新人combinergn这个函数小弟我不会用啊求不出来两多边形的交集

CRgn   rgnA, rgnB, rgnC;

VERIFY(rgnA.CreateRectRgn( 50, 50, 150, 150 ));
VERIFY(rgnB.CreateRectRgn( 100, 100, 200, 200 ));
VERIFY(rgnC.CreateRectRgn( 0, 0, 50, 50 ));

int nCombineResult = rgnC.CombineRgn( &rgnA, &rgnB, RGN_OR );
ASSERT( nCombineResult != ERROR && nCombineResult != NULLREGION );

CBrush br1, br2, br3;
VERIFY(br1.CreateSolidBrush( RGB(255, 0, 0) ));  // rgnA Red
VERIFY(pDC->FrameRgn( &rgnA, &br1, 2, 2 ));
VERIFY(br2.CreateSolidBrush( RGB(0, 255, 0) ));  // rgnB Green
VERIFY(pDC->FrameRgn( &rgnB, &br2, 2, 2 ));
VERIFY(br3.CreateSolidBrush( RGB(0, 0, 255) ));  // rgnC Blue
VERIFY(pDC->FrameRgn( &rgnC, &br3, 2, 2 ));