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SetLayeredWindowAttributes function

Applies to: desktop apps only

Sets the opacity and transparency color key of a layered window.

Syntax

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BOOL WINAPI SetLayeredWindowAttributes(
  __in  HWND hwnd,
  __in  COLORREF crKey,
  __in  BYTE bAlpha,
  __in  DWORD dwFlags
);

Parameters

hwnd [in]

Type: HWND

A 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.

Windows 8: The WS_EX_LAYERED style is supported for top-level windows and child windows. Previous Windows versions support WS_EX_LAYERED only for top-level windows.

crKey [in]

Type: COLORREF

A COLORREF structure 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 generate a COLORREF, use the RGB macro.

bAlpha [in]

Type: BYTE

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.

dwFlags [in]

Type: DWORD

An action to be taken. This parameter can be one or more of the following values.

Value	Meaning
LWA_ALPHA 0x00000002	Use bAlpha to determine the opacity of the layered window.
LWA_COLORKEY 0x00000001	Use crKey as the transparency color.

Return value

Type:

Type: BOOL

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.

Remarks

Note that once SetLayeredWindowAttributes has been called for a layered window, subsequent UpdateLayeredWindow calls will fail until the layering style bit is cleared and set again.

For more information, see Using Layered Windows.

Requirements

Minimum supported client	Windows 2000 Professional
Minimum supported server	Windows 2000 Server
Header	Winuser.h (include Windows.h)
Library	User32.lib
DLL	User32.dll

The thing that is not obvious from this documentation is the behavior of a window that uses both LWA_ALPHA and LWA_COLORKEY together. When used together, everything in the window that matches the crKey color will be completely transparent. The *rest* of the window will be alpha-blended using bAlpha.

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