The “Atkinson” circle algorithm
After the groundwork done for rectangle drawing, the rest of geometric shapes were pretty easy to add. To make sure the results of drawing operations would look exactly like on a real Mac, the Midpoint circle algorithm was implemented as closely to what Atkinson used as possible
The fun thing is, that a bunch of QuickDraw primitives actually share this method, so not only did this allow drawing Circles, it also enabled drawing ovals (basically stretched circles, thus this is why overly long framed ovals on Mac tend to have holes on their silhouette), and the infamous round rectangles. Also drawing arcs benefits from this, but we won’t be needing them at least for some time so we won’t touch them at this point.
Here’s also a neat video of round rectangle clipping: