Embedding data in material

摘要

A spatial domain image I produced by a source 1 is combined with watermark data Ri to produce a spatial domain watermarked image I'. The watermarked image is produced by an embedder 3 according to the equation Ci'=Ci + alpha . Ri where Ci and Ci' are wavelet transform coefficients of the image, and alpha is a scaling factor, alpha is chosen so that the watermark is imperceptible in the image and to resist removal of the watermark by unauthorised processing. It is desirable that alpha has the smallest value which achieves that. If alpha is too big the watermark is perceptible in the image; if it is too small the mark may not survive processing of the image. alpha is determined from a trial decoding of the image I in a decoder 4. The decoding is that which would be used to decode the watermarked image I'. A value alpha ' is produced by a calculator S3-S8, to which an offset value is added by an adder S9 to produce alpha . This produces values of alpha over the image, which are used to scale the data Ri so as to conceal the data. An image is one example of material to which the invention is applicable. The step of producing modified coefficient values Ci may not use coefficients of magnitude greater than a threshold T and does not use corresponding information symbols Ri. Alternatively, a threshold Tclip may be set. The scaling factor alpha is calculated using clipped coefficient values and coefficients Ci of magnitude less than Tclip.