如何使用GDI绘制半透明矩形

/*使用GDI绘制半透明矩形*/
void CDirectXDraw::DrawHalfOpacityRect(HDC hdc,CRect rect)
{
	CDC dc;
	dc.Attach(hdc);

	CDC cdc; 
	cdc.CreateCompatibleDC( &dc );

	CBitmap bitmap, *pOldBitmap;
	bitmap.CreateCompatibleBitmap( &dc, rect.right, rect.bottom );
	CRect src( rect );
	src.OffsetRect( CSize( -rect.left, -rect.top ) );
	pOldBitmap = cdc.SelectObject( &bitmap );
	cdc.FillSolidRect( src, RGB( 0, 0, 0 ) ); //透明色

	if( ::AlphaBlend == 0 )
	{
		dc.BitBlt( rect.left, rect.top, rect.right - rect.left, rect.bottom - rect.top, &cdc, src.left, src.top, SRCINVERT );

	}else{

		BLENDFUNCTION bf;    
		memset( &bf, 0, sizeof( bf ) );
		bf.SourceConstantAlpha = 0x3f; //透明程度//值越大越不透明
		bf.BlendOp = AC_SRC_OVER;

		::AlphaBlend( dc.GetSafeHdc(), rect.left, rect.top, rect.right - rect.left, rect.bottom - rect.top, 
			cdc.GetSafeHdc( ), src.left, src.top, src.right - src.left, src.bottom - src.top, bf );  
	}

	cdc.SelectObject( pOldBitmap );

	dc.Detach();
}