Opencv 3.1 Tutorial Optical flow (calcOpticalFlowFarneback)

Opencv simple C++ tutorial and code to achieve optical flow and farneback optical flow of moving an object in OpenCV video. Let's check the video example and the achieved result on my blog. Working and well-described code is included. 

optical flow Farneback

Optical Flow Farneback parameters remarks

You need 2 images at least to calculate optical flow, the previous image (prevgray) and the current image (img). !! The previous image must be initialized first !! Both images have to be grayscale. 

The result is stored in flowUmat which has the same size as inputs but the format is CV_32FC2

calcOpticalFlowFarneback(prevgray, img, flowUmat, 0.4112281.20);
  • 0.4- Image pyramid or simple image scale
  • 1 is the number of pyramid layers. 1 means that flow is calculated only from the previous image. 
  • 12 is the window size. Flow is computed over the window larger value is more robust to the noise. 
  • 2 mean number of iterations of the algorithm
  • 8 is polynomial degree expansion recommended value is 5 - 7
  • 1.2 standard deviation used to smooth used derivatives recommended values from 1.1 - 1,5

calcOpticalFlowFarneback(prevgray, img, flowUmat, 0.4, 1, 12, 2, 8, 1.2, 0);

calcOpticalFlowFarneback Example

My gopro video is captured at Old Town Square in Prague.

Opencv Tutorial C++ code calcOpticalFlowFarneback

#include "opencv2\highgui.hpp"
#include "opencv2\imgproc.hpp"
#include "opencv2\objdetect\objdetect.hpp"
#include "opencv2/video/tracking.hpp"
#include <vector>
#include <stdio.h>
#include <Windows.h>
#include <iostream>
using namespace cv;
using namespace std;

int main(int argcconst char** argv)
 // add your file name
 VideoCapture cap("yourFile.mp4");
 Mat flow, frame;
 // some faster than mat image container
 UMat  flowUmat, prevgray;
 for (;;)
  bool Is = cap.grab();
  if (Is == false) {
    // if video capture failed
   cout << "Video Capture Fail" << endl;
  else {
   Mat img;
   Mat original;
   // capture frame from video file
   cap.retrieve(img, CV_CAP_OPENNI_BGR_IMAGE);
   resize(img, img, Size(640480));
   // save original for later
   // just make current frame gray
   cvtColor(img, img, COLOR_BGR2GRAY);
   // For all optical flow you need a sequence of images.. Or at least 2 of them. Previous
                           //and current frame
   // if there is no current frame
   // go to this part and fill previous frame
   //else {
   // img.copyTo(prevgray);
   //   }
   // if previous frame is not empty.. There is a picture of previous frame. Do some 
//optical flow alg. 
   if (prevgray.empty() == false ) {
    // calculate optical flow 
    calcOpticalFlowFarneback(prevgray, img, flowUmat, 0.4112281.20);
    // copy Umat container to standard Mat
        // By y += 5, x += 5 you can specify the grid 
    for (int y = 0; y < original.rows; y += 5) {
     for (int x = 0; x < original.cols; x += 5)  {
              // get the flow from y, x position * 10 for better visibility
              const Point2f flowatxy =<Point2f>(y, x) * 10;
              // draw line at flow direction
       line(original, Point(x, y), Point(cvRound(x + flowatxy.x), cvRound(y + flowatxy.y)), Scalar(255,0,0));
       // draw initial point
       circle(original, Point(x, y), 1Scalar(000), -1);
    // draw the results
    namedWindow("prew", WINDOW_AUTOSIZE);
    imshow("prew", original);
    // fill previous image again
   else {
    // fill previous image in case prevgray.empty() == true
   int key1 = waitKey(20);
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  • Unknown
    Unknown September 28, 2016 at 6:55 PM

    Hi Vladimir, I try to run your code, however it reported undeclared identifier: UMat. Is it only for GPU? Could you please inform how to run your code if I only have CPU? Thanks.

    • Unknown
      Unknown October 3, 2016 at 1:19 AM

      Hello. :)

      I am just a guest.

      GMat is defined in the OpenCV 3.1.

    • Unknown
      Unknown October 4, 2016 at 9:40 AM

      Thank you Juhong! Is UMat only for GPU?

    • Munesh
      Munesh June 9, 2017 at 8:19 AM

      Umat is basically for OpenCL/ CPU whereas GpuMat is meant for allocating device memory in Nvidia-based GPUs.

  • Anonymous
    Anonymous November 20, 2016 at 10:55 PM

    can you please tell how to calculate velocity using optical flow

    • Unknown
      Unknown November 23, 2016 at 8:11 AM

      yes find out the centroid and fps. do perspective projection and calculate speed

    • Unknown
      Unknown April 24, 2017 at 10:18 AM

      hello, can you share code for velocity calculation using optical flow....thanks

  • Priya
    Priya January 20, 2017 at 11:27 PM

    I do agree with all the ideas you have presented in your post. They’re really convincing and will certainly work. Still, the posts are very short for newbies. Could you please extend them a little from next time?..Keep this great work..
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  • Unknown
    Unknown May 5, 2017 at 3:21 AM

    Thanks for sharing the very useful info about clanguage and please keep updating........

  • Unknown
    Unknown July 14, 2017 at 6:29 AM

    Thanks for sharing the c++ tutorial with us and I have learned new programming skills especially on Optical Flow Farneback parameters remarks. I will bookmark this site so that I can practice writing the shared programs on my own especially when I have finish accessing Guide on Assignment Writing.

  • Unknown
    Unknown July 25, 2017 at 3:04 AM

    Can anybody give me same code of Optical Flow with Opencv2, Python Language

  • Unknown
    Unknown October 10, 2017 at 4:22 AM

    hey bro can you you help same project in java

    • Vl
      Vl November 27, 2017 at 12:03 PM

      Sorry man, not a java person.

  • vaiybora
    vaiybora November 27, 2017 at 7:48 AM

    This blog is awesome and very informative keep Sharing this type of blog.

    แตกใน xxx

    • Vl
      Vl November 27, 2017 at 12:03 PM

      Thanks, i will keep sharing and please share my blog. That helps to doing this. :)

  • Sant
    Sant December 14, 2017 at 11:37 AM

    If i give any video file as input to videoCapture then "cap.grab();" is returning false always. the code is working for vidoeCapture(0) (From webcam). need some help to debbug!!!

  • Anonymous
    Anonymous January 22, 2018 at 11:29 PM

    It's very interesting, can you introduce me?

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  • Unknown
    Unknown March 27, 2018 at 2:46 AM

    Hey Man its a nice tutorial but how to modify the code for occlusion culling?
    It would be nice if you share your thoughts!

  • Unknown
    Unknown October 22, 2018 at 12:19 AM

    This is very much interesting, thanks for sharing your ideas with us.
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