line art on banknotes and the like is subtly altered to steganographically encode plural - bit digital information . for example , the lines &# 39 ; widths or spacings can be locally changed so as to slightly modulate the apparent luminance of the document . the encoded information can be decoded using suitably - equipped processing equipment and used to identify a document as a banknote . various anti - counterfeiting actions &# 39 ; can then be taken .