A water marked image frame for conveying a payload data word is generated by combining at least one image frame with a two-dimensional water mark pattern. Comprising a plurality of regions, each including a two dimensional water mark block selected from a predetermined set of possible water mark blocks to represent the value of the associated payload data symbol. The payload data is detected by correlating each region of the water marked image frame with each block from of the set of water marked blocks, to form for each region a set of probability surfaces, each of which is combined with a distortion probability value for each region and for each of the water mark blocks which may be present in the region thus forming for each region, for each of the possible values of the payload symbol a probability value that that region includes that symbol value and therefrom generating an estimate of the payload data.