Mobile phones can be modified to act as pens that record messages written in the air that can then be sent as emails.

Researchers at Duke University wrote software for a Nokia N95 phone that interprets the movements of the phone - measured by its on-board accelerometer - as letters. The notes written this way can be packaged up as the text of email messages and sent to the user as a reminder.

The example used by the authors of a paper about the PhonePoint Pen has a person writing the airport parking garage level and section number and emailing it to himself so he can find his car when he returns from his trip. The researchers presented the paper at the recent Sigcomm conference in Barcelona.

For more lengthy writing, PhonePoint developers envision a phone with its camera pointed at a keyboard printed on a sheet of paper being able to record what users 'type' on the paper keyboard.

Writing in thin air has some challenges. The accelerometer in the phone detects motion in three directions, but can't detect rotation. So if writers rotate their wrists while writing, the phone won't know it, and its interpretation of the motions it does detect will be off.

To deal with this, the researchers recommend holding the phone like a pen and imagining one corner as the tip, and then not rotating that hand while writing. Alternatively, the researchers call for breaking down the elements of a letter and pausing after each part is drawn to allow the phone to start afresh with a new sense of position when the next stroke is drawn. Users have to be careful not to rotate the phone in the middle of a stroke.

Researchers say the experimental technology could be enhanced by creating a mechanism to give users a real-time display of what they just wrote. They are working on filtering out motions unrelated to writing, such as the movements of a person while on a train that is lurching side to side.

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