What touch pads use and also we did is capacitive sensors, but they don't really measure if something touches normally, they just give a high response if you are very close. Touchpads are covered by a insulating film.
But, if you already know the x,y position then one option could potentially be to make the keys of a conductive plastic and measure the change in capacitance when they get touched. You just need "one" wire that goes to every key. Treat all keys as one sensor.
With optical tracking, one can, for example, have all fingers resting on home row, with one finger tapping. But it's hard to distinguish actually touching from merely being near the surface, so one exaggerates the taps to rise above tracking noise. Which is sort of ok for taps, but is a pretty bad user experience for say finger-stroking scrolling.
So the objective is to have per-finger high-quality is-it-touching-or-not-quite touch determination, on a good keyboard. Perhaps made easier by having high-quality XY position available from the optical tracking.
A keyboard where every keycap was a multitouch touchpad would work... but seems problematic.
Monotouch pads are more feasible, but imagine home row fingers now shifted half a key to the left, so finger tips touch multiple keys, and keys are touched by multiple fingers. Now detect tapping, when the tapped key remains continuously touched, and the tapping finger may be in contact with other adjacent fingers.
One could wire together all keys, if say you have finger-tip electrodes. And wired fingers have the bonus of providing good touch determination for finger-finger contact, instead of just finger-key. But the pragmatics of electrodes on skin are messy, and gloves badly sacrifice typing experience.
Ah well. Thanks. Perhaps the Tactual PRISM keyboard will work out.
