Fwd: Re: Openmoko Bug #1194: Touchscreen Jitter

Carl-Daniel Hailfinger c-d.hailfinger.devel.2006 at gmx.net
Wed Jun 18 15:48:10 CEST 2008

On 18.06.2008 00:44, Andy Green wrote:
> So if the "selfselection" of samples from the firehose don't make it a
> nonsense to look at it this way, it means we get excursions in the first
> coordinate of up to +/- 20% at about 10% probability.
> What we could do is refuse to accept samples too far outside a running
> average into the average reported back to userspace....

Beware of that approach. While it would be doable, it is certainly not
always obvious how to do it correctly. If the stylus is moved too fast,
all new samples will be outside the cleaned running average. So if you
compare against the cleaned running average, fast movements will result
in a hang of the fast-moving axis. If you compare against the uncleaned
running average, similar stuff can happen and some really strong
measurement errors will hang the axis with the errors.

Having a log at high speed without discarded values would help finding a
pattern. The results you posted look like using the median every 5
samples with 5 times the normal sample rate should give you pretty good
input for your averaging function. That depends on the following error
characteristic, though: There may be no more than 2 too-high and no more
than 2 too-low samples in any given 5 subsequent samples. Best case of
that function is giving correct output in the presence of up to 80%
jittered samples, worst case is correct output up to 40% jittered
samples. Added benefit is instant answer to sudden movements of the
stylus. Finding the median of n samples is more expensive than
averaging, though.


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