reading two webcams in opencv

Question

If I run this code on a PC with two webcams attached it runs fast (or fast enough) without any issues:

import cv2

cap0 = cv2.VideoCapture(0)
cap1 = cv2.VideoCapture(1)

while True:
    ret0, img0 = cap0.read()
    print('read 0')
    ret0, img1 = cap1.read()
    print('read 1')

When I run the same code on Rpi3B+, I get this output. It cannot read the second camera and even if the first camera is being read, it's so slow it's unusable:

read 0
[ WARN:[email protected]] global /tmp/pip-wheel-efxaz4j7/opencv-python_bedc0fac27944da0921e079da44d32bf/opencv/modules/videoio/src/cap_v4l.cpp (1000) tryIoctl VIDEOIO(V4L2:/dev/video1): select() timeout.
read 1

Somehow OpenCV on RPi cannot properly do cv2.VideoCapture on two cameras. I've tried using threads and QThreads, but that doesn't help. If you change the loop to capture and release the camera in every loop step, it works: but it's too slow (< 1 FPS):

import cv2

while True:
    cap0 = cv2.VideoCapture(0)
    ret0, img0 = cap0.read()
    cap0.release()
    print('read 0')
    cap1 = cv2.VideoCapture(1)
    ret1, img1 = cap1.read()
    print('read 1')
    cap1.release()

Are there any tricks to overcome the limitations of OpenCV on RPi?

Update: I degraded resolution from 640x480 (default) to 320x240 per this question and it seems to work. However, that resolution may not be usable. I'm basically just asking for one frame at a time. It's confusing to me why reducing resolution makes such a huge difference.

cap0.set(3,320)
cap0.set(4,240)
cap1.set(3,320)
cap1.set(4,240)

Answer 1

Try using using grab() and retrieve() instead of read() in multi-camera setups, see the OpenCV documentation

I have used this technique to handle 2 cameras in a single thread on a Pi 3, see this blog post