Color image processing apparatus capable of suppressing moire

Claim 1

A color image processing apparatus, comprising: an image data generating circuit for generating multivalued image data for each color in accordance with density of each color of magenta (M), cyan (C) and yellow (Y); an M-screen threshold value storing circuit for storing a plurality of first threshold values for comparison with said multivalued image data of magenta (M); a C-screen threshold value storing circuit for storing a plurality of second threshold values for comparison with said multivalued image date of cyan (C); a Y-screen threshold value storing circuit for storing a plurality of third threshold values for comparison with said multivalued image data of yellow (Y); a comparing circuit for comparing said multivalued image data outputted from said image data generating circuit with said first to third threshold values outputted from said M-, C- and Y- screen storing circuits to output a comparison result in the form of a binary signal; a threshold value generating circuit for generating said first second, and third threshold values; and a control circuit for controlling writing and reading of said first to third threshold values into/from said M-, C- and Y- screen threshold value storing circuits; wherein said M-screen threshold value storing circuit and said C-screen threshold value storing circuit store said first and second threshold values respectively assigned to each pixel of an M multi-cell and a C multi-cell constituted by square matrixes having screen angles of ±tan- 1 (q/p) and each side of β·p·q (β is an integer), respectively, and p and q are any values of (11,3), (15,4) and (19,5), and said M multi-cell and said C multi-cell respectively consist of N (=p 2 , +q 2 ) sub-cells arrayed continuously in directions of said screen angles of ±tan -1 (q/p) and having threshold values in accordance with the predetermined number of gradations, respectively; wherein said Y screen threshold value storing circuit stores said third threshold value assigned to a Y multi-cell constituted by a square matrix having a screen angle of 45 degrees and each side of β·p·q, and said Y multi-cell consists of 2(p-q) 2 sub-cells arrayed continuously in a direction of said screen angle of 45 degrees and having threshold values in accordance with the predetermined number of gradations; wherein said first to third threshold values are repeatedly outputted in a cycle of β·p·q in synchronism with said multivalued image data of each color in accordance with control of said control circuit; and wherein said threshold value generating circuit comprises: coordinate data generating means for generating coordinate data X and Y of coordinates (X, Y) within a square region where a surplus region is added to image regions of said M multi-cell, said C multi-cell and said Y multi-cell: first coordinate transformation means for transforming said coordinate data X and Y into coordinate data x and y of coordinates (x, y) on a virtual orthogonal screen plane in accordance with the affine transformation to output said coordinate data x and y; second coordinate transformation means for transforming said coordinate data x and y into coordinate data x' and y' within a square region of -j≦x≦j and -j≦x≦j (j is an integer) to generate flags which discriminate the regions of said sub-cells of said M multi-cell, said C multi-cell and said BK multi-cell in association with said coordinate data x' and y'; threshold value output means responsive to said coordinate data x' and y' for outputting a threshold value t on the basis of t=k·(|x'|+|y'|)/2 where k is an integer; memory means for storing said threshold value t outputted from said threshold value output means in association with said flag: threshold value replacing means for selecting said threshold value t associated with said flag within said region from said memory means to replace said threshold value t with another threshold value so as to express a smooth halftone; and writing means for allowing said another threshold value replaced by said threshold value replacing means to be stored in each of said M-, C- and Y- screen threshold value storing circuits.