Re: Touch screens and wxPython

From: Gene Mosher <gene_at_viewtouch_dot_com>
Date: Tue Dec 02 2003 - 14:26:36 CST

Douglas W. Jones wrote:

> The fact that a touch-screen emulates a mouse does not imply
> that you can ignore the difference.

This is correct but it may not matter. X handles touches by default as
emulation of mouse button 1.

> With a mouse, clicks are unambiguous. They design the switches
> with hysteresis so that you have to push relatively hard to make it
> click, and then you can let up quite a bit before it unclicks. This gives
> you tactile feedback.Touch screens offer no tactile feedback, and if
> there is hysteresis in their response to a touch, it is done in software
> or firmware not in the springwork of the switches itself.
> The result is that, for many naive touch-screen interfaces, multiple
> clicks are a problem. If you program it so that each click toggles
> the selection, and if the touch screen has a simple threshold, you
> get really wierd behavior.

The solution we're using provides configuration options of visual and
subtle audio effects to indicate selections and provide confirmation to
users. The design of the gui can also include provisions to preclude or
filter multiple touches, essentially rejecting errors that would result
from multiple touches.

> I've seen at least one voting system (ES&S) vendor who faced this problem
> and solved it badly. Instead of adding hysteresis (difficult), they
> added a timer. Only touches lasting longer than 0.2 seconds were
> significant. The problem with this is that if you don't get immediate
> response, it's easy to think you're not touching hard enough, so while
> you wait for it to respond, you push harder and harder. By the end of
> voting one ballot, my fingertip was in pain! You can train people not
> to push so hard, but voting shouldn't require training!

This exemplifies bad software design. If the touchscreen technology is
surface wave, about $100 more, then no pressure is needed.

> Another way to deal with this is to have the push-to-select rule
> different.
> Design the user interface so that multiple pushes on one button always
> have the same effect as a single push on that button. Touch the
> candidate
> name to select, touch again does nothing, only touching a different
> name (or between names) will deselect.

This configurable feature is supported in our touchscreen gui
application framework.

> Resolution is another problem. Fingers are blunt, and the point of
> highest
> pressure or the centroid of the pressure function is what the touch
> screen
> presents.

If it's resistive, then this is true. There is a sensitivity setting
which can deal with this somewhat.
If the touchscreen technology is capacitive or surface wave then this
issue goes away.

> Two people pointing in what appears to be identical ways may
> actually touch different points because their fingers have callouses that
> are arranged differently or because their nails are trimmed differently.

Believe me, some of the young ladies who are hostesses and are using our
touchscreen systems in restaurants have nails that are long enough and
sharp enough to kill. We have lots of experience in devising ways to
deal with things like this.

> Also, touch screens are subject to false touches. You're officially
> touching with your index finger, but your pinky drags lightly on the
> screen
> at some point, and suddenly your selections are messed up. Setting a
> high
> pressure threshold or a time limit solves this, but that has its own
> problems, as I already mentioned.

There are many techniques which deal effectively with this behavior and
prevent it from causing problems.

> Finally, there's the issue of calibration. Some touch screen
> technologies
> use a relative calibration model, where the screen doesn't necessarily
> give absolute position output. If your touch moves a cursor, this can be
> a minor issue. The cursor shows you what you're doing. If, on the other
> hand, you want a cursorless interface, you need to calibrate the screen,
> probably when you open the polls.

We used to calibrate every sensor, and often. We don't need to do this
any more, however. We can do it, but we don't need to. Touchscreen
technology used to just be atrociously finicky. It's quite mature and
reliable these days. We could even make it so the users would actually
recalibrate the screen without knowing it.

> This creates a security issue! Miscalibration has been blamed on at
> least
> one real election problem. If you run the calibration script and it
> tells
> you to touch the center of the screen and then each corner, and you,
> instead,
> do your touching in a rectangle over on the low side of the screen,
> staying
> well away from the top corners, then from then on, the software will bias
> all touches to the top, making it potentially very difficult to vote
> for a candidate listed near the bottom of the screen. When it comes
> time to
> charge someone with impropriety, is there any evidence? Not if nobody
> was
> watching the improper setup, all you have to say is: Well, it looks
> like,
> somehow, the touch screen got out of calibration.

I have hundreds of systems in use and I must say that calibration is not
an issue in today's technology. I have no trouble calls in three years
dealing with this issue. In the past everything you heard was true.
Doctors, pilots and nuclear plant personnel are using touchscreens, and
that's because the technology is pretty smooth after more than 30 years
of efforts by many good engineers. None of this is in argument of
moving elections support people out of the voting area, people who can
be trained to deal with the issues that are raised as systems are
brought into use. Your comments are all very relevant and insightful,
however, to be sure.

> Doug Jones

Gene Mosher
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Received on Wed Dec 31 23:17:02 2003

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