'Cannot allocate memory' - OpenCV multiple webcam stream

Question

I'm trying to make a live stereovision setup using OpenCV in C++ and two webcams. It is possible to seperately get frames from the two webcams. However, when I try to access them simultaneously in threads, I get a runtime error:

VIDIOC_STREAMON: Cannot allocate memory
OpenCV Error: Assertion failed (size.width>0 && size.height>0) in imshow, file /home/lorre851/Downloads/opencv-3.1.0/modules/highgui/src/window.cpp, line 281
terminate called after throwing an instance of 'cv::Exception'
  what():  /home/lorre851/Downloads/opencv-3.1.0/modules/highgui/src/window.cpp:281: error: (-215) size.width>0 && size.height>0 in function imshow

My code is as follows:

#include <iostream>
#include <thread>
#include "opencv2/opencv.hpp"

using namespace std;
using namespace cv;

void stream(int camera) {
    VideoCapture cap(camera); // open the default camera
    if(cap.isOpened()) { // check if we succeeded
        while(true) {
            Mat frame;
            cap >> frame; // get a new frame from camera
            imshow("Stream " + to_string(camera), frame);
            if (waitKey(30) >= 0) break;
        }
    }
}

int main() {
    thread cam1 (stream, 1);
    thread cam2 (stream, 2);


    cam1.join();
    cam2.join();

    return 0;
}

Anybody got any idea what could be causing this? I'm using CLion on Ubuntu 15.10 LTE.

UPDATE 1: I'm using index 1 and 2 for the camera's because I have a built-in camera in my laptop (0) and two USB camera's (1 and 2) plugged in. The USB camera's are the target hardware here.

UPDATE 2: Putting both camera feeds in one thread (see code below) works just fine (assuming your USB ports use separate busses, otherwise you'll get a 'NO SPACE LEFT ON DEVICE' error), but the delay between the two frames is noticeable, which is not ideal for a stereovision setup.

cv::VideoCapture camera0(0);
cv::VideoCapture camera1(1);

if( !camera0.isOpened() ) return 1;
if( !camera1.isOpened() ) return 1;
cv::Mat3b frame0;
cv::Mat3b frame1;

while(true) {
    camera0 >> frame0;
    camera1 >> frame1;
    if(mat_is_empty(frame0)) cout << "SKIPPED FRAME IN 0";
    else cv::imshow("Stream 0", frame0);

    if(mat_is_empty(frame1)) cout << "SKIPPED FRAME IN 1";
    else cv::imshow("Stream 1", frame1);

    int c = cvWaitKey(40);

    //exit the loop if user press "Esc" key  (ASCII value of "Esc" is 27)
    if(27 == char(c)) break;
}

Answer 1

Maybe, I have stupid answer. Your code works fine..

Count cameras from zero.

 thread cam1 (stream, 0);    THIS works
// thread cam2 (stream, 1);  I dont have second camera

First of all, I am on windows machine. My cameras are count from 0,1,2 I do not have second web camera.

In the picture i just use your code and paste cam(0) my web camera and for the second camera i use testing rtsp stream.

#include <iostream>
#include <thread>
#include "opencv2/opencv.hpp"

using namespace std;
using namespace cv;

void stream(int camera) {
    VideoCapture cap;
    if (camera == 0) {
        cap.open(0); // open the default camera
    }
    if (camera == 1) {
        cap.open("rtsp://mpv.cdn3.bigCDN.com:554/bigCDN/definst/mp4:bigbuckbunnyiphone_400.mp4"); // open the default camera
    }



if (cap.isOpened()) { // check if we succeeded
        while (true) {
            Mat frame;
            cap >> frame; // get a new frame from camera
            imshow("Stream " + to_string(camera), frame);
            if (waitKey(30) >= 0) break;
        }
    }
}

int main() {
    thread cam1(stream, 0);
    thread cam2(stream, 1);


    cam1.join();
    cam2.join();

    return 0;
}