I didn't read it in detail but I did find this version of the same anecdote followed by a commentary on the original ROM code: http://wg20.criticalcodestudies.c

I didn't read it in detail but I did find this version of the same anecdote followed by a commentary on the original ROM code: http://wg20.criticalcodestudies.com/index.php?p=/discussion/80/code-critique-roundrects-apple-quickdraw-source-code http://wg20.criticalcodestudies.com/index.php?p=/discussion/... the underlying algorithm to draw an octant of an arc may have been https://en.wikipedia.org/wiki/Midpoint_circle_algorithm https://en.wikipedia.org/wiki/Midpoint_circle_algorithm You "just" need to draw 2 octants of an oval at each corner of the rectangle, plus the remainder of the 4 sides of the rectangle, and correctly account for fill/edge flags while doing so. This likely leveraged the existing "region" code in the Apple QuickDraw, but to be sure you'd have to get down into the code or find a more complete commentary.