SetLayeredWindowAttributes function
Advanced GDI routines
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function SetLayeredWindowAttributes(  
   hWnd: hWnd;       // handle to the layered window  
   crKey: COLORREF;  // specifies the color key  
   bAlpha: Byte;     // value for blend function  
   dwFlags: LongInt  // action  
): LongInt;  

The SetLayeredWindowAttributes function sets the opacity and transparency color key of a layered window.  

[in] Handle to the layered window. A layered window is created by specifying WS_EX_LAYERED when creating the window with the CreateWindowEx function or by setting WS_EX_LAYERED via SetWindowLong after the window has been created.  
[in] Pointer to a COLORREF value that specifies the transparency color key to be used when composing the layered window. All pixels painted by the window in this color will be transparent. To convert TColor value to the COLORREF, use ColorToRGB function.  
[in] Alpha value used to describe the opacity of the layered window. Similar to the SourceConstantAlpha member of the BLENDFUNCTION structure. When bAlpha is 0, the window is completely transparent. When bAlpha is 255, the window is opaque.  
[in] Specifies an action to take. This parameter can be one or more of the following values.  
LWA_COLORKEYUse crKey as the transparency color.      
LWA_ALPHAUse bAlpha to determine the opacity of the layered window.     

Return Values
If the function succeeds, the return value is nonzero.  
If the function fails, the return value is zero. To get extended error information, call GetLastError.  

Windows NT/2000: Requires Windows 2000 or later.  
Windows 95/98: Unsupported.  
tip If you call this function on Win95, or on Win98 with AW_BLEND parameter — nothing happends.  

See also
acWin2kEffects component.  
acMakeTranslucentWindow and acMakeOpaqueWindow procedures.