Michael Abrash, an industry legend who's worked on games like Quake and Doom explains the complex problems that were encountered while making Quake and how Carmack and himself found the solutions to those problems. For example, in a video excerpt, he talks about how the potential visible set can be built using calculations i.e. how much area of a level would be visible while standing at a particular location in that level.

An excerpt:

The year is 1997 and a top programmer named Michael Abrash has recently left id Software. As per id Software and John Carmack’s open policy, Mike is allowed to deliver a speech on the internal coding methods that were used in solving complex problems faced while making the famous first person shooter that changed the way we play FPS games today – Quake. Unbound by any NDAs, Mike reveals the exact ideas and reasoning behind the choices made back then.

For the complete audio presentation and 2 video excerpts, hit up the following link:

Mike Abrash's 1997 Quake Keynote Speech