A
data processing method for detecting a
watermark codeword in a received
image signal representing a watermarked image is described. The watermarked image comprises a plurality of
signal samples each of which represents a pixel value of the image, to which a
watermark codeword coefficient value has been added. The method comprises the steps of
low pass filtering the received
image signal to generate, for each
signal sample, a local mean value, subtracting the local mean from the received
image signal to generate a residual
signal comprising, for each signal sample, a
residual value, and determining, from the residual signal, a local standard deviation for each signal sample. The method also comprises the step of generating, for each signal sample, a
watermark strength value providing an estimate of the magnitude with which the watermark codeword coefficient has been added to the received image signal sample. The method also comprises the steps of generating, for each signal sample, a first
gaussian likelihood function describing the likelihood of the watermark codeword coefficient embedded into the signal sample being positive, the first
gaussian likelihood function having a mean defined by the sum of the local mean value and the watermark strength value and a standard deviation defined by the local standard deviation and generating, for each signal sample, a second
gaussian likelihood function describing the likelihood of the watermark codeword coefficient embedded into the signal sample being negative, the second gaussian likelihood function having a mean defined by the difference between the local mean value and the watermark strength value and a standard deviation defined by the local standard deviation. When the above likelihood functions have been generated, the method can detect a watermark in the received watermark
image based on the probability of the watermark codeword coefficients added to each signal sample being one of positive or negative, the probability being calculated from the first and second likelihood functions. This provides an estimate of the positioning of the watermark within the received watermark image.