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

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;
}

A better way to do it is do it while allocating memory to the pixels only once. so the only copy done here is the one that made by BitBlt

int main()

{

    int x_size = 800, y_size = 600; // <-- Your res for the image





    HBITMAP hBitmap; // <-- The image represented by hBitmap

    Mat matBitmap; // <-- The image represented by mat





    // Initialize DCs

    HDC hdcSys = GetDC(NULL); // Get DC of the target capture..
    HDC hdcMem = CreateCompatibleDC(hdcSys); // Create compatible DC 






    void *ptrBitmapPixels; // <-- Pointer variable that will contain the potinter for the pixels









    // Create hBitmap with Pointer to the pixels of the Bitmap
    BITMAPINFO bi; HDC hdc;
    ZeroMemory(&bi, sizeof(BITMAPINFO));
    bi.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
    bi.bmiHeader.biWidth = x_size;
    bi.bmiHeader.biHeight = -y_size;  //negative so (0,0) is at top left

    bi.bmiHeader.biPlanes = 1;

    bi.bmiHeader.biBitCount = 32;
    hdc = GetDC(NULL);
    hBitmap = CreateDIBSection(hdc, &bi, DIB_RGB_COLORS, &ptrBitmapPixels, NULL, 0);
    // ^^ The output: hBitmap & ptrBitmapPixels


    // Set hBitmap in the hdcMem 
    SelectObject(hdcMem, hBitmap);



    // Set matBitmap to point to the pixels of the hBitmap
    matBitmap = Mat(y_size, x_size, CV_8UC4, ptrBitmapPixels, 0);
    //                ^^ note: first it is y, then it is x. very confusing

    // * SETUP DONE *




    // Now update the pixels using BitBlt
    BitBlt(hdcMem, 0, 0, x_size, y_size, hdcSys, 0, 0, SRCCOPY);


    // Just to do some image processing on the pixels.. (Dont have to to this)
    Mat matRef = matBitmap(Range(100, 200), Range(100, 200));
    //                              y1    y2            x1     x2
    bitwise_not(matRef, matRef); // Invert the colors in this x1,x2,y1,y2




    // Display the results through Mat
    imshow("Title", matBitmap);

    // Wait until some key is pressed

    waitKey(0);


    return 0;


}

Note that no error handling done here to make it simple to understand but you have to do error handling in your code!

Hope this helps