Smartpen Technology

3.2 Digital Paper

The pen relies on special "digital paper" that can be printed using a standard printer.
Each sheet of this paper is covered with a special background comprising a large array of very
small dots. The dots are originally arranged in a rectangular grid, each one placed .3
millimeters from its four neighbors. However, each dot is offset a small amount in one of the
four cardinal directions, and thus contains two bits of information. The field of vision of the
camera in the pen covers a circle with a radius of roughly one centimeter, allowing it to cover
a total area of roughly 300 square millimeters. Each dot covers .09 square millimeters (as
they are on average .3 millimeters apart), so the average image contains around 3000 dots.
Each dot carries two bits of information, and thus the total information stored in the image is
around 6000 bits, or nearly a kilobyte. This is enough information for the pen to uniquely
identify its position on the page.

Enabling Collaboration in the Sketching Domain
by Jesse Michael Smithnosky
Submitted to the Department of Electrical Engineering and Computer
Science
in partial fulfillment of the requirements for the degree of
Master of Engineering in Electrical Engineering and Computer Science
at the MASSACHUSETTES INSTITUTE May 19, 2005
Copyright 2005 Jesse Michael Smithnosky. All rights reserved.
The author hereby grants to M.I.T permission to reproduce and distribute
publicly paper and electronic copies of this thesis and to grant others the right to
do so.

http://dspace.mit.edu/bitstream/handle/1721.1/33362/62458663.pdf?sequence=1