Azimuth sets the facing of the nib shape
Aliases: azimuth · nib orientation · brush facing
What it is
Azimuth is the facing of the pen axis projected on the screen plane: which way the pen is lying. The wide edge of a flat nib, a calligraphic chisel, or a rectangular highlighter has to follow that facing, or the side-stroke points the wrong way. It is not roll (rotation about the shaft). Many pens report tilt and azimuth but not whether the barrel was twisted in the fingers; a highlighter’s wide edge then follows only the lie-down direction.
Why it happens
The digitizer projects the shaft onto the plane and yields a 0–360° heading. The brush aligns the long axis or wide edge of a non-round footprint (ellipse, rectangle, cut chisel) to that heading. When grip changes, the wrist usually sweeps the shaft across the plane, azimuth follows, and the wide edge paints a “face” in the calligraphic sense. A round footprint hides even a perfect azimuth. If the engine uses path tangent instead—“the chisel faces wherever the stroke is going”—that is travel direction, not shaft direction. People can pull in one direction while the pen lies in another; the two headings should stay apart. Without roll, twisting the barrel to change the chisel does nothing; the user can only change which side the pen lies toward. Protocols often pack azimuth with tilt: as tilt nears zero the projection degenerates and azimuth becomes mathematically arbitrary, which needs its own handling.
Studying it
With a fixed chisel brush, have people change only lie-down direction at constant tilt, drawing a cross and a fan sweep. Compare reported azimuth with the on-screen long axis. Add a “follow path tangent” control to see whether the chisel wrongly tracks travel.
Independent variables: footprint (disk / ellipse / rectangle), azimuth source (shaft projection vs path tangent), presence of roll. Dependent measures: angular error between long axis and true lie-down, whether barrel twist changes facing, and whether the chisel “follows the pen”.
High-speed video or a marker on the barrel helps separate true azimuth from motion direction.
Where it stops holding
Round pencils and ball-like tools do not need azimuth; wiring it in only adds noise. Near-vertical writing makes azimuth unusable: freeze the last valid facing or fall back to a disk, rather than spinning the chisel. Left- and right-hand lie-down distributions differ; a default facing trained on right-hand data will punish left-handers. Hardware that does not report azimuth can only offer a fixed-angle chisel, which suits a stamp, not calligraphy that turns with grip.
Applying it
- Bind the wide edge of chisels, washes, and calligraphic nibs to shaft azimuth, not to stroke tangent.
- Ignore azimuth on round tools so noise does not micro-rotate the contour.
- When azimuth is briefly invalid, keep the last facing and show a short preview tick for the current wide edge.
- How to check: keep the nib almost still and lie the pen toward up, down, left, and right; the wide edge should face those four directions. Then travel along a diagonal with lie-down held constant; the wide edge should not twist with the path.