Body-locked content moves with the person but not with head turn

What it is

A tool tray hovers about an arm’s length in front of the chest: walk to the other end of the room and the tray comes along; turn the face toward a window and the tray stays on the chest, leaving the view. That is body-locked content. The reference frame is the torso (or the pelvis, or the waist), not the room and not the headset. It travels with the person, yet a head turn can look it into or out of the field of view.

Why it happens

Body-lock parents the interface to torso heading. In translation the torso and the headset move together, so the content keeps its pose relative to the body and is not walked away from the way a plaque on a wall would be. In yaw, head and torso split: a neck turn changes headset heading while the torso still faces the walk direction, so the panel slides relative to the view and can leave through the periphery. That is the geometry of glancing at a wrist or a belt — the hand is still at the hip; the head has to turn to it.

Where torso heading comes from decides whether the lock is real. Most headsets do not track the torso; implementations approximate it with the low-frequency part of headset yaw, a hip tracker, or the locomotion direction. If the approximation leaks, a glance at a side wall twists the tray a little — and that is no longer “does not follow the head,” it is head motion leaking into the body frame.

Studying it

Have people stand still and turn only the head, then keep the head aligned with the torso and turn the whole body. A body-locked panel should leave the centre of the view in the first case and stay in front of the torso in the second. A contrast task: retrieve a named tool from the tray after it has left the view, and record how large a head turn is required.

Independent variables: reference node (head / chest / waist), whether in-place head turns are allowed, whether walking accompanies the task. Dependent variables: fraction of time the panel occupies the view, head-turn angle to find the target, the subjective report “it follows me versus it is glued to my head.”

If headset yaw is used as a stand-in for the torso, “body-lock” and head-lock become indistinguishable under head turns. Establish the source of the torso signal before comparing lock modes.

Where it stops holding

Seated with the back locked to a chair, the torso barely moves independently of the head, so body-lock and head-lock converge. On a device with no torso tracking that treats headset yaw as body heading, so-called body-lock drags the interface during a glance. Fast turns and running lag the torso estimate and sling the panel in an arc beside the body — that is estimator error, not a property the lock is supposed to have. A watch-style surface on the wrist is arm-referenced, not torso-referenced; do not collapse it with a chest tray into one mode.

Applying it

• Put tools, inventory, and secondary status that must travel with the person but need not be stared at into the space in front of the torso or at the waist, so a head turn can look them away.
• Do not use unfiltered headset yaw as torso heading. The torso signal has to be separable from glances and nods.
• On first entry, ask the person to look to the side: the panel should slide out of the view centre; then have them turn the body: the panel should remain in front. If the two reactions swap, the reference frame is wired wrong.
• How to check: first-person video. In head-only-turn clips the panel must be able to leave the centre of the frame; in body-turn clips with the head still relative to the torso, the panel must stay in front of the body.

Related

• Same group: N3.01.1 World-locked content stays fixed in the environment · N3.01.3 Head-locked content stays in the same place in the view · N3.01.4 Head-lock is the most uncomfortable mode for prolonged use
• Nearby: N3.08 World-locked, Body-locked, and Head-locked · N2.07 The Body as Reference Frame
• Search terms: body-locked · torso-referenced UI · body-anchored interface