What is output from OpenCV's Dense optical flow (Farneback) function? Cpp - c++

Question

First of all sorry for my bad english.
So, im using dense optical flow, GunnarFarneback speciffically. The code works fine.
Here's the code:
#include "opencv2/highgui.hpp"
#include "opencv2/imgproc.hpp"
#include "opencv2/objdetect/objdetect.hpp"
#include "opencv2/video/tracking.hpp"
#include <vector>
#include <iostream>
using namespace std;
using namespace cv;
// Display the results of the matches
//
int main(int argc, char** argv)
{
cv::Mat res, img1, img2, img2Original, img2OriginalC;
cv::VideoWriter writer;
cv::VideoCapture cap;
//cv::VideoCapture cap("prueba1.mp4");
// cap.open(std::string(argv[1]));
cap.open(0);
cv::namedWindow("cat", cv::WINDOW_AUTOSIZE);
cap >> img1;
cvtColor(img1, img1, COLOR_BGR2GRAY);
double fps = cap.get(cv::CAP_PROP_FPS);
//cv::Size tamano((int)cap.get(cv::CAP_PROP_FRAME_WIDTH), (int)cap.get(cv::CAP_PROP_FRAME_HEIGHT));
Size S = Size((int) cap.get(CV_CAP_PROP_FRAME_WIDTH),(int) cap.get(CV_CAP_PROP_FRAME_HEIGHT));
writer.open("mouse.avi", CV_FOURCC('M', 'J', 'P', 'G'), fps, S);
for (;;) {
cap >> img2;
if (img2.empty()) break;
img2.copyTo(img2OriginalC);
cvtColor(img2, img2, COLOR_BGR2GRAY);
img2.copyTo(img2Original);
cv::calcOpticalFlowFarneback(img1, img2, res, .4, 1, 12, 2, 8, 1.2, 0);
for (int y = 0; y < img2.rows; y += 5) {
for (int x = 0; x < img2.cols; x += 5)
{
// get the flow from y, x position * 3 for better visibility
const Point2f flowatxy = res.at<Point2f>(y, x) * 1;
// draw line at flow direction
line(img2OriginalC, Point(x, y), Point(cvRound(x + flowatxy.x), cvRound(y + flowatxy.y)), Scalar(255, 0, 0));
// draw initial point
circle(img2OriginalC, Point(x, y), 1, Scalar(0, 0, 0), -1);
}
}
img2Original.copyTo(img1);
imshow("cat", img2OriginalC);
writer << img2OriginalC;
cout <<"Fps"<<endl;
cout <<fps<< endl;
for( int i=3; i>=0; i--)
{
//cout<< res << endl;
//return 0;
}
if (cv::waitKey(1) == 27) break;
}
cap.release();
return 0;
}
i think i know whats the output matrix but im not sure at all. Also,i dont know what does the values mean, finally does anyone knows how to use that output data??
Thanks