Occlusion range changes with grip and handedness
Aliases: grip · handedness · occlusion range
What it is
Finger occlusion is not a fixed circle: it changes with grip, handedness, finger, screen size, and location. A right thumb often approaches from lower right, while left-hand or two-handed index use differs. Applying one posture's offset rule to everyone can hide feedback for others.
Why it happens
Wrist rotation, device support, and approach direction determine finger and palm projection, and the key variable here is the angle at which the finger approaches the screen: in one-handed grip, the thumb almost always sweeps toward the target at a shallow angle from below or below-the-side of the device, while in two-handed grip the opposite index finger typically comes down steeply from above the screen. These two approach directions define "safe, unoccluded" positions that point in nearly opposite directions, so any offset hard-coded to a single direction can suit one posture only at the cost of the other — there is no single offset direction that serves both at once. Some touch sensors report the orientation of the contact ellipse's major axis, which is itself a direct clue to approach angle, and could in principle be used to infer offset direction in real time rather than depending on a static left-/right-hand toggle in a settings screen.
Studying it
Cover right and left hands, one-thumb and two-index grips, portrait and landscape, and device sizes; record occlusion, feedback visibility, and errors. Let people hold devices naturally rather than only in prescribed laboratory posture. Concretely, test the four combinations — left-hand thumb, right-hand thumb, left-hand index, right-hand index — separately for whether the offset direction is correct, rather than testing only whether a generic "supports left/right hand" toggle exists.
Where it stops holding
Every user need not have a precisely tracked occlusion contour; excessive personalization is unstable and surprising. Safer designs offer visible zones, flippable offsets, and multimodal feedback rather than fixing critical content on one side.
Applying it
- Let movable previews or magnifiers switch sides or choose safe placement from gesture direction, and where feasible infer offset direction in real time from contact-ellipse orientation rather than relying solely on a static handedness setting.
- Test critical work across handedness and common grips, not only right-handed desktop tapping.
- How to check: test the four combinations — left-hand thumb, right-hand thumb, left-hand index, right-hand index — separately for whether offset feedback lands in the correct visible direction; a failure in any one combination means the current scheme only serves part of the user base.