It only applies to objects inside reachable space
Aliases: out-of-reach cannot grab · arm-length limit · near-field grab
What it is
The sculpture across the room cannot be taken, no matter how straight the elbow or how high the heel. Direct grab only applies to objects inside reachable space: acquisition is defined inside the volume the arm can arrive at. Outside that volume there is no such act as grabbing. The far field is not grab done poorly; grab is simply not defined there.
Reach here means space the hand can touch, not space it can touch comfortably, and not space the eyes can see. Comfort and sight are other volumes that intersect this one; they are not drawn by this leaf.
Why it happens
Grabbing requires the hand and the object to close in the same place. The arm is a kinematic chain of finite length, the shoulder an approximate origin, the end effector sweeping a pose-dependent envelope. Once the object’s coordinate sits outside that envelope, closing happens in air and the object does not accrue to the hand. Walking the envelope toward the object is moving the body, not lengthening grab.
So direct grab is a near-field technique. It consumes the shell the body already occupies, not the rest of the room. Making a far object grabbable demands that the person first move, or that the object be quietly “placed” in the hand — which is teleport or distant manipulation, not direct grab. Insist on grab for the far field and people will lean and tiptoe through a string of null grasps until they quit.
Studying it
Place the same objects at within arm’s length, just reachable, and clearly out of reach, offer only direct grab, and watch when people walk, quit, or push to a physiological limit.
Independent variables: distance from object to shoulder, whether stepping is allowed, sitting versus standing. Dependent variables: acquisition success, peak extension (shoulder–elbow angles), null grasps, the moment locomotion is recruited.
The “just reachable” band is the most informative: success drops, posture goes to an ugly extreme, yet people still judge “this is something I should take with my hand.” If they are still grabbing air when the object is clearly out of reach, the system has not stated the range. Allowing a step walks the envelope away; the measurement is then whether movement is accepted as a legal range extension, not grab range itself.
Where it stops holding
When the world is miniaturised, or person or object is scaled, the far field can become near and grab range follows the scale rather than breaking. Magnetic grab, ray-reel, and a long virtual tool give the hand an extension; that has already left direct grab. Sitting clips a slice of the envelope with thighs and chair back — the same object reachable standing is unreachable sitting; range tracks pose. A child’s envelope and a long-armed adult’s differ by tens of centimetres; a single world radius for the “grabbable zone” will short-change both. This leaf does not handle “I walked out of a world-locked panel.” That is a person leaving an object fixed in the world. Here the only question is whether, right now, the kinematic chain can arrive at this coordinate.
Applying it
- Offer grab only on objects the arm can arrive at. Farther than that envelope, use another acquisition method; do not put a grab glyph on something out of reach.
- Place grabbables inside the envelope of the current pose, not on the limit surface that is only hit standing, locked-out, on tiptoe.
- When an object is about to fall outside the envelope, fail in a way that reads as out of reach — not as a close that silently takes nothing.
- How to check: have people grab the same set sitting and standing, and measure each object’s distance from the shoulder. Anything repeatedly grabbed empty while sitting is outside grab’s applicable range — move it in, or take grab off it. Do not expect one more stretch.