Tango is the "correct" way to do this and will give essentially the most robust set of data back to whoever is looking. A SLAM image + depth based hybrid approa

Tango is the "correct" way to do this and will give essentially the most robust set of data back to whoever is looking. A SLAM image + depth based hybrid approach will always give you more robust data. But just plain old analysis of "parallax" (things closer move faster etc) can quickly give you a workable understanding of the general geometry of a space. The new MR headsets like Magic Leap and HoloLens likely use both types of sensors (like the kinect camera) to determine this stuff. But Snap just wants the widest push possible, and since it is not mounted on your eyes, the 'pixel stick' being off is not as much of an issue.