On lots of multitouch devices, input suffers from jitter: The actual touch location is reported imprecisely and changes from frame to frame. This has obvious negative effects, since it’s much harder to hit a target this way. For years, people have been telling me that a lowpass filter would help. In its simplest form, a lowpass filter averages together the location values from the last few frames. This removes most of the jitter – because the jitter is random, there’s a good chance that the errors in successive frames cancel each other out. On the other hand, it adds latency because the software is not using the latest data. This tradeoff didn’t seem like a good one to me, so I didn’t add a jitter filter to libavg.
However, at this year’s CHI conference, Géry Casiez and coauthors published a paper on a 1€ Filter. This filter is based on an extremely simple observation: Precise positions are only important when the user is moving his finger slowly, while latency is important at fast speeds. So, the solution to the dilemma I described in the first paragraph is to build a filter that adjusts its latency depending on speed. Their filter is extremly simple to implement, and the results are really nice.
libavg can now process the touch input positions using this filter. The filter parameters are configurable in avgrc, and there’s a configuration utility (
avg_jitterfilter.py) that helps in finding correct filter values. The complete implementation is in the
libavg_uilib branch – I’ll merge it to trunk in the next few weeks.