A re-entry grace period can keep engagement after brief tracking loss

What it is

Once a user is engaged, a brief tracking loss caused by a cuff crossing the lens, a slight head turn, or someone walking through is not a deliberate exit. A re-entry grace period is a window measured in milliseconds to a few seconds: if the skeleton or hand reappears inside the window, the system still honors the existing engagement and does not demand the enter action again. Grace answers “is the engagement relation still alive,” not “should the in-flight dragged value be zeroed.” That second question is how to treat a continuous operation while tracking is gone.

Why it happens

Vision tracking drops frames to occlusion, motion blur, and the edge of the frustum. Writing “no hand in this frame” as disengagement makes every blink-length drop require a new hand raise, dwell, or wake word, so engagement is more brittle than the user’s intent. Grace is hysteresis on the engagement flag: a timer starts when loss begins, engagement clears only when the timer expires, and a hand returning into range cancels the timer. This is independent of how one enters—the enter action is still required when not engaged—and only protects an engagement already made. The timer must be bound to a person (or a hand); otherwise B’s hand can “step into” A’s engagement while A is dropped.

Studying it

During an already-engaged task, insert repeatable occlusions (a card, a walking confederate, dropping N frames) from 50 ms to several seconds. Dependent measures: after the occlusion, is the user still treated as engaged, are they asked to repeat the enter action, is the task interrupted. Stair-step the occlusion duration to estimate the upper bound at which most people still feel they never left. Do not measure grace before engagement exists; that test is false entry, not re-entry.

Where it stops holding

Contact sensors on a wearable or controller rarely produce “the hand is still there, the signal is not,” so grace is almost unused. Purely visual desktop modules and long-range cameras drop often; without grace, a continuous task longer than a few seconds cannot finish. Grace does not replace a distinctive enter action: the window opens only for someone already engaged. With several people present, grace must carry identity, or the window becomes whoever reaches in first takes over.

Applying it

• Give the engagement flag its own loss timer, configured separately from freeze policy on continuous controls.
• While grace is running, use a low-interruption cue (engagement halo goes translucent, does not extinguish) so the user can see “you still count,” rather than a silent wait.
• Calibrate the window on occlusions that actually happen in the venue (sleeves, people walking through, looking aside), not on lab video where the hand stays square to the camera.

Related

• Same group: C4.19.2 An overly long grace keeps the system live after the user has abandoned the action · C4.19.3 An overly short grace treats brief occlusion as deliberate disengagement · C4.19.4 Whether re-entry requires repeating the enter action depends on whether the grace has expired
• Adjacent: C4.16 Disengagement and timeout · C4.23 Handling tracking loss
• Search: re-entry grace · hysteresis · tracking dropout