Fingertip contact gives pinch a tactile endpoint, so onset and offset beat a wave

What it is

An air pinch brings thumb pad to another finger and then apart. At contact, skin itself supplies a tactile edge: the person knows “that one went down,” and the system gets a kinematic discontinuity much steeper than a wave. Wave onset and offset have to be guessed from speed valleys or dwell. Pinch onset and offset are a pair that can be aligned: close and open. Headset air-tap and finger-click use the same edge with a shorter stroke.

Why it happens

Two fingers in contact temporarily close the open kinematic chain. Before contact, fingertip distance falls continuously; at contact, relative speed collapses and contact force fires cutaneous mechanoreceptors as a clear on/off. For the person this is close to a key bottoming out. For visual tracking, the fingertip-distance curve shows a detectable knee. A wave has no such knee; its edges are estimated from whole-arm speed, which varies widely. Pinch moves the edge from shoulder–elbow scale down to fingertip scale. Everyday arm-swing less often completes a full close–open, so the false-fire surface is narrower than a large wave. The edge is still not a physical key: force never reaches a device. The device is only looking.

Studying it

Cut the same command with a pinch edge and with a wave speed-valley, and compare segmentation hits, early/late commits, and idle-swing false fires. Record the fingertip-distance curve against a user key-press of “I closed,” and measure the time gap. Factors: pinch speed, gloves, camera azimuth relative to the hand. Gloves strip the cutaneous edge but keep the distance knee, separating proprioceptive from visual contribution. Do not control with a touchscreen tap: that has device-side contact sensing; pinch does not.

Where it stops holding

When fingertips are occluded or the back of the hand faces the camera, the visual knee vanishes while the tactile edge remains: people think they pressed, the system has no event. Stability is conditional. Wet fingers, long nails, prosthetic tips weaken or remove the cutaneous edge. A large, slow open-to-close pinch dulls the knee and the advantage approaches a wave. Near-field headsets looking at the palm see the biggest gain. Distant room-scale skeletons often cannot resolve fingertips, so pinch degrades to “the hand moved.” Do not claim the same onset quality for far full-body tracking.

Applying it

• Prefer a pinch edge for discrete confirm, rather than adding a dwell window to a wave.
• Detect the knee on the fingertip-distance curve; do not only classify a single frame that “looks like a pinch.” A single frame treats a near-miss as already down.
• Accept on four groups: gloved and not, palm to camera and back of hand to camera. If the back-of-hand group collapses, say so; do not publish only the palm-group number.

Related

• Same group: C4.06.2 Pinch splits into enter, hold, and release for selection and continuous control · C4.06.3 A short pinch equals a click on the pointed target · C4.06.4 Recognition fails when fingertips are occluded or the hand faces away
• Adjacent: C4.03 Gesture onset and offset detection · C3.01 Tap
• Search: air pinch · air tap · tactile delimiter