Velocity must be judged together with distance

What it is

Speed alone treats a fast in-place jitter as a swipe; distance alone treats a slow drag across half the screen as a page turn. The stable rule is joint velocity and distance: for example, commit a discrete jump only if lift-off is fast enough and the contact travelled enough arc. Fast-but-tiny or far-but-already-stopped stay pans. A single gate breaks on both spoof trajectories.

Why it happens

Instantaneous velocity is inflated by touch noise and a micro-fling in the last frames; integrated distance says whether position actually changed. The two are nearly independent: short-fast is tremor, long-slow is following, long-fast is a swipe. Joint rules are often AND (both gates to swipe) or partitioned (past half a page even if slow; otherwise must be fast). Pagers commonly use “halfway OR fast,” which is a product choice that must be explicit—not a lone speed constant. Distance should be millimetres or a fraction of page width so PPI does not warp a pixel gate.

Studying it

Build four trajectory classes: short-fast, short-slow, long-fast, long-slow, and read the label. The factor is rule shape (speed only / distance only / AND / OR). Measures are per-class error. Treat “past halfway” as one distance yardstick. Do not use random scribble as the set—the four classes should be balanced.

Where it stops holding

Continuous scrolling has no “one step” to commit, so the joint gate shrinks and inertia cares mostly about lift-off speed. A very short target (drag a slider to the end) will fight a distance gate via endpoint hitting. System gestures sometimes use speed alone because an edge band already stands in for distance. Joint judgement is completeness of the classifier, not flicker near the cut.

Applying it

• Write two swipe gates: minimum arc (mm or page fraction) and minimum lift-off speed, and say AND or OR.
• Use short-fast jitter and long-slow drags as required counterexamples; a speed-only gate must fail both before joint gates are justified.
• Log lift-off speed and integrated distance together so a regression can see which leaf of the gate is killing strokes.

Related

• Same group: C3.29.1 The swipe-versus-pan velocity cut decides a jump or a follow · C3.29.3 Near the cut, the same stroke flickers between two readings · C3.29.4 Velocity thresholds must be recalibrated across size and resolution
• Adjacent: C3.05 Swipe · C3.19 Gesture conflicts and disambiguation
• Search: velocity-distance · joint threshold · flick detection