Pressure sensitivity

A contact is not a point. A pen touching a tablet reports how hard it is pressing, how far it is leaning and in which direction, how far it has been rolled about its own axis, and, on pens with a barrel control, how hard that is being squeezed. Pointer events carry all five as ordinary properties on every move: pressure from 0 to 1, tiltX and tiltY in degrees, twist in degrees of barrel rotation, and tangentialPressure. Nothing about them is exotic. They arrive on the same event a mouse move does, which is exactly why they are so easy to forget about.

What an application does with them is mostly typography for the hand. Pressure drives stroke width and opacity, so a line can start thin, swell through the middle of a gesture, and taper as the pen lifts. Tilt turns a round nib into a chisel or spreads a shading brush. Twist rotates a flat nib the way a calligrapher rotates a real one. Tangential pressure is the airbrush trigger. The mapping is almost never linear: a pressure response curve is the setting artists actually spend time on, because the interesting range of a human hand is not evenly spread across the axis, and the same raw values feel completely different once the curve is bent.

Two neighbours are worth keeping apart. Pressure is a continuous axis, sampled on every move and meaningful along its whole length. Force touch is a threshold laid on top of that axis: cross one line and a discrete command fires, which makes it a button rather than a dial. Haptic feedback is the other side of the exchange entirely, an output the device produces rather than an input it reads, and pairing the two is what makes a threshold feel like a click.

The design consequence is blunt. Most pointers have no pressure at all. A mouse reports a constant while it is down, plenty of touchscreens report the same, and every one of those readers has to be able to reach the command anyway. Pressure is therefore an enrichment and never a requirement: it may make a line prettier, and it may not be the only way to make a line thick. It is also completely undiscoverable, since nothing on screen suggests that pressing harder would do anything, which is why tools that take it seriously put the current value on screen while the pen is down and let the reader see the axis they are playing.