id Software last night released the engine source code for Doom 3 under a GPL3 license, letting all and sundry get their hands dirty and bend it to their will. You can download the code here at Shacknews. Let the coding commence!

So what can we expect to see this code used for? When previous id engines were open-sourced, the first new releases were typically simple patches fixing long-standing bugs and niggles, but once the base source is up to scratch, the exciting work begins.

Graphics overhauls are always popular, cramming years of new rendering techniques into aging engines. Mods can become standalone games--though this is trickier for those relying heavily on Doom 3's assets, as those still cannot be legally redistributed. In the long run, we'll likely also see the source used to make brand new games.

You may very well be wondering how id overcame the legal hurdle caused by 'Carmack's Reverse.' As ever, chief technomancer John Carmack was only too happy to explain.

"Where it can't be shown the player is outside a shadow volume, I used the "preload" technique from http://kb.cnblogs.com/a/28036/," he said on Twitter, adding, "The preload shadow technique may have some other utility--you can use it with a rear clip plane projection, unlike depth-fail." And now you know.