Thief is being built using a modified version of Unreal Engine 3, not Unreal Engine 4, Eidos Montreal has confirmed.
Despite being one of the first next-gen titles to be revealed, the game's producer Stephane Roy explained that the decision to stick with Unreal Engine 3 rather than opt for Epic's next-gen Unreal Engine 4 was a matter of timing and "flexibility".
"Keep in mind when we started, Unreal 4 was [still in] development," Roy explained to VideoGamer.com.
"Something really important for us is the flexibility to modify the tools, for example, and Unreal is flexible on this side.
"We worked a lot to improve the tools for the artists and the designers. The game is big, we have to produce a lot of assets. Light and shadow gameplay is amazingly important on our side so we worked very hard to boost the rendering aspect of the engine, how to deal with the light. So I think it's a question of flexibility. With Unreal it's possible to do it. So yes, it's Unreal 3, but [there are] some modifications."
The decision to build on UE3 may come as a surprise to some fans, with Epic billing Unreal Engine 4 as the engine best-suited to next-generation platforms.
Nevertheless, Roy claims that the use of UE3 will still provide Eidos with the opportunity to deliver the game in the way it had hoped.
"The main goal is to give you the game experience we want to provide," he continues. "It's really important for us. It's the immersion. There is no compromise about the quality of the game we want to give you to make sure that you are going to have fun and that you are going to believe in this universe.
"That said, is it possible to have the same quality of product on today's generation? If the answer is yes, I am open to consider the question [why it's next-gen only]. But right now, we feel that the next-gen gives us the maximum of the game experience we want to give you."
Thief launches on PC, PlayStation 4 and "other next-generation platforms" next year. Square-Enix claims that the next-gen versions will look 'almost identical' to the PC version.
Source: VideoGamer.com Interview