opnecv tracking dots

i want to put dots to cordinates to a video frame which i determine and track them like opencv sample "lk demo"

i didnt understand the sample. which functions put the dots and track them

thanks for suggestions

/* Demo of modified Lucas-Kanade optical flow algorithm.
   See the printf below */

#ifdef _CH_
#pragma package <opencv>
#endif

#define CV_NO_BACKWARD_COMPATIBILITY

#ifndef _EiC
#include "cv.h"
#include "highgui.h"
#include <stdio.h>
#include <ctype.h>
#endif

IplImage *image = 0, *grey = 0, *prev_grey = 0, *pyramid = 0, *prev_pyramid = 0, *swap_temp;

int win_size = 10;
const int MAX_COUNT = 500;
CvPoint2D32f* points[2] = {0,0}, *swap_points;
char* status = 0;
int count = 0;
int need_to_init = 0;
int night_mode = 0;
int flags = 0;
int add_remove_pt = 0;
CvPoint pt;


void on_mouse( int event, int x, int y, int flags, void* param )
{
    if( !image )
        return;

    if( image->origin )
        y = image->height - y;

    if( event == CV_EVENT_LBUTTONDOWN )
    {
        pt = cvPoint(x,y);
        add_remove_pt = 1;
    }
}


int main( int argc, char** argv )
{
    CvCapture* capture = 0;

    if( argc == 1 || (argc == 2 && strlen(argv[1]) == 1 && isdigit(argv[1][0])))
        capture = cvCaptureFromCAM( argc == 2 ? argv[1][0] - '0' : 0 );
    else if( argc == 2 )
        capture = cvCaptureFromAVI( argv[1] );

    if( !capture )
    {
        fprintf(stderr,"Could not initialize capturing...\n");
        return -1;
    }

    /* print a welcome message, and the OpenCV version */
    printf ("Welcome to lkdemo, using OpenCV version %s (%d.%d.%d)\n",
        CV_VERSION,
        CV_MAJOR_VERSION, CV_MINOR_VERSION, CV_SUBMINOR_VERSION);

    printf( "Hot keys: \n"
            "\tESC - quit the program\n"
            "\tr - auto-initialize tracking\n"
            "\tc - delete all the points\n"
            "\tn - switch the \"night\" mode on/off\n"
            "To add/remove a feature point click it\n" );

    cvNamedWindow( "LkDemo", 0 );
    cvSetMouseCallback( "LkDemo", on_mouse, 0 );

    for(;;)
    {
        IplImage* frame = 0;
        int i, k, c;

        frame = cvQueryFrame( capture );
        if( !frame )
            break;

        if( !image )
        {
            /* allocate all the buffers */
            image = cvCreateImage( cvGetSize(frame), 8, 3 );
            image->origin = frame->origin;
            grey = cvCreateImage( cvGetSize(frame), 8, 1 );
            prev_grey = cvCreateImage( cvGetSize(frame), 8, 1 );
            pyramid = cvCreateImage( cvGetSize(frame), 8, 1 );
            prev_pyramid = cvCreateImage( cvGetSize(frame), 8, 1 );
            points[0] = (CvPoint2D32f*)cvAlloc(MAX_COUNT*sizeof(points[0][0]));
            points[1] = (CvPoint2D32f*)cvAlloc(MAX_COUNT*sizeof(points[0][0]));
            status = (char*)cvAlloc(MAX_COUNT);
            flags = 0;
        }

        cvCopy( frame, image, 0 );
        cvCvtColor( image, grey, CV_BGR2GRAY );

        if( night_mode )
            cvZero( image );

        if( need_to_init )
        {
            /* automatic initialization */
            IplImage* eig = cvCreateImage( cvGetSize(grey), 32, 1 );
            IplImage* temp = cvCreateImage( cvGetSize(grey), 32, 1 );
            double quality = 0.01;
            double min_distance = 10;

            count = MAX_COUNT;
            cvGoodFeaturesToTrack( grey, eig, temp, points[1], &count,
                                   quality, min_distance, 0, 3, 0, 0.04 );
            cvFindCornerSubPix( grey, points[1], count,
                cvSize(win_size,win_size), cvSize(-1,-1),
                cvTermCriteria(CV_TERMCRIT_ITER|CV_TERMCRIT_EPS,20,0.03));
            cvReleaseImage( &eig );
            cvReleaseImage( &temp );

            add_remove_pt = 0;
        }
        else if( count > 0 )
        {
            cvCalcOpticalFlowPyrLK( prev_grey, grey, prev_pyramid, pyramid,
                points[0], points[1], count, cvSize(win_size,win_size), 3, status, 0,
                cvTermCr开发者_如何学Citeria(CV_TERMCRIT_ITER|CV_TERMCRIT_EPS,20,0.03), flags );
            flags |= CV_LKFLOW_PYR_A_READY;
            for( i = k = 0; i < count; i++ )
            {
                if( add_remove_pt )
                {
                    double dx = pt.x - points[1][i].x;
                    double dy = pt.y - points[1][i].y;

                    if( dx*dx + dy*dy <= 25 )
                    {
                        add_remove_pt = 0;
                        continue;
                    }
                }

                if( !status[i] )
                    continue;

                points[1][k++] = points[1][i];
                cvCircle( image, cvPointFrom32f(points[1][i]), 3, CV_RGB(0,255,0), -1, 8,0);
            }
            count = k;
        }

        if( add_remove_pt && count < MAX_COUNT )
        {
            points[1][count++] = cvPointTo32f(pt);
            cvFindCornerSubPix( grey, points[1] + count - 1, 1,
                cvSize(win_size,win_size), cvSize(-1,-1),
                cvTermCriteria(CV_TERMCRIT_ITER|CV_TERMCRIT_EPS,20,0.03));
            add_remove_pt = 0;
        }

        CV_SWAP( prev_grey, grey, swap_temp );
        CV_SWAP( prev_pyramid, pyramid, swap_temp );
        CV_SWAP( points[0], points[1], swap_points );
        need_to_init = 0;
        cvShowImage( "LkDemo", image );

        c = cvWaitKey(10);
        if( (char)c == 27 )
            break;
        switch( (char) c )
        {
        case 'r':
            need_to_init = 1;
            break;
        case 'c':
            count = 0;
            break;
        case 'n':
            night_mode ^= 1;
            break;
        default:
            ;
        }
    }

    cvReleaseCapture( &capture );
    cvDestroyWindow("LkDemo");

    return 0;
}

#ifdef _EiC
main(1,"lkdemo.c");
#endif

---

the first iteration of the algo: you just locate some features you would like to track. They are stored in a point array, and they are marked by green dots on the image for you to follow. Then, on the subsequent iterations, the algo uses an optical flow function to track the movements of the points.

cvGoodFeaturesToTrack and cvFindCornerSubPix are initializing the points you folow, cvCalcOpticalFlowPyrLK tracks the movements of the given points and cvCircle put the green dot where the points are.

Hope this helps.

---

The short answer - you cannot track just any point, it should be a special point that has gradient in two directions and possibly some other qualities (stability, good localization, absence of close neighbors that are also good points).

The reason for having two gradient directions is so-called “aperture problem” - it is impossible to find the exact motion vector of a point with single gradient since motion along the gradient doesn’t change the image (if one to see it through a small aperture or opening).

For this reason you cannot really select points and have to choose them among the one offered by cvGoodFeaturesToTrack() function. You however have a choice to try to find a closest good point to the location of your mouse click as your mouse callback specifies.