Eye tracker

Here is an interesting idea:

The first article is about tricking your eye into seeing impossible color combinations. (Red-Green, Yellow-Blue, etc). The key is to use an eye tracker to always show the same parts of your eyes the same colors.
The second article is on eye tracking with open CV. It would take a bit of work, but I think this whole system could be done on iPad/etc.

http://www.lifeslittlemysteries.com/2069-forbidden-colors-red-green.html
http://opencv-code.com/Real_Time_Eye_Tracking_and_Blink_Detection

Once you have the eyes, You can determine your position relative to the camera (and thus iPad screen) using some simple trig. X and Y for sure... Z would be a little more tricky, but essentially use pairwise distance/scale change.

The next step would be to determine the direction of the eyes themselves. I'm not entirely sure what the best method would be to do that. Any ideas? 

Once you had a general purpose library to give you this field of view information, I bet you could come up with a whole bunch of applications.

Optical illusions seem like the obvious first class of products, but I wonder if you could create an entirely new style of UI? For example a lot of UI's currently hide menu bars unless the cursor has entered a region... But the optical illusion idea seems to indicate a whole new class of objects can be drawn that aren't really there. More vivid colors for sure...perhaps some form of 3D that doesn't require glasses.

-JD

---Update---
Apparently you want "Gaze Tracking".

This guy apparently nailed it
http://www.youtube.com/watch?v=iQyG8eFJGgQ

And here is another project with source that _may_ be what we need:
http://www.codeproject.com/Articles/26897/TrackEye-Real-Time-Tracking-Of-Human-Eyes-Using-a

I think these people are utilizing a much simpler approach to detection, by using nightvision mode of a camera, and the iris' high reflectivity, etc.. But after that I suppose all the angle calculations would be the same.

http://www.gazegroup.org/downloads/23-gazetracker


Some papers:
http://books.google.com/books?id=k0VGgU0lHroC&pg=PA117&lpg=PA117&dq=calculate+eye+gaze+calculation&source=bl&ots=RQZLpULcAU&sig=AzWUX6VeRXON6cAvkrKQK7mObH8&hl=en&sa=X&ei=3gsfT82BBeSjmQXc5cHIDg&ved=0CEAQ6AEwAQ#v=onepage&q=calculate%20eye%20gaze%20calculation&f=false

http://www.google.com/url?sa=t&rct=j&q=calculate%20eye%20direction%20of%20view&source=web&cd=1&ved=0CCMQFjAA&url=http%3A%2F%2Fwww.cs.clemson.edu%2F~hmurphy%2Fpapers%2Fbrmic%2FBET102cr.pdf&ei=9ggfT7_JFqSfmQXsnvXADg&usg=AFQjCNHAuzIpjDXxWruUt0Aft5GbL8aD8Q&cad=rja