Digital paper

Simplified principle of the Anoto digital pen.
The camera identifies a 6×6 matrix of dots, each displaced from the blue grid (not printed) in one of 4 directions.
The combinations of relative displacements of a 6-bit de Bruijn sequence between the columns, and between the rows gives its absolute position on the digital paper.

Digital paper, also known as interactive paper, is patterned paper used in conjunction with a digital pen to create handwritten digital documents.[1] The printed dot pattern uniquely identifies the position coordinates on the paper. The digital pen uses this pattern to store handwriting and upload it to a computer.