Tilt angle simulates a side-stroke
Aliases: pen tilt · side-stroke · chisel stroke
What it is
Tilt is the angle between the pen axis and the display normal. Bound to a brush, it can mimic the side-stroke of a brush, chisel marker, or oil bristle: the more the pen lies down, the flatter the contact ellipse and the greater the effective width. That is not the same as pressing harder to inflate a radius. Side-stroke changes the projection of the nib on the plane; width can grow with force held constant.
Why it happens
The digitizer estimates shaft pose from magnetic field direction, asymmetric capacitive coupling, or an inertial unit in the pen, yielding inclination from the surface normal. The ink engine maps that angle onto the brush footprint: near upright, a disk about the nib radius; as tilt grows, an ellipse whose major axis follows the lie-down direction. Calligraphic side-stroke, a wide wash, and a highlighter’s flat head all depend on that geometry. Force can still modulate ink amount, but flattening of the contour is driven by tilt. If the engine only scales a disk isotropically by tilt, the stroke thickens with no facing and reads as pressure. The footprint must be a directed ellipse or a flat polygon so the wide edge follows the shaft. When sample rate is low, a fast roll of the pen lags the ellipse facing and the side-stroke twists at corners.
Studying it
Ask people to draw parallel lines with force held as steady as possible while changing only tilt, then a stroke that must move from centre-stroke to side-stroke. Compare an inclinometer or the pen’s tilt report with the on-screen axis ratio of the footprint.
Independent variables: tilt range, footprint model (isotropic scale vs directed ellipse), whether pressure is also live. Dependent measures: correlation of axis ratio with tilt, whether width still changes when force is constant, and whether the tool “feels like a flat nib”.
Lock pressure to separate side-stroke from force. Mean stroke width alone mixes the two channels.
Where it stops holding
A capacitive stylus or a thin pen that does not report pose has no usable tilt; side-stroke can only be faked with pressure or a fixed brush. Near vertical, the tilt estimate itself is unstable, so side-stroke should not change sharply in that band. Some writing grips stay almost upright and never enter the side-stroke region; a centre-stroke disk is then the safer default. Binding tilt to a non-shape action (canvas zoom) fights drawing habits.
Applying it
- For side-stroke tools, use a directed ellipse or a flat nib so the wide edge follows lie-down; do not treat tilt as a second pressure slider.
- Leave an upright dead band for ordinary handwriting so width does not breathe while writing.
- Preview the current footprint facing so people can see which way the stroke will fatten when the pen lies down.
- How to check: hold force roughly constant and lay the pen from upright to about 45°. The ink should flatten, with the wide edge aligned to the shaft. Then lock tilt and change only force: width should follow pressure, and the footprint should not rotate.