New details from polygon:
They narrowed it down to two game engines.
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"We finally have enough manpower enough to put out this teaser and do these experiments, these tests I want to do," he said. However, Kojima wouldn't say what game engine his team has settled on. After a worldwide tour of video game studios, and with the help of technical adviser Mark Cerny, Kojima said they'd settled on two possible game engines, one of which was used to develop Death Stranding's teaser.
"There's this planning phase, when I try to do some [technical] tests to see what can be done, and what can't be done," Kojima said. While "there are many fantastic engines all over the world," he said, Kojima Productions is looking for two specific things for its future games.
"One, something that is capable of portraying all the visuals of this world I want to portray," Kojima said. "I also want to have as many new game elements and unseen game elements, and I'm looking for something that makes that possible to achieve. There were many candidates and from all of them, we narrowed the list to two strong candidates. One of these engines was used for a teaser, the other engine we have been continuing to do mechanics tests on. The names I cannot disclose. But we have to settle down soon and decide."
They narrowed it down to two game engines.