An encoding data processing apparatus is arranged to generate a marked copy of an image by introducing a code word into a copy of the image. The apparatus comprises a code word generator operable to generate the code word. The encoding apparatus includes an image perception analyser and a strength adapter. The image perception analyser is operable to receive the image and to generate weighting factors. A combiner is operable to combine the weighted code word with the image. The image perception analyser includes a contrast masking engine, a smoothness compensation engine and a compensation combiner. The weighting factor has an effect of reducing the code word coefficients for image parts having lower relative contrast measures. A compensation combiner is operable to combine the compensation factors with the weighting factor. The likelihood of a code word being perceivable when the code word is combined with an image is reduced.