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MouseLight - Utility to Make Spot Light Based on Mouse Moving

, 15 Aug 2008 CPOL 25K 989 24
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MouseLight support for your presentation
MouseLight Sample - Click to enlarge image


Sometimes it's difficult to tell an audience to look at a particular area of the screen. Whether it's for a product demo, presentation, screencast or a lecture. MouseLight tries to eliminate this problem.

This utility puts a "spotlight" and dims the surrounding area which enables your audience to effortlessly follow your path of movement on the screen.


Our goal solves the problem that we need to know the 2 concepts given below:

  • LayeredWindow - Windows APIs support a layered window made transparent for a dialog. We will create a dialog with size equal to desktop and use this technique to blur the surrounding area.
  • Region - We can change the shape of the dialog based on Region. With this technique, we can make a spotlight with different shapes such as Round, etc.

How To Make SpotLight

First, we will create a dialog with a size equal to desktop window. We must go to the resource mode of that dialog, set properties of dialog - border = none, so this dialog will be removed in the title bar. And we will blur this dialog by making transparent with specified percent rate. So we still see different windows although our dialog is topmost, as you see the code below (also see my 2 WndShadow h/cpp files):

    // We get address of function from WindowAPI
    HMODULE hSysDll = LoadLibrary(_T("USER32.DLL"));
    s_UpdateLayeredWindow = 
    // If the import did not succeed, probably layered window is 
    // not supported by current OS
    if (NULL == s_UpdateLayeredWindow)
        return false;

    s_SetLayeredWindowAttributes = 

    // After that, we call this function to make transparent to that dialog
    /* Windows need to be layered to be made transparent. This is done
    * by modifying the extended style bits to contain WS_EX_LAYARED. */

                 GWL_EXSTYLE , 
                 GetWindowLong(hWnd, GWL_EXSTYLE) | WS_EX_LAYERED);

    if (GetLastError())
        return FALSE;

    /* Now, we need to set the 'layered window attributes'. This
    * is where the alpha values get set. */
    return s_SetLayeredWindowAttributes (hWnd, 
                                       RGB(255, 0, 255), 

To make spotlight, we use the Region technique. Create a memory DC/Bitmap with a size equal to desktop window and draw Round, etc. at some position and then convert BitmapToRegion (you can see my 2 Region cpp/hpp files) and set Region to our dialog as below:

    // Draw mem dc to make region
    m_cVideoMemDC.FillSolidRect(m_rcVideoWindow, COLOR_WHITE);
    // Draw round
    CRect cRectRound(0, 0,,;
    m_cRoundVideoMemDC.FillSolidRect(cRectRound, COLOR_WHITE);
    m_cRoundVideoMemDC.RoundRect(cRectRound, cRectRound.CenterPoint());
                            &m_cRoundVideoMemDC, 0, 0, SRCCOPY);
    // Draw mem dc to update on dialog background
    // Draw round
    CRect cRectRound2(0, 0,,;
    cRectRound2.DeflateRect(cRectRound2.Width()/4, cRectRound2.Height()/4);
    m_cRoundBgMemDC.RoundRect(cRectRound2, cRectRound2.CenterPoint());
                         	&m_cRoundBgMemDC, 0, 0, SRCCOPY);

    // Make region
    HRGN hRgn = BitmapToRegion(m_cVideoMemDC.m_hDC, 0, 0, 
                                m_rcVideoWindow.Width(), m_rcVideoWindow.Height(), 
                                RGB(0,0,0), RGB(20,20,20));
    int iRes = this->SetWindowRgn(hRgn, FALSE);

    // Notify to draw

SpotLight will move to follow the mouse position changes. In my old version, I used Mouse event as left clickup/down and mousemove on Dialog to move SpotLight. Now, I wrote a DLL to monitor MouseMove action (also see SetWindowHook in MSDN).

// Install hook
InstallHook(this->m_hWnd, WM_MOUSEMOVE);

// Wait to receive msg from mouse
LRESULT CMouseLightDlg::OnMouseHook(WPARAM wParam, LPARAM lParam)
    if (wParam == WM_MOUSEMOVE)
        if (g_bSaved == FALSE)
            g_bSaved = TRUE;
        CPoint Point;
        if (Point != g_cSavedPoint)
            if (this->IsWindowVisible())
            g_cSavedPoint = Point;

    return S_OK;

We can also draw an image background on that dialog and an image around SpotLight. So it is easy for an audience to look at a particular area of the screen. (Also see another article here.)

Points of Interest

Windows APIs support SetLayeredWindowAttributes function to make a dialog transparent. So we can use this function to make our application "beautifully".


  • Version 
    • Use the MouseHook technique to monitor MouseMove action
  • Version
    • Allow to zoom in/out by Ctrl + MouseMove 
    • Look better


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

James Duy Trinh (VietDoor)
Software Developer (Senior)
Vietnam Vietnam
Bachelor of Natural Science University, HCMC VietNam

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