Mat hwnd2mat(HWND hwnd) {
    HDC hwindowDC, hwindowCompatibleDC;
    int height, width, srcheight, srcwidth;
    HBITMAP hbwindow;
    Mat src;
    BITMAPINFOHEADER  bi;
    hwindowDC = GetDC(hwnd);
    hwindowCompatibleDC = CreateCompatibleDC(hwindowDC);
    SetStretchBltMode(hwindowCompatibleDC, COLORONCOLOR);
    RECT windowsize;    // get the height and width of the screen
    GetClientRect(hwnd, &windowsize);
    srcheight = windowsize.bottom;
    srcwidth = windowsize.right;
    height = windowsize.bottom / 1;  //change this to whatever size you want to resize to
    width = windowsize.right / 1;
    src.create(height, width, CV_8UC4);
    // create a bitmap
    hbwindow = CreateCompatibleBitmap(hwindowDC, width, height);
    bi.biSize = sizeof(BITMAPINFOHEADER);    //http://msdn.microsoft.com/en-us/library/windows/window/dd183402%28v=vs.85%29.aspx
    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;
    SelectObject(hwindowCompatibleDC, hbwindow);
    StretchBlt(hwindowCompatibleDC, 0, 0, width, height, hwindowDC, 0, 0, srcwidth, srcheight, 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
    DeleteObject(hbwindow);
    DeleteDC(hwindowCompatibleDC);
    ReleaseDC(hwnd, hwindowDC);
    return src;
}

int main() {
    HWND hwndDesktop = GetDesktopWindow();
    Mat src = hwnd2mat(hwndDesktop);
    imwrite("output.bmp", src);
    return 0;
}