Opencv cannot run as usual

[Buruxianian]

Question

Unable to obtain frame by running nicegui-main\examples\opencv_webcam\main.py

To solve the problem, I compared two simplified scripts. But I don't know how to solve it.
The first script was unable to retrieve frames, as detailed below：

import base64
import cv2
from nicegui import ui

video_capture = cv2.VideoCapture(0)
ui_interactive_image = ui.interactive_image()

def lazy_update() -> None:
    global video_capture, ui_interactive_image
    ret, frame = video_capture.read()
    if ret and ui_interactive_image is not None:
        _, imencode_image = cv2.imencode('.jpg', frame)
        base64_image = base64.b64encode(imencode_image)
        base64_image_string = 'data:image/jpg;base64,' + base64_image.decode('ascii')
        ui_interactive_image.source = base64_image_string
        
ui.timer(interval=2, callback=lazy_update)
ui.run()

The second script can retrieve frames, as detailed below：

import base64
import cv2
from nicegui import ui

ui_interactive_image = ui.interactive_image()

def lazy_update() -> None:
    global ui_interactive_image
    video_capture = cv2.VideoCapture(0)

    ret, frame = video_capture.read()
    if ret and ui_interactive_image is not None:
        _, imencode_image = cv2.imencode('.jpg', frame)
        base64_image = base64.b64encode(imencode_image)
        base64_image_string = 'data:image/jpg;base64,' + base64_image.decode('ascii')
        ui_interactive_image.source = base64_image_string
    video_capture.release()
ui.timer(interval=2, callback=lazy_update)

ui.run()

[rodja]

Have you seen and tried our OpenCV webcam example?

[Buruxianian]

I have tested it and cannot obtain frames.
It's strange that I can run the code below normally, but I can't use run opencv with nicegui.

import cv2
cap = cv2.VideoCapture(0)
if not cap.isOpened():
    print("error")
else:
    while True:
        ret, frame = cap.read()
        if not ret:
            break
        
        
        cv2.imshow("Frame", frame)
        if cv2.waitKey(1) == ord('q'):
            break
    
    cap.release()
    cv2.destroyAllWindows()

[Buruxianian]

The problem has been resolved.

ui.run(reload=False)

After turning off the reload, it will run normally

[jiang131072]

So is this a bug? or should we update the OpenCV exmple ?

[rodja]

Can anyone else reproduce the problem from @jiang131072 with Linux?

[rodja]

Another idea: Try replacing the top-level video_capture assignment with

# OpenCV is used to access the webcam.
def setup() -> None:
    global video_capture
    video_capture = cv2.VideoCapture(0)

app.on_startup(setup)

That way, the VideoCapture device will only be allocated when the server really starts, and not once when the main reloader process starts and a second time when it starts the server.

[jiang131072]

That works.

[rodja]

Great. I updated the example in PR #3619 with a slightly improved variant of the on_startup technique to avoid the global variable. Does it work for you, too?

[jiang131072]

yes.