Systems and methods that determine a path of a pen tip as the pen tip is moved across a document are described. The document is watermarked with a maze pattern from which encoded position information is determined. A sequence of images is captured by a camera that is located in a pen. The path of the...http://www.google.co.uk/patents/US7570813?utm_source=gb-gplus-sharePatent US7570813 - Strokes localization by m-array decoding and fast image matching
