Optical Flow using ROS OpenCV [closed]

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I'm currently working on obstacle avoidance of an UAV(ArDrone). I'm using calcOpticalFlowFarneback() method to calculate the optical flow. But when I run my code, optical flow command is not executing wherelse the video stream is perfectly allright. I'm new to both opencv and ros. Any help to solve this problem will be grateful. Thanks in advance. Here is my project code.

`#include <ros/ros.h>
#include <image_transport/image_transport.h>
#include <sensor_msgs/image_encodings.h>
#include <cv_bridge/cv_bridge.h>
#include <cv_bridge/CvBridge.h>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/opencv.hpp>

using namespace cv;
using namespace std;
static const char WINDOW[]="OUTPUT";
namespace enc = sensor_msgs::image_encodings;
image_transport::Publisher image_pub;


void drawOptFlowMap(const cv::Mat& flow,
                    cv::Mat& flowmap,
                    int step,
                    const cv::Scalar& color
                   ) 
{
float ttc, suml=0, sumr=0, sumc=0, Rvmag, Cvmag, Lvmag;
    for(int y = 0; y < flowmap.rows; y += step)
    {
        for (int x=0;x<flowmap.cols*0.33;x += step)
        {
            const cv::Point2f& fxy = flow.at<cv::Point2f>(y, x);
            cv::line(flowmap,
                             cv::Point(x,y),
                             cv::Point((x+fxy.x),(y+fxy.y)),
                 CV_RGB(255,0,0), 2);
            //cv::circle(flowmap, cv::Point(x,y), 0.1, color, 1);
            Lvmag = sqrt((int)((fxy.x)*(fxy.x)+(fxy.y)*(fxy.y)));
            suml+=Lvmag;
            //ttc= 1/sum * 100;
        }

        for (int x=flowmap.cols*0.33;x<flowmap.cols*0.33*2;x += step)
        {
            const cv::Point2f& fxy = flow.at<cv::Point2f>(y, x);
            cv::line(flowmap,
                             cv::Point(x,y),
                             cv::Point((x+fxy.x),(y+fxy.y)),
                 CV_RGB(0,255,0), 2);
            //cv::circle(flowmap, cv::Point(x,y), 0.1, color, 1);
            Cvmag = sqrt((int)((fxy.x)*(fxy.x)+(fxy.y)*(fxy.y)));
            sumc+=Cvmag;
        }   

            for (int x=flowmap.cols*0.33*2; x<flowmap.cols; x += step)
        {

            const cv::Point2f& fxy = flow.at<cv::Point2f>(y, x);
            cv::line(flowmap,
                             cv::Point(x,y),
                             cv::Point((x+fxy.x),(y+fxy.y)),
                 CV_RGB(0,0,255), 2);
            //cv::circle(flowmap, cv::Point(x,y), 0.1, color, 1);
            Rvmag = sqrt((int)((fxy.x)*(fxy.x)+(fxy.y)*(fxy.y)));
            sumr+=Rvmag;

        }


    }   

        float sum = suml+sumr+sumc;
        ttc= 1/sum


        if (ttc < 0.04)
        {
            if (suml<sumr && suml<sumc){
            cout<< "TURN LEFT" <<endl;}
            else if(sumr<suml && sumr<sumc){
            cout<<"TURN RIGHT" <<endl;
            }
        }
        else{
            cout<<"MOVE FORWARD" <<endl;
        }


}

void obstacleAvoidance(cv::Mat& image)
{    
    Mat imageA, imageB;
    // get a image from camera



    double pyr_scale = 0.5;
    int levels = 3;
    int winsize = 5;
    int iterations = 1;
    int poly_n = 5;
    double poly_sigma = 1.1;
    int flags = 0;




    cv::resize(image,image, cv::Size(240,160));

    cvtColor(image,imageA, CV_BGR2GRAY);
    imageB = imageA.clone();

        Mat flow = Mat(imageA.size(), CV_32FC2);

        /* Do optical flow computation */

        calcOpticalFlowFarneback( 
            imageB, 
            imageA,
            flow,
            pyr_scale,
            levels,
            winsize,
            iterations,
            poly_n,
            poly_sigma,
            flags
            );

        drawOptFlowMap(flow, image, 20, CV_RGB(0,255,0));

        imshow("Output", image);
        waitKey(100);

        imageB = imageA.clone();

    //image_pub.publish(cv_ptr->toImageMsg());
}

void process(const sensor_msgs::ImageConstPtr& original_image)
{
   cv_bridge::CvImagePtr cv_ptr;

try
{
cv_ptr = cv_bridge::toCvCopy(original_image, enc::BGR8);
}
catch (cv_bridge::Exception& e ...