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People ask about corner rounding scripts from time to time here at Typophile. I have heard about several people writing these scripts, but I couldn't find any that were publicly available, so I wrote one for some projects I was working on last year. I dusted it off recently, cleaned it up a little and thought I'd put it out there. It's pretty simple – only 90 lines of python - but it can be easily hacked to support a variety of projects. For example, I modified it to curve the same point across several masters.
It seems like corner rounding is a common enough operation in type design that it is a good candidate for a new tool in FontLab.