Patent ID: 7639391

Claim:
An image forming apparatus for digitally reproducing a color image using a screen set consisting of a halftone screen for each color, the screen set comprising: a first-color halftone screen having an associated first halftone pitch; a second-color halftone screen having an associated second halftone pitch measured in a corresponding direction to the first halftone pitch, a third-color halftone screen; and a fourth-color halftone screen, wherein two first screen vectors, one in each of the first and second-color halftone screens, each first screen vector being in a spatial frequency domain defined by a basis vector of a halftone dot pattern of the respective halftone screen, are parallel to each other, two second screen vectors, one in each of the first and second-color halftone screens, each second screen vector being in a spatial frequency domain defined by a basis vector of a halftone dot pattern of the respective halftone screen, are not parallel to each other, two second screen vectors, one in each of the third and fourth-color halftone screens, each second screen vector being in a spatial frequency domain defined by a basis vector of a halftone dot pattern of the respective halftone screen, are parallel to each other, and two first screen vectors, one in each of the third and fourth-color halftone screens, each first screen vector being in a spatial frequency domain defined by a basis vector of a halftone dot pattern of the respective halftone screen, are not parallel to each other.