Difference in output with waitKey(0) and waitKey(1)

Question

I've just begun using the OpenCV library for Python and came across something I didn't understand.

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

When I use cv2.waitKey(1), I get a continuous live video feed from my laptops webcam. However when I use cv2.waitKey(0), I get still images. Every time I close the window, another one pops up with another picture taken at the time. Why does it not show as a continuous feed?

Answer 1

From the doc:

1.waitKey(0) will display the window infinitely until any keypress (it is suitable for image display).

2.waitKey(1) will display a frame for 1 ms, after which display will be automatically closed. Since the OS has a minimum time between switching threads, the function will not wait exactly 1 ms, it will wait at least 1 ms, depending on what else is running on your computer at that time.

So, if you use waitKey(0) you see a still image until you actually press something while for waitKey(1) the function will show a frame for at least 1 ms only.

Answer 2

waitKey(0) will pause your screen because it will wait infinitely for keyPress on your keyboard and will not refresh the frame(cap.read()) using your WebCam. waitKey(1) will wait for keyPress for just 1 millisecond and it will continue to refresh and read frame from your webcam using cap.read().

More clearly, Use debugger in your code.When using waitKey(0) in the while loop, the debugger never crosses this statement and does not refresh the frame and hence the frame output seems stable.Does not move. Where as with waitKey(1), the debugger will cross the code after pausing at

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

for 1 milli second.