OPENCV Desktop Capture

Question

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

Hello,

Can anyone explain to me how I would use this code in OpenCV to capture the desktop screen? I've been trying to get it to work for about 30-45 minutes but my screen doesn't capture anything when I run it.

In my application from main I have the follow three statements

HWND hwndDesktop = GetDesktopWindow();
hwnd2mat(hwndDesktop);
imshow("output", src);

i'm calling the function hwnd2mat that is found in the link above. i'm a noob.

Thanks to anyone who answers.

Solution 1

upon OP's comment i think there is still a need to explain how to capture desktop like a video stream

#include "opencv2/imgproc.hpp"
#include "opencv2/highgui.hpp"
#include <Windows.h>
#include <iostream>

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/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;

    // 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;
}

int main(int argc, char **argv)
{
    HWND hwndDesktop = GetDesktopWindow();
    namedWindow("output",WINDOW_NORMAL);
    int key = 0;

    while( key != 27 )
    {
        Mat src = hwnd2mat(hwndDesktop);
        // you can do some image processing here
        imshow("output", src);
        key = waitKey(60); // you can change wait time
    }

}

EDIT: you can find a sample code for screen capturing and saving as a video here

Solution 2

It seems you forgot to capture the return of hwnd2mat():

HWND hwndDesktop = GetDesktopWindow();
Mat src = hwnd2mat(hwndDesktop);
imshow("output", src);
waitKey(0);

Solution 3

I would make hwnd2mat a little bit more specific and address Stepan Yakovenko's comment by adding definition and references to "valueOfChangeTheSizeOfTextAppsAndOtherItemsInWindowsDisplaySettings".

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);

     float valueOfChangeTheSizeOfTextAppsAndOtherItemsInWindowsDisplaySettings = 1.5;
     srcheight = (int)((float)windowsize.bottom * valueOfChangeTheSizeOfTextAppsAndOtherItemsInWindowsDisplaySettings);
     srcwidth = (int)((float)windowsize.right * valueOfChangeTheSizeOfTextAppsAndOtherItemsInWindowsDisplaySettings);
     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;

     // 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;
 }

Comments

[Stepan Yakovenko]

Please note that this will grab only piece of screen on Windows 10 if you run it in 175% large fonts mode...

[Yoda]

Does it use DWM or GDI ?

[Burak]

@Yoda GetDC requires GDI