samples/cpp/lkdemo.cpp

An example using the Lucas-Kanade optical flow algorithm

#include "opencv2/video/tracking.hpp"
#include "opencv2/imgproc.hpp"
#include "opencv2/videoio.hpp"
#include "opencv2/highgui.hpp"
 
#include <iostream>
#include <ctype.h>
 
using namespace cv;
using namespace std;
 
static void help()
{
 // print a welcome message, and the OpenCV version
 cout << "\nThis is a demo of Lukas-Kanade optical flow lkdemo(),\n"
 "Using OpenCV version " << CV_VERSION << endl;
 cout << "\nIt uses camera by default, but you can provide a path to video as an argument.\n";
 cout << "\nHot 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" << endl;
}
 
Point2f point;
bool addRemovePt = false;
 
static void onMouse( int event, int x, int y, int /*flags*/, void* /*param*/ )
{
 if( event == EVENT_LBUTTONDOWN )
 {
 point = Point2f((float)x, (float)y);
 addRemovePt = true;
 }
}
 
int main( int argc, char** argv )
{
 VideoCapture cap;
 TermCriteria termcrit(TermCriteria::COUNT|TermCriteria::EPS,20,0.03);
 Size subPixWinSize(10,10), winSize(31,31);
 
 const int MAX_COUNT = 500;
 bool needToInit = false;
 bool nightMode = false;
 
 help();
 cv::CommandLineParser parser(argc, argv, "{@input|0|}");
 string input = parser.get<string>("@input");
 
 if( input.size() == 1 && isdigit(input[0]) )
 cap.open(input[0] - '0');
 else
 cap.open(input);
 
 if( !cap.isOpened() )
 {
 cout << "Could not initialize capturing...\n";
 return 0;
 }
 
 namedWindow( "LK Demo", 1 );
 setMouseCallback( "LK Demo", onMouse, 0 );
 
 Mat gray, prevGray, image, frame;
 vector<Point2f> points[2];
 
 for(;;)
 {
 cap >> frame;
 if( frame.empty() )
 break;
 
 frame.copyTo(image);
 cvtColor(image, gray, COLOR_BGR2GRAY);
 
 if( nightMode )
 image = Scalar::all(0);
 
 if( needToInit )
 {
 // automatic initialization
 goodFeaturesToTrack(gray, points[1], MAX_COUNT, 0.01, 10, Mat(), 3, 3, 0, 0.04);
 cornerSubPix(gray, points[1], subPixWinSize, Size(-1,-1), termcrit);
 addRemovePt = false;
 }
 else if( !points[0].empty() )
 {
 vector<uchar> status;
 vector<float> err;
 if(prevGray.empty())
 gray.copyTo(prevGray);
 calcOpticalFlowPyrLK(prevGray, gray, points[0], points[1], status, err, winSize,
 3, termcrit, 0, 0.001);
 size_t i, k;
 for( i = k = 0; i < points[1].size(); i++ )
 {
 if( addRemovePt )
 {
 if( norm(point - points[1][i]) <= 5 )
 {
 addRemovePt = false;
 continue;
 }
 }
 
 if( !status[i] )
 continue;
 
 points[1][k++] = points[1][i];
 circle( image, points[1][i], 3, Scalar(0,255,0), -1, 8);
 }
 points[1].resize(k);
 }
 
 if( addRemovePt && points[1].size() < (size_t)MAX_COUNT )
 {
 vector<Point2f> tmp;
 tmp.push_back(point);
 cornerSubPix( gray, tmp, winSize, Size(-1,-1), termcrit);
 points[1].push_back(tmp[0]);
 addRemovePt = false;
 }
 
 needToInit = false;
 imshow("LK Demo", image);
 
 char c = (char)waitKey(10);
 if( c == 27 )
 break;
 switch( c )
 {
 case 'r':
 needToInit = true;
 break;
 case 'c':
 points[0].clear();
 points[1].clear();
 break;
 case 'n':
 nightMode = !nightMode;
 break;
 }
 
 std::swap(points[1], points[0]);
 cv::swap(prevGray, gray);
 }
 
 return 0;
}