The SetWindowRgn function sets the window region of a window. The window region determines the area within the window where the operating system permits drawing. The operating system does not display any portion of a window that lies outside of the window region
int SetWindowRgn(
HWND hWnd, // handle to window whose window region is to be set
HRGN hRgn, // handle to region
BOOL bRedraw // window redraw flag
);
Parameters
hWnd : Handle to the window whose window region is to be set.
hRgn : Handle to a region. The function sets the window region of the window to this region.
If hRgn is NULL, the function sets the window region to NULL.
bRedraw : Boolean value that specifies whether the operating system redraws the window after setting the window region. If bRedraw is TRUE, the operating system does so; otherwise, it does not. Typically, you set bRedraw to TRUE if the window is visible.
Return Values
If the function succeeds, the return value is nonzero.
If the function fails, the return value is zero.
Remarks
If the bRedraw parameter is TRUE, the system sends the WM_WINDOWPOSCHANGING and WM_WINDOWPOSCHANGED messages to the window.
The coordinates of a window's window region are relative to the upper-left corner of the window, not the client area of the window.
After a successful call to SetWindowRgn, the operating system owns the region specified by the region handle hRgn. The operating system does not make a copy of the region. Thus, you should not make any further function calls with this region handle. In particular, do not close this region handle.
To obtain the window region of a window, call the GetWindowRgn function.