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 object in opencv video. Lets checkt the  video example and the achieved result on my blog. Working and well describe code is included. 

Optical Flow Farneback parameters remarks

You need 2 images at least to calculate optical flow, the previous image (prevgray) and current image (img). 

!! The previous image must be initialized first !! 

Both images have to be gray scale. 

Result is computer in flowUmat which has same size as inputs but format is CV_32FC2

0.4- image pyramid or simple image scale

1 is number of pyramid layers. 1 mean that flow is calculated only from previous image. 

12 is win size.. Flow is computed over the window larger value is more robust to the noise. 

2 mean number of iteration of algorithm

8 is polynomial degree expansion recommended value are 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

#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 argc, const 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;

break;

}

else {

Mat img;

Mat original;

// capture frame from video file

cap.retrieve(img, CV_CAP_OPENNI_BGR_IMAGE);

resize(img, img, Size(640, 480));

// save original for later

img.copyTo(original);

// 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, 0.4, 1, 12, 2, 8, 1.2, 0);

// copy Umat container to standard Mat

flowUmat.copyTo(flow);

                           

// 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 = flow.at<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), 1, Scalar(0, 0, 0), -1);

}

}

                                 // draw the results

namedWindow("prew", WINDOW_AUTOSIZE);

imshow("prew", original);

                                 // fill previous image again

img.copyTo(prevgray);

}

else {

                                 // fill previous image in case prevgray.empty() == true

img.copyTo(prevgray);

}

int key1 = waitKey(20);

}

}

}