Are you familiar with Carmack at all? Like, ever talked with him?

He's friendly and is exactly the sort of engineer that helps teach others how to do things--just look at the GPL releases of all the old Quake stuff when it came out (back in the 90s!).

> I find it hard to imagine that an industry veteran like Carmack would write code for another company while employed with another and proving some bit of proprietary advice was incorporated into the product would be difficult.

Back in the day id programmers shared snippets of code with whoever they wanted - they didn't need to ask any corporate overseer for permission. There were a few little bits found when the GarageGames/Tribes engine code was first released, for example. (pretty sure they got those lines of code from Brian Hook, but it's been a long time...)

It seems like Carmack produced Oculus demos of Id software before he left Id. May be he worked with the Oculus guys to support that effort and talked them into making changes to make those demos work. It would be rather unsurprising to me, close working relationships between game studios and hardware vendors (like graphics card, and console companies) happen all the time.

Not unprecedented though. Carmack did once advise Ken Silverman on using sectors with his 3D engine. Which later became the Build Engine.

Correct me, Build engine is 2.5D. Isn't it?

The exact nature of various FPS gaming engines from that era kind of gives me a massive headache.

Still, the point stands that this isn't unprecedented. John Carmack's has, in the past, just helped out potential competitors before.