Christopher Heiny heiny at
Tue Apr 10 16:54:58 CEST 2007

On Tuesday 03 April 2007, ground control picked up the following 
transmission from Florent THIERY:
> > Tapping two fingers on and off on my Mac laptop (old G4 model) I
> > can see behavior that matches this, although the pointer algorithm
> > it uses is trying to figure out a single point.
> >
> > So we will not see clearly defined bounding box limits. The point
> > will skate around within the limits depending on relative pressure.
> >
> > The first finger will set a clear start point, the second finger
> > will make that point shoot off towards it, but it will not go all
> > the way to the second touch. The effect should be to oscillate
> > along the line between the two end points, and it wont return to
> > the position of the first touch.
> On my own touchpad (thinkpad), it's almost the same, except that if i
> release the 1st finger, the cursor goes to the effective 2nd finger
> position. And i'd add that the oscillation is pretty steady on mine:
> almost none.

If you are using a capacitive touchpad on that Mac or ThinkPad, the 
behavior can NOT be used to infer anything about the behavior of a 
resistive touchpad.  The data used to compute the finger position, and 
the algorithms that operate on that data are completely different 
between resistive and capacitive touch solutions.

> Which gives us 2 sure elements:
> * the pointer won't come OUT of this box
> * the displacement speed from point 1 (single finger) to point 2 is
> constant, and hopefully will be detectable

It is not safe to assume that these are "sure elements".  These two 
behaviors depend on where the two fingers are, and which finger 
location algorithm the touchpad uses, and which capacitive sensing 
technology the touchpad uses.

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