Embedding data in material

Apparatus for embedding data in material, comprising

• a transformer for producing transform coefficients Ci representing a transform of the material, and
• a combiner for combining the coefficients Ci with data symbols Ri to produce modified coefficients Ci′ where
• Ci′=Ci+αi Ri
• the apparatus further comprising
• a calculator for calculating αi for each unmodified coefficient Ci as a function F{Cn}i of a predetermined set {Cn}i of transform coefficients Cn which set excludes the coefficient Ci, wherein the coefficients are serially ordered and the coefficients Cn are coefficients preceding coefficient Ci.

A method comprising the steps of:

• embedding data in first material to produce second material in which the data is embedded;
• removing the data from the second material to produce recovered material;
• comparing the first and recovered material to determine any differences and locations of differences therebetween;
• and
• storing corrections which correct the said differences and data identifying the said locations in the first material at which the differences occur.

Apparatus comprising:

• an embedder for embedding data in first material to produce second material in which data is embedded;
• a remover for removing the data from the second material to produce recovered material;
• a comparator for comparing the first and recovered material to determine the differences, and locations of differences, therebetween;
• and
• a store for storing data identifying the said locations and corrections which correct the said differences.