Carmack’s Reverse: a FOSS patent case study

Just a quick one on the subject of open source and patents. John Carmack is well known in gaming circles as the lead programmer behind such classic PC and console games as Castle Wolfenstein, Doom (and sequels) and Quake (and most of its sequels). Carmack and his company id software are the originators of the ‘First Person Shooter’ genre of game which has in turn spawned such gigantic franchises as Call of Duty and Halo. As well as being technical pioneers, id has an interesting policy of releasing their old engine technology (the software which renders the game’s video and audio) as open source under the GNU GPL v2. This allows students of gaming software development to look at how real commercial games software is written, and also allows the games to be ported to new hardware platforms by volunteers. As the art and sound assets are not included with the code, this also generates a small market for licences to old id games – games which may well not run on more modern operating systems – in order to get the game data for use with the aforementioned ports.

Rage, the id game which uses version five of the id rendering engine (id tech 5, as it is known) has just been released. This is the point at which the source to the previous engine would usually be released as open source. However in this case there is a problem. Back when id tech 4 was being written for the game Doom 3 over the period 2000-2004, many games developers were looking into what were then cutting edge graphical technologies for inclusion in their games. One such technology was stencil shadowing, which accurately projects shadows from moving objects onto surrounding surfaces based upon the light sources which are illuminating them. This is hard work even for the specialist graphics hardware in PCs and consoles, and so any algorithmic optimisations that are possible are highly valued by the industry. So, various developers hit upon the same optimisation around the year 2000. This optimisation has come to be known as Carmack’s Reverse, even though it was first presented by William Bilodeau and Michael Songy of Creative Labs back in 1998. John Carmack discovered it independently some time later, and was perturbed to discover that Creative had already patented the process.

Faced with a choice between licensing the patent from Creative or making his code sub-optimal, Carmack decided to strike a deal with Creative that allowed him to use the technology at no cost. Perhaps coincidentally, id also agreed to use sound rendering technology by Creative in their game.

So the game was shipped and everyone was happy, until half a decade later when it came time to ship the code as open source. Obviously whatever deal was agreed between Creative and id did not involve making code available that embodied Creative’s patent under an open source licence for the whole world to use. In a tweet on the subject Carmack explained:

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.

It will be interesting to see how Carmack replaces the code, but the issue is also of interest because it illustrates the importance of keeping good records of the inbound IP in a software project. id’s lawyers caught what could have been an expensive potential infringement of Creative’s rights when they ‘skittishly’ requested that Carmack rewrite the code.

3 thoughts on “Carmack’s Reverse: a FOSS patent case study

  1. Pingback: Patents Roundup: Bad System, Clear Symptoms | Techrights

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