Palm rejection depends on a pen prior or on contact-area classification

What it is

Palm rejection has to separate “the pen that is writing” from “the palm that came down with it”. Two usual sources of evidence: a pen prior (a paired pen already in the near field or just reporting coordinates) and contact area / shape (the palm is a large flat blob, the nib a small round point). Neither is stable alone. Without a pen signal, area will kill a fat fingertip; with only a pen signal, a lingering palm after the pen leaves can still be treated as a finger.

Why it happens

An active pen has its own digitizer channel, so the touch controller can ask whether a pen is present—hover or contact. If it is, large, slow contacts near the nib are labelled palm and kept out of gestures and ink. That is a prior: trust the pen, then interpret the capacitive image around it. Passive setups have no pen ID and must cluster by area, eccentricity, duration, and relative motion: a large, almost still blob is palm; small and fast is finger or nib. Classifiers are often thresholds with hysteresis, or a shallow firmware model. The pen prior helps most in the first few hundred milliseconds, when the palm is already down and area has not yet seen a trajectory. Area classification still blocks a resting hand when the pen is away. Fusion needs time: keep a short “palm may still be there” suppression after the pen leaves, without making it a permanent swallow of fingers. Edge distortion inflates a fingertip toward palm size, so area-only rejection fails near the bezel.

Studying it

Log the pen channel, raw capacitive contacts, and class labels together. Tasks: palm down then nib, hover then palm, finger-only taps, and pinch with the other hand while holding the pen.

Independent variables: pen-presence prior on/off, area threshold, suppression window after the pen leaves. Dependent measures: palm-as-finger counts, finger-as-palm counts, missed ink at stroke start, pinch failures.

Plot labels on a timeline to see whether errors cluster at start, lift, or corners.

Where it stops holding

A passive capacitive stylus has no independent prior and lives on area; fat fingers and gloves overlap the palm. When the pen battery dies the prior vanishes; without a fallback to area, a resting palm immediately scribbles. Two pens in range (or a pen plus a capacitive stylus) break a unique-pen prior. Wet hands change the blob and drift area thresholds.

Applying it

• With an active pen, take presence as the first evidence and area/motion as support. With no pen, switch to a more conservative area rule rather than keeping the large suppression zone from “pen in range”.
• Log classification failures: which class was rejected, at start or lift.
• Do not reject by area alone on corners and large buttons; add a location prior.
• How to check: palm-then-pen, pen-then-palm, finger taps with no pen, and opposite-hand pinch while holding the pen. Count false positives (palm acts) and false negatives (finger or pinch swallowed). Kill the pen battery and check that finger taps return.

Related

• Same group: C5.04.1 Palm contact is inevitable while writing · C5.04.3 Rejection direction follows handedness · C5.04.4 Over-rejection swallows intentional finger input
• Adjacent: C2.09 Accidental Touch and Palm Rejection · C5.03 Pen Hover Preview
• Search: palm classifier · stylus presence · contact area