[3] At the QuakeCon 2007, John Carmack, the lead graphics engine developer at id, said to LinuxGames: "I mean, I won't commit to a date, but the Doom 3 stuff will be open source".

[6] On November 16, 2011, Carmack announced on Twitter that he's writing new code for Doom 3's open source release, because "lawyers are still skittish about the patent issue around 'Carmack's reverse'".

[13] The original requirement of id Tech 4 was that it needed a high-end graphics processing unit (GPU) with fully programmable vertex and pixel shaders, such as the Nvidia GeForce 3 or ATI Radeon 8500, with at least 64 MB of VRAM.

While the Radeon 9700's DirectX 9.0 features are not necessary to render the game, its advanced architecture, 256-bit memory bus, and efficiency were needed to run Doom 3 at high detail and playable speed.

[15] id Tech 4 resulted in the obsolescence of DirectX 7 graphics chips such as the widespread GeForce 2 and Radeon 7200, as well as older chipsets such as RIVA TNT2 and Rage 128, and software rendering (with an integrated Intel GMA).

There have been cases of enthusiasts forcing Doom 3 to run on unsupported graphics chips, such as the long obsolete Voodoo 2, but these are unable to render the per-pixel lighting and bump mapping.

[20] The original version of the id Tech 4 engine was designed for somewhat dark environments and was criticized for its perceived inability to handle extremely large daytime outdoor areas.

By painting a single massive texture (32,768×32,768 pixels, though it has been extended to larger dimensions in recent versions of the MegaTexture technology) covering the entire polygon map and highly detailed terrain, the desired effects can be achieved.

The MegaTexture can also store physical information about the terrain such as the amount of traction in certain areas or indicate what sound effect should be played when walking over specific parts of the map.

[26][27] id Tech 4 has a comprehensive scripting language that can be used when creating mods, and is used in Doom 3 to control monsters, weapons, and map events.

However, the unexpectedly long development time going into Doom 3 from 2002-04 meant that they could not put up competition to Epic Games's Unreal Engine 2 during that period.

While id Tech 4 had taken a new direction with its dynamic per-pixel lighting, this unconventional feature had steeper hardware requirements and was initially only useful in "spooky games" (until the MegaTexture addition), whereas an increasing number of developers preferred conventional engines that could render large outdoor areas.