How do I make Xglamo read directly from the touchscreen?

Nelson Castillo nelsoneci at
Sat Dec 27 02:36:58 CET 2008

On Fri, Dec 26, 2008 at 8:08 PM, Werner Almesberger <werner at> wrote:
> Nelson Castillo wrote:
>> That is because when we get an IRQ we gather as many samples
>> as the filters need in order to produce a point, and only then we check
>> if the touchscreen is still pressed.
> Hmm, but is this a bug or a feature ? :-) Samples you get when the
> touch screen is no longer pressed don't seem to be particularly
> useful, or are they ? What happens in this case ? Do you discard
> the event ?

A feature I would say :-)  This is even better than what we have in the
stable kernel because we discard noise. Sometimes the last few events
get discarded because they do not provide reliable data, for instance
because not much pressure is applied.

I tried to use a heuristic: discarding the first N and the last M events.
And still that did not work. We could not guess the values of M and N because
we don't know in advance much much noise we are going to get, it
depends of how much
pressure is applied, how fast the pen is moving, etc.

The "group" filter discards a lot of noise. Sometimes it discards a lot
of samples (say 50 adc conversions) in a click. This might sound
crazy, but what is
crazy is the data that we are discarding... Right now it seems we are not
experiencing bogus clicks and this was an issue before.

I had a few samples of what the data looks like but I made a mistake,
so I removed the data. I should measure and upload the data again to illustrate
this point. The hardware is reporting a lot of nonsense that we manage
to discard.

I'll do it...

Group filter => discard  noise (sometimes A LOT of noise that doesn't
reach user-space).
Median + mean => discard some noise, smooth samples

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