A mid-air swipe is a round trip: a throw-out and a return

What it is

A mid-air swipe is not the friction path of a touchscreen swipe. It is a round trip: the hand throws out along a direction, then returns along a nearly opposite path to rest. The throw-out supplies direction and speed; the return recovers the limb so it does not park at the far point. Without the return, the next swipe has no start, and the recognizer treats a hanging hand as not yet finished. A swipe carries direction (left / right / up / down). It does not depend on fingertip closure or on how many fingers are up. Drawing it as a one-way arrow throws away half the action.

Why it happens

A swipe is ballistic: accelerate, peak, decelerate, then reverse. Direction is encoded on the throw-out's principal axis. Amplitude usually sits below a full-arm wave and above a fingertip pinch. With no surface friction, there is no tactile tick of “sliding down a list,” so the system has to read a speed peak and a direction histogram. The return is almost necessarily a mirror of the throw-out; if the vocabulary also holds the opposite swipe, the return is bait for that token. The round trip must therefore be internal structure of one event, not two independent swipes. Unlike a palm flip, a swipe should carry little rotation; a simultaneous wrist roll sends the classifier into the rotation family.

Studying it

Annotate swipe as two timestamps, throw-out and return, and watch recognizer output on each stretch. Measures: direction accuracy, reverse false fires on the return, whether reset is possible between two swipes. Factors: direction (four-way), amplitude, whether return is required. Have people throw out and not return, and complete the round trip, to separate hangs from double fires. A touchscreen swipe is not a gold-standard control: it has a contact path and a lift edge that mid-air does not.

Where it stops holding

A continuous fling in one direction (shooing something along) is not a round-trip swipe; the definition has no return. Use repeated pulses or rate control; do not force a round-trip template. In headset near-field a finger flick can still be called a swipe; the return may be too short to see, but the definition must still say “spring back,” or the next flick has no zero. Large living-room waves are more socially visible; tiny near-field flicks hide better and demand the opposite sensing resolution. Pinch-drag also has out and back, with a closed hold in the middle; that is not a swipe.

Applying it

• Write swipe as “throw-out + return + rest,” and read direction only from the throw-out.
• If the vocabulary holds left/right or up/down pairs, regression must include no-return and full-return, and count reverse false fires on purpose.
• Run teaching animation until the hand is back at rest. A one-way arrow poster must not be the only teaching artifact.

Related

• Same group: C4.07.2 A forearm-only flick is possible; shortness does not guarantee stable recognition · C4.07.3 Teaching must keep direction, sequence, and reset · C4.07.4 Whether firing waits for the return is an implementation choice
• Adjacent: C4.03 Gesture onset and offset detection · C3.05 Swipe
• Search: mid-air swipe · flick and return · ballistic gesture