Bringing Phase Sync to Mobile VR

Phase Sync is a frame timing management technology that we first introduced in the Oculus PC SDK, which can help minimize pose latency by dynamically adjusting frame timing. We are excited to announce that Phase Sync is now available on the Oculus Mobile SDK and has been integrated into UE4 and Unity.

Unlike the Oculus PC SDK, the Oculus Mobile SDK has been using fixed-latency mode to manage frame timing since its inception. The philosophy behind fixed-latency mode is to finish everything as early as possible to avoid stale frames. It achieves this goal well, but with our release of Quest 2, which has significantly more CPU and GPU compute than our original Quest, a lot of apps can finish rendering their frames earlier than planned. As a result, we tend to see more “early frames”' in Quest 2’s VrApi log line. The SDK does already have some ability to control latency under the current fixed-latency mode; e.g. extra latency mode / non-extra latency mode settings. However, these are not flexible enough for developers to handle a dynamic workload, which means that if developers disable extra-latency mode but the app takes longer than expected to finish, the app generates a lot of stale frames, which negatively impacts the user experience since it can cause frame judder.

Compared with fixed-latency mode, Phase Sync handles frame timing adaptively according to the app's workload. The aim is to have the frame finish rendering right before our compositor needs the completed frame, so it can save as much latency as possible, and also not missing any frames. The difference between Phase Sync and fixed-latency mode can be illustrated in the following graph on a typical multi-threaded VR app.

We can see that Frame N’s complete time is a lot closer to its compositing time under Phase Sync, which is the source of Phase Sync’s latency savings. The above is just a simplified illustration of how Phase Sync works, and more technical details can be found in our Oculus Connect 4 Phase Sync talk.

Phase Sync is available to developers with our V23 Mobile SDK as an opt-in feature, and we have also integrated it into the latest UE4 and Unity versions for developers to enable it easily.

To enable it in UE4, download and build UE4.25 from our github, and check the “Phase Sync” check box under Project Settings->OculusVR.

To use Phase Sync in Unity, you must be using the Oculus XR Plugin, under the XR Plugin Management system. Phase Sync is supported on Unity version 2019.4 and beyond, and inside Unity, the Oculus XR Plugin version 1.7.0 and beyond. You can select the following checkbox to enable it in the Oculus-specific settings:

Once you have successfully enabled Phase Sync in your app, you can confirm that it works by running“adb logcat -s VrApi”. There should be a component labeled “Lat=-1”, which means that the latency is managed dynamically by Phase Sync. In the fixed-latency mode, the value would be “Lat=1” ( extra latency mode ) or “Lat=0” ( non-extra latency mode ) under non-phase sync.

You can also read the “Prd=?” value to compare your application’s latency difference before and after enabling Phase Sync. Prd is a direct measure of the prediction latency that your app incurs, so under phase sync, this value will be lower.

Phase Sync will be the default Vr Timing management method in our OpenXR implementation. However, VrApi developers need to adopt the new frame APIs ( VrApi_WaitFrame / VrApi_BeginFrame / VrApi_SubmitFrame2 ) to enable it. Please reference our developer documentation for details.

We have enabled Phase Sync on a couple of Oculus apps, for which the latency savings have been fairly impressive. For example, we were able to achieve a 10 milliseconds latency reduction in Oculus Home with Quest 2, and an 8 milliseconds reduction with Quest.

We encourage every in-development app to enable it, especially if your app is latency sensitive (if it uses hand tracking, for example). Below are a few additional tips to help inform your decisions.

Please let us know if you see any abnormal app behaviors when you enable Phase Sync in your app.

There are quite a few concepts in this post that we didn’t give too much detail about, as they have been covered in our previous blog posts.

For Rift Phase Sync: Oculus Connect 4 | Deep Dive into the Oculus Rift SDK

For extra latency: Understanding Gameplay Latency for Oculus Quest, Oculus Go and Gear VR

For VrApi log: OVR Metrics Tool + VrApi: What Do These Metrics Mean?

For late latching: UE4 Late Latching for Oculus Quest: Design, Benefits and How to Get Started