退出功能在使用Python的OpenCV中不起作用

Question

I just want to capture one frame, save it to test.png and then exit the code. In this program exit() doesn't work, I have to use CTRL+C from the terminal every time.

import cv2

#cv2.namedWindow("window")
cap = cv2.VideoCapture(0)

if cap.isOpened(): # if we are able to capture the first frame.
    val, frame = cap.read()
else:
    val = False
while val:
    #cv2.imshow("window", frame)
    cv2.imwrite('test.png',frame)
    val, frame = cap.read()
    key = cv2.waitKey(20)
    if key == 27: # exit on ESC
        break

cap.release
cv2.destroyAllWindows()
exit()

Thanks in advance.

Answer 1

Try adding & 0xFF to the wait key:

key = cv2.waitKey(20) & 0xFF

You also need an active window:

Note The function only works if there is at least one HighGUI window created and the window is active. If there are several HighGUI windows, any of them can be active.

Answer 2

The method waitkey(t) waits t in milliseconds, so your code is waiting 20 ms for a key press for each loop. Considering that your

cv2.imwrite('test.png',frame)

might take some time to write to file, are you perhaps pressing your exiting key at the wrong moment?

Answer 3

cv::waitKey only works if any openCV window is present and maybe active.

Since you commented #cv2.namedWindow("window") and #cv2.imshow("window", frame) out there is no waitKey time and no chance to fetch the key.

Does it work if you activate namedWindow and imshow?

To be sure additionally try

if key > 0: # exit on ESC
    break

to cancel on ANY keypress (but you still need an active openCV window)

To capture a single frame try this:

import cv2

cap = cv2.VideoCapture(0)

val = False
maxTry = 100 # maximum number of tries to capture a frame from an opened device
cTry = 0

if cap.isOpened(): # if we are able to capture the first        frame.
    while (!val) and (cTry < maxTry)
        val, frame = cap.read()
        cTry = cTry + 1
else:
    val = False
if val:
    cv2.imwrite('test.png',frame)
else:
    print "No image captured"

cap.release
exit()

I am not a python programmer so please forgive me any syntax errors (and give me a hint to correct them)

Answer 4

for python3

cv2.imshow('imafast', frame)
if cv2.waitKey(25) & 0xFF == ord('q'):
    break