The 'regions' used in QuickDraw are an amazing (for their time) data structure, a testament to the genius of Bill Atkinson. I strongly recommend reading the patent to see how cool and useful they were to the original Mac windowing system (http://patentdetails.patenttools.com/patent/04622545?searchi... click on the 'download pdf' link in the top left).
I've been intending to write a blog post retrospective on the workings and algorithms of the quick draw library, but regions seemed too big a topic to start with. You've given me an idea though: I'll have have a go at RoundRects first and see if there is any interest.
In a semi-related matter, does anyone know of the algorithm Bill used to draw RoundRects 'blisteringly fast'? http://www.folklore.org/StoryView.py?project=Macintosh&s...