Screening techniques by identification of constant grey components

• US 4,730,221 A
• Filed: 10/16/1986
• Issued: 03/08/1988
• Est. Priority Date: 10/16/1986
• Status: Expired due to Term

• Current Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Original Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Inventors: Roetling, Paul G.
• Primary Examiner(s): Goldberg, E. A.
• Assistant Examiner(s): SHAW, CLIFFORD C
• Application Number:
  US06/919,521
• Time in Patent Office: 509 Days
• Field of Search: 358/296, 358/298, 358/280, 358/285, 358/300, 358/302, 358/283
• US Class Current: 358/3.01
• CPC Class Codes: H04N 1/4055 producing a clustered dots or a size modulated halftone pattern

Abstract

Identification of constant grey components in electronic screening by first determining from the continuous-tone input, the maximum value of grey, the minimum value of grey and the average value. The average value is used to determine how many white bits are to be turned on in the halftone dot generated. Since "Moire" problems only occur only from the fluctuating part of the input, the three grey ranges corresponding to the minimum, the maximum and the fluctuating portions of the grey found in the area are then treated separately. The minimum and maximum grey levels identify constant levels within the grey area. That is, every point is at least as light as the minimum and every point is no lighter than the maximum. A template black dot corresponding to the amount of black in the maximum light grey area is generated and correspondingly a template white dot corresponding in size to that for the minimum grey level is generated, and compared to those actual dots created by ordinary halftoning. The ordinary halftone process produces black in at least the area of the template black dot and white in at least the area of the template white dot. The fluctuating grey levels between these two control the remaining dot area as to which is black and which is white.

Claims

1. A method for creating on a light sensitive recording medium a halftone reproduction of an original image comprising the steps of:

(a) providing an electronic signal representing the average grey scale of said original image over a halftone dot period;

(b) providing a plurality of electronic signals representing details of said original image over said halftone dot period;

(c) providing a halftone screen function in electronic form;

(d) combining said halftone screen signals of (c) with said original image signals of (b) to provide a plurality of sum functions;

(e) finding the;

maximum and minimum grey scale values over said halftone dot period;

(f) generating black or white template dots correspond to the maximum and minimum grey scale values;

(g) placing any generated white dots in the halftone dot period;

(h) decreasing from the allowable areas where white dots can be placed; and

(i) locating the rest of the dots in the remainder of the halftone dot period space.

2. A method for creating a halftone reproduction of an original image by identification of constant grey components comprising the steps of:

providing electronic signals representing the grey scale values of said original image over a halftone dot period;

finding the maximum and minimum grey scale values over said halftone dot period;

generating black or white or both template dots corresponding to the minimum and maximum grey scale values;

placing any generated white dots in the halftone dot period and subtracting from the total number of half tone dots that need to be generated in the halftone period as computed from the average grey scale value;

eliminating from the allowable areas in which white dots can be placed; and

placing the rest of the dots in the remainder of the halftone dot period space.

3. A method for creating a halftone reproduction of a portion of an original image by identification of constant grey components comprising:

scanning a portion of the image to determine the maximum and minimum grey scale levels within said portion,

generating contrast dots corresponding to at least one of the maximum and minimum grey scale values;

locating the constant dots in the portion of the image; and

placing the rest of the dots in the remaining part of said image portion.