Encoding method for generating a data-bearing halftone image, and decoding method for decoding the data-bearing image

A method for generating a data-bearing halftone image from a grayscale image, said method comprising the steps of:

• converting a grayscale image into a halftone image by screening the grayscale image using a predetermined clustered-dot threshold matrix that includes a plurality of grayscale threshold values, the halftone image having a plurality of image cells each corresponding to the predetermined clustered-dot threshold matrix in size;
• selecting, from the halftone image, at least one of the image cells to be a carrier cell according to a set of reference dot patterns that corresponds with the predetermined clustered-dot threshold matrix, the carrier cell having a dot pattern identical to one of the reference dot patterns;
• and
• generating a data-bearing halftone image according to a plurality of sets of encoding dot patterns, each of the sets being associated with a code;
• wherein, in the step of generating a data-bearing halftone image, the dot pattern of the carrier cell is replaced by a specified one of the encoding dot patterns of one of the sets of the encoding dot patterns, and the data-bearing halftone image thus generated is encoded with a code associated with said one of the sets of the encoding dot patterns;
• wherein each of the dot pattern of the carrier cell, the reference dot patterns and the encoding dot patterns consists of a plurality of dots, at least one of which is a first-tone dot and each of the rest of which is a second-tone dot;
• wherein the specified one of the encoding dot patterns and the dot pattern of the carrier cell have the same number of the first-tone dot(s).