http://blog.jerrynixon.com/2012/12/walk ... -xaml.html
In particular, in the Comments after the post, the author, Jerry Nixon, gives a link to the Cartesian coordinates math that's used
http://codepaste.net/j7839b

The graphics are nothing fancy, but it's a start if you happen to be making knobs for a Windows Store Metro app or somesuch.
