OpenCV  4.10.0-dev
Open Source Computer Vision
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An example using drawContours to clean up a background segmentation result

#include <stdio.h>
#include <string>
using namespace std;
using namespace cv;
static void help(char** argv)
"This program demonstrated a simple method of connected components clean up of background subtraction\n"
"When the program starts, it begins learning the background.\n"
"You can toggle background learning on and off by hitting the space bar.\n"
"%s [video file, else it reads camera 0]\n\n", argv[0]);
static void refineSegments(const Mat& img, Mat& mask, Mat& dst)
int niters = 3;
vector<vector<Point> > contours;
vector<Vec4i> hierarchy;
Mat temp;
dilate(mask, temp, Mat(), Point(-1,-1), niters);
erode(temp, temp, Mat(), Point(-1,-1), niters*2);
dilate(temp, temp, Mat(), Point(-1,-1), niters);
findContours( temp, contours, hierarchy, RETR_CCOMP, CHAIN_APPROX_SIMPLE );
dst = Mat::zeros(img.size(), CV_8UC3);
if( contours.size() == 0 )
// iterate through all the top-level contours,
// draw each connected component with its own random color
int idx = 0, largestComp = 0;
double maxArea = 0;
for( ; idx >= 0; idx = hierarchy[idx][0] )
const vector<Point>& c = contours[idx];
double area = fabs(contourArea(Mat(c)));
if( area > maxArea )
maxArea = area;
largestComp = idx;
Scalar color( 0, 0, 255 );
drawContours( dst, contours, largestComp, color, FILLED, LINE_8, hierarchy );
int main(int argc, char** argv)
bool update_bg_model = true;
CommandLineParser parser(argc, argv, "{help h||}{@input||}");
if (parser.has("help"))
return 0;
string input = parser.get<std::string>("@input");
if (input.empty());
if( !cap.isOpened() )
printf("\nCan not open camera or video file\n");
return -1;
Mat tmp_frame, bgmask, out_frame;
cap >> tmp_frame;
printf("can not read data from the video source\n");
return -1;
namedWindow("video", 1);
namedWindow("segmented", 1);
cap >> tmp_frame;
if( tmp_frame.empty() )
bgsubtractor->apply(tmp_frame, bgmask, update_bg_model ? -1 : 0);
refineSegments(tmp_frame, bgmask, out_frame);
imshow("video", tmp_frame);
imshow("segmented", out_frame);
char keycode = (char)waitKey(30);
if( keycode == 27 )
if( keycode == ' ' )
update_bg_model = !update_bg_model;
printf("Learn background is in state = %d\n",update_bg_model);
return 0;
Designed for command line parsing.
Definition utility.hpp:820
n-dimensional dense array class
Definition mat.hpp:812
MatSize size
Definition mat.hpp:2160
bool empty() const
Returns true if the array has no elements.
Class for video capturing from video files, image sequences or cameras.
Definition videoio.hpp:731
virtual bool open(const String &filename, int apiPreference=CAP_ANY)
Opens a video file or a capturing device or an IP video stream for video capturing.
virtual bool isOpened() const
Returns true if video capturing has been initialized already.
std::shared_ptr< _Tp > Ptr
Definition cvstd_wrapper.hpp:23
#define CV_8UC3
Definition interface.h:90
void imshow(const String &winname, InputArray mat)
Displays an image in the specified window.
int waitKey(int delay=0)
Waits for a pressed key.
void namedWindow(const String &winname, int flags=WINDOW_AUTOSIZE)
Creates a window.
void drawContours(InputOutputArray image, InputArrayOfArrays contours, int contourIdx, const Scalar &color, int thickness=1, int lineType=LINE_8, InputArray hierarchy=noArray(), int maxLevel=INT_MAX, Point offset=Point())
Draws contours outlines or filled contours.
Ptr< BackgroundSubtractorMOG2 > createBackgroundSubtractorMOG2(int history=500, double varThreshold=16, bool detectShadows=true)
Creates MOG2 Background Subtractor.
int main(int argc, char *argv[])
Definition highgui_qt.cpp:3
"black box" representation of the file storage associated with a file on disk.
Definition core.hpp:102
STL namespace.