adrian cockcroft adrian.cockcroft at
Tue Apr 3 18:04:20 CEST 2007

This is key:- "Pressure has almost no effect on a single touch, but
not so on a double touch. The relative pressures will cause a
significant skewing effect towards the harder touch. You can easily
move the pointer along the line between your two fingers by changing
the relative pressure."

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.

If we capture a clear single touch, and an average position of
oscillation, then we can take the average oscillation to be the center
of the bounding box, and project an estimate of the opposite corner
where the second touch should be. With the right filtering and
limiting algorithm it should be possible to get the effect we want. If
we can give visual feedback on the screen showing the touch points and
bounding box it may help the user control the input better.

Challenges: In comparison with a true dual touch input device, its
going to react more slowly as the algorithm will need to gather more
data to decide where the pointer should be. Some of the faster moving
single touch gestures may be hard to distinguish from multi touch.


On 4/3/07, Florent THIERY <fthiery at> wrote:
> >With that being said I have been thinking about
> > multi-touch lately and after some things on the wiki page
> > I noticed
> > that is mentioned the zoom feature of the iphone.
> as an example. The aim of the question is to discover how a monotouch
> screen behaves when you try to use it as multi touch: is it
> determinist, or erratical?
> If somebody could try reproducing various "strange" usages of the
> screen and report any deterministic/detectable behaviour, we might
> find ourselves additional input methods based on "guess hacks".
> For instance, i think we can use cursor jumps as inputs; example:
> P**************+
> *                     2 *
> *                        *
> *  1                    *
> - **************N
> Zoom +
> P **************+
> *  1                     *
> *                         *
> *                    2   *
> - **************N
> Next page/item
> It would be some timing-based detection, like non-linear handgesture
> recognition, done with 1 or 2 fingers.
> We have to:
> 1- experiment the touchscreen with all imaginable gestures (ex: 2
> finger sliding, ...)
> 2- select the interesting behaviours
> 3- evaluate their determinism, eventually low level hacks on the kernel module
> 4- integrate them into openmoko finger-based controls
> > So my question now is do we have access to the bounding box?  If we can get
> > at the coordinates of the bounding box can we not figure out if the bounding
> > box is shrinking or growing?
> Yes, that is an open question :)
> > Also if we have
> > access to the bounding box we could with some practice figure out when users
> > are trying to rotate an image.
> Indeed: if the "bounding box" is accurate, what if:
> - you narrow the 2 fingers? <----- zoom
> - rotate the box? <---- rotation / linear map rotation....
> - 2 parallel fingers (horizontal) sliding down = flat bounding box
> (almost a line) = scrolling down
> ... But, i'm not sure i understood what a bouncing box is: if all of
> this was true, then the touchscreen would report 2 points / an area...
> It would be great if people having access to hardware could try doing
> some detailed testing / reporting. But, it can wait until the next
> shipping phase is over :)
> > the
> > high resolution of the neo this should help a great deal.
> Well, even with high resolution, our eyes don't have better zooming :)
> Especially on mobility situation...
> >.  You press the zoom button then click an area to enlarge.  Just
> > curious how difficult would this be?
> Well, it's quite easily doable if the webbrowser / image displayer
> implements zooming !
> > looking at the iphone and it doesn't seem to be very multi touch but as it
> > has not come out I could be wrong.  It seems to only be able to accept 2
> > simultaneous inputs.
> Hey, that's just what's needed :) How many free space do you have on
> such a screen?
> Ciao
> Florent
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