Patent ID: 7898692

Claim:
An image forming method for digitally reproducing a moiré-free color halftone image using a different halftone screen for each of five colors, the method comprising: using a color halftone screening apparatus to perform: defining a first color halftone screen having a first fundamental frequency vector V a1 =(V a1 , θ a1 ), and second fundamental frequency vector V a2 =(V a2 , θ a2 ); defining a second color halftone screen having a first fundamental frequency vector V b1 =(V b1 , θ b1 ), and second fundamental frequency vector V b2 =(V b2 , θ b2 ); defining a third color halftone screen having a first fundamental frequency vector V c1 =(V c1 , θ c1 ), and second fundamental frequency vector V c2 =(V c2 , θ c2 ), where the vector angles obey the ordering θ ai >θ bi >θ ci ; selecting the values of the fundamental frequency vectors to identify combinations that do not produce objectionable moiré by satisfying the following thresholds: | V a1 −V b1 +V c2 |≦V low or | V a1 −V b1 +V c2 |≧V high and | V a2 −V b2 −V c1 |≦V low or | V a2 −V b2 −V c1 |≦V high , where V low <0.5 cycle/inch and V high >50 cycles/inch; defining a fourth color halftone screen having a first fundamental frequency vector V d1 =(V d1 , θ d1 ), and second fundamental frequency vector V d2 =(V d2 , θ d2 ); defining a fifth color halftone screen having a first fundamental frequency vector V e1 =(V e1 , θ e1 ), and second fundamental frequency vector V e2 =(V e2 , θ e2 ); and, selecting the values of the fundamental frequency vectors for the fourth screen and fifth screen to satisfy V d1 , V d2 [V a1 , V c1 , V b2 ], V d2 ≠V d1 V e1 , V e2 [V b1 , V a2 , V c2 ], V e2 ≠V e1 ; so that the set of five different halftone screens used in combination, in an image forming apparatus, produce moiré-free halftone output.