Method and apparatus for processing a stream of image pixel data to determine the presence of high frequency half-tone data. The arrangement applies the function ##EQU1## to a stream of image pixel data. The presence of minimums of .phi.(k) at times indicative of predetermined half-tone image frequencies...http://www.google.co.uk/patents/US4811115?utm_source=gb-gplus-sharePatent US4811115 - Image processing apparatus using approximate auto correlation function to detect the frequency of half-tone image data
