WO2001030065A1

WO2001030065A1 - Halftoning method and system for improved color printing

Info

Publication number: WO2001030065A1
Application number: PCT/US2000/028583
Authority: WO
Inventors: Harry Bowers
Original assignee: CV US Inc
Current assignee: CV US Inc
Priority date: 1999-10-15
Filing date: 2000-10-16
Publication date: 2001-04-26
Anticipated expiration: 2002-04-15
Also published as: AU8025200A; AU8025300A; WO2001029517A1

Abstract

A halftoning method and system is provided that reduces visible artifacts for a printing system (10) having a limited number of available color inks. For each color value to be printed, the color is formed by a distribution of color dots from nearest neighbor color values (20). When printing a color that is one of the available color inks, the color is also formed by a distribution of color dots from a nearest neighbor color values. In this manner, the transition from an area printed with an available color ink and a similar color ink is visually smoother and with less visual artifacts since both colors are printed with a similar distribution of nearest color values.

Description

HALFTONING METHOD AND SYSTEM FOR IMPROVED COLOR PRINTING

Cross-Reference To Related Applications This application claims the benefit of U.S. Provisional Application No.

60/159,606 filed October 15, 1999.

Field Of The Invention

The present invention relates to the color printing arts. It finds particular application to a halftoning method and system for improving color reproduction by smoothing the transition between printed colors. It will be appreciated that the present invention will find application to any type of printing system that has a limited number of inks.

Background Of The Invention

If a printing system can print more than four (4) inks, the visual result is greatly improved. However, the half-toning method becomes more complex and more difficult to implement. An error diffusion half-toning method is often chosen when there are more than four printing inks available. A problem with error diffusion printing with multicolors is the half-toning transitions between the colors of the ink. These transitions are not smooth and produce undesirable artifacts.

For example, to print a 24-bit color Co, classical error diffusion looks at the color C₀. It is unlikely that any of the available printing colors is actually Co- Error diffusion chooses the nearest ink color (C_n) to Co (the real color) and prints a dot with that color. The error between the color printed and the desired color is then subtracted from the 4 adjacent pixels C_ls C₂, C₃, C₄, this compensates for the error introduced when printing with C_n. For example:

Let Ci , C₂ , C₃ , C₄ be new adjusted colors. Then:

where.

The above method creates undesirable half-toning artifacts for color between the pure ink colors. Suppose, a printing system can print with white ink, 50% gray ink and black ink. If a 50% gray area is to be printed using classical error diffusion, the result is a solid 50% gray ink. There is no error because the color requested is exactly one of the available inks. If we wanted to print 49% gray, the result would be 98% of 50% gray ink and 2% black ink dots. The 2% of black dots on a background of 50% gray are easily apparent to the naked eye and appear as an artifact. The classic method displays an abrupt visual change from 50% gray to 49% gray because the 2% black dots on the solid 50% gray ink are visually apparent.

The present invention provides a new and unique halftoning method that cures the above problems and others.

Summary Of The Invention

In accordance with the present invention, a new and unique method of printing with a printing device that includes a set of available color inks is provided. In response to an instruction to print a first color where the first color is not one of the available color inks, a combination of nearest color values is outputted in a halftone pattern that produces the first color. In response to an instruction to print the first color where the first color is one of the available color inks, the one available color ink is outputted with a combination of nearest color values in a halftone pattern that produces the first color. In accordance with another aspect of the present invention, a printing device is provide. The printing device includes a set of available color inks. A halftoning pattern combines the available color inks to produce a gamut of color values. The halftoning pattern includes a pattern of color dots for producing each of the available color inks by combining a plurality of different color dots formed by a combination the available color ink and color values nearest to the available color ink.

One advantage of the present invention is that the transition between printed colors appear smoother and artifacts are less visible.

Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.

Brief Description Of The Drawings

The following is a brief description of each drawing used to describe the present invention, and thus, are being presented for illustrative purposes only and should not be limitative of the scope of the present invention, wherein:

Figure 1 is a diagram of a printing system and process in accordance with the present invention; and

Figure 2 illustrates how available ink colors are printed in accordance with the present invention.

Detailed Description Of The Preferred Embodiments

With reference to Figure 1, an overall color printing process is shown in accordance with the present invention. In a printing device 10, there are one or more ink colors typically contained in ink cartridges or the like. The available ink colors are limited in number, for example, ink color 1, ink color 2...ink color N. The available inks are also referred to as pure ink colors herein. Typical printers have red, blue, green and black inks, or cyan, magenta, yellow and black inks. Of course, any combination of ink colors can be present. A gamut of colors are produced by combining the available color inks in a predetermined ratio to obtain each individual color in the gamut. The ratios are defined by a halftoning algorithm 15 where each pixel of color is produced by a halftone dot pattern as is known in the art.

To print a color X, the halftone algorithm selects a halftone pattern nearest to the value of color X and the appropriate ink colors are combined according to the halftone pattern to form the color X. The halftone pattern represents a combination 20 of color X and/or nearest neighbor colors. The number of nearest neighbor colors can be at least one and as many as desired based on desired results. If color X is the same color as one of the available ink colors 1, 2,...N, however, the available ink color is not printed alone. Rather, as shown in Figure 2, the available ink color is printed by combining it with colors of nearest neighbor values just as a non-available ink color is printed. Thus, even though the pure ink is available, it is produced by a plurality of different color dots. The halftone pattern for the available ink color includes a distribution of different color dots, some slightly darker than the available color and some slightly lighter than available color. When the resulting dot is viewed by the human eye, it will look like the pure color. By not printing pure ink colors alone, visual changes between adjacent colors will be reduced, thus, visual artifacts will be reduced. In other words, when printing a pure ink color value and a pure ink color value + 1 side-by-side, the transition between them is smoother because both are printed with similar or equivalent dot distributions of neighboring color values.

By way of another example, assume the printing device includes ink colors black, white, and 50% gray. The printing device receives instructions to print 50% gray. The present method does not allow a solid color area to be printed with pure ink color 50% gray. Rather, to print the gray, a combination of black, white and 50% gray is used. For example, an even distribution of 1/3 black, 1/3 white and 1/3 50% gray. Of course, other distributions can be used. If a 49% gray color is to be printed, the result is a combination of black, white and 50% gray dots, which give the appearance of a 49% gray color. As a result, solid color areas of pure ink are eliminated which reduces artifacts between the transition of pure ink color to non-pure ink color. There is no visible abrupt change from 50% gray to 49% gray because both are made up of a distribution of black, white and gray dots.

With the present invention, halftone patterns for color values that exactly match an available ink color are not solid areas of that ink color. Rather, the available ink colors are formed with a distribution of that color and of nearest color values using error diffusion principles. In this manner, every halftone pattern has a similar or equal number of different color dots.

The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon a reading and understanding of this specification. It is intended to include all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalence thereof.

Claims

I claim:

1. A method of printing with a printing device that includes a set of available color inks, the method comprising the steps of: in response to an instruction to print a first color where the first color is not one of the available color inks, outputting a combination of nearest color values in a halftone pattern that produces the first color; and in response to an instruction to print the first color where the first color is one of the available color inks, outputting the one available color ink with a combination of nearest color values in a halftone pattern that produces the first color.

2. The method of printing as set forth in claim 1 wherein the outputting a combination includes an even distribution of nearest color values.

3. The method of printing as set forth in claim 1 wherein the outputting the one available color ink includes outputting an even distribution of the one available color ink and the nearest color values.

4. The method of printing as set forth in claim 1 wherein the outputting the one available color ink includes outputting a similar distribution of the one available color ink and the nearest color values.

5. A halftoning pattern for a printing device having a limited number of available ink colors comprising: a first halftone pattern for color values that are different from the available color inks being formed of a combination of dots of nearest color values; and a second halftone pattern for color values that match the available color inks being formed of a combination of dots of the matching available color ink and dots of nearest color values.

6. The halftoning pattern as set forth in claim 5 wherein the first halftone pattern and the second halftone pattern include an equal distribution of the nearest color values.

7. A printing device comprising: a set of available color inks; and a halftoning pattern for combining the available color inks to produce a gamut of color values, the halftoning pattern including a pattern of color dots for producing each of the available color inks by combining a plurality of different color dots formed by a combination of the available color ink and color values nearest to the available color ink.

8. The printing device as set forth in claim 7 wherein the plurality of different color dots that form the available color ink mcludes an even distribution of the different color dots.