content: import java.util.ArrayList;
  import org.opencv.core.*;
  import org.opencv.highgui.HighGui;
  import org.opencv.imgproc.Imgproc;
  import org.opencv.video.Video;
  import org.opencv.videoio.VideoCapture;
  
  
  class OptFlowDense {
      public void run(String[] args) {
          String filename = args[0];
          VideoCapture capture = new VideoCapture(filename);
          if (!capture.isOpened()) {
              //error in opening the video input
              System.out.println("Unable to open file!");
              System.exit(-1);
          }
  
          Mat frame1 = new Mat() , prvs = new Mat();
          capture.read(frame1);
          Imgproc.cvtColor(frame1, prvs, Imgproc.COLOR_BGR2GRAY);
  
          while (true) {
              Mat frame2 = new Mat(), next = new Mat();
              capture.read(frame2);
              if (frame2.empty()) {
                  break;
              }
              Imgproc.cvtColor(frame2, next, Imgproc.COLOR_BGR2GRAY);
  
              Mat flow = new Mat(prvs.size(), CvType.CV_32FC2);
              Video.calcOpticalFlowFarneback(prvs, next, flow,0.5,3,15,3,5,1.2,0);
  
              // visualization
              ArrayList<Mat> flow_parts = new ArrayList<>(2);
              Core.split(flow, flow_parts);
              Mat magnitude = new Mat(), angle = new Mat(), magn_norm = new Mat();
              Core.cartToPolar(flow_parts.get(0), flow_parts.get(1), magnitude, angle,true);
              Core.normalize(magnitude, magn_norm,0.0,1.0, Core.NORM_MINMAX);
              float factor = (float) ((1.0/360.0)*(180.0/255.0));
              Mat new_angle = new Mat();
              Core.multiply(angle, new Scalar(factor), new_angle);
  
              //build hsv image
              ArrayList<Mat> _hsv = new ArrayList<>() ;
              Mat hsv = new Mat(), hsv8 = new Mat(), bgr = new Mat();
  
              _hsv.add(new_angle);
              _hsv.add(Mat.ones(angle.size(), CvType.CV_32F));
              _hsv.add(magn_norm);
              Core.merge(_hsv, hsv);
              hsv.convertTo(hsv8, CvType.CV_8U, 255.0);
              Imgproc.cvtColor(hsv8, bgr, Imgproc.COLOR_HSV2BGR);
  
              HighGui.imshow("frame2", bgr);
  
              int keyboard = HighGui.waitKey(30);
              if (keyboard == 'q' || keyboard == 27) {
                  break;
              }
              prvs = next;
          }
          System.exit(0);
      }
  }
  
  public class OpticalFlowDenseDemo {
      public static void main(String[] args) {
          System.loadLibrary(Core.NATIVE_LIBRARY_NAME);
          new OptFlowDense().run(args);
      }
  }
  