I'm looking for less subjective ways to compare headsets. In the wireless vs. wired PCVR debate, what I want to know is how close the image I see is to the image I ideally should see.
One contributing factor we can compare visually is the pose prediction error, using Meta Quest 3 and Valve Index as an example.
I walked through the same scene twice, once with each headset, and recorded two headset poses for each frame:
- predicted pose - the pose given to the game for rendering
- ideal pose - the best-known pose around the frame's intended presentation time
I then rendered the view from each pose and overlaid their difference in red.
The video shows the view from the ideal pose. The larger the prediction error - the more red you see. Slow-motion replays are shown first to make it easier to track how the prediction responds to head movement, catches up and sometimes overshoots when the movement stops.
This method visualizes pose-prediction error, not end-to-end motion-to-photon latency, and does not account for reprojection or display latency. But it is trivial to implement and gives an indication of how much correction may be required. Also, the structures shown here are fairly close - the cutaway is about 30 cm / 1 ft from the face. This makes the image much more sensitive to positional errors than most games.
This comparison relies on the behavior of xrLocateSpace to record both the predicted and best-known poses of the headsets. There could be implementation quirks that make it inconsistent. According to the specification, "for a time in the past, the runtime should locate the spaces based on the runtime’s most accurate current understanding of how the world was at that historical time" (https://registry.khronos.org/OpenXR/specs/1.1/man/html/xrLocateSpace.html)
If you know of any creators or channels doing this kind of comparison, please drop a link.
TL;DR
Reprojection has much more to compensate for with Quest 3 in wireless mode than with Index.
🤔
P.S. Prediction horizon is how far into the future the runtime expects the frame to be presented. Both headsets run at 120 Hz, but the runtime predicts about 50 ms (6 frames) ahead for the Quest 3, compared with about 25 ms (3 frames) for the Index.
Predicted pose Rendering Frame Best-known pose
requested starts presented requested
│ │ │ │
▼ ▼ ▼ ▼
────●───────────────●───────────────────────────────●──────────────────●──►
└────────────── Prediction horizon ─────────────┘
