Record multiple videos from webcam stream in OpenCV C++ - c++

Question

I want to record selected frames as multiple videos from webcam. I tried the following code to start recording a video on a key press and stop recording that video with a different key press. I want to record multiple such videos. But the recorded video files are empty. I can run its equivalent Python code successfully, but I want the same in C++. Can you please help me to correct my mistake?
#include <iostream>
#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
using namespace std;
using namespace cv;
int main(){
// Create a VideoCapture object and use camera to capture the video
VideoCapture cap(0);
// Check if camera opened successfully
if(!cap.isOpened()){
cout << "Error opening video stream" << endl;
return -1;
}
// Default resolutions of the frame are obtained.
int frame_width = cap.get(cv::CAP_PROP_FRAME_WIDTH);
int frame_height = cap.get(cv::CAP_PROP_FRAME_HEIGHT);
bool recording = false;
int videono = 1;
VideoWriter video("dummy.avi", cv::VideoWriter::fourcc('M','J','P','G'), 10, Size(frame_width,frame_height));
video.release();
while(1)
{
Mat frame;
// Capture frame-by-frame
cap >> frame;
// If the frame is empty, break immediately
if (frame.empty())
break;
// Display the resulting frame
imshow( "Frame", frame );
// Press ESC on keyboard to exit
char c = (char)waitKey(1);
if( c == 27 )
break;
// Press s on keyboard to start recording
if( c == 115 and !recording)
{
char path[100];
sprintf(path, "%d.avi", videono);
std::cout << "recording started for " << path << "\n";
videono += 1;
VideoWriter video(path, cv::VideoWriter::fourcc('M','J','P','G'), 10, Size(frame_width,frame_height));
recording = true;
}
if( recording )
video.write(frame);
// Press x on keyboard to stop recording
if( c == 120)
{
std::cout << "recording finished.\n";
recording = false;
video.release();
}
}
// release the video capture and write object
cap.release();
// Closes all the frames
destroyAllWindows();
return 0;
}

Answer 1

Instead of re-creating a new VideoWriter every time and deleting it immediately like this
{
VideoWriter video(path, cv::VideoWriter::fourcc('M','J','P','G'), 10, Size(frame_width,frame_height));
} // <-- deleted here, because it's going out of scope
You should just use the 'open' function on the existing VideoWriter.
So something like this:
#include <iostream>
#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
using namespace std;
using namespace cv;
int main(){
// Create a VideoCapture object and use camera to capture the video
VideoCapture cap(0);
// Check if camera opened successfully
if(!cap.isOpened()){
cout << "Error opening video stream" << endl;
return -1;
}
// Default resolutions of the frame are obtained.
int frame_width = cap.get(cv::CAP_PROP_FRAME_WIDTH);
int frame_height = cap.get(cv::CAP_PROP_FRAME_HEIGHT);
bool recording = false;
int videono = 1;
VideoWriter video;
while(1)
{
Mat frame;
// Capture frame-by-frame
cap >> frame;
// If the frame is empty, break immediately
if (frame.empty())
break;
// Display the resulting frame
imshow( "Frame", frame );
char c = (char)waitKey(1);
if( c == 27 ) // Press ESC on keyboard to exit
break;
if( c == 115 and !recording) // Press s on keyboard to start recording
{
char path[100];
sprintf(path, "%d.avi", videono);
std::cout << "recording started for " << path << "\n";
videono += 1;
video.open(path, cv::VideoWriter::fourcc('M','J','P','G'), 10, Size(frame_width,frame_height));
recording = true;
}
if( recording )
video.write(frame);
if( c == 'x') // Press x on keyboard to stop recording
{
std::cout << "recording finished.\n";
recording = false;
video.release();
}
}
cap.release();// release the video capture and write object
destroyAllWindows(); // Closes all the frames
return 0;
}