The CombineRgn function combines two regions and stores the result in a third region. The two regions are combined according to the specified mode.
int CombineRgn(
HRGN hrgnDest, // handle to destination region
HRGN hrgnSrc1, // handle to source region
HRGN hrgnSrc2, // handle to source region
int fnCombineMode // region combining mode
);