How to write a video? - visual-studio-2010

Question

I'm using opencv with visual studio 2010 in Windows 7 with 32 bit OS.... While running the sample program of People detection, it shows the output video playing in a window... But I'm unable to open the output video, stored in a particular location... Kindly help me... Thankyou...
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/objdetect/objdetect.hpp"
#include "opencv2/highgui/highgui.hpp"
#include <opencv2/core/core.hpp>
#include <iostream>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
using namespace cv;
using namespace std;
int main(int argc, char** argv)
{
Mat img; char _filename[1024];
HOGDescriptor hog;
hog.setSVMDetector(HOGDescriptor::getDefaultPeopleDetector());
namedWindow("people detector", 1);
CvCapture *cap=cvCaptureFromFile("E:/Phase_I_output/2.walk.avi");
img=cvQueryFrame(cap);
for(;;)
{
img=cvQueryFrame(cap);
if(img.empty())
break;
fflush(stdout);
vector<Rect> found, found_filtered;
double t = (double)getTickCount();
int can = img.channels();
hog.detectMultiScale(img, found, 0, Size(8,8), Size(32,32), 1.05, 2);
t = (double)getTickCount() - t;
printf("tdetection time = %gms\n", t*1000./cv::getTickFrequency());
size_t i, j;
for( i = 0; i < found.size(); i++ )
{
Rect r = found[i];
for( j = 0; j < found.size(); j++ )
if( j != i && (r & found[j]) == r)
break;
if( j == found.size() ) found_filtered.push_back(r);
}
for( i = 0; i < found_filtered.size(); i++ )
{
Rect r = found_filtered[i];
r.x += cvRound(r.width*0.1);
r.width = cvRound(r.width*0.8);
r.y += cvRound(r.height*0.07);
r.height = cvRound(r.height*0.8);
rectangle(img, r.tl(), r.br(), cv::Scalar(0,255,0), 3);
}
Size size2 = Size(640,480);
int codec = CV_FOURCC('M', 'J', 'P', 'G');
VideoWriter writer2("E:/Phase_I_output/video_.avi",codec,50.0,size2,true);
writer2.open("E:/Phase_I_output/video_.avi",codec,15.0,size2,true);
writer2.write(img);
imshow("people detector", img);
if(waitKey(1) == 27)
break;
}
std::cout << "Completed" << std::endl ;
waitKey();
return 0;
}

Answer 1

You should initialize the videowriter before the infinite loop, and release the videowriter (not necessary with the C++ API) and the videocapture once there are no more frame to grab :
Size size2 = Size(640,480);
int codec = CV_FOURCC('M', 'J', 'P', 'G');
VideoWriter writer2("E:/Phase_I_output/video_.avi",codec,50.0,size2,true);
writer2.open("E:/Phase_I_output/video_.avi",codec,50.0,size2,true);
for(;;){
//do your stuff
//write the current frame
writer2.write(img);
}
cvReleaseVideoWriter( writer2 );
cvReleaseCapture( &cap );
You should also use the C++ API of openCV. I believe every function beginning with 'cv' is part of the C API (and no longer supported). Check the openCV documentation to find the corresponding C++ function.
For example :
img=cvQueryFrame(cap);
Will become :
cap >> img;
Edit
I corrected your code to use the C++ API of openCV, and it's working fine (the people detection seems to give false positive though). Here is the code :
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/objdetect/objdetect.hpp"
#include "opencv2/highgui/highgui.hpp"
#include <opencv2/core/core.hpp>
#include <iostream>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
using namespace cv;
using namespace std;
int main(int argc, char** argv)
{
Mat img;
string _filename;
_filename = "path/to/video.avi";
HOGDescriptor hog;
hog.setSVMDetector(HOGDescriptor::getDefaultPeopleDetector());
namedWindow("people detector", WND_PROP_AUTOSIZE);
VideoCapture cap = VideoCapture(_filename);
if(!cap.isOpened()){
cout<<"error opening : "<<_filename<<endl;
return 1;
}
Size size2 = Size(640,480);
int codec = static_cast<int>(cap.get(CV_CAP_PROP_FOURCC));
double fps = cap.get(CV_CAP_PROP_FPS);
VideoWriter writer2("../outputVideo_.avi",codec,fps,size2,true);
for(;;)
{
cap >> img;
if(img.empty()){
cout<<"frame n° "<<cap.get(CV_CAP_PROP_FRAME_COUNT)<<endl;
break;
}
fflush(stdout);
vector<Rect> found, found_filtered;
double t = (double)getTickCount();
int can = img.channels();
hog.detectMultiScale(img, found, 0, Size(8,8), Size(32,32), 1.05, 2);
t = (double)getTickCount() - t;
printf("tdetection time = %gms\n", t*1000./cv::getTickFrequency());
size_t i, j;
for( i = 0; i < found.size(); i++ )
{
Rect r = found[i];
for( j = 0; j < found.size(); j++ )
if( j != i && (r & found[j]) == r)
break;
if( j == found.size() ) found_filtered.push_back(r);
}
for( i = 0; i < found_filtered.size(); i++ )
{
Rect r = found_filtered[i];
r.x += cvRound(r.width*0.1);
r.width = cvRound(r.width*0.8);
r.y += cvRound(r.height*0.07);
r.height = cvRound(r.height*0.8);
rectangle(img, r.tl(), r.br(), Scalar(0,255,0), 3);
}
//writer2.write(img);
writer2 << img;
imshow("people detector", img);
if(waitKey(1) == 27)
{
break;
}
}
writer2.release();
cap.release();
cout << "Completed" << endl ;
waitKey();
destroyAllWindows();
return 0;
}