https://techcrunch.com/2018/06/28/niantics-latest-acquisition-lets-ar-pokemon-hide-behind-the-real-world/

It seems like a simple concept: when rendering something in augmented reality (like, say, a Pikachu in Pokémon Go) and something in actual reality (like, say, a human, or a car, or a planter) passes in front of it, make the rendered object appear to be “behind” the real one.

In practice, it’s pretty damn hard. A device would need the ability to tell which pixels are close, or far, or somewhere in between. While adding more cameras (or lasers!) to the mix can help, achieving that sense of depth from the single rear RGB camera found on most smartphones — and doing it fast — is a helluva task. That’s why objects in most augmented reality apps tend to just sort of float in front of anything that gets too close.