An encoding
data processing apparatus is operable to introduce a visible
watermark into a compression encoded
video image represented by a compression encoded bit
stream having a predetermined format. The format of the compression encoded bit
stream provides image frames, which are represented by macroblocks, each
macroblock representing a block of pixels from the
image frame. The macroblocks are formed into groups of macroblocks which are contained in fixed portions of the compression encoded bit
stream, the fixed data portions being able to accommodate a predetermined fixed amount of data. The encoding
data processing apparatus being operable to parse the compression encoded bit stream to identify spare
data capacity in each frame provided from each of the fixed data portions as a result of the fixed data portion not being completely filled with the macroblocks. The encoding
data processing apparatus is operable to determine whether an additional amount of data required to represent the visible
watermark can be accommodated in each
image frame. If the additional data can be accommodated, the encoding data
processing apparatus is operable to change selected macroblocks in the
image frame to represent the visible
watermark, to generate a bit pattern representing the macroblocks in the original image frame which have changed, and to insert the additional data into the fixed data portions of the image frame. A visibly watermarked bit stream can therefore be produced in which the watermark changes on a frame by frame basis, if the frames can accommodate the data required to represent the watermark. In one example the visible watermark represents a time code.