31. The apparatus according to claim 17 , wherein the processing unit is configured to include in the free cells one or more of the following: padding cells, border cells and metadata cells.
17. An apparatus for generating a readable matrix code image encoding a message based on an input image and a readable matrix coding specification: a memory unit capable of storing the input image and the readable matrix coding specification; a processing unit capable of: applying a function patterns specification to function areas of the readable matrix code image; applying a code word specification to code word areas of the readable matrix code image, the code word areas including free areas and derived areas in an extent which is in compliance with the code word specification; wherein the free areas include free cells and derived cells, and the processing unit is configured to apply a visual perceptual similarity criterion when processing the free cells, such that the appearance of the free cells, when compared to respective areas of the input image, comply with a visual perceptual similarity criterion, and wherein the processing unit is configured to process the derived cells, such that the derived cells form, together with the free cells, providing a valid code word.