Why is the lines of optical flow are not drawn in my code

Question

I'm trying to use the optical flow, but optical flow lines are not drawn and instead only points, what's the problem ? Here is the source code of the project. Looked through the debugger. GDB shows that always p0.x = p1.x and p0.y = p1.y. but why ? Sorry for my bad English.

#include "opencv/cv.h"
#include "opencv2/core/core.hpp"
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/objdetect/objdetect.hpp>
#include <opencv2/opencv.hpp>

#include <iostream>
#include <stdio.h>

std::vector<cv::Point2f> corners;
std::vector<cv::Point2f> corners_b;
double qualityLevel = 0.01;
double minDistance = 10;
int blockSize = 3;
bool useHarrisDetector = false;
double k = 0.04;
int maxCorners = 200;
int maxTrackbar = 100;

void MotionDetection(cv::Mat frame1, cv::Mat frame2)
{
    cv::Mat prev, next;
    cvtColor(frame1, prev, CV_BGR2GRAY); 
    cvtColor(frame2, next, CV_BGR2GRAY); 
    goodFeaturesToTrack( prev, 
            corners,
            maxCorners,
            qualityLevel,
            minDistance,
            cv::Mat(),
            blockSize,
            useHarrisDetector,
            k );
    cornerSubPix(prev, 
            corners,
            cvSize( 10, 10 ) ,
            cvSize( -1, -1 ), 
            cvTermCriteria( CV_TERMCRIT_ITER | CV_TERMCRIT_EPS, 20, 0.03 ) );
    std::vector<uchar> features_found;
    features_found.reserve(maxCorners);
    std::vector<float> feature_errors;
    feature_errors.reserve(maxCorners);
    calcOpticalFlowPyrLK(prev, next, corners, corners_b, features_found, 
            feature_errors, cvSize( 10, 10 ), 5, cvTermCriteria( CV_TERMCRIT_ITER | CV_TERMCRIT_EPS, 20, 0.3 ), 0);
    IplImage g = next;
    for( int i = 0; i < maxCorners; ++i )
    {
        CvPoint p0 = cvPoint( cvRound( corners[i].x ), cvRound( corners[i].y ) );
        CvPoint p1 = cvPoint( cvRound( corners_b[i].x ), cvRound( corners_b[i].y ) );
        cvLine( &g, p0, p1, CV_RGB(255,0,0), 3, CV_AA );
    }
    cv::Mat rs(&g);
    imshow( "result window", rs );  
    int key = cv::waitKey(5);
}

int main(int argc, char* argv[])
{
    cv::VideoCapture cap(0); 
    if(!cap.isOpened())              
    {
        std::cout<<"[!] Error: cant open camera!"<<std::endl;
        return -1;
    }
    cv::Mat edges;
    cv::namedWindow("result window", 1);
    cv::Mat frame, frame2;
    cap >> frame;
    while(1)
    {
        cap >> frame2;
        MotionDetection(frame, frame2);
    }
    return 0;
}

Answer 1

in you Main function frame is clone frame2. I think, that cap >> frame2; frame2.copyTo( frame ); instead of cap >> frame; thats all