Angular tolerance on a swipe sets the misclassification rate

What it is

A swipe is labeled “left” or “up” by whether the path’s angle to a main axis falls inside a tolerance sector. Widen the sector and diagonal page-flips, scrolls, and dismissals impersonate each other. Narrow it and a slightly bent wrist is read as no gesture. Misclassification is largely a function of this angle gate. Distance and speed can pass; one wrong angle drops the whole command in the wrong slot.

Why it happens

A thumb traces an arc in the lower half of the screen; an index finger coming from the side also bends. Geometric “horizontal” is not a straight line for a hand. Recognizers usually take the angle of the first displacement, or the principal direction of the whole path, and slice 360° into sectors with dead zones between them. Too thin a dead zone and a 20° bias jumps to the neighbor (a horizontal swipe becomes a vertical scroll). Too fat and many sincere swipes fall through. Once direction is locked to a sector, a later bend is often ignored—that stabilizes the command and also stops someone who changes their mind from turning “left” into “up.”

Studying it

Have people swipe in a named sector and also err by 15°, 30°, and 45°. Draw a confusion matrix: left / right / up / down / null. Independent variables include sector half-angle, using an onset window versus a whole-path fit, and thumb versus index. Dependent measures are neighbor-swap rate and the null rate of “I swiped and nothing happened.” Overall accuracy can bless a collapsed model that classifies everything as one direction.

Where it stops holding

A surface with only one meaningful direction (archive left only) can open the sector toward a half-circle; error becomes “was this a swipe at all,” not “wrong direction.” Knobs and dials want continuous angle; sectoring chops fine adjustment into commands. An edge-origin back gesture is constrained by origin; its “inward” is not simply one of four sectors. Left-handers mirror the arc; a sector calibrated on the right hand systematically hurts them.

Applying it

• Leave a dead zone between neighboring commands; do not bisect 0–90° with two gestures. Aim the primary sector at that hand’s natural arc, not the geometric screen axis.
• Lock direction from a short onset, so late jitter cannot reclassify. If the finger stops mid-way, allow the swipe to abort rather than freeze a wrong sector.
• Build a four-direction-plus-null confusion matrix with a thumb pass and an index pass. Wherever two directions swallow each other, widen their dead zone; do not tighten velocity—that does not fix angle.

Related

• Same group: C3.05.1 A swipe is jointly classified by direction, distance, and velocity · C3.05.3 A swipe-triggered action should be undoable by a reverse swipe · C3.05.4 A swipe leaves no trace and needs a visual hint that it exists
• Adjacent: C3.07 Pan · C3.28 Gesture direction locking
• Search: swipe angle · direction sector · gesture confusion matrix