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You can now do something very similar on the iPhone 4 because of the front facing camera. Imagine if you can hit-track when someone is looking at an ad. You could have an advertising business model at cost-per-glance. This works great if the ads are mainly for branding impact.

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Yes. Very yes. I can't upvote this comment enough.

Advertising pricing with cost-per-glance (or eyeball, or something) is a big dream of ours. Front-facing cameras on mobile devices excite us no end.

Would it work with such a small screen?

People typically hold the iPhone closer to their face than they hold their laptop. Since accuracy in eye tracking depends on the angle between the calculated gaze vector and the true gaze vector, this means that error will be reduced.

The amazingly high pixel density on the iPhone 4 means that that the calculated gaze focus may be more pixels away from the real point - but we're working on some ways to compensate for that.

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