hwnd (HWND) - input

      The window handle.

    hrgn (HRGN) - input

      Region handle to receive the current visible region.

    ulretn (ULONG) - returns

      RGN_ERROR

        An error occurred.
      RGN_NULL
        The window currently has no visible area on screen.
      RGN_RECT
        The region contains a series of rectangular areas.
      RGN_COMPLEX
        The region contains a complex shape.

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