Color image processing apparatus capable of suppressing moire

• US 5,572,600 A
• Filed: 06/22/1994
• Issued: 11/05/1996
• Est. Priority Date: 06/22/1993
• Status: Expired due to Term

• Current Assignee: NEC Corporation
• Original Assignee: NEC Corporation
• Inventors: Ito, Miki, Kaji, Mitsuo, Tajima, Joji, Ito, Akira, Sato, Youichi
• Primary Examiner(s): Boudreau, Leo
• Assistant Examiner(s): KELLEY, CHRISTOPHER S
• Application Number:
  US08/263,687
• Time in Patent Office: 867 Days
• Field of Search: 382/162, 382/163, 382/275, 358/454, 358/533
• US Class Current: 382/163
• CPC Class Codes: H04N 1/52 Circuits or arrangements for halftone screening

Abstract

An image data generating circuit of a color image processing apparatus generates 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 generating circuit outputs a plurality of first threshold values, a C-screen threshold value generating circuit outputs a plurality of second threshold values, and a Y-screen threshold value generating circuit for outputting a plurality of third threshold values. A comparing circuit compares the multivalued image data outputted from the image data generating circuit with the first to third threshold values outputted from the M-, C- and Y- screen generating circuits for each color to output a comparison result in the form of a binary signal. The first and second threshold values is 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, respectively. For preventing coarse moire, p and q are any value of (11,3), (15,4) and (19,5), and the M multi-cell and the C multi-cell consist of N (=p² +q²) sub-cells arrayed continuously in the directions of the screen angles of ±tan^-1 (q/p) and having threshold values in accordance with the predetermined number of gradations. The third threshold value is 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. The Y multi-cell consists of 2(p-q)² sub-cells arrayed continuously in the direction of the screen angle of 45 degrees. The first to fourth threshold values are repeatedly outputted in a cycle of β·p·q in synchronism with the multivalued image data of each color.

Claims

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 (=p², +q²) 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.

2. A color image processing apparatus as claimed in claim 1, wherein said second coordinate transformation means comprises:

first adding/subtracting means for adding 2j to said coordinate data x outputted from said first coordinate transformation means until x becomes -j or more while x is smaller than -j, and for subtracting 2j from x until x becomes j or less while x is j or more;

first counting means for decreasing count according to adding operation and increasing count according to subtracting operation of said first adding/subtracting means;

second adding/subtracting means for adding 2j to said coordinate data y outputted from said first coordinate transformation means until y becomes -j or more while y is smaller than -j and for subtracting 2j from y until y becomes j or less while y is j or more; and

second counting means for decreasing count according to adding operation and increaing count according to subtracting operation of said second adding/subtracting means.