How deformation speed relates to perceptibility
Aliases: shape-change latency · slow actuation · motion perceptibility · rise time
What it is
A bump that swells over thirty seconds is filed as “the table was always like this.” One that pops in eighty milliseconds is filed as vibration, not shape. Deformation speed and perceptibility means a geometry change is read as “state changed” only inside a window that is neither too slow nor a mere impulse. This is not about how fine a pin array can be. It is about whether the motion can be read as a shape event.
Why it happens
Vision is insensitive to slow change without a transient at the boundary—the same family of mechanisms as change blindness on a table: without a sudden contour jump, people edit the new shape into “it was always so.” Touch reads shape more by contour following and static press; a very fast impulse is taken mostly by the Pacinian channel and feels like “a buzz,” not “this got taller.” Deformation therefore has two shutters: fast enough to become a vibration event, slow enough to become unnoticed drift. In the band between, the motion profile itself is the signal—rise, stop, new height held—and all three must last long enough for the result to be registered as new geometry. When a hand is already on the surface, speed also becomes force: too abrupt is a shove; too slow is “probably nothing moving.”
Studying it
Present the same target height at different rise times, under looking, not looking, and a hand resting lightly on the surface; measure whether people notice the shape change and what they call it.
Independent variables: rise time (tens of milliseconds to tens of seconds), a pause in the middle, contact present or not, a concurrent sound. Dependent measures: detection rate, report category (vibration / shape / nothing), gaze or touch time needed to detect.
Measuring only with direct gaze under lab lighting overestimates slow deformation. Harsher: put the change in peripheral vision or during a primary task. Code impulsive pulses separately from “rise to a height and hold”; they are not one condition.
Where it stops holding
Pneumatics, SMAs, and thermal expansion may already sit slower than the perceptible window; animation curves will not rescue “it never changed.” Phone actuators often sit faster than the shape window and can only be haptic symbols, not new geometry. In the dark the visual window closes and only the speed–force relation in the hand remains; gloves narrow both windows. A safety alert that uses slow swell may finish after the time to respond has passed. Ambient vibration (car, boat) can mask mid-speed deformation as noise.
Applying it
- For deformation as state reading, put rise, stop, and hold inside the time that can still be filed as shape: fast enough to notice, slow enough not to feel like a knock.
- Do not use a slow swell that must be watched to the end for alerts; a short impulse may mean “attend,” but then leave a still new height.
- When a hand may rest on the surface, cap rise force so it is not read as the system shoving the person off.
- Verify: play the target deformation in peripheral vision and during a primary task; ask what just changed. Answers of “a buzz” or “nothing” mean change the speed or add a hold. Replay in gloves and in the target vibration.