Exit function doesn't work in OpenCV with python

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.

try puttin a print statement on the key variable before your if statement, this way you will find if the break is reached (which is not). You can update you question then. — Bouramas
– Bouramas, Commented Jan 26, 2017 at 11:10
Thus you see the issue @amolkulkarni . Thats why the break statement is not reached. — Bouramas
– Bouramas, Commented Jan 26, 2017 at 13:46

Micka · Accepted Answer · 2017-01-27 07:41:17Z

1

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)

edited Jan 27, 2017 at 7:41

answered Jan 26, 2017 at 15:45

Micka

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2 Comments

MateuszB Over a year ago

That's basically what I suggested and apparently it still doesn't work.

claw107 Over a year ago

it works when I remove the comment #. But as i have mentioned in the question, i want it to capture one frame and exit.

NAmorim · Accepted Answer · 2017-01-26 14:33:41Z

0

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?

answered Jan 26, 2017 at 14:33

NAmorim

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Comments

MateuszB · Accepted Answer · 2017-01-26 14:35:40Z

0

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.

edited Jan 26, 2017 at 14:35

answered Jan 26, 2017 at 13:15

MateuszB

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5 Comments

MateuszB Over a year ago

Does any other key work? Instead of comparing with 27 try: ord('q')

MateuszB Over a year ago

Do one more test. Show image with cvw.imshow. Maybe the window capture the keystrokes.

claw107 Over a year ago

Nothing. Still the same

MateuszB Over a year ago

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.

eric Over a year ago

This is a really good reason that opencv should provide a way to make a window active programmatically!

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