There's always in-house ways to deal with graphics drivers and certain effects. Remember how Source 1 was the only engine which was able to render HDR with measly ATI 9600XT, without a performance penalty most of the time?
You carry this know how in evolved form for generations, and it gets buried under as the foundation of the new things you're building on top of it.
This is what I remember from a friend who implemented his own game engine and created a company for it, and half the woes were making the graphics drivers and processors behave as they said in their manuals.
ID's engines are always called "The world's most expensive xcopy". You always get the source, but you never share it.
Same model. Get the license, get the source.
See the relevant page at: https://dev.epicgames.com/documentation/en-us/unreal-engine/...
From that page:
> This page provides detailed instructions for subscribers to download the source code for Unreal Engine (UE) from the Unreal Engine GitHub repository, and to get started working with the code.
(Emphasis mine)
* The NDA'ed console bits are hidden of course but all the PC code is there. Not just release snapshots either, you can see all of Epics commits in real time.
It’s just free as in beer.
“Click through EULA” is the mammoth in the room.
If this apocryphal story is true, they might only have a license to it (making it a derivative work) and depending on the terms of their licensing agreement from id they might not be able to do that without having their legal people talk to ZeniMax’s (they bought out id software and are their parent company now) lawyers, which runs to thousands of dollars in pure costs even if everyone is totally cooperative and on board and wants to make it happen immediately.
Do you think they will release it at any point? Maybe there are licensing issues where they don't have the rights to all of it and couldn't easily opensource it. Or maybe there is in fact still too much secret sauce left there?