Bad VR is easy to spot. But it can be hard to understand.

At the Game Developers Conference this week in San Francisco, we’re releasing a new frame capture analysis tool for virtual reality, called FCAT VR, to help VR developers, press and enthusiasts analyze the performance and quality of the experience in VR.

Traditional performance measurement tools such as FRAPS measure what’s happening on the desktop monitor rather than what’s happening on the VR headset. They focus solely on frame rate. They don’t measure stutter, hitching or latency — which have a big impact on a VR experience.

So, stutter and latency are critical to measure, particularly in VR, as they can degrade the experience to the point of motion sickness.

Given the limitation of the existing tools, the common evaluation method is to experience VR and see how it feels. Does it hitch? Is there stutter? Pan the view around and feel how smooth it is in motion. However, this method leaves much room for interpretation.

FCAT VR takes the guesswork out of VR performance testing with an objective, data-based process. It’s based on FCAT, a frame capture analysis tool we released in 2013 to benchmark graphical quality in games. FCAT VR builds on this to provide comprehensive performance measurement for frame time and stutter on the VR headset without the need for special external capture hardware. The tool supports both Oculus Rift and HTC Vive.

Under the Hood

Rendering in VR is a lot more complicated and demanding than traditional rendering. The GPU has to render to a higher resolution. A high-quality VR experience requires very high, consistent frame rate, as well as very low latency between head motion and display output. And frames have to be pre-distorted or “warped” to offset the effect of looking at the display through an optical lens.

High latency caused by issues such as longer than expected frame time, inconsistent frame rate, frame drops and warp misses will degrade a VR experience. To ensure that FCAT VR reflects the real VR experience, the tool captures and tracks four key performance metrics:

Frame Time — Since FCAT VR provides detailed timing, it’s possible to measure the time it takes to render each frame. The lower the frame time, the more likely it is that the app will maintain a frame rate of 90 frames per second needed for a quality VR experience. Measurement of frame time also allows an understanding of the PC’s performance headroom above the 90 fps VSync cap employed by VR headsets.

Since FCAT VR provides detailed timing, it’s possible to measure the time it takes to render each frame. The lower the frame time, the more likely it is that the app will maintain a frame rate of 90 frames per second needed for a quality VR experience. Measurement of frame time also allows an understanding of the PC’s performance headroom above the 90 fps VSync cap employed by VR headsets. Dropped Frames — Whenever the frame rendered by the VR game arrives too late for the headset to display, a frame drop occurs. It causes the game to stutter and increases the perceived latency which can result in discomfort.

Whenever the frame rendered by the VR game arrives too late for the headset to display, a frame drop occurs. It causes the game to stutter and increases the perceived latency which can result in discomfort. Warp Misses — A warp miss occurs whenever the runtime fails to produce a new frame (or a re-projected frame) in the current refresh interval. The user experiences this miss as a significant stutter.

A warp miss occurs whenever the runtime fails to produce a new frame (or a re-projected frame) in the current refresh interval. The user experiences this miss as a significant stutter. Synthesized Frames — Asynchronous Spacewarp (ASW) is a process that applies animation detection from previously rendered frames to synthesize a new, predicted frame. If FCAT VR detects a lot of ASW frames, we know a system is struggling to keep up with the demands of the game. A synthesized frame is better than a dropped frame, but isn’t as good as a rendered frame.

Capture Performance, Analyze Results

FCAT VR includes two parts: FCAT VR Capture is a simple software utility to capture detailed performance metrics without need for specialized capture hardware. FCAT VR Analyzer provides an easy way to visualize and analyze the captured data in a graphical user interface.

The tool is available for download now on GeForce.com.

We all know what good VR “feels” like, but, until now, quantifying that feel was a challenge. Our FCAT VR tool gives developers, gamers and press a tool they can use to better understand what they’re seeing. The result will be more immersive VR.

For more details, see our in-depth article on GeForce.com, or download FCAT VR for yourself.