Air Link Framerate Insurance (AFI)

The freedom to feel truly immersed in VR—without the confines of wires—is one of the key features of Quest 2. Air Link recently brought that freedom to PC VR by using your wireless network to stream PC VR content to a Quest. However, sometimes your wireless network can experience some degradation, which will in turn impact your Air Link experience with dropped frames and lag.

We’re working to mitigate network interference. We’ve recently released an experimental feature called Air Link Framerate Insurance (AFI) that should smooth over some of the visual artifacts of network interference. This article explains the technique and how to enable it.

Asynchronous Spacewarp (ASW) was originally released for the Oculus Rift in 2016 and was intended to enable lower spec PCs to run VR content. The key was augmenting the rotational correction of Asynchronous Timewarp (ATW) with an additional in-image warp correction using motion vectors derived from consecutive frames. More information can be found here.

The original ASW was later augmented with app-reported depth information and better motion vector filtering and released as ASW 2.0, which remains state-of-the-art.

However, once your VR rendering has left the PC and encountered Wi-Fi turbulence, you no longer have the resources of PC ASW. This is where AFI kicks in.

When your wireless signal becomes erratic, this shows up on the headset as late or dropped frames. In order to keep showing you VR, the headset keeps applying a rotational correction in order to forward-project previous frames to match your head’s current rotation; however, closer objects are subject to parallax motion due to head translation and moving objects are no longer being corrected. You see these artifacts as lurches or judders in near or moving objects. These artifacts can negatively affect your VR experience.

These are the exact artifacts ASW was designed to mitigate; however, traditional “Half-Rate” ASW cannot be used in the most obvious use case for a Quest-side ASW in a VR streaming application: mitigating frame drops caused by network transmission spikes.

On Air Link, dynamic bitrate helps to ensure that the only time a network spike causes frame drops is when it is either unprecedented by past network behavior or is not mitigated by reducing video frame size. Enabling half-rate ASW requires two-way communication between PC and HMD, making it useless during a network spike; it also requires reliable, on-time delivery of every other frame for quality, which an unstable network does not guarantee. Finally, unlike dynamic bitrate, half-rate ASW adds input delay when used for an extended period.

For Air Link, we needed a technique that could apply ASW-style warp correction instantly and on-demand to short, anomalous bursts of late frames. We call this technique Air Link Framerate Insurance, or AFI.

When enabled, AFI will apply similar techniques as ASW 1.0, but on-demand and on Quest. The past couple of frames that have been submitted to the headset are submitted to the GPU and used to compute motion vectors that are then applied to extrapolate head motion and animation and thus smooth out the reprojected frame.

If your wireless connection is good enough, you should never see AFI, but on the chance it occurs, AFI will hopefully smooth things out for you.

Note this technology is different from Application SpaceWarp (AppSW). AppSW is a developer-facing technology for optimizing native Quest apps’ performance and requires game engine integration. AFI is focused on mitigating Air Link transmission interference and works for all Link apps automatically when enabled.

By default, AFI is disabled for all users. To turn it on, follow these instructions:

Framerate Insurance can help you with intermittent hiccups in your wireless transmission. However, if your network conditions are so poor that you’re dropping multiple frames in a row, we’ll stop using ASW for those frames, since its approximation gets worse the further the reprojection time becomes from the last good frame.

In addition, the Optical Flow algorithm that computes the relative frame motion isn’t perfect. If you have textures with repeated patterns in the frame, this can confuse the algorithm, resulting in localized errors. These appear simply as wobbles in your rendered image. These artifacts existed in PC ASW 1.0, and the assumption is that they are less disagreeable than the potential full screen judder of a non-ASW frame.

Running AFI with 120Hz is not supported in this release. Doing so causes significant judder and latency artifacts. We’re addressing this bug in a future release.

We’re excited for you to try out this experimental feature. Please share your experience in the Developer Forums, and let us know if Framerate Insurance improved your Air Link experience.