opencv 4.0 can't save video - c++

Question

I am trying to record my desktop screen and save it into a video using opencv videoWriter but always end up having a 6kb video which is not even playable.
Here is my code for the same, I'm first creating mat object for the screen and then writing them into the file.
#include "pch.h"
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <iostream>
#include <stdio.h>
#include <opencv2/objdetect/objdetect.hpp>
#include <Windows.h>
using namespace std;
using namespace cv;
int height;
int width;
Mat hwnd2mat()
{
// returning Mat object for screen and working fine as I'm showing it into a window
}
void CaptureScreen()
{
int nScreenWidth = GetSystemMetrics(SM_CXSCREEN);
int nScreenHeight = GetSystemMetrics(SM_CYSCREEN);
HWND hDesktopWnd = GetDesktopWindow();
int key = 0;
string filename = "D:/outcpp.avi";
cv::Size targetSize = cv::Size(320, 240);
VideoWriter video(filename, cv::VideoWriter::fourcc('M', 'J', 'P', 'G'), 10, targetSize);
while (1)
{
Mat frame = hwnd2mat();
cv::Mat image = frame;
cv::Mat dst;
cv::resize(image, dst, targetSize);
if (dst.empty())
break;
video.write(dst);
imshow("Frame", dst);
char c = (char)waitKey(1);
if (c == 27)
break;
}
video.release();
destroyAllWindows();
//readVideo(filename);
}
int main(int argc, char **argv)
{
CaptureScreen();
return 0;
}

Answer 1

You are calling a function that, according to its signature, returns something, but actually doesn't. This causes undefined behaviour according to the C++ standard, so your program is buggy.
Please also enable warnings, because the compiler would normally have told you exactly that.

Answer 2

Try different fourcc code with *.mp4, which is better anyway, and run without showing the image. Make sure video is opened and released successfully.
void hwnd2mat(cv::Mat &mat)
{
int w = mat.cols;
int h = mat.rows;
auto hdc = GetDC(0);
auto memdc = CreateCompatibleDC(hdc);
auto hbitmap = CreateCompatibleBitmap(hdc, w, h);
auto holdbmp = SelectObject(memdc, hbitmap);
BitBlt(memdc, 0, 0, w, h, hdc, 0, 0, SRCCOPY);
BITMAPINFOHEADER bi = { sizeof(bi), w, -h, 1, 24 };
GetDIBits(hdc, hbitmap, 0, h, mat.data, (BITMAPINFO*)&bi, 0);
SelectObject(memdc, holdbmp);
DeleteDC(memdc);
DeleteObject(hbitmap);
ReleaseDC(0, hdc);
}
int main()
{
int w = GetSystemMetrics(SM_CXSCREEN);
int h = GetSystemMetrics(SM_CYSCREEN);
cv::Size size = cv::Size(w, h);
Mat frame(size, CV_8UC3);
double fps = 10.0;
//string filename = "d:\\outcpp.avi";
//auto fourcc = cv::VideoWriter::fourcc('M', 'J', 'P', 'G');
string filename = "d:\\outcpp.mp4";
auto fourcc = cv::VideoWriter::fourcc('m', 'p', '4', 'v');
VideoWriter video(filename, fourcc, fps, frame.size());
if(!video.isOpened())
{
cout << "codec failed\n";
return 0;
}
while(GetAsyncKeyState(VK_ESCAPE) == 0)
{
cout << ".";
hwnd2mat(frame);
video.write(frame);
Sleep(int(1000.0/fps));
}
video.release();
cout << "\nreleased\n";
return 0;
}