Re: Questions about Multi-touch Protocal

[Peter Hutterer <peter.hutterer@who-t.net>]

On Wed, Jan 06, 2010 at 09:49:58AM +0900, Joonyoung Shim wrote:
> >> First, what is the purpose of ABS_MT_TOUCH_MAJOR event? I wonder how
> >> it is used at userspace side.
> > 
> > In a graphical program it can of course be used as is, but in the specific
> > application of creating gesture events, the TOUCH and WIDTH parameters can be
> > used to, e.g., approximate finger pressure or distinguish between index finger
> > and thumb. With the addition of the MINOR parameters, one can also distinguish
> > between a sweeping finger and a pointing finger, and with ORIENTATION, one can
> > detect twisting of fingers.
> > 
> >> Should the ABS_MT_TOUCH_MAJOR be the mininum set of events for multi
> >> touch? I'm not sure whether this question is valid because of first
> >> question.
> > 
> > Each finger packet should have an ABS_MT_TOUCH_MAJOR value. In can be computed
> > from various types of sensory input, e.g., the size of the contact area or a
> > pressure sensor.
> > 
> 
> Hmm, I think ABS_MT_TOUCH_MAJOR is similar to ABS_PRESSURE, of course it
> can be used other purposes. If the device doesn't support the pressure,
> the driver doesn't report ABS_PRESSURE, so if the device cannot get a
> touching size of the finger, how can the driver report it?

Note that ABS_MT_TOUCH_MAJOR complements ABS_PRESSURE but cannot substitute
it. The nature of our fingures usually mean that with increasing pressure
the touch size increases as the finger flattens. This is only useful as a
relative indicator though, as fingers differ greatly in size.

Cheers,
  Peter