Large translations and scales readily cause discomfort
Aliases: large optic flow · full-page scale · self-motion illusion
What it is
A full-page scale or a large translation paints optic flow on the retina that looks like self-motion, while the inner ear says the person is still sitting. The conflict yields nausea, vertigo or headache, not “more depth.” The risk is large-field optic flow, not the duration number as such: a short, violent full-screen scale can still trigger it. Whether the OS reduced-motion switch is on is a separate question — people who never declared can still be hit by large flow.
Why it happens
Visual–vestibular conflict has a classic shape: vision reports the world moving (or the self moving), vestibular and proprioceptive signals report a still head. Large numbers of pixels translating or scaling in a common direction most resemble a lunge forward or a sudden approach, and the visual system explains it as vection. In some people the conflict becomes an autonomic reaction; a headset is not required.
Scale is easy to underestimate: in a mock it is “focus emphasis,” on the retina it is field-wide expansion, equivalent to an object lunging at the face. Travel confined to a small control keeps optic flow local and conflict weak; once the background comes along, it becomes whole-field flow.
Studying it
Compare small-object travel, full-screen translation and full-screen scale on vection ratings, nausea scales and early-termination rates. Independent variables: fraction of the visual field in motion, speed, whether the scale origin sits at fixation. Dependent variables: vection, discomfort, requests to stop. Always allow immediate stop, and screen out people in an active migraine.
Short lab exposures underestimate accumulation: real use is repeated entry. Report total exposure, not only single-clip duration.
Where it stops holding
- A small object moving on a stable background usually does not have enough flow to take this path.
- When the user is the one scrolling or dragging, the motion is active and vestibular expectation partly aligns, so discomfort is lower than an equivalent passive clip; passive full-screen scale remains high risk.
- Headsets and wide monitors have larger fields of view, so the same pixel travel is a higher angular speed and the threshold is lower.
Applying it
- Take full-page scale and full-screen translation out of default transitions; use a local object or a fade.
- If large travel is unavoidable, shrink the field it covers (do not take the background with it), lower speed, and offer an immediate skip.
- Check: on the target device, have vestibular-sensitive colleagues walk the transition repeatedly. Dizziness or a request to stop means it should not be the default.