Yep, I’ve been surfin’ matterport code way deep for a time, so I can confirm @makc3d is right on this: it is envmaps projected onto the mesh. This solution seems to be mixed with another on areas of envmap that exhibit discontinuity from user view. As for the transition, as suggested by @pailhead the interpolation is not linear, and it is not getting calculated naivly on cpu but with a dedicated shader, so it is hardware accelerated.
As @makc3d is telling you, glsl is the way to go if you want to get close to this.