Why must I not delay VideoCapture read() calls?

Python
1

Hi

I am using a usb camera.

The code below behaves as I expected.
Because this is common usage.

while True:
    ret, img = cap.read()
    cv2.imshow('Video', img)

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

but,
There is something like inferring the image acquired after cap.read() by deep learning.
I reproduced this reasoning time using sleep().

while True:
    ret, img = cap.read()
    time.sleep(3)
    cv2.imshow('Video', img)

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

I thought that cap.read() is executed every 3 seconds, so I can take a picture of that moment every 3 seconds.

However! !
The photo will appear when waiting for 3 seconds. In cap.read (), it will be like a 3-second picture of sleep is held in FIFO.

Why is this?

I raised my right hand in front of the camera.
The next cap.read is 3 seconds later.
I lowered my right hand during these three seconds.
cap.read is an image of still raising the right hand.

Why is this?

2

Hi,
Wht’s your camera?
How do you open it (souce code)?

1 Like
3

assuming, this is linux, and using the default v4l backend,
there is indeed a fifo queue

you could try to restrict it to a single image:

vcap.set(CAP_PROP_BUFFERSIZE, 1)
1 Like
4

@berak

Thank you for your advice.
There were many interesting articles when I checked with CAP_PROP_BUFFERSIZE.
I think I can solve this problem.

5

“common” doesn’t mean good. that code lacks all error checking.

  • assert cap.isOpened() before the loop
  • if not ret: break in the loop
1 Like
6

A more generic solution would be to read all frames as they come, keep track of elapsed time between interesting frames and discard the rest.

1 Like