虚幻引擎
Compared to the preceding and widely used id Tech 3 ("Quake III Arena" engine) and id Tech 2 ("Quake II" engine), id Tech 4 has had less success in licensing to third parties. This is especially apparent in comparison to its closest contemporaries, Unreal Engine 2 (2002) and Unreal Engine 3 (2006) from Epic Games. id Software was unwilling to license their newest engine before its "parent game" "Doom 3" was completed. However the unexpected 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. Many who licensed Unreal Engine 2 were thus able to make the switch to Unreal Engine 3 more easily.