env/opencv-4.8.0/samples/cpp/fback.cpp

#include "opencv2/video/tracking.hpp"
#include "opencv2/imgproc.hpp"
#include "opencv2/videoio.hpp"
#include "opencv2/highgui.hpp"

#include <iostream>

using namespace cv;
using namespace std;

static void help(char** argv)
{
    cout <<
            "\nThis program demonstrates dense optical flow algorithm by Gunnar Farneback\n"
            "Mainly the function: calcOpticalFlowFarneback()\n"
            "Call:\n"
        <<  argv[0]
        <<  "This reads from video camera 0\n" << endl;
}
static void drawOptFlowMap(const Mat& flow, Mat& cflowmap, int step,
                    double, const Scalar& color)
{
    for(int y = 0; y < cflowmap.rows; y += step)
        for(int x = 0; x < cflowmap.cols; x += step)
        {
            const Point2f& fxy = flow.at<Point2f>(y, x);
            line(cflowmap, Point(x,y), Point(cvRound(x+fxy.x), cvRound(y+fxy.y)),
                 color);
            circle(cflowmap, Point(x,y), 2, color, -1);
        }
}

int main(int argc, char** argv)
{
    cv::CommandLineParser parser(argc, argv, "{help h||}");
    if (parser.has("help"))
    {
        help(argv);
        return 0;
    }
    VideoCapture cap(0);
    help(argv);
    if( !cap.isOpened() )
        return -1;

    Mat flow, cflow, frame;
    UMat gray, prevgray, uflow;
    namedWindow("flow", 1);

    for(;;)
    {
        cap >> frame;
        cvtColor(frame, gray, COLOR_BGR2GRAY);

        if( !prevgray.empty() )
        {
            calcOpticalFlowFarneback(prevgray, gray, uflow, 0.5, 3, 15, 3, 5, 1.2, 0);
            cvtColor(prevgray, cflow, COLOR_GRAY2BGR);
            uflow.copyTo(flow);
            drawOptFlowMap(flow, cflow, 16, 1.5, Scalar(0, 255, 0));
            imshow("flow", cflow);
        }
        if(waitKey(30)>=0)
            break;
        std::swap(prevgray, gray);
    }
    return 0;
}