Tracking failure requires a safe fallback
Aliases: full-resolution fallback · tracking-loss degrade · conservative island
What it is
Foveated rendering stakes high-quality pixels on a coordinate the tracker reports. When tracking fails—lids closed, spectacle glare, direct sun, a loosened headset, the face leaving the tracking box—the island stops in the wrong place or vanishes. Safe fallback means that once failure is detected, painting switches immediately to a method that no longer depends on gaze: full resolution, a large island centered on head direction, or uniform medium quality, rather than keeping the last valid sample as the foveation center.
Why it happens
Failure is more dangerous than delay. Delay at least still follows true gaze; failure delivers full resolution to a coordinate that is no longer true, and the fovea sees peripheral-grade quality. Pupil loss, a confidence crash, binocular disagreement, and sample holes are machine-detectable. Fallback chooses between harms: full resolution fills the GPU, drops frames, and can hurt comfort; continuing on a wrong gaze point blurs the center. On most headsets a brief frame drop costs less than lasting central blur, especially while walking or manipulating.
Fallback must also be able to climb back. Snapping to a small island the instant confidence returns flashes the quality boundary. A hysteresis or a soft recovery of island radius avoids oscillating on blinks, which are periodic, lawful failures.
Studying it
Manufacture failure: occlude one eye, switch to reflective glasses, slide the headset up a few millimeters, kill the IR illuminators. Record time to detect, whether fallback fires, reported blur and dropped frames. Contrasts: no fallback (keep the last sample), cut to full resolution, cut to a large head-centered island. Include blinks as “lawful failure” and count how often a normal blink is treated as a fault that needs full resolution—false alarms eat the savings. GPU occupancy during fallback must be logged too, or “clearer” hides frame drops.
Where it stops holding
A device that always paints full resolution does not need this fallback and also never saved foveation’s power. Laboratory trackers that only work in indoor IR fail less often than outdoor headsets; “almost never fails” from the lab is not a product assumption. Monocular users, a prosthetic eye, or severe ptosis make “binocular disagreement” itself a false alarm. For photosensitive epilepsy, a quality flash at fallback can be more dangerous than blur and needs a cross-fade, not a hard cut.
Applying it
- Threshold tracking confidence: below it, switch within a few frames to a head-centered large island or full resolution; do not keep foveating on a stale gaze point.
- Hold or slightly enlarge the island across a blink; do not cut to full-screen quality on every blink. Reserve the full fallback for sustained loss.
- Verify under reflective glasses, outdoor hotspots, a deliberately moved headset, and normal blinks that the fovea does not sit in lasting blur and that blinks do not collapse frame rate.