Hiding information inside structured shapes
This paper describes a new technique for embedding a message within structured shapes. It is desired that any changes in the shape owing to the embedded message are invisible to a casual observer but detectable by a specialized decoder. The message embedding algorithm represents shape outlines as a set of cubic Bezier curves and straight line segments. By slightly perturbing the Bezier curves, a single shape can spawn a library of similar-looking shapes each corresponding to a unique message. This library is efficiently stored using Adaptively Sampled Distance Fields (ADFs) which also facilitate rendering of the modified shapes at the desired resolution and fidelity. Given any modified shape, a forensic detector applies Procrustes analysis to determine the embedded message. Results of an extensive subjective test confirm that the shape modifications are indeed unobtrusive. Further, to test the recovery of the message bits in noisy physical environments, a text document is put through a print-photocopy-scan process. Message recovery is found to be stable even after multiple rounds of photocopying.