trying to run 6 USB cameras simultaneously on a single Raspberry Pi 3B+ with add-on 4 port USB hub with external power

Question

We are trying to simultaneously run 6 cameras on a Raspberry Pi 3B+ with an add-on 4 port USB hub that has an external power supply. The cameras are set at 15 FPS with a colour image size of 640 X 480. I found that it just works properly when I opened 2 cameras simultaneously. When I add a third camera I get this error message: "select timeout".

Below is the test code I'm using :

import cv2
import sys
print sys.argv
if len(sys.argv) < 2:
    print "Usage : > python video.py [camno1] [camno2]"
    sys.exit(0)
cam = []
cap = []
for i in range(len(sys.argv) - 1):
    cam.append(int(sys.argv[i+1]))
    cap.append(cv2.VideoCapture(cam[i]))
while True:
    for i in range(len(cam)):
        print "cam"+str(i)+" read"
        ret, frame = cap[i].read()
        if ret :
            cv2.imshow("frame"+str(i), frame)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break
for i in range(len(cam)):
    cap[i].release()

My goal is to get a still image from each camera, one-by-one, and I don't need higher than 15 FPS. When I try to open/read and close each camera, one-by-one, it takes too long to start them all (around 1.5 sec per camera) so that the total time is more than 10 secs. So I am looking for a solution to either keep all 6 cameras open at the same time or to speed up the: open/read/close times for each camera so that my total rotation time is around one second. Anyone have any ideas?

Answer 1

640x480 Pixel at 24 Bit (guessing here) has an uncompressed size of 1 MB. At 15 FPS that would be 15 MB/s per camera. Six cameras for a total of 90 MB/s. That would be outside the realm of the possible. Expect a net data rate in the range of 35 to 40 MB/s at best (see here and here).

If we, however, assume a compression ratio of 1:10 (which would seem reasonable for JPG) that could be as less as 9 MB/s and well within a reasonable range for the Pi's USB 2.0 ("Hi-Speed"). But it all depends on bandwith requirements of the cameras (compression setting, colordepth) and protocol overhead. Not to mention the processing of the captured images - I take it the Pi is supposed to do something meaningful with those images.

Answer 2

This intrigued me, so I tried a script that looks like

fswebcam -d /dev/video0 p0.jpg &
sleep 0.9
fswebcam -d /dev/video1 p1.jpg &
sleep 0.9
fswebcam -d /dev/video2 p2.jpg &
sleep 0.9
fswebcam -d /dev/video3 p3.jpg &

I was using bog-standard Chinese 640x480 USB cameras.

This was on a Pi 2 and 0.9 seconds is as low as I could go. Anything less and the instances start tripping over each other. Maybe it could be less on a later zippier Pi.

However if you installed a clusterhat (clusterhat.com) that would let you fire off four cameras at once - one on each of the attached Pi Zeros. That plus two cameras one second apart on the "main" pi should meet your requirements for 1.5 sec total sweep time. AFAICT taking a pic on a connected Pi Zero would not load the main system at all, so they could be (effectively) simultaneous.