2.waitKey(0) & 0xFF
    ret, frame = cap.read()
    cv2.imshow('frame', frame)
    cv2.waitKey(0)
    if key == ord('q'):
        cap.release()
        cv2.destroyAllWindows()
        break
    if key == ord('c'):
        cv2.imwrite('capture.jpg', frame)
cap.release()
cv2.destroyAllWindows()

I think it might be this as when you do the imwrite you are effectively breaking out of the while loop (albeit slightly) to do something else with opencv.