Capturing video from two cameras in OpenCV at once

Question

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.

Answer 1

Using OPENCV and two standard USB cameras, I was able to do this using multithreading. Essentially, define one function which opens an opencv window and VideoCapture element. Then, create two threads with the camera ID and window name as inputs.

import cv2 import threading  class camThread(threading.Thread):     def __init__(self, previewName, camID):         threading.Thread.__init__(self)         self.previewName = previewName         self.camID = camID     def run(self):         print "Starting " + self.previewName         camPreview(self.previewName, self.camID)  def camPreview(previewName, camID):     cv2.namedWindow(previewName)     cam = cv2.VideoCapture(camID)     if cam.isOpened():  # try to get the first frame         rval, frame = cam.read()     else:         rval = False      while rval:         cv2.imshow(previewName, frame)         rval, frame = cam.read()         key = cv2.waitKey(20)         if key == 27:  # exit on ESC             break     cv2.destroyWindow(previewName)  # Create two threads as follows thread1 = camThread("Camera 1", 1) thread2 = camThread("Camera 2", 2) thread1.start() thread2.start() 

Great resource for learning how to thread in python: https://www.tutorialspoint.com/python/python_multithreading.htm