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processing multiple cams is not much different from a single cam, you just have to be aware, that there's bandwith limits from the usb bus. if you experience trouble there

• don't plug all your cams into the same usb bus
• reduce the resolution (see below)

#include "opencv2/opencv.hpp"
using namespace cv;

#include <iostream>
using namespace std;

int main(int argc, char **argv)
{
    int N = 4; // num cams:
    Size S(320,240);
    vector<VideoCapture> caps(N);

    for (int i=0; i<N; i++)
    {
        if ( ! caps[i].open(i) )
        {
            cerr << "capture " << i << " won't open !" << endl;
            return -1;
        }
        cerr << "cap " << i << " : " << caps[i].isOpened() << endl;

        // lower resolution:
        caps[i].set(CAP_PROP_FRAME_WIDTH,  S.width);
        caps[i].set(CAP_PROP_FRAME_HEIGHT, S.height);
    }

    while(1)
    {
        for (int i=0; i<N; i++)
        {
            Mat frame;
            if ( ! caps[i].read(frame) )
            {
                cerr << "could not read image from capture " << i << endl;
                return -1;
            }

            //
            // process frame from capture i ...
            //
            // show it:
            imshow(format("cap%d",i), frame);
        }
        if (waitKey(10)==27) break;
    }
}