In an interview with Edge, Infinity Ward's Mark Rubin has simply revealed that Respawn's TItanfall will run at 720p.
Asked if Call of Duty could one day run natively at 1080p on Xbox One as Ghosts does on PS4 he remarked:
"I don’t know but overall I do know that other companies have been saying that they’re also 720 on Xbox One – BF4 was 720, Titanfall has already said they’re going to be 720 on Xbox One so it seems to be the dominant direction at the moment, but so much can change."
Titanfall isn't due to be released until Spring, 2014 so the game's resolution may have changed by then but for now it seems the title, which will be exclusive to Xbox 360, Xbox One, and PC for its lifetime, will run at 720p, most likely upscaled to 1080p.
Respawn has said that they will make games for PS4 eventually so those titles may run at 1080p compared to the Xbox One version or perhaps the studio will have learned how to get their titles running natively at 1080p on Xbox One by then.
Titanfall, which is seen as a major scoop for Microsoft, will likely provoke more debate over the so-called 'Resolutiongate' issue surrounding the Xbox One if Rubin's comments prove true. He has also explained the resolution difference for Call of Duty: Ghosts.
