Pressure maps to stroke width, opacity, or flow
Aliases: pressure mapping · stroke width · opacity pressure · ink flow
What it is
Pressure mapping turns the normal force at the nib into a continuous stroke parameter. The usual destinations are stroke width, opacity, and flow (pigment deposited per distance travelled). Pressure here is an analogue channel, not a contact switch. Width change from a tilted chisel or side-stroke is a different channel and should not share the same knob as force.
Why it happens
A digitizer turns nib force into a sample stream. An EMR stylus modulates a resonant circuit with an in-pen force element; an active electrostatic stylus typically uses a strain gauge or FSR in the tip. Reports are a 0–1 value or an ADC integer. The ink engine then drives the brush: width changes geometric radius; opacity changes alpha per deposit, so harder press looks more solid; flow changes pigment rate along arc length, so the same path can be gone over to build density, as with an airbrush or watercolour, rather than fattening the outline in one pass. The three mappings look unlike one another on the same force sequence. Width mapping inflates the contour; opacity keeps the contour and varies depth; flow couples time and speed—moving fast without enough force starves the stroke of ink. Engines interpolate radius or alpha per sample, then spline or stamp. Averaging force over a whole stroke erases on-stroke pressure variation.
Studying it
Compare an external force plate or the pen’s raw force channel with what appears on screen. Ask people to draw straight lines, tapered presses, and overpainted patches at named force levels (very light / medium / very heavy).
Independent variables: mapping type (width / opacity / flow), force-to-parameter curve, nib material, writing speed. Dependent measures: error between instructed force and produced parameter, fraction of the force range actually used, whether overpainting reaches a target density, and whether the tool “feels like that pen”.
Log speed. Flow mapping looks starved when the hand moves fast; width mapping shows error mainly on the contour. Static press-and-hold points miss continuous press–release.
Where it stops holding
A capacitive stylus with no force sensor is a contact switch; none of the three mappings apply. The same pen may bind force to different parameters in different apps, so “press harder to go thicker” is not a portable rule. If a dead zone swallows the lightest force, hairlines vanish; if the mapping saturates early, extra force in the upper range does nothing. Medium metaphor (nib pen, marker, airbrush) decides whether width, opacity, or flow is the right destination; an airbrush implemented as pure width will feel wrong.
Applying it
- Choose a default mapping from the medium: nib/pencil to width, marker to opacity, airbrush/watercolour to flow. State in the tool what force changes; do not only claim “pressure support”.
- For width, expose visible min/max radius; for flow, let overpainting accumulate to opaque so the stroke is not a permanent wash.
- Apply force per sample, not as a stroke-wide average, so press–release still tapers or darkens.
- How to check: one stroke from hairline to bold, and a lightly overpainted patch. Compare the recording with nib press and confirm contour or density changes continuously rather than jumping between two steps.