When I run the following code, I get the error

edit

The correct code should be as follow:

//You should include correct hpp file


#include "opencv2/core/core.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/video/background_segm.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include "vector"

using namespace cv;
using namespace std;

int main(int argc, char *argv[])

{
    Mat frame;
    Mat back;
    Mat fore;
    VideoCapture cap;
    //open video
    cap.open("test.avi");

    //Background Model
    BackgroundSubtractorMOG2 bg;
    vector<vector<Point> > contours;

    namedWindow("Input");
    namedWindow("Frame");
    namedWindow("Foreground");
    namedWindow("Background");
    for(;;)
    {
        //read frame
        if(!cap.read(frame))
        {
            return 0;
        }
        imshow("Input",frame);
        //get foreground
        bg(frame,fore,0.01);
        imshow("Foreground",fore);
        //get background
        bg.getBackgroundImage(back);
        //erode image
        erode(fore,fore,Mat());
        //dilate image
        dilate(fore,fore,Mat());
        //find contours
        findContours(fore,contours,CV_RETR_EXTERNAL,CV_CHAIN_APPROX_NONE);
        //draw contours to frame
        drawContours(frame,contours,-1,Scalar(0,0,255),2);
        imshow("Frame",frame);
        imshow("Background",back);

        if(waitKey(33) >= 0)
        {
            break;
        }

    }
    cap.release();
    return 0;

}

Hope can help you.

oh, that's a fun one ;)

the culprit is bg.getBackgroundImage(back);

i tested this with 2.4.2, there we end up in an empty method in the BackgroundSubtractor base class. (in 2.4.9 it throws a not impl exception)

so your "back" Mat is left untouched, still empty, and imshow does not like it. (complains of an empty image, that's your exception)

BackgroundSubtractorMOG2 can do that. (that's the good news)

the background image has to get expensively reconstructed from a Model, that's not a simple member access