Patent ID: 7679787

Claim:
An image forming method for digitally reproducing a moiré-free color halftone image using an enhanced halftone screen set consisting of a halftone screen for each of N colorants, N>4, the method comprising: using a color halftoning screening apparatus to perform: defining rosette fundamental frequency vectors (V R1 , V R2 ) that satisfy a length and sum requirement to meet visual acceptability standards according to |V R1 |>f min , |V R2 |>f min , and |V R1 ±V R2 |>f min ; defining N halftone screens for colorants i=1, N, respectively possessing first and second frequency vectors (V i1 , V i2 ), where no two screens possess identical fundamental frequency vector pairs; selecting fundamental frequency vectors for the N halftone screens according to: (V i1 , V i2 )=(m i1 V R1 +m i2 V R2 , n i1 V R1 +n i2 V R2 ) for integer m's and n's, where at least one fundamental frequency vector or its conjugate satisfies one of the following: V ik =V R1 , V ik =V R2 , and |V ik |>2max [|V R1 |, |V R2 |]; such that the set of N different halftone screens used in combination produces a moiré-free halftone output; and using said N halftone screens, as further specified by said selected fundamental frequency vectors, in an image forming apparatus for digital reproduction of a moiré-free color halftone image.