The Google Daydream team has posted an in-depth technical review of the Equi-Angular Cubemap technique they developed for streaming higher quality VR video.
In the post, "Bringing Pixels Front and Center in VR Video", Daydream engineer Chip Brown describes the challenges of streaming 2D videos in a bandwidth constrained VR world, the history of projection, and a description of the Equi-Angular Cubemap technique. Ultimately the algorithm leads to a marked visual improvement over the equirectangular projection traditionally used for 360 video.
From the post:
The traditional cubemap has samples of varying length depending on the sample’s location on the cube face. EAC is specifically constructed to keep these lengths equal, creating uniformly allocated pixels. For all the reasons that cartography is hard, this 2D picture does not extend perfectly into 3D: if you choose to preserve one important feature of the mapping you invariably give up something else. Check out Conformal Mapping and Equal Area Projections for more information on preserving some characteristics at the cost of others. The EAC formula is mathematically precise in 2D, but only an approximation of an equal angle pixel distribution in 3D, albeit a pretty good approximation with manageable distortion.