如何获得在滚动查看器上放置的画布的可见边界

我把画布放在scrollviewer上，以便允许滚动。是否有可能得到画布可见部分的坐标?

我正试着那样计算

leftBorder = ScrollViewer1.HorizontalOffset;
rightBorder = ScrollViewer1.ViewportWidth - ScrollViewer1.HorizontalOffset;
topBorder = ScrollViewer1.VerticalOffset;
bottomBorder = ScrollViewer1.ViewportHeight - ScrollViewer1.VerticalOffset;