使用OpenCV的2个摄像头（用于立体视觉）的视频，但其中一个是滞后的

Question

I'm trying to create Stereo Vision using 2 logitech C310 webcams. But the result is not good enough. One of the videos is lagging as compared to the other one.

Here is my openCV program using VC++ 2010:

#include <opencv\cv.h>
#include <opencv\highgui.h>
#include <iostream>

using namespace cv;
using namespace std;

int main()
{
    try
    {
        VideoCapture cap1;
        VideoCapture cap2;

        cap1.open(0);
        cap1.set(CV_CAP_PROP_FRAME_WIDTH, 1040.0);
        cap1.set(CV_CAP_PROP_FRAME_HEIGHT, 920.0);

        cap2.open(1);  
        cap2.set(CV_CAP_PROP_FRAME_WIDTH, 1040.0);
        cap2.set(CV_CAP_PROP_FRAME_HEIGHT, 920.0);
        Mat frame,frame1;

        for (;;)
        {
            Mat frame;
            cap1 >> frame;

            Mat frame1;
            cap2 >> frame1;

            transpose(frame, frame);
            flip(frame, frame, 1);

            transpose(frame1, frame1);
            flip(frame1, frame1, 1);

            imshow("Img1", frame);
            imshow("Img2", frame1);

            if (waitKey(1) == 'q')
                break;
        }

        cap1.release();
        return 0;
    }
    catch (cv::Exception & e)
    {
        cout << e.what() << endl;
    }
}

How can I avoid the lagging?

Answer 1

you're probably saturating the usb bus.

try to plug one in front, the other in the back(in the hope to land on different buses),

or reduce the frame size / FPS to generate less traffic.

Answer 2

I'm afraid you can't do it like this. The opencv Videocapture is really only meant for testing, it uses the simplest underlying operating system features and doesn't really try and do anything clever.

In addition simple webcams aren't very controllable of sync-able even if you can find a lower level API to talk to them.

If you need to use simple USB webcams for a project the easiest way is to have an external timed LED flashing at a few hertz and detect the light in each camera and use that to sync the frames.

Answer 3

I know this post is getting quite old but I had to deal with the same problem recently so...

I don't think you were saturating the USB bus. If you were, you should have had an explicit message in the terminal. Actually, the creation of a VideoCapture object is quite slow and I'm quite sure that's the reason of your lag: you initialize your first VideoCapture object cap1, cap1 starts grabbing frames, you initialize your second VideoCapture cap2, cap2 starts grabbing frames AND THEN you start getting your frames from cap1 and cap2 but the first frame stored by cap1 is older than the one stored by cap2 so... you've got a lag.

What you should do if you really want to use opencv for that is to add some threads: one dealing with left frames and the other with right frames, both doing nothing but saving the last frame received (so you'll always deal with the newest frames only). If you want to get your frames, you'll just have to get them from theses threads.

I've done a little something if you need here .