This function is similar to WinQueryUpdateRegion It returns a region which could be NULL, rectangular, or complex, and represents the visible region of the window.

#define INCL_WINMESSAGEMGR /* Or use INCL_WIN, INCL_PM, */
#include <os2.h>

HWND     hwnd;    /*  The window handle. */
HRGN     hrgn;    /*  Region handle to receive the current visible region. */
ULONG    ulretn;

ulretn = WinQueryVisibleRegion(hwnd, hrgn);