I didn’t see another posting of this in the forums after a quick search.
So I thought I’d share.
id’s awesome as always! 
Time to parrrrtay! open’s a cold one Wohoo!
Now to see if I can find their code for shadow volumes. :3
edit: nooooooooooooooooooooooooooooooo~'s! 
Carmack is a definite champ the game dev community.
The very reason it was held back by the lawyers I believe.
Still though, the public get access to a pretty good, albeit old engine; I’m sure some programmer(s) out there will add their own implementation of shadow volumes.
Carmack made some minor changes to the code before releasing so the shadow stuff is both included and kosher for the lawyers. The dumb thing about that whole ordeal was Creative Labs owns the software patent, but the actual method is usually called Carmack’s Reverse.
maybe BGE can learn some stuf ^^
I’m obviously not a programmer.
But, I hope that there is maybe some way to bring a lot of idtech 4 code into the bge in some way. Wouldn’t it be nice to just import code between engines like models?
with code it’s never copy and paste. I’m a coder and when clients says “isn’t it just copy and paste” it never works 
but you could learn some from doom3 engine.
I would almost start from scratch and build a blenderdoomengine and learn all the neat stuf they’ve done. maybe not all but some could be used with blender as good as BGE is.
don’t forget that this engine is kind of old. it was designed for directx 8 class hardware. nobody uses stencil shadows anymore in the meantime and i am quite sure it still uses some kind of visibility calculation system that is easier to use with the doom3 level editor instead of a 3d editor like blender.
For the shadow volumes you can read the substituted implementation here: https://github.com/TTimo/doom3.gpl/blob/master/neo/renderer/tr_stencilshadow.cpp
Some pretty recent games were made with it. None really garnered much interest after release. But I think it needs to be said that even an old engine like 4 could do some great things if used properly. A great engine don’t a good game make.
I’m very interested in seeing what people do with it.