first order low-pass filter
^^^^^^^^^^^^^^^^^^^^^^^^^^^
(Known as PT1-Element; related to exponential decay) 

  fac = exp(-dt*T1);
  output = fac*output + (1-fac)*input;

dt: time passed since last calculation
T1: time constant, think "time it takes to react". After this time,
    about 2/3 of the difference between input and output is gone.

(event-based calculation; in the not-event-based version, dt would be
constant, thus fac would also be constant.)

If the input is a position, then
  filtered_velocity = input - output
This is better than the straight-forward approach:
  velocity = (new_input - old_input) / dt
Because if dt is varying and very small, all kind of artefacts
and noise are amplified.