Opencv 3.1 Tutorial Optical flow (calcOpticalFlowFarneback)

Farneback Optical flow

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 describe 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 window size. Flow is computed over the window larger value is more robust to the noise. 
  • 2 mean number of iteration 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

Mine 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);


  1. 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.

    1. Hello. :)

      I am just a guest.

      GMat is defined in the OpenCV 3.1.

    2. Thank you Juhong! Is UMat only for GPU?

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

  2. can you please tell how to calculate velocity using optical flow

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

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

  3. 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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  6. 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.

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

  8. hey bro can you you help same project in java

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  11. 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!!!

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

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