opencv - Pausing and restarting a video in Python

Question

I have several video and I want to go through them frame by frame, and annotate some of them by pressing a keyboard key (which depends on the frame). For many of the frames I won't press any key. Here is what I have so far:

import numpy as np
import cv2

cap = cv2.VideoCapture('video.mp4')

frame_number = []
annotation_list = []

i = 0
while(True):        
    # Read one frame.
    ret, frame = cap.read()

    # Show one frame.
    cv2.imshow('frame', frame)

    # Set the time between frames in miliseconds
    c = cv2.waitKey(500)
    i = i + 1

    try:
        annotation_list = annotation_list + [chr(c)]
        frame_number = frame_number + [i]
    except:
        continue

So this is showing each frame for 0.5 seconds, and associates to each frame where I press a button, the given letter. What I need now is an option such that for a given frame I can stop the video at that frame for as long as I need, by pressing "Space" for example, in order to think about how to annotate it, then press "Space" again to continue the video, once I decide how to annotate. How can I add this pause/continue option? Thank you!

Answer 1

You can implement a pause/resume feature by determining what key was pressed from the return value of cv2.waitKey(). To pause the video, you can pass no parameter (or 0) to cv2.waitKey() which will wait indefinitely until there is a key press then it will resume the video. From the docs:

cv2.waitKey() is a keyboard binding function. Its argument is the time in milliseconds. The function waits for specified milliseconds for any keyboard event. If you press any key in that time, the program continues. If 0 is passed, it waits indefinitely for a key stroke. It can also be set to detect specific key strokes like, if key a is pressed etc which we will discuss below.

To determine if the spacebar was pressed, we can check if the returned value is 32. If this key was pressed then we indefinitely pause the frame until any key is pressed then we resume the video. Here's an example:

import cv2

cap = cv2.VideoCapture('video.mp4')
if not cap.isOpened():
    print("Error opening video")

while(cap.isOpened()):
    status, frame = cap.read()
    if status:
        cv2.imshow('frame', frame)
    key = cv2.waitKey(500)

    if key == 32:
        cv2.waitKey()
    elif key == ord('q'):
        break

---

In the future if you want to perform some action after pressing a key, you can determine the "key code" with this script:

import cv2

# Load a test image
image = cv2.imread('1.jpg')

while(True):
    cv2.imshow('image', image)
    key = cv2.waitKey(1)
    # 'q' to stop
    if key == ord('q'):
        break
    # Print key 
    elif key != -1:
        print(key)