Contour comparison in OpenCV (Convertion from C to C++)

Question

I am still new in C++ and now I need to convert some parts from this old program of mine from C to C++ because I want to apply BackgroundSubtractorMOG2 in my program since it only available in C++. Basically this program will detect contours from a video camera based on background subtraction and choose the largest contours available.

I have a problem particularly on this part (taken from the old program ):

double largestArea = 0;                    //Const. for the largest area
CvSeq* largest_contour = NULL;             //Contour for the largest area
while (current_contour != NULL){           //If the current contour available
    double area = fabs(cvContourArea(current_contour,CV_WHOLE_SEQ, false));   //Get the current contour's area as "area"    
    if(area > largestArea){            //If "area" is larger than the previous largest area
        largestArea = area;
        largest_contour = current_contour; 
    }
    current_contour = current_contour->h_next;  //Search for the next contour
}

This part is where the program will scan each contour available as current_contour , find its area and compare it to previous largest contour. My question is how to get the current_contour , its area and jump to the next contour in C++? Also, what is indicated by contours.size() in C++? Is it the number of contours scanned or the total area of the contours?

This is what I've done so far:

for(;;)
{
    cap >> frame; // get a new frame from camera
    if( frame.empty() )
            break;
    image=frame.clone();
    mog(frame,foreground,-1);

    threshold(foreground,foreground,lowerC,upperC,THRESH_BINARY);
    medianBlur(foreground,foreground,9);
    erode(foreground,foreground,Mat());
    dilate(foreground,foreground,Mat());

    findContours(foreground,contours,CV_RETR_LIST,CV_CHAIN_APPROX_SIMPLE);  

    if(contours.empty())
        continue;

//Starting this part
    double largest_area = 0;
    for(int i= 0; i < contours.size(); i++){
        double area = contourArea(contours);
        if(area >= largest_area){
            largest_area = area;
            largest_contours = contours;
        }
    }
//Until this part

    drawContours(image,largest_contours,-1,Scalar(0,0,255),2);

    imshow( "Capture",image );
    imshow("Contours",foreground);

    if(waitKey(30) >= 0) break;

}

Thanks in advance.

PS: The old program got some bugs in it but the algorithm works just fine. Free to as me if you need the updated program. Currently using OpenCV 2.4.3 + VS C++ 2010 Exp.

EDIT:

Thanks to everybody who're trying to help me but I already got the answer which is from here . Still, for those how still don't know: OpenCV in C IS NOT EXACTLY THE SAME AS OpenCV in C++ .

Answer 1

This is a part of the code, where I am finding all contours on image and calcilate their perimeter and area:

IplImage* bin = cvCreateImage( cvGetSize(_image), IPL_DEPTH_8U, 1);
cvConvertImage(_image, bin, CV_BGR2GRAY);
cvCanny(bin, bin, 50, 200);
CvMemStorage* storage = cvCreateMemStorage(0);
CvSeq* contours=0;

//Number of all contours on image @contoursCont@
int contoursCont = cvFindContours( bin, storage,&contours,sizeof(CvContour),CV_RETR_LIST,CV_CHAIN_APPROX_SIMPLE,cvPoint(0,0));
assert(contours!=0);

// iterate through all contours --> current = current->h_next
 for( CvSeq* current = contours; current != NULL; current = current->h_next )
 {
     //calculate perimeter and area of each contour
     double area = fabs(cvContourArea(current));
     double perim = cvContourPerimeter(current);
     cvDrawContours(_image, current, cvScalar(0, 0, 255), cvScalar(0, 255, 0), -1, 1, 8);
     //the rest code 
  }

From OpenCV documentation:

The function cvFindContours retrieves contours from the binary image and returns the number of retrieved contours. The pointer CvSeq* contours=0 is filled by the function. It will contain a pointer to the first outermost contour or NULL if no contours are detected (if the image is completely black). Other contours may be reached from first_contour using the h_next and v_next links.