1. A method of transforming a first image, which is based on an array of pixels and in which each pixel takes one of two tone values, into a second image which is based on the array of pixels and in which each pixel takes one of two tone values, the second image including additional data, the method including: determining a plurality of data storage pixels in said array using a random number generator based on a seed; for each of said data storage pixels, setting a tone value of the second image according to a respective binary digit of the additional data; setting the tone values of second pixels of the second image, which are not said data storage pixels, to the respective tone values of the first image; and a compensation step wherein, for data storage pixels for which said tone value of the first image is a first tone value which is different from said set tone value of the second image, if a predefined neighbourhood of the data storage pixel contains a second pixel for which the first image has a tone value different from said first tone value, setting the tone value of said second pixel in the second image to said first tone value.