C++ GetDC All Monitors

Question

Basically, I'm making something that imitates a screen melting effect, but I can only get it working on my primary monitor. I've looked up as much as I could and there was only one forum on GetDC for all monitors but it was to no use, all it done was make a rectangle from my primary monitor to my secondary monitor with the effect still only working on my primary monitor. This is the thread I read: GetDC(NULL) gets primary monitor or virtual screen?

LRESULT CALLBACK Melter(HWND hWnd, UINT Message, WPARAM wParam, LPARAM lParam) {
    switch (Message) {
        case WM_CREATE: {
            HDC Desktop = GetDC(HWND_DESKTOP);
            HDC Window = GetDC(hWnd);

            BitBlt(Window, 0, 0, ScreenWidth, ScreenHeight, Desktop, 0, 0, SRCCOPY);
            ReleaseDC(hWnd, Window);
            ReleaseDC(HWND_DESKTOP, Desktop);

            SetTimer(hWnd, 0, Interval, 0);
            ShowWindow(hWnd, SW_SHOW);

            break;
        }
        case WM_PAINT: {
            ValidateRect(hWnd, 0);
            break;
        }
        case WM_TIMER: {
            HDC Window = GetDC(hWnd);
            int uX = (rand() % ScreenWidth) - (150 / 2), uY = (rand() % 15), Width = (rand() % 150);

            BitBlt(Window, uX, uY, Width, ScreenHeight, Window, uX, 0, SRCCOPY);
            ReleaseDC(hWnd, Window);

            break;
        }
        case WM_DESTROY: {
            KillTimer(hWnd, 0);
            PostQuitMessage(EXIT_SUCCESS);
            break;
        }
        return EXIT_SUCCESS;
    }

    return DefWindowProc(hWnd, Message, wParam, lParam);
}

The line I changed was HDC Window = GetDC(Window) to HDC Window = GetDC(NULL) and then some other stuff like the RECT. It'd be great if someone could help me, thanks :)

PS, ScreenWidth = 3600, ScreenHeight = 1080 whilst PMScreenWidth = 1920, PMScreenHeight = 1080. PM as in Primary Monitor, so I've got all the stuff in that function set to ScreenWidth/ScreenHeight so it's the width/height of all monitors. Still doesn't work though.

Barmak Shemirani Barmak Shemirani · Accepted Answer · 2018-11-16T08:35:30

GetDC(HWND_DESKTOP) (same as GetDC(0)) already returns the DC for all monitors. The problem with above code is mainly with the usage of BitBlt and choosing the coordinates. See the MCVE below that addresses the question.

Don't draw in response to WM_CREATE, it's just going to get erased in WM_PAINT or when background is erased.

Don't call ValidateRect in response to WM_PAINT. If you want to erase the window then just use FillRect, or force repaint from a command or another route.

Use GetSystemMetrics(SM_CXVIRTUALSCREEN) and GetSystemMetrics(SM_CYVIRTUALSCREEN) to return width and height for the virtual monitor.

Also be sure the process is DPI aware. For testing you can call SetProcessDPIAware(); at the start of the program. Ideally DPI awareness should be set in manifest file.

int uX = (rand() % ScreenWidth) - (150 / 2);
int uY = (rand() % 15);
int Width = (rand() % 150);
BitBlt(Window, uX, uY, Width, ScreenHeight, Window, uX, 0, SRCCOPY);

Above code is copying bits from client DC the the same client DC, it's not going to do anything. Presumably you want to copy from desktop DC to client DC.

Moreover, the coordinates are basically picked at random. It assumes the primary monitor is on top-left. If uX is greater than window's own width, it will not get copied, unless the window stretches the whole virtual monitor.

LRESULT CALLBACK Melter(HWND hWnd, UINT Message, WPARAM wParam, LPARAM lParam) {

    switch(Message) 
    {
    case WM_PAINT: 
    {
        PAINTSTRUCT ps;
        auto hdc = BeginPaint(hWnd, &ps);
        RECT rc;
        GetClientRect(hWnd, &rc);

        HDC hdesktop = GetDC(0);

        int screenx = GetSystemMetrics(SM_XVIRTUALSCREEN);
        int screeny = GetSystemMetrics(SM_YVIRTUALSCREEN);
        int screenw = GetSystemMetrics(SM_CXVIRTUALSCREEN);
        int screenh = GetSystemMetrics(SM_CYVIRTUALSCREEN);

        StretchBlt(hdc, 0, 0, rc.right, rc.bottom,
            hdesktop, screenx, screeny, screenw, screenh, SRCCOPY);

        ReleaseDC(0, hdesktop);

        EndPaint(hWnd, &ps);
        break;
    }

    case WM_DESTROY: 
        PostQuitMessage(0);
        break;
    }

    return DefWindowProc(hWnd, Message, wParam, lParam);
}

C++ GetDC All Monitors

2 Answers