Anchor sharing between multiple (perhaps similar) devices

[NoTuxNoBux]

I realize the explainer currently still mentions persistent and shared anchors are outside of the scope, but since persistent anchors have been added since, I wanted to bring this up explicitly since I couldn't find a ticket that did already and we - as in the company I work for - notice anchor sharing becoming more relevant in the XR applications we build, especially for co-located shared experiences where e.g. two HMD users are in the same room looking at the same visual content in the same place visually (for example on the table in the room).

I realize this isn't an easy matter since spatial anchors tend to be opaque to the underlying platform and sharing them between different types of devices is usually impossible, but this would be useful even if it were restricted to single types of devices, such as between HoloLenses or Quests.

For the Meta Quest 3, we currently do this with Unity and Meta's SDK's, which is unfortunately specific to the Quest 3, and also uses Meta's cloud to share the anchors. With WebXR there seems to currently be no path forward since there is no direct support for sharing nor does there appear to be any way to serialize an anchor to some internal opaque format that we could then transfer over the network and hand to another device to restore.

An alternative might include having a physical item that can be found by multiple devices individually, such as a QR code, but unfortunately support for something like QR codes, image recognition or object detection is also few and far in between, and not part of the WebXR specification either.