OpenCV hangs on VideoCapture grab() - c++

Question

I am trying to write a program to capture images from 2 webcams simultaneously (or near simultaneously), but sometimes when I would run my program it would start to hang. What I mean by that is it would drop the FPS so low, that there would be a good 5 - 10 seconds between each image capture. I decided to make a much more sparse program that uses the code I thought might be causing the problem so I could isolate the source. Sure enough, my small program is causing problems but I am stumped as for what is causing them. Most of the time it will run without fault, but sometimes it will exhibit the same symptoms of hanging anywhere from 10 seconds to 1 minute into running the code. No errors are raised, but from the output of my program I am confident that VideoCapture's grab() is the line slowing down.
I am running this in OS X, with two external webcams through a USB hub, OpenCV version 10.4.11_1, and in C++. I don't think the USB hub is causing the problem. Quite frankly, it is so slow to tell when it will and will not freeze that it is difficult to troubleshoot. I would get rid of the USB hub, but I need it in the end and I know bandwidth is not the issue. I can run multiple (I have tried 4) instances of a different OpenCV test program that captures from a single camera with all cameras attached through the USB hub.
I wonder if there is an internal buffer in the VideoCapture class that is filling up, or some other internal issue because I can't seem to find much documentation on VideoCapture's grab() function and find out what it is actually taking so long to do.
Thanks for reading my lengthy description. Here is my code:
int main(){
VideoCapture vc1(1);
VideoCapture vc2(2);
Timer tmr;
Mat img1;
Mat img2;
namedWindow("WINDOW1", CV_WINDOW_NORMAL);
namedWindow("WINDOW2", CV_WINDOW_NORMAL);
waitKey(1);
int count = 0;
while (true){
tmr.reset();
vc1.grab();
vc2.grab();
cout << "Double grab time(" << ++count << "): " << tmr.elapsed() << endl;
tmr.reset();
vc1.retrieve(img1);
vc2.retrieve(img2);
cout << "Double retrieve time: " << tmr.elapsed() << endl;
imshow("WINDOW1", img1);
imshow("WINDOW2", img2);
if (waitKey(25) == 27){
cout << "Quit" << endl;
break;
}
}
return 0;
}
using this timer class from a SO post:
class Timer
{
public:
Timer() : beg_(clock_::now()) {}
void reset() { beg_ = clock_::now(); }
double elapsed() const {
return std::chrono::duration_cast<second_>
(clock_::now() - beg_).count(); }
private:
typedef std::chrono::high_resolution_clock clock_;
typedef std::chrono::duration<double, std::ratio<1> > second_;
std::chrono::time_point<clock_> beg_;
};
and compiled with:
clang++ `pkg-config --libs --cflags opencv` -o test test.cpp
I just can't image I am the only one who has or will run into this, so if I find anything out I will be sure to post it. In the mean time I would be eternally grateful for some help.
Thanks

Answer 1

I have a partial solution in case anyone else runs into this problem. I was able to stop my program from freezing by using different webcams. Originally I used two webcams called "Creative Live! Cam Chat HD, 5.7MP" which otherwise seem to work perfectly, however after replacing them with two Logitech c920s I was able get it to work. (or so it seems. I have been using them now for about 1.5 months and the one time I saw it freeze like it did before was when I was adding code to resize the video based on CLI input in a multithreaded program. I was also getting seg. faults so it is not exactly strong evidence.)
If I find out why the Logitech cameras work when the others didn't I will post a reply, but my advice would be to try using different webcams if anyone runs into a similar problem.