Как сделать скриншот полноэкранного приложения?

BITMAPINFOHEADER createBitmapHeader(int width, int height)
{
    BITMAPINFOHEADER  bi;
 
    // create a bitmap
    bi.biSize = sizeof(BITMAPINFOHEADER);
    bi.biWidth = width;
    bi.biHeight = -height;  //this is the line that makes it draw upside down or not
    bi.biPlanes = 1;
    bi.biBitCount = 32;
    bi.biCompression = BI_RGB;
    bi.biSizeImage = 0;
    bi.biXPelsPerMeter = 0;
    bi.biYPelsPerMeter = 0;
    bi.biClrUsed = 0;
    bi.biClrImportant = 0;
 
    return bi;
}
 
/**
 * Capture a screen window as a matrix.
 *
 * @param hwnd : window handle.
 *
 * @return Mat (Mat of the captured image)
 */
Mat captureScreenMat(HWND hwnd)
{
    Mat src;
 
    // get handles to a device context (DC)
    HDC hwindowDC = GetDC(hwnd);
    HDC hwindowCompatibleDC = CreateCompatibleDC(hwindowDC);
    SetStretchBltMode(hwindowCompatibleDC, COLORONCOLOR);
 
    // define scale, height and width
 
 
    int screenx = GetSystemMetrics(SM_XVIRTUALSCREEN);
    int screeny = GetSystemMetrics(SM_YVIRTUALSCREEN);
 
    int screen_main_x = GetSystemMetrics(SM_CXSCREEN);
    int screen_main_y = GetSystemMetrics(SM_CXSCREEN);
 
    RECT rect;
    if (GetWindowRect(hwnd, &rect))
    {
        width = rect.right - rect.left;
        height = rect.bottom - rect.top;
    }
    // create mat object
    cout << "wid: " << width << " hei: " << height << endl;
    cout << "wid: " << screenx << " hei: " << screeny << endl;
    width = screen_main_x-100;
    height = screen_main_y-100;
    src.create(height, width, CV_8UC4);
    
    // create a bitmap
    HBITMAP hbwindow = CreateCompatibleBitmap(hwindowDC, width, height);
    BITMAPINFOHEADER bi = createBitmapHeader(width, height);
 
    // use the previously created device context with the bitmap
    SelectObject(hwindowCompatibleDC, hbwindow);
 
    // copy from the window device context to the bitmap device context
    StretchBlt(hwindowCompatibleDC, 0, 0, width, height, hwindowDC, screenx, screeny, width, height, SRCCOPY);  //change SRCCOPY to NOTSRCCOPY for wacky colors !
    GetDIBits(hwindowCompatibleDC, hbwindow, 0, height, src.data, (BITMAPINFO*)&bi, DIB_RGB_COLORS);            //copy from hwindowCompatibleDC to hbwindow
 
    // avoid memory leak
    DeleteObject(hbwindow);
    DeleteDC(hwindowCompatibleDC);
    ReleaseDC(hwnd, hwindowDC);
 
    return src;
}
 
 
 
 
int main()
{
 
    HWND hwnd = FindWindow(0, WINDOWNAME);
 
        // get frame from the game
        frame = captureScreenMat(hwnd);
        cvtColor(frame, frame, COLOR_RGBA2RGB);