Convert Canvas to PNG on Rails, server side
===

I know there are ways to convert a canvas to a PNG image on (some) browser clients into a encoded Base64 string, but I'm searching for a way that is a) server side and b) doesn't use image magic.

If there isn't a direct conversion library somewhere, is there some sort of "standard" for ruby / rails image creation and manipulation?  I found Cairo, but I'm confused on what exactly this gets me.