John Carmack, the founder of id Software and well known open-source advocate, tweeted tonight, "Lawyers are still skittish about the patent issue around "Carmack's reverse", so I am going to write some new code for the doom3 release."
Carmack's Reverse refers to a stencil buffer implementation of shadow volumes, which became a technique to become widely popular among video games following the 2004 release of Doom 3. Around 1999, a patent was filed that covers a "Method for rendering shadows using a shadow volume and a stencil buffer" and was then issued by the USPTO in 2002. John Carmack had independently conceived a similar algorithm to what's covered by the patent, which he created in the year 2000 while working on Doom 3. The patent in question is owned by Creative Labs.
A decade later this shadow volume implementation is causing a last-minute problem for the open-sourcing of Doom 3 / id Tech 4. Hopefully this new code that John Carmack is writing for the open-source code-base will satisfy the company's lawyers without sacrificing the engine's performance or visual quality.
There's no word on when the workaround to the patent issue will be ready, but hopefully it will be soon. As of the end of October, the Doom 3 source-code was ready to ship and just waiting this legal approval, but now there's this last minute legal problem. Hopefully no other problems arise.
It was this summer when John Carmack announced the Doom 3 source-code would be released in 2011 following the release of id Software's Rage game, which is the inaugural title on the id Tech 5 engine. Earlier versions of the id Tech engine were open-sourced once they reached the end of their commercial usefulness, and now with id Tech 5 being out there, id Tech 4 is going to be joining the open-source party.
The release of the Doom 3 / id Tech 4 source-code will immediately make it one of the most advanced open-source games available for Linux and other platforms. It's expected the code -- like earlier id Software code drops -- will be under the GNU GPL.
If you haven't run Doom 3 before under Linux, the Linux client is first-rate and has been supported since 2004 shortly after the Windows launch. Doom 3 can also work just fine under FreeBSD using the Linux binary compatibility layer. A Mac OS X client for Doom 3 also exists.
Next year we will hopefully see the Rage Linux game client (in binary form) as the first AAA game release under Linux in quite a while. Work on Doom 4 is also underway by id Software.
For those wishing to do some more reading about "Carmack's Reverse" until the Doom 3 source-code is released, see the shadow volume Wikipedia page and the patent causing the problems.
(For those that do not currently follow me on Twitter, as mentioned there, benchmarks of Doom 3 from the open-source Mesa / Gallium3D drivers on a range of hardware under Linux will be published in the very near future.)