Yufei

I would also love to see a virtual measure tape style line node. But it's been a long time since last commit. I may as well just implement myself and...

如果背景圆大于或等于前景圆就会出现这个问题。我在pull #26 中修复了。 

这样就可以做成小圆在大圆里动，像这样。但是这个暂时还不完美，背景里的圆的直径是作者写死的，所以要改直径一定要在init里改才行。