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

Syntax

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

Parameters

hwnd

[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.

crKey

[in] 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] 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] Specifies an action to take. This parameter can be one or more of the following values.

LWA_COLORKEY

Use crKey as the transparency color.

LWA_ALPHA

Use bAlpha to determine the opacity of the layered window.

Return Value

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.

Function Information

Minimum DLL Version	user32.dll
Header	Declared in Winuser.h, include Windows.h
Import library	User32.lib
Minimum operating systems	Windows 2000

See Also

Windows Overview, Using Layered Windows, AlphaBlend, CreateWindowEx, SetWindowLong, TransparentBlt, UpdateLayeredWindow, COLORREF, RGB