Touchscreen requirements

[John Watlington]

OLPC is looking to add multi-touch to our interface over
the next year --- it is certainly necessary for a tablet.
But "multi-touch" describes a huge range of parameters.
Before taking a first pass at a spec. document, I'd like to
stir up some discussion.   Here are the parameters that
I think should be specified.   Feel free to comment on them
as well as suggest others!

Cheers,
wad

-----------------
1) Number of simultaneous touches:
 The number of simultaneous touches that can be tracked.
For W7, this is two.   I believe OLPC is looking for more.

2) Behavior when number of simultaneous touches is exceeded:
 If the number of simultaneous touches is exceeded, what happens ?
I suggest that the "oldest" touch be forgotten and no longer tracked,
but have seen other behaviors as well.

3) Palm rejection:
 A number of vendors include "palm rejection" algorithms in their
controllers.   I'm not sure how I feel about this --- I would prefer to
push this information higher in the stack before discarding it...

4) "Sensor size":
  This applies to multizone resistive touchscreens, which may be
thought about as a number of small touchscreens, each capable of
a single touch.    Two touches cannot be detected in any one zone,
so this affects how close buttons which might be pressed simultaneously
(think piano keys) can be placed to one another.   W7 specs 1 in. x1 in. max.
I believe this needs to be closer to 1 cm x 1cm max.

5) Resolution:
 Do we need to have a touch resolution equal to the screen size ?

6) Scan rate:
  The number of times a second that the touch controller
can identify and report a touch.    W7 specifies 50 Hz minimum,
which seems a little high.

7) Robustness:
  This is usually specified as the number of presses in one spot with a contact
area of either 8mm (finger) or 0.8mm (stylus).   Industry standard for resistive
(single or multizone) seems to be around 80K, which is too low for our needs
(we try to reach a 2000 day lifetime).  But the one vendor supporting 250K
touches was unusable by a bare finger (needed fingernail or stylus).

8) UV resistance:
  Since this touchscreen is on top of a sunlight readable display, it will need
to be UV resistant.   Our current standard (for the display) is no significant
change (>5%) in optical properties after 4000 hours of full sunlight UV
irradiation.

9) Humidity, temperature:
  Same as the XO:  operation from 0 to 50C, in RH up to 95%.

Regards,
wad