New Prototype Screen Lock Program: zedlock

Clarke Wixon cwixon at
Fri Oct 12 02:27:27 CEST 2007

I have put together a proof-of-concept "screen lock" prototype intended 
to prevent unintended touchscreen inputs while the phone is pocketed, 
against your ear, etc.  The program is called "zedlock" and it's 
available at my wiki page:

There are also screenshots there.

The prototype requires python, pygtk, and pycairo, but the final version 
will be in C.

I'm interested in comments and thoughts from the community.

The README follows:


Zedlock 0.0.1 PROTOTYPE
Copyright (C) 2007 Clarke Wixon

How it works

This is a screen-lock program in prototype form. To run it, just run from a terminal in the directory where it's installed.

It's really simple. To unlock, just trace a large 'Z' with a single 
stroke of your finger or stylus, anywhere on the screen. It must be BIG 
ENOUGH (about half the smallest screen dimension in both height and 
width, approx. 2cm on the Neo), it must be drawn in the CORRECT SEQUENCE 
(from top-left to bottom-right), and it must be QUICK ENOUGH (completed 
in less than one second). Within these parameters, a fair amount of 
variation (i.e., sloppiness) is tolerated. (As a result, you can make 
some non-Z patterns that fool it.)

Who's Zed?

Zed's dead, baby, Zed's dead.

No, really, why Zed?

There are a bunch of things out there already called "zlock" or the 
like. I selected the 'Z' glyph because it's fast and easy to make 
without looking, and its four points are easy to pull out of a stream of 
touchscreen data -- they are simply the maxima and minima of the 
functions (x+y) and (x-y). I don't need to keep a linked list of 
intermediate points or anything like that; these points are updated 

Why would I want this?

You don't need to be looking at the screen to unlock it. It is pretty 
unlikely to unlock accidentally (by your face while you are on the 
phone, in your pocket, etc.). But it's simple and fast enough to perform 
frequently and easily, and to become second-nature.

Plus it looks nice: gtk+ and cairo are used for fluid anti-aliased 
graphics. Visual feedback is provided after a stroke is completed: an 
openmoko-orange line shows where you made your stroke, and if it finds 
the 'Z' pattern, it is highlighted in white for a moment.  If your 
stroke doesn't meet the criteria, you get a big red 'X' on the screen 
and a three-second lockout.

With the iPhone and the Qtopia phone platform, you need to be looking at 
the touchscreen to unlock it. Their sliders and animated keys are cool, 
but this is better.

Remember, this is a prototype

To install it, copy and zedlock.png to your home directory 
(or a subdirectory), make sure is executable, and run it from 
the directory you installed it in. The program depends on python-pygtk 
and python-pycairo. Note that it doesn't actually do anything all that 
functional yet -- it is simply a demonstration program. This prototype 
doesn't actually lock or unlock anything; it just clears the screen 
after each stroke (and the subsequent visual feedback) so you can try it 
again and again and again.

The program should be more-or-less QVGA-friendly (well, almost, just 
change two global variables) and orientation-friendly already, but YMMV.

Yes, it's slow, especially to start up. But that's embedded python for 
ya. Plus I had to do some weird hacky things in cairo to get it to 
render predictably. This is my first cairo project and my first python 
program, SO DON'T GIVE ME ANY GRIEF! There are probably terrible, ugly, 
spaghetti-code-monsters in there if you care to look. They'll go away 
over time.  :)

Future Plans

After the featureset solidifies a bit more, I will rewrite it in C 
(which I'm much more comfortable with), optimize it (a lot!), and add 
some features:

1) It will be rotationally insensitive so you don't need to look at it 
to determine portrait vs. landscape before you unlock. This will be 
pretty easy to add.

2) The text on there is statically-coded right now, and fixing that will 
be trivial, so don't complain about it now. It doesn't actually tell you 
the GSM/GPRS status or even the real time of day. Yes, wise guy, IT'S 
ALWAYS 5:35 pm HERE.

3) Eventually it will get a battery/charging state icon, GSM signal 
strength bars, and a ringer volume indicator, so you can see those 
things while the screen remains locked. I don't want to make the 
interface too cluttered, but those items seem essential.

4) Ideally this will slip right into the OpenMoko power management 
scheme. Preferably neod and/or the dialer will *lock* the screen and 
require an unlock program (such as zedlock) in the following circumstances:

  * immediately after an incoming call is answered
    (to prevent touchscreen input from the user's face or ear)

  * immediately after an outgoing call is dialed

  * on request
    (via a lock button or menu, before you put it in your pocket)

  * upon an automatic return-from-suspend
    (e.g., a datebook alarm, incoming phone call, SMS, etc.)

  * but *not* following a manual return-from-suspend

Note that this concept of screen-lock is different from the 
power-management concept of screen blanking. Screen LOCK should probably 
occur more often, and is intended primarily to prevent accidental 
user-interface manipulation while the touchscreen is against the user's 
face, in the user's pocket, or otherwise subject to spurious inputs. It 
is not intended to save power.

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