Pixels are classified as belonging to or not belonging to a circular pattern. The method calculates the mean intensity QO of a block of pixels centred on pixel PO, and the mean intensites Qi of peripheral pixel blocks surrounding the central region around PO; the differences Δi between QO and Qi are calculated, and the ratio of a value characteristic of the differences Δi and the dispersion in the differences Δi is thresholded to give determine whether or not pixel PO falls within a circular pattern.