Making any application transparent in Windows 2000/XP

• Download demo executable - 7 Kb
• Download source - 12 Kb

Introduction

There are many articles that demonstrate how you can use the new layering features of Windows 2000 or Windows XP to create applications that are translucent. This article adds to that and shows how you can use the same features to turn any application transparent even if you do not have sources for it.

Using this "WinTrans" application you will be able to select any currently running application and turn it transparent by dragging and dropping the wand (icon at the top-left corner) on the title bar of that application. You can also control the transparency using the slider. "WinTrans" has an interface very much like SPY and also demonstrates the usage of Win32 APIs to locate a window under the mouse pointer and extract details like the class associated with it, the caption of the window, etc.

"WinTrans" comes in handy if you want to work on a maximized window and at the same time keep a watch on some other application running in the background.

Background

In Windows 2000 and Windows XP a new function named SetLayeredWindowAttributes has been added to User32.dll. To use this function an application needs to set the WS_EX_LAYERED (0x00080000) bit in the window style either while creating it or later using the SetWindowLong function. Once this bit is set any application can call this function passing a handle to a window and make either the whole window or a particular color on the window transparent. This function takes the following arguments.

• HWND hWnd: Handle to the window
• COLORREF col: Color to be made transparent
• BYTE bAlpha: If this value is 0 the window becomes completely transparent and if it is 255 it becomes opaque
• DWORD dwFlags: If this flag is 1 then only the color col is made transparent. If it is 2 then the whole window becomes transparent as dictated by the bAlpha value
  

Using the code

We begin by defining the following member variables of the main dialog class in WinTransDlg.h

bool m_bTracking;   //  will be true when the mouse is 
                    //  being tracked
HWND m_hCurrWnd;    //  Handle to the window over which 
                    //  the mouse was last present
HCURSOR m_hCursor;  //  The wand cursor

We also define a function pointer that points to the SetLayeredWindowAttributes function. This function is defined in the User32.dll.

//  Global definition
typedef BOOL (WINAPI *lpfn) (HWND hWnd, COLORREF cr, 
              BYTE bAlpha, DWORD dwFlags);
lpfn g_pSetLayeredWindowAttributes;

In the OnInitDialog event handler for the dialog we get the address of the SetLayeredWindowAttributes function and store it in g_pSetLayeredWindowAttributes. We also load the wand cursor and store the handle in m_hCursor

BOOL CWinTransDlg::OnInitDialog()
{
    ....
    //  get the function pointer for SetLayeredWindowAttributes 
    //  in User32.dll
    HMODULE hUser32 = GetModuleHandle(_T("USER32.DLL"));
    g_pSetLayeredWindowAttributes = (lpfn)GetProcAddress(hUser32,
               "SetLayeredWindowAttributes");
    if (g_pSetLayeredWindowAttributes == NULL)
        AfxMessageBox (
            "Layering is not supported in this version of Windows",
             MB_ICONEXCLAMATION);

    //  Load the wand cursor
    HINSTANCE hInstResource = AfxFindResourceHandle(
         MAKEINTRESOURCE(IDC_WAND), RT_GROUP_CURSOR);
    m_hCursor = ::LoadCursor( hInstResource, MAKEINTRESOURCE(IDC_WAND) );
    ...
}

We then define handler for the WM_LBUTTONDOWN, WM_LBUTTONUP and WM_MOUSEMOVE events. For the handler for WM_LBUTTONDOWN we do the following

void CWinTransDlg::OnLButtonDown(UINT nFlags, CPoint point)
{
    ...
    SetCapture();      // make mouse move events to 
                       // be directed to this window
    m_hCurrWnd = NULL; // Currently no window is to be made transparent
    m_bTracking = true;     // set the tracking flag
    ::SetCursor(m_hCursor); // turn the mouse pointer into the wand cursor
    ...
}

For mouse move event handler the following code is used

void CWinTransDlg::OnMouseMove(UINT nFlags, CPoint point)
{
    ...
    if (m_bTracking)
    {
        ...
        //  convert mouse coordinates to screen
        ClientToScreen(&point);
        ...
        //  get the window at the mouse coords
        m_hCurrWnd = ::WindowFromPoint(point);
        ...
        // Show details of the window like class, caption, etc.
        ...
    }
    ...
}

As long as the left mouse button is clicked anywhere inside the main dialog and is not released the mouse pointer will change into the wand and the details of the window beneath the pointer will be displayed on the WinTrans dialog

When the button is released the event handler for WM_LBUTTONUP is called.

void CWinTransDlg::OnLButtonUp(UINT nFlags, CPoint point)
{
    ...
    //  stop tracking the mouse
    ReleaseCapture();
    m_bTracking = false;

    //  If the window under the mouse is not of this 
    //  application we toggle its
    //  layer style flag and apply the alpha as set by the slider control
    if (g_pSetLayeredWindowAttributes && m_hCurrWnd != m_hWnd)
    {
        ::SetWindowLong(m_hCurrWnd, GWL_EXSTYLE,
                        GetWindowLong(m_hCurrWnd, 
                        GWL_EXSTYLE) ^ WS_EX_LAYERED);
        g_pSetLayeredWindowAttributes(m_hCurrWnd, 0,
                        (BYTE)m_slider.GetPos(), LWA_ALPHA);

        ::RedrawWindow(m_hCurrWnd, NULL, NULL,
                       RDW_ERASE | RDW_INVALIDATE | 
                       RDW_FRAME | RDW_ALLCHILDREN);
    }
    ...
}

Points of Interest

This application works correctly only when the wand is dropped on the title bar of another application or on the body of a dialog based application. If for example the wand is dropped on the body of notepad it will not work

To remove the transparent effect you can simply drag-n-drop the wand again on that application. Since in OnLButtonUp we toggle the WS_EX_LAYERED bit the transparency effect will also be toggled

TransWand does not work on the command window

History

• v1.0 This is the initial version.
• v1.1 Added Undo All button and check box to clear the transparency effect on all windows when WinTrans is closed