ARKit with multiple users

As some newer answers have added, multiuser AR is a headline feature of ARKit 2 (aka ARKit on iOS 12). The WWDC18 talk on ARKit 2 has a nice overview, and Apple has two developer sample code projects to help you get started: a basic example that just gets 2+ devices into a shared experience, and SwiftShot, a real multiplayer game built for AR.

The major points:

1. ARWorldMap wraps up everything ARKit knows about the local environment into a serializable object, so you can save it for later or send it to another device. In the latter case, "relocalizing" to a world map saved by another device in the same local environment gives both devices the same frame of reference (world coordinate system).

2. Use the networking technology of your choice to send the ARWorldMap between devices: AirDrop, cloud shares, carrier pigeon, etc all work, but Apple's Multipeer Connectivity framework is one good, easy, and secure option, so it's what Apple uses in their example projects.

3. All of this gives you only the basis for creating a shared experience — multiple copies on your app on multiple devices all using a world coordinate system that lines up with the same real-world environment. That's all you need to get multiple users experiencing the same static AR content, but if you want them to interact in AR, you'll need to use your favorite networking technology some more.

   Apple's basic multiuser AR demo shows encoding an ARAnchorand sending it to peers, so that one user can tap to place a 3Dmodel in the world and all others can see it. The SwiftShot game example builds a whole networking protocol so that all users get the same gameplay actions (like firing slingshots at each other) and synchronized physics results (like blocks falling down after being struck). Both use Multipeer Connectivity.

_{(BTW, the second and third points above are where you get the "2 to 6" figure from @andy's answer — there's no limit on the ARKit side, because ARKit has no idea how many people may have received the world map you saved. However, Multipeer Connectivity has an 8 peer limit. And whatever game / app / experience you build on top of this may have latency / performance scaling issues as you add more peers, but that depends on your technology and design.)}

Original answer below for historical interest...

This seems to be an area of active research in the iOS developer community — I met several teams trying to figure it out at WWDC last week, and nobody had even begun to crack it yet. So I'm not sure there's a "best way" yet, if even a feasible way at all.

Feature points are positioned relative to the session, and aren't individually identified, so I'd imagine correlating them between multiple users would be tricky.

The session alignment mode gravityAndHeading might prove helpful: that fixes all the directions to a (presumed/estimated to be) absolute reference frame, but positions are still relative to where the device was when the session started. If you could find a way to relate that position to something absolute — a lat/long, or an iBeacon maybe — and do so reliably, with enough precision... Well, then you'd not only have a reference frame that could be shared by multiple users, you'd also have the main ingredients for location based AR. (You know, like a floating virtual arrow that says turn right there to get to Gate A113 at the airport, or whatever.)

Another avenue I've heard discussed is image analysis. If you could place some real markers — easily machine recognizable things like QR codes — in view of multiple users, you could maybe use some form of object recognition or tracking (a ML model, perhaps?) to precisely identify the markers' positions and orientations relative to each user, and work back from there to calculate a shared frame of reference. Dunno how feasible that might be. (But if you go that route, or similar, note that ARKit exposes a pixel buffer for each captured camera frame.)

Good luck!

Now, after releasing ARKit 2.0 at WWDC 2018, it's possible to make games for 2 to 6 users.

For this you need to use ARWorldMap class. By saving world maps and using them to start new sessions, your iOS application can now add new Augmented Reality capabilities: multiuser and persistent AR experiences.

AR Multiuser experiences. Now you may create a shared frame of a reference by sending archived ARWorldMap objects to a nearby iPhone or iPad. With several devices simultaneously tracking the same world map, you may build an experience where all users (up to 6) can share and see the same virtual 3D content (use brand new Pixar's USDZ file format for 3D in Xcode 10 and iOS 12).

session.getCurrentWorldMap { worldMap, error in     guard let worldMap = worldMap else {        showAlert(error)        return    }}let configuration = ARWorldTrackingConfiguration()configuration.initialWorldMap = worldMapsession.run(configuration)

AR Persistent experiences. If you save a world map and then your iOS application becomes inactive, you can easily restore it in the next launch of app and in the same physical environment. You can use ARAnchors from the resumed world map to place the same virtual 3D content (in USDZ or DAE format) at the same positions from the previous saved session.

Not bulletproof answers more like workarounds but maybe you'll find these helpful. All assume the players are in the same place.

1. DIY ARKit sets up it's world coordinate system quickly after the AR session has been started. So if you can have all players, one after another, put and align their devices to the same physical location and let them start the session there, there you go. Imagine the inside edges of an L square ruler fixed to whatever available. Or any flat surface with a hole: hold phone agains surface looking through the hole with camera, (re)init session.

2. Medium Save the player aligning phone manually, instead detect a real world marker with image analysis just like @Rickster described.

3. Involved Train an Core ML model to recognize iPhones and iPads and their camera location. Like it's done with human face and eyes. Aggregate data on a server, then turn off ML to save power. Note: make sure your model is cover-proof. :)