模仿QQ截图时的灰色透明背景

void CClipScreenToolDlg::ChangeBackGround(HDC &hDC,CRect rect)
{
	m_hGrayBitmap=CreateCompatibleBitmap(hDC,m_nGrayWidth,m_nGrayHeight);
	HDC hMemDC=CreateCompatibleDC(hDC);
	SelectObject(hMemDC,m_hGrayBitmap);

	BLENDFUNCTION blend;
	memset(&blend,0,sizeof(blend));
	blend.BlendOp=AC_SRC_OVER;
	blend.SourceConstantAlpha=ALPHA_GRAY;//透明度

	if (rect.left==rect.right||rect.top==rect.bottom)
	{
		AlphaBlend(hDC,0,0,m_xScreen,m_yScreen,hMemDC,0,0,m_nGrayWidth,m_nGrayHeight,blend);
	}else{
		//【除去矩形后，alpha混合其余四个区域都灰化】
		int x1 = rect.left < rect.right  ? rect.left   : rect.right;
		int y1 = rect.top  < rect.bottom ? rect.top    : rect.bottom;
		int x2 = rect.left < rect.right  ? rect.right  : rect.left;
		int y2 = rect.top  < rect.bottom ? rect.bottom : rect.top;

		if (y1 > 0)
		{
			AlphaBlend(hDC,0,0,m_xScreen, y1,hMemDC,0,0,m_nGrayWidth,m_nGrayHeight, blend);
		}

		if (y2 < m_yScreen)
		{
			AlphaBlend(hDC,0,y2,m_xScreen, m_yScreen-y2,hMemDC,0,0,m_nGrayWidth,m_nGrayHeight, blend);
		}

		if (x1 > 0)
		{
			AlphaBlend(hDC,0,y1,x1, y2 - y1,hMemDC,0,0,m_nGrayWidth,m_nGrayHeight, blend);
		}

		if (x2 < m_xScreen)
		{
			AlphaBlend(hDC,x2,y1,m_xScreen - x2, y2 - y1,hMemDC,0,0,m_nGrayWidth,m_nGrayHeight, blend);
		}
	}
	DeleteDC(hMemDC);
}void CClipScreenToolDlg::ChangeBackGro