A digital watermarking scheme for binary document image was proposed.First

a binary document image was segmented into individual characters.Then

based on the multiplicative transformation model

watermark was embedded by flipping the boundary points of each character.Moreover

a strategy is proposed for flipping the boundary points to ameliorate the visual imperceptibility of the watermarked image.Extensive experimental results show that the proposed scheme is effective to resist print-and-copy

it can achieve the purpose of hiding information in paper documents and can be used for information tracking.