Touch feedback must clear the finger’s occlusion
Aliases: finger occlusion · callout offset · Shift technique
What it is
Hold a key and draw the magnified preview in the middle of the keycap: the pad swallows it. The pixels exist; the retina never gets them. Lift the preview above the fingertip and the character becomes a checkable confirmation. Occlusion-aware feedback means the confirmation of a touch cannot be drawn on the contact — it has to sit outside the pad’s projection.
The failures are local: a map ripple drawn at the pin’s feet, a colour value written on the slider thumb, a swipe-to-delete label covered by the thumb. People are looking. The place they are looking is under an opaque piece of flesh.
Why it happens
The pad is an opaque mask of roughly ten to fourteen millimetres, and on contact it covers the target’s centre. A mechanical reading of co-location sends the only confirmation into that mask. Vision has no channel through the finger, so the confirmation has to be offset: out along the finger’s long axis, usually toward the top or the ulnar side, where the fovea can take it.
Offset direction follows handedness and grip. A right thumb arriving from the lower right points up-left; a preview above and slightly left of the target is less likely to be covered again by the palm. The left hand reverses it. A callout that always sits dead above the target, indifferent to which hand, will crawl back under the palm in left-handed or landscape grips.
Studying it
The classic move is close to Shift: the target is hidden under the finger, a second offset preview is opened, and the test is whether the selection can still be confirmed. Over-shoulder video at the moment of contact, frame by frame, shows whether the feedback sits outside the pad’s outline.
Independent variables: direction and distance of the preview from the contact, left versus right hand, portrait versus landscape. Dependent variables: whether the current target can be read during the press, error rate on release, how often people lift the finger to see (and miss).
Do not ask “did it feel pressed?” — touch already supplied that. What is missing is visual confirmation of which target.
Where it stops holding
Mice and pointing sticks have no such mask; a hover card can open on the target. A stylus occludes far less than a pad, so the offset can be shorter, but not zero. System keyboards that already show a key preview will double-image if the app draws another. Feedback that is a full-page jump, or a control morph large enough to spill past the finger, is no longer an occlusion problem. Gloves enlarge the mask; an offset that used to clear will be covered again.
Applying it
- For keys, colour picking, map pins, and live slider values, lift the preview or number above the contact (or out along the finger), not centred on the target.
- Record a press for each hand and each orientation, and check that the preview does not dive under the palm.
- Open long-press menus upward from the contact, not as a ring around it — the lower half is eaten by the palm.
- How to check: film the contact frame over the shoulder, trace the pad, and ask whether every confirmation pixel sits outside that outline. Any critical letter still inside fails the claim.