An issue surrounding a patent for a specific shading technique has held up a promised open source release for the code behind Doom 3, as programmer John Carmack goes back to make modifications.

Carmack promised at an August QuakeCon keynote speech that the open source release for Doom 3 would come after the company's October release of Rage.

But Carmack explained in a tweet yesterday that the "lawyers are still skittish about the patent issue," referring to a Creative Labs patent on a 3D shading technique known as "depth fail."

Creative Labs filed for a patent on the technique in 1999, though the process became widely known as "Carmack's Reverse" after the Doom 3 programmer independently discovered and publicized it during the game's development.

Id used the technique under a license from Creative for Doom 3's 2004 release, but Carmack will have to "write some new code" to get around the legal issues for the open source release, he tweeted.

In a 2004 forum discussion, Carmack suggested he had devised a separate shading method that got around the patent issues but that resulted in a "speed hit" on performance.