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Since @kirtai asked... How'd I design a USB drawing tablet? Or a touchpad or touchscreen?
As I've said earlier (for the latter) the raw sensor would be a grid of capacitors impacted by the proximity of your finger. With some circuitry (shift registers) to send voltages along the rows to see the capacitance-drop on the columns.
Due to the large quantity of raw input I'd use the DSP I designed to extract touches to report to the CPU. With a 1st pass locating 2D runs of non-zero values.
1/?
I'd compare to previous results to give each finger a stable ID within a gesture. And I'd locate the max-value within a touch to derive angle & pressure.
We'd want to expose that extra data to the CPU for the sake of drawing editors (a Krita port) for a natural way for artists to supply richer input, though otherwise it'd mostly be used to statistically verify that it is real input rather than accidental noise.
Our DSP would be much better to this analysis than our CPU.
2/4?
@alcinnz
random silly thought . . . could it distinguish between fingertips and wandering kittens . . . .
Basically that verification ensures that the gestures fit formulas describing what's realistic.
A drawing tablet is a large touchpad or touchscreen, though typically we find it more ergonomic to keep the screen external. Including a stylus with a capacitive tip.
Maybe we'd include additional buttons, etc representing useful artistic actions.
It'd upload (when polled) all this data to the computer classified (via its constant metadata) under Usage Page hex-0D "Digitizers Page".
3/4
> A drawing tablet is a large touchpad or touchscreen
On devices marketed as drawing tablets, the stylus functionality is usually technologically separate from the touch functionality. There are a few stylus technologies that use the touchscreen (I believe most USI-branded tablets and the apple pencil do this), but if you get a cheap USB drawing tablet, it will almost always not have any touch capabilities because the actual "drawing" part is implemented with some variant of EMR technology. I'd be surprised if there were any non-tablet-computer stylus input devices that use capacitive sensing for the stylus.
There are a lot of reasons for this: better accuracy/precision, lower noise (and thus lower latency), sometimes allows for a battery-less stylus, don't need to implement palm rejection, tilt/rotation detection, probably more that I'm not aware of.
Before Wacom's patents expired, most non-Wacom drawing tablets used an active EMR tech that requires the stylus to have batteries, but these days, since the basic patents have expired, there are plenty that don't.
A cool project that implements and explains the tech: https://patchouli.readthedocs.io/en/latest/
Also honorable mention to the non-EMR non-capacitive Nintendo DS. And also light pens.
The raw, if parsed, USB HID input would be exposed to the focused window for a Krita port (operating within our OS's paradigm) to transfer this data to a sprite rendered via one of our 2 GPUs. With routine screenshots freezing it in place.
This screenshot would be saved to disk along with your edit history.
And I'd probably add additional logic for rendering finer levels of detail in the appropriate tiles, when the hardware's not otherwise busy.
4/4 Fin!
cc: @kirtai
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