Reach travels with the person; a world-fixed panel can be walked out of

What it is

A virtual keyboard sits on a table in the middle of the room. Teleport two steps and it is still at the old coordinates; the hands no longer reach it. Body-relative reach is a volume glued to the body. A panel nailed in world coordinates does not travel with that volume. Leave the placement pose and the direct-manipulation set can go empty — the widget did not vanish; the person took their hands away.

Why it happens

The arm's kinematic envelope is shoulder-origin, and the shoulder follows the trunk. Translate or teleport and the whole envelope moves. A world-locked interface writes vertices in room coordinates and stays when the person does not. Once the two frames disagree, the keyboard that was at the chest is a board two metres behind. Direct grab needs the hand to arrive; if it cannot, that path closes. People walk the envelope back over the panel, or switch to a ray that does not care about arm length.

Walking out does not need a stride. A half-step sideways or a ninety-degree turn can take a narrow panel out of the forward hand sector. Follow the shoulder or waist instead and the reach relation holds, at the cost of losing “that board lives in that corner” as a shared referent. Which frame to nail to is a bet on whether the person will leave the volume.

Studying it

Finish a block of direct actions on a world-locked panel, then leave the placement with a teleport or a real walk, and ask for the next action. Code walk-back, switch to far-field pointing, and abandon. The control is the same panel following shoulder or waist.

Independent variables: locking frame, distance and yaw of the leave, whether a ray fallback is offered. Dependent variables: time to re-acquire direct operation, walk-backs, mode switches, task breaks.

The first action after leaving is the observation: feet, ray, or an empty grab. An empty grab means the person still predicted “it should be in my hands.”

Where it stops holding

In a small room where people barely leave the stance, world-lock and body-follow nearly coincide and walking out is rare. When several people must point at the same board, world-lock is the premise, and the walk-out risk is paid for with a larger board or a summon-back. Continuous walking hides the problem more than teleport: people drift out and do not discover the empty hand on a single frame. Seated in a fixed chair, walking out almost never happens; a swivel can still rotate a side-locked board out of the envelope.

Applying it

• Let frequent direct-manipulation panels follow shoulder or waist, or offer one-button recall to the chest. Do not weld the main keyboard to the world pose of first placement.
• When a world-locked surface leaves arm length, show that it is out of operating distance and offer a walk-back cue or a far-field entry. Do not leave people grabbing air.
• Place world-locked panels where the person will stand next, not where the nose happened to point at open.
• How to check: after the first block, teleport or walk them away and watch the next move. If most people empty-grab and stall, reach and panel have already diverged — recall or switch channel, rather than hoping they walk back.

Related

• Same group: N2.10.1 The usable zone is the intersection of view and reach · N2.10.2 Cost is asymmetric across the body midline · N2.10.3 Upper bound is set by the shoulder; lower bound by the cost of bending
• Nearby: N2.06 Reach Space · N3.01 Locking Modes
• Search terms: body-relative reach · world-locked · egocentric workspace