Creating Holes in a Window

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Let's say you have to create a window inside a window or a hole, just to look outside and say: "Hi" ;-).

It's not so difficult to accomplish - actually, it's pretty easy. The keyword is SetWindowRgn - which sets the window border to the specified CRgn. You can also invert the CRgn (RGN_XOR) and create a hole inside the window. I've created 2 functions for this:

• void CreateHole(CRgn& rgn) - creates the hole in the shape of CRgn
• void ClearHoles() - clear the holes and return the window to its usual shape

void CHolesDlg::CreateHole(CRgn& rgn)
{
    CRgn WindowRgn;
    CRgn HoleRgn;
    static CRgn ThisRgn;

    CRect WindowRect;
    static bool Start = true;
    GetWindowRect (WindowRect);   // Get the Window rect
    WindowRgn.CreateRectRgn (0,0,WindowRect.Width (), WindowRect.Height());

    // initialize the hole region
    HoleRgn.CreateRectRgn (0,0,0,0);
 
    if (Start)
    {
       // initialize ThisRgn
       ThisRgn.CreateRectRgn (0,0,0,0);
       // First copy the region himself in ThisRgn
       ThisRgn.CopyRgn (&rgn);
    }
    else
    {
       // Other times, Add the new region to ThisRgn
       ThisRgn.CombineRgn (&ThisRgn, &rgn, RGN_OR);
    }
 
    Start = false;
    // create the border of the hole(s) with ThisRgn and set it in that
    // border by reversing it
    HoleRgn.CombineRgn (&ThisRgn, &WindowRgn, RGN_XOR);
    SetWindowRgn ((HRGN__*)HoleRgn.m_hObject, TRUE);
}

void CMyControl::ClearHoles()
{
    CRect WindowRect;
    CRgn WindowRgn;
    GetWindowRect (WindowRect);
    WindowRgn.CreateRectRgn (0,0,WindowRect.Width (), WindowRect.Height());
    // Get back to the classic border
    SetWindowRgn ((HRGN__*)WindowRgn.m_hObject, TRUE);
}

void CMyControl::OnClear()
{
    // TODO: Add your control notification handler code here
    CString s;
    GetDlgItem (IDC_CLEAR)->GetWindowText (s);
    if (s == "Clear Holes")
    {
       GetDlgItem (IDC_CLEAR)->SetWindowText ("Show Holes");
       ClearHoles();
    }

    if (s == "Show Holes")
    {
       GetDlgItem (IDC_CLEAR)->SetWindowText ("Clear Holes");
       ShowHoles();
    }
}

In my example, I show 4 shaped holes in a dialog-based application (VC++ 6.0, Windows 98). It should work well for other controls as well.

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