1.4k words | Dan Hollick
Touch screens.
A capacitive touch screen locates fingers by measuring tiny changes in an invisible grid of electric fields.
A capacitive touch screen locates fingers by measuring tiny changes in an invisible grid of electric fields. The apparent simplicity comes from a set of carefully chosen representations, transformations and physical assumptions working together.
Transparent electrodes
Rows and columns of conductive material sit above the display while remaining almost invisible.
This is one part of a longer chain: finger becomes field disturbance becomes controller becomes x / y point. The useful abstraction hides the physical work, but the underlying constraints still shape the software built above it.
Capacitance
A nearby finger changes the charge stored at intersections in the electrode grid.
The implementation is full of compromises. Precision, speed, storage and energy rarely improve together, so practical systems choose the errors people are least likely to notice.
Multi-touch
Controllers scan the grid repeatedly and group nearby changes into stable touch points.
Once this layer is visible, familiar design conventions stop looking arbitrary. They are accumulated responses to the capabilities and limits of the machinery below.
A visual study based on the original chapter. Text is condensed and rewritten.