Fallback can mean shrinking to a simpler gesture set or switching to another modality

What it is

Fallback is not one road. One road shrinks the gesture set: keep coarser, less confusable motions (a big wave as “next,” park rotation and multi-finger for now). Another switches modality: hand focus to a button, a remote, voice, or a controller. Both count as predefined paths. They serve the same collapsing recognizer by different means. Which to pick depends on which channel is still alive on site, and whether the user’s hands can still make a large motion.

Why it happens

A smaller vocabulary eases the classifier: fewer classes, fewer mid-range scores, fewer Midas-style candidates. It still needs the camera to see a large enough motion blob. If the camera is already blind (lens covered, lights out), shrinking the vocabulary does nothing and the modality must change. Switching costs attention moved onto another set of controls, and the current task state has to travel with it, or the user starts from scratch. The two paths can be sequenced: shrink the vocabulary for a few seconds, then hand off to buttons if that does not hold. “There is a voice wake word” is not already a fallback—if voice has not been given an equivalent for this step, the switch is only another place to fail.

Studying it

Under the same injected failure, compare shrink-only, button-only, shrink-then-switch, and no path. Measure completion, time to change channel, wrong actions, and “I know what to use now” ratings. Inject both camera failure and “camera alive but fine motion is dead,” because they license different paths. Record whether state crossed the channel: after shrinking, is focus still on the original object; after switching to buttons, is the progress bar still at the original value.

Where it stops holding

When both hands are occupied or impaired, shrinking the vocabulary may still demand an arm wave; that path does not hold for those users and the modality should switch immediately. In a noisy plant, a voice switch will fail; prefer a button or a physical knob. In a headset, “switch to the controller” assumes the controller is already in hand; rummaging for it is not instant fallback. Shrinking to one motion that now carries every consequence can raise the cost of a false fire; pair it with a stricter confirm.

Applying it

• Name a default path for “fine motion unreliable but large motion remains” and another for “the visual channel is dead.” Do not prepare only one of them.
• On a switch, take the current object and uncommitted values along. When shrinking the vocabulary, highlight only the motions that are still live.
• Accept two injections: noisy keypoints should shrink, not die; covering the lens should surface another reachable modality, not keep teaching a more canonical pinch.

Related

• Same group: C4.25.1 When recognition fails, the system should follow a predefined fallback rather than going silent · C4.25.3 Fallback should not happen silently; users need to know they are in a degraded state · C4.25.4 Without a predefined fallback, edge cases present as random system failure
• Adjacent: C4.28 Necessity of non-gesture paths · C4.14 Gesture vocabulary size limits
• Search: reduced vocabulary · modality switch · fallback path