OPENCV C++: Sorting contour by using their areas - c++

Question

Few days back ive found a code which is sorting a contours inside vector by using their areas.But i could not understand the code very well especially in comparator function. Why does the contour 1&2 are convert into Mat in the contourArea parameter? Can anyone explain me the whole code in step by step. Your explanation will be usefull for my learning process
// comparison function object
bool compareContourAreas ( std::vector<cv::Point> contour1, std::vector<cv::Point> contour2 )
{
double i = fabs( contourArea(cv::Mat(contour1)) );
double j = fabs( contourArea(cv::Mat(contour2)) );
return ( i < j );
}
[...]
// find contours
std::vector<std::vector<cv::Point> > contours;
std::vector<cv::Vec4i> hierarchy;
cv::findContours( binary_image, contours, hierarchy, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE,
cv::Point(0, 0) );
// sort contours
std::sort(contours.begin(), contours.end(), compareContourAreas);
// grab contours
std::vector<cv::Point> biggestContour = contours[contours.size()-1];
std::vector<cv::Point> smallestContour = contours[0];

Answer 1

Your input image is binary, so it only exists of 0 and 1. When you use cv::findContours it searches for points with the value '1' and are touching other 1's, this makes a contour. Then puts all contours in std::vectorstd::vector<cv::Point> contours.
When you would take a grid and draw the points of one contour in it, you will get a 2D array of points, this basically is a one-channel cv::Mat.
In 'compareContourAreas' you take two contours out of your vector 'contours' and compare the absolute sum of all of the points in the contour. To add all the points you use contourArea, which needs a cv::Mat as input, so you first need to convert your contour-vector of points to a cv::Mat.
Then with the sort function you sort all the contours from small to big.