When pressure-fitting parameters are not shared across apps, people must retune in every app

What it is

A force band is a property of the person, not of a notes app. If the curve lives only in one app’s private settings, every new drawing, markup, or signing tool demands another tune—and the tunes still disagree, because each app’s default dead zone and width range also differ. Non-sharing produces repeated labour plus a split feel, not “each app may have its own brush style”. Brush style may differ; normalisation from force to light/heavy should be the same.

Why it happens

If the OS or pen driver offers a system pressure curve, apps read the same normalised force and bind it to their brushes. If each app reads raw ADC and fits on its own, each person has a mutually unrecognised curve per app. The second pattern is common: apps want control of drawing feel and privatise fitting with it. Ordinary force that meant medium width in app A becomes maximum in app B; people think B’s pen is broken and walk through calibration again. Different width ranges (8 px max in A, 40 px in B) will look different even with a shared curve, but that is the brush; what is unshared is the ADC-to-0–1 layer. With no system profile, apps should at least import/export a curve, or read a stylus setting in system accessibility. An account login that does not sync this block adds another round on a new device.

Studying it

After calibrating in app A, have people immediately draw the same target thicknesses in app B with no recalibration. Compare system-level sharing, app-private curves, and an importable file.

Independent variables: curve shared across apps, whether default width ranges match, import available. Dependent measures: hit rate in B, whether people open B’s calibration, whether they say “these are two different pens”.

Control brush look: even with a shared curve, first bring B’s max width in line with A, or range and mapping cannot be told apart.

Where it stops holding

A professional painting app may keep an override (an airbrush wants another curve) but should inherit the system curve by default; override must be explicit. If the OS has no stylus settings namespace, apps cannot share from nowhere; an export file is the practical fallback. Web apps live in a sandbox per session, so both persistence and sharing are harder and need site-level config or a browser permission.

Applying it

• Prefer reading a system- or account-level force normalisation; the app only binds 0–1 to its own brush range.
• If a private curve is required, offer “import from system / from another app” and mark the setting “this app only”.
• Do not advertise “calibrate once, use everywhere” while writing the curve into the app sandbox.
• How to check: calibrate in A, open B, do not tune B; light/medium/heavy should still land on B’s own thin/medium/bold (max width may differ). If B saturates and A does not, the mapping is not shared. After import they should align.

Related

• Same group: C5.09.1 Natural grip force varies; the same curve is too soft or too hard · C5.09.2 Fitted curves should persist as user settings · C5.09.3 Calibration samples a few force levels
• Adjacent: C5.01 Pressure Sensitivity · C5.07 Division of Labor Between Pen and Finger
• Search: per-app calibration · shared pressure profile · input settings silo