Information encoding methods, and sets of graphical codes

A method of defining a set of graphical codes comprises the steps of: defining a size and shape of a two-dimensional barcode and of each element making up the bar code; defining a set comprising all possible graphical codes conforming to the definition of the size and shape of the bar code; identifying some of the bar codes as invalid codes, according to a number of predefined rules; and identifying other of the bar codes as valid codes. The predetermined rules comprise at least one of the following rules. A valid bar code may not have a row or column which consists of elements of a same value. A valid code has a predetermined number (or numbers) of elements of a particular type. A valid bar code may not be rotationally symmetrical with any other valid bar code. Each valid bar code must differ from each other valid bar code in at least a predetermined number of elements. The graphical codes may be used to retrieve further information relating to information or advertisements presented in a visible message.