Motion detection with OpenCV c++

Question

I'm trying to play with my webcam and OpenCV. I follow this tuto : http://mateuszstankiewicz.eu/?p=189. But the only result I have is one red border and I don't understand why. Could anyone help me to make it right and fix this ?

Here is my code :

#include "mvt_detection.h"


Mvt_detection::Mvt_detection()
{

}

Mvt_detection::~Mvt_detection()
{
}

cv::Mat Mvt_detection::start(cv::Mat frame)
{
    cv::Mat back;
    cv::Mat fore;
    cv::BackgroundSubtractorMOG2 bg(5,3,true) ;
    cv::namedWindow("Background");
    std::vector<std::vector<cv::Point> > contours;

    bg.operator ()(frame,fore);
    bg.getBackgroundImage(back);
    cv::erode(fore,fore,cv::Mat());
    cv::dilate(fore,fore,cv::Mat());
    cv::findContours(fore,contours,CV_RETR_EXTERNAL,CV_CHAIN_APPROX_NONE);
    cv::drawContours(frame,contours,-1,cv::Scalar(0,0,255),2);
    return frame;
}

Here is a screenshot of what our cam returns :

I tried on two other video from there and there and there is the same issue.

Thanks for the help :).

Answer 1

As @Lenjyco said, we fixe the problem.

@Micka had a good idea :

Firstly the BackgroundSubtractorMOG2 as to be instancied only ONCE.

We instantiate it in the constructor and play with the Hystory and Threashold:

Mvt_detection::Mvt_detection()
{
    bg = new cv::BackgroundSubtractorMOG2(10, 16, false);
}

10 : the number of image the backgound look back to compare.

16 : the threshold level (blur)

This way, we are now able to detect motion.

Thank you !

Answer 2

I have used the following code which is similar to yours and it is working well. I am also taking the inputs from my webcam. In your code, i didnt find any imshow() and waitkey. Try to use them. My code is following:

#include "opencv2/core/core.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/video/background_segm.hpp"
#include "opencv2/highgui/highgui.hpp"
#include <stdio.h>

#include <iostream>
#include <vector>

using namespace std;
using namespace cv;

int main()
{

    VideoCapture cap;
    bool update_bg_model = true;

    cap.open(0);
    cv::BackgroundSubtractorMOG2 bg;//(100, 3, 0.3, 5);
    bg.set ("nmixtures", 3);
    std::vector < std::vector < cv::Point > >contours;

    cv::namedWindow ("Frame");
    cv::namedWindow ("Background");

    Mat frame, fgmask, fgimg, backgroundImage;

    for(;;)
    {
        cap >> frame;
        bg.operator()(frame, fgimg);
        bg.getBackgroundImage (backgroundImage);
        cv::erode (fgimg, fgimg, cv::Mat ());
        cv::dilate (fgimg, fgimg, cv::Mat ());

        cv::findContours (fgimg, contours, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_NONE);
        cv::drawContours (frame, contours, -1, cv::Scalar (0, 0, 255), 2);

        cv::imshow ("Frame", frame);
        cv::imshow ("Background", backgroundImage);


        char k = (char)waitKey(30);
        if( k == 27 ) break;

    }

    return 0;
}

Answer 3

Problem fixed, putting BackgroundSubtractorMOG2 in my object's field and initialise it in constructor make him work well.