截图软件截图区域以外背景变灰的实现--AlphaBlend

zqp2013 · 发表于 2015-01-20 09:53:01

OnPaint()方法中：
         // ------------画黑色背景----------------
                     COLORREF bgColor = RGB(0, 0, 0);
                     char alpha = (255 * 30) / 100;
                     int screenX = GetSystemMetrics(SM_CXSCREEN);
                     int screenY = GetSystemMetrics(SM_CYSCREEN);

                     CDC memdc;
                     memdc.CreateCompatibleDC(&dc);
                     CBitmap bmp, *pOldBitmap;
                     bmp.CreateCompatibleBitmap(&dc, screenX, screenY);
                     pOldBitmap = memdc.SelectObject(&bmp);
                     // 拷贝背景位图到内存DC
                     memdc.BitBlt(0, 0, screenX, screenY, &dc, 0, 0, SRCCOPY);

                     CRgn rgnScreen, rgnTracker;
                     rgnScreen.CreateRectRgn(0, 0, screenX, screenY);
                     CRect rect = m_rectTracker.m_rect;
                     rect.DeflateRect(-1, -1, -1, -1);
                     rgnTracker.CreateRectRgn(rect.left, rect.top, rect.right, rect.bottom);

                     // 屏幕区域减去截图区域
                     rgnScreen.CombineRgn(&rgnScreen, &rgnTracker, RGN_DIFF);

                     CBrush brush;
                     brush.CreateSolidBrush(bgColor);
                     // 指定区域填充背景色
                     memdc.FillRgn(&rgnScreen, &brush);
                     brush.DeleteObject();

                     BLENDFUNCTION bf;
                     bf.BlendOp = AC_SRC_OVER;
                     bf.BlendFlags = 0;
                     bf.SourceConstantAlpha = alpha;
                     bf.AlphaFormat = 0;
                     // 内存DC透明映射到绘图DC
                     dc.AlphaBlend(0,0,screenX,screenY,&memdc,0,0,screenX,screenY,bf);
                     memdc.SelectObject(pOldBitmap);

效果图：