Pressure curves must be adjustable to match force habits
Aliases: pressure curve · force habit · pressure transfer function
What it is
A pressure curve is the transfer function between reported force and the brush parameter: dead zone, mid-range slope, and saturation. A factory curve is cut for a notional average user, but grip-force habits differ widely. The same curve makes some people hit maximum width before they feel they have pressed, and leaves others unable to go thick no matter how hard they push. The curve has to be editable, or the channel asks the person to adapt to the sensor’s range. How an individual fit is sampled, saved, and shared across apps is a separate layer.
Why it happens
The function usually has a start dead zone (treat force below a floor as zero, to ignore nib weight and noise), an adjustable mid slope or polyline, and a saturation point beyond which the parameter stops rising. Too steep, and ordinary force fills the range, leaving two usable steps; too shallow, and ordinary force sits in the hairline band, so bold strokes need extra push. The shape need not be linear: a log or gamma-like curve spends resolution on light force, which suits people who live in hairlines; an S-curve boosts the middle. A smaller dead zone is not always better—zero dead zone turns the impact spike at contact into a blot. Adjustability lines up a person’s sense of light / medium / heavy with thin / medium / thick on the brush; it does not change the hardware. The sensor range stays; what changes is which band of force is treated as the working interval.
Studying it
Have people do the same press–release tasks (tapers, graded hatching, a requested “medium” stroke) on a locked curve and on an adjustable one. Record force histograms, where they parked the curve, and error against a target width after adjustment.
Independent variables: default curve shape, whether dead zone / slope / cap can be edited, presence of a live preview. Dependent measures: whether force mass moves back into the working band from the extremes, retries to hit a requested look, and “too soft / too hard” ratings.
Do not only ask whether people “like pressure”. Lock the curve, then unlock it, and see whether the same person’s force range was being wasted in saturation.
Where it stops holding
Curve editing without a preview is blind; people blame the pen. Some tools bind force to a non-continuous action (“hard press confirms delete”); an adjustable drawing curve would wreck that threshold, and those uses should not reuse the paint curve. Users with limited strength may need a very steep curve to reach full width; that is range fitting, not cheating. A factory curve still matters: the first stroke has to look like a line. Adjustability is the correction.
Applying it
- Expose at least dead zone, overall hardness (slope), and a force cap, and draw a live taper with the same test brush while the user edits.
- Park the default in the middle of typical force, so the factory curve neither saturates nor produces only hairlines.
- Make “this curve is too hard / too soft” a findable setting, not an expert page only illustrators open.
- How to check: light-handed and heavy-handed people each try a set of target thicknesses. If one end saturates and the other cannot make a hairline on the default, both ends should be reachable after adjustment, and the ink should visibly follow the edit.