The glasses app uses the smartphone's front-facing TrueDepth camera to map the user's face and create an ideal fit for a new set of frames.
Warby Parker using the iPhone X TrueDepth camera is super super smart: pic.twitter.com/LgHpO8hfUx— Joanna Stern (@JoannaStern) November 7, 2017
Apple's Face ID authentication works by projecting 30,000 dots on the surface of a person's face, accurately mapping its curvature and unique features.
The camera's sensors also capture the data in three dimensions, and it's this technology in particular that the glasses app uses to recommend to the user a series of frames that it thinks will fit their facial structure.
The only failing of the app is that it doesn't (yet) place the spectacles on the user's face, Snapchat-style, to let the customer see what they look like wearing them.
Apple's ARKit augmented reality framework would seem to be the obvious solution to this lack. We've reached out to Warby Parker to ask if they have any plans to make use of it, and will update this article if we hear back.