Synchronization tolerance of a multi-finger tap
Aliases: chord tolerance · two-finger coincidence · staggered down
What it is
Two fingers almost never land in the same millisecond. A synchrony window is how far apart they may land and still count as one multi-finger tap. Too narrow, and a sincere two-finger tap splits into a one-finger tap plus a late bump. Too wide, and an accidental second contact after a one-finger tap impersonates the chorded command.
Why it happens
The recognizer opens a window on first-finger down. A second down inside it, and both lifting promptly, emits the multi-finger tap. If the first finger has already lifted past the tap timeout before the second arrives, a one-finger tap fires first and the late finger becomes another tap or a miss. The window also has to watch movement slop: a second finger that arrives with obvious displacement looks like a pinch start, not a tap. Thumb–index asynchrony is tens of milliseconds; motor-impaired or older users can run to one or two hundred. Calibrating on gamers throws the latter group away.
Studying it
Inject two-finger downs at mechanical offsets (0, 30, 80, 150, 300 ms) and plot recognition. Then have people two-finger tap naturally, measure the spontaneous offset distribution, and place the window at a high percentile, not the mean. The false-trigger condition is a one-finger tap followed by a palm or other finger brushing the glass. Report hits and false fires separately; accuracy alone hides the cost of a wide window.
Where it stops holding
A stylus-plus-finger “two-contact” is more staggered and often not a tap intent. A system three-finger gesture’s window is set by the system; the app cannot change it. Rhythm games treat synchrony as skill and may tighten the window on purpose; a tool shortcut should not.
Applying it
- Place the two-finger tap window at a high percentile of natural landing offset, and delay committing the one-finger tap until the window has missed a second finger.
- If the second finger arrives with displacement beyond tap slop, go to pinch; do not also fire a tap command.
- Have people do twenty slightly messy two-finger taps, then one-finger tap and brush with another finger. The first set should mostly succeed; the second should not fire the shortcut.