OpenCV multiple cameras single image - c++

Question

I'm trying to access multiple usb-cameras in openCV with MacOS 10.11.
My goal is to connect up to 20 cameras to the pc via USB Quad-Channel extensions and take single images. I do not need to have live streaming.
I tried with the following code and I can take a single image from all cameras (currently only 3, via one usb controller).
The question is: does opencv stream a live video from the usb cameras all the time, or does grab() stores an image on the camera which can be retrieve with retrieve() ?
I couldn't find the information, wether opencv uses the grab() command on it's internal video buffer, or on the camera.
int main(int argument_number, char* argument[])
{
std::vector<int> cameraIDs{0,1,2};
std::vector<cv::VideoCapture> cameraCaptures;
std::vector<std::string> nameCaptures{"a","b","c"};
//Load all cameras
for (int i = 0;i<cameraIDs.size();i++)
{
cv::VideoCapture camera(cameraIDs[i]);
if(!camera.isOpened()) return 1;
camera.set(CV_CAP_PROP_FRAME_WIDTH, 640);
camera.set(CV_CAP_PROP_FRAME_HEIGHT, 480);
cameraCaptures.push_back(camera);
}
cv::namedWindow("a");
while(true) {
int c = cvWaitKey(2);
if(27 == char(c)){ //if esc pressed. grab new image and display it.
for (std::vector<cv::VideoCapture>::iterator it=cameraCaptures.begin();it!=cameraCaptures.end();it++)
{
(*it).grab();
}
int i=0;
for (std::vector<cv::VideoCapture>::iterator it=cameraCaptures.begin();it!=cameraCaptures.end();it++)
{
cv::Mat3b frame;
(*it).retrieve(frame);
cv::imshow(nameCaptures[i++], frame);
}
}
}
return 0;
}

Answer 1

Could you please make the statement more clear. You only want the frame from the feed or you want the streams to be connected all the time.
Opencv camera capture is always in running mode unless you release the capture device. So if you say you want only one frame from a device then its better to release this device once you retrieve the frame.
Another point is instead of using grab or retrieve in a multi cam environment, its better to use read() which combines both the above methods and reduces the overhead in decoding streams. So if you want say frame # 2sec position from each of the cams then in time domain they are pretty closely captured as in say the frame x from cam1 captured # 2sec position then frame x at 2.00001sec and frame x from cam3 captured at 2.00015sec..(time multiplexing - multithreading - internal by ocv)
I hope I am clear in explanation.
Thanks