Wayland Relative Pointer API Progress

Pekka Paalanen ppaalanen at gmail.com
Mon Apr 20 01:48:58 PDT 2015

On Mon, 20 Apr 2015 10:13:34 +0200
Michal Suchanek <hramrach at gmail.com> wrote:

> On 20 April 2015 at 09:36, Pekka Paalanen <ppaalanen at gmail.com> wrote:
> > On Sun, 19 Apr 2015 09:46:39 +0200
> > Michal Suchanek <hramrach at gmail.com> wrote:
> >
> >> So the device is always absolute and interpretation varies.
> >
> > I disagree.
> >
> > Let's take a mouse, optical or ball, doesn't matter. What you get out
> > is a position delta over time. This is also know as velocity. Sampling
> > rate affects the scale of the values, and you cannot reasonably define
> > a closed range for the possible values. There is no home position. All
> There is a home position. That is when you do not move the mouse. The
> reading is then 0.

That is not a unique position, hence it cannot be a home position. That
is only a unique velocity. By definition, if your measurement is a
velocity, it does not directly give you an absolute position.

When we talk about absolute, we really mean absolute position.

> > A mouse could be an absolute device only if you were never able to lift
> > it off the table and move it without it generating motion events. This
> > is something you cannot do with an absolute device like a joystick.
> You are too much fixed on the construction of the sensor. Mouse is a
> velocity sensor similar to some nunchuck or whatever device with
> reasonable precision accelerometer. That you can and do lift it off
> the table is only relevant to how you use such sensor in practice.

Accelerometers measure acceleration. Acceleration, like velocity, is
not a position. It does not give you an absolute position directly.


More information about the wayland-devel mailing list