Image processing apparatus using approximate auto correlation function to detect the frequency of half-tone image data

摘要

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 <IMAGE> to a stream of image pixel data. The presence of minimums of phi (k) at times indicative of predetermined half-tone image frequencies is detected and an output signal indicative of the presence or absence of minimums at the predetermined half-tone image frequencies is provided. More particularly, the two's complement of the stream of image pixel data is added to each of a plurality of differently delayed signals derived from the original stream of image pixel data. The absolute value is taken of the resulting function. The signal is summed for a selected number of pixels, corrected for noise, and reviewed for minimums. Minimums occurring at selected positions in the pixel stream indicate certain frequencies of the data. An output is created based on frequency indications.