How do you capture video from two or more cameras at once (or nearly) with OpenCV, using the Python API?
I have three webcams, all capable of video streaming, located at /dev/video0, /dev/video1, and /dev/video2.
Using the tutorial as an example, capturing images from a single camera is simply:
import cv2
cap0 = cv2.VideoCapture(0)
ret0, frame0 = cap0.read()
cv2.imshow('frame', frame0)
cv2.waitKey()
And this works fine.
However, if I try to initialize a second camera, attempting to read() from it returns None:
import cv2 cap0 = cv2.VideoCapture(0) cap1 = cv2.VideoCapture(1) ret0, frame0 = cap0.read() assert ret0 # succeeds ret1, frame1 = cap1.read() assert ret1 # fails?!
Just to ensure I wasn’t accidentally giving OpenCV a bad camera index, I tested each camera index individually and they all work by themselves. e.g.
import cv2 #cap0 = cv2.VideoCapture(0) cap1 = cv2.VideoCapture(1) #ret0, frame0 = cap0.read() #assert ret0 ret1, frame1 = cap1.read() assert ret1 # now it works?!
What am I doing wrong?
Edit: My hardware is a Macbook Pro running Ubuntu. Researching the issue specifically on Macbooks, I’ve found others that have run into this problem too, both on OSX and with different types of cameras. If I access the iSight, both calls in my code fail.
Advertisement
Answer
Yes you’re definitely limited by the USB bandwidth. Attempting to read from both devices at full-rez you probably got error:
libv4l2: error turning on stream: No space left on device
VIDIOC_STREAMON: No space left on device
Traceback (most recent call last):
File "p.py", line 7, in <module>
assert ret1 # fails?!
AssertionError
And then when you reduce the res to 160×120:
import cv2 cap0 = cv2.VideoCapture(0) cap0.set(3,160) cap0.set(4,120) cap1 = cv2.VideoCapture(1) cap1.set(3,160) cap1.set(4,120) ret0, frame0 = cap0.read() assert ret0 # succeeds ret1, frame1 = cap1.read() assert ret1 # fails?!
now it seems to work! I bet you have both cams connected on the same USB card. You can run lsusb command to make sure, and it should indicate something like:
Bus 001 Device 006: ID 046d:081b Logitech, Inc. Webcam C310 Bus 001 Device 004: ID 0409:005a NEC Corp. HighSpeed Hub Bus 001 Device 007: ID 046d:0990 Logitech, Inc. QuickCam Pro 9000 Bus 001 Device 005: ID 0409:005a NEC Corp. HighSpeed Hub Bus 001 Device 003: ID 0409:005a NEC Corp. HighSpeed Hub Bus 001 Device 002: ID 1058:0401 Western Digital Technologies, Inc. Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
(Note both cameras on same bus.) If possible, you can add another USB card to your machine to gain more bandwidth. I’ve done this before in order to run multiple cams at full resolution on a single machine. Albeit that was a tower workstation with available motherboard slots, and unfortunately you may not have that option on a MacBook laptop.