Does touchpads have buttons?

Marius Gedminas marius at
Mon Sep 29 07:33:17 PDT 2008

On Mon, Sep 29, 2008 at 03:25:26PM +0200, Simon Thum wrote:
> Søren Hauberg wrote:
> > This stuff seems to only be in git. It's not in any releases, right?
> Yes. It needs a current server also.
> > The information I seem to get from the kernel is (BTN_TOUCH, 1) when
> > the stylus is pressed to the screen, and (BTN_TOUCH, 0) when the
> > stylus is removed. When I move the stylus around (when its pressed
> > onto the screen), I don't get any BTN_TOUCH's.
> Then you don't need any hysteresis :) I'd guess the HW does that 
> internally then.
> If you're curious anyway, from wikipedia: Hysteresis represents states, 
> and the characteristic curve shape is sometimes remiscent of a two-value 
> state, also called a bistable state. The hysteresis curve really 
> contains infinitely many states, but a simple application is to let the 
> threshold regions (usually to the left and to the right) represent 
> respectively the on and off states. In this way, the system can be 
> regarded as bistable.

In practical terms it means that if you get a range of pressure values,
you set two watermarks on it: a low one and a high one.  When the
pressure rises above the high watermark, you interpret it as a touch, when
the pressure drops below the low watermark, you interpret it as a
release.  This way if the pressure fluctuates only slightly, you won't
get a series of unintentional presses and releases.

Marius Gedminas
This sentence contradicts itself -- no actually it doesn't.
                -- Douglas Hofstadter
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