How to capture the desktop in OpenCV (ie. turn a bitmap into a Mat)?

Question

I want to use OpenCV to process my desktop as if it were a video stream.
I am familiar with OpenCV.
I am not familiar with the Windows API. I realize there are other ways to capture the screen, but for the purposes of my question, I need it to be done using OpenCV.

Here is my (super naive) code:

HWND hDesktopWnd;
HDC hDesktopDC;
hDesktopWnd=GetDesktopWindow();
hDesktopDC=GetDC(hDesktopWnd);

// get the height and width of the screen
int height = GetSystemMetrics(SM_CYVIRTUALSCREEN);
int width = GetSystemMetrics(SM_CXVIRTUALSCREEN);

// create a bitmap
HBITMAP hbDesktop = CreateCompatibleBitmap( hDesktopDC, width, height);

Mat src(height,width,CV_8UC4);
src.data = (uchar*)hbDesktop;

imshow("output",src);  //fails :(

There are similar questions on StackOverflow, but they are either for the old-style OpenCV, or for Android operating system.
I'm on windows 7 64x
Opencv 2.4.3

Thanks anyone who can answer this question.

Answer 1

After MUCH trial and error, I managed to write a function to do it. here it is for anyone else who might want it:

#include "stdafx.h"
#include "opencv2/core/core.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <opencv2/highgui/highgui.hpp>
#include <Windows.h>
#include <iostream>
#include <string>
using namespace std;
using namespace cv;

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/2;  //change this to whatever size you want to resize to
    width = windowsize.right/2;

    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;

    // 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, 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

    // avoid memory leak
    DeleteObject (hbwindow); DeleteDC(hwindowCompatibleDC); ReleaseDC(hwnd, hwindowDC);

    return src;
}