Combined color halftoning

Details Combined color halftoning Combined color halftoning

1999-04-28

2002-03-26

Grant, II, Jerome (Department: 2624)

Image analysis

Color image processing

Color correction

C358S534000

Reexamination Certificate

active

06363172

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a method and apparatus for reproducing full-color images on output devices through electronic image processing and, more particularly, to a method and apparatus for improving the quality of the output of full-color images reproduced on output devices such as color printers and displays, for example, that reproduce colors using a comparatively small number of intensity or density levels of primary colorants by using halftoning techniques to combine the number of dots of each primary colorant so that an output display level, or dot level, is formed having the different colorant dots substantially homogeneously distributed.
BACKGROUND OF THE INVENTION
In full-color images, the color value of each pixel may be specified by 24 bits which can uniquely specify each of over 16 million different colors. These images are typically displayed on output devices such as a color printer, for example, using primary colorants having a comparatively small number of intensity or density values to produce color.
For example, an ink jet color printer commonly combines cyan, magenta, and yellow inks, and optionally black ink, in varying proportions to produce the illusion of many of these 16 million colors when viewed at a normal viewing distance. The proportions of ink are varied by placing patterns of discrete amounts of each colorant over selected areas of the printed page.
As is common for a binary ink jet or laser printer, a dot of a specific colorant can either be placed or not placed at a given pixel location in a rectangular array. This produces a pattern that may be highly visible to the human eye.
Various halftoning algorithms have previously been used to produce patterns that are pleasing to the human eye. These algorithms have been traditionally applied independently for each colorant. While this results in dot patterns for each colorant being visually pleasing, the overall pattern of the dots is not normally pleasing when dots from all of the colorants are placed on the page. This is because the distributions of dots of the two or more colorants are not selected together to be visually pleasing.
The halftoning techniques of Blue Noise Mask and Void and Cluster Mask are known as point algorithms because the decision to place or not to place dots at a specific location (x, y) on an image plane depends only upon the color values at that location. With a grayscale image system using an 8-bit value to specify the grayscale image information and printing this image on a printer with a binary black printhead, the minimum grayscale value of 0 at a specific pixel location means that a dot should not be placed at the specific pixel location, and the maximum grayscale value of 255 at a pixel location means that a dot should be placed at the specific pixel location.
Each of the Blue Noise Mask and the Void and Cluster Mask consists of a large two-dimensional table of numbers, typically 128×128 or 256×256 square pixels, which are used to halftone full color images. The mask is tiled across a document so that every pixel location has associated with it a threshold value T(x, y) in the range of 0 to 255 from a mask or a matrix.
To decide whether or not to place a dot at a pixel location (x, y), the grayscale value of the image at that location, I(x, y), is compared against the threshold value T(x, y). If I(x, y)>T(x, y), a dot is placed at that location (x, y);
otherwise, a dot is not placed at that location. The values of T(x, y) are chosen so that for any grayscale value I between 0 and 255, a pleasing pattern of dots results over a wide area provided that the masks are properly constructed.
The construction of a Blue Noise mask is discussed in “A Modified Approach to the Construction of a Blue Noise Mask,” Dr. Kevin J. Parker,
Journal of Electronic Imaging
, January 1994. The construction of a Void and Cluster Mask is discussed in “Void and Cluster Halftoning Technique,” Robert Ulichney,
Proceedings of the SPIE
, Febuary 1993.
These masks have been used in several different ways to halftone color images. To correlate two colorants q and r, for example, at a pixel location (x, y), the image values I
q
(x, y) and I
r
(x, y) are compared against the same threshold value T(x, y). To decorrelate the two colorants q and r, one image value I
q
(x, y) is compared against the threshold value T(x, y) in the same manner as when correlating the two colorants, but the second colorant image value is compared against a different or inverted threshold value T(x+a, y+b); this implies that the original mask used to halftone the colorant q is shifted a pixels in the x direction and b pixels in the y direction to halftone the colorant r. To anticorrelate the two colorants q and r, one colorant image value I
q
(x, y) is compared against the threshold value T(x, y) in the same manner as when correlating the two colorants, but the second colorant image value I
r
(x, y) is compared against a different threshold value 255−T(x, y).
In general, when halftoned by each of these three techniques, the pattern of dots for each individual colorant is visually pleasing. However, the pattern of dots formed by combining the dots of each of the color planes is not necessarily visually pleasing because no effort is made to insure that the dots of each of the different color planes are distributed relative to the dots of the other color planes. Examples of producing a color composed of two colorants with each of these three methods follow.
Table 1 is assumed to represent an 8×8 square pixel mask for either a Blue Noise mask or a Void and Cluster mask.
TABLE 1
0
168
48
220
72
244
100
248
84
148
116
20
228
12
152
60
196
32
184
128
68
188
124
224
56
216
112
44
200
96
36
172
164
8
136
212
28
232
204
104
108
180
80
160
88
132
4
236
144
24
240
52
192
64
156
76
252
92
140
120
16
208
40
176
A grayscale, binary, printing process forms a black and image by either placing a dot or not placing a dot of black ink at each printable pixel location. An input value of I=0 at a location (x, y) represents the lightest printable color, which is produced by printing no dot at the location (x, y). An input value of I=255 at the pixel location (x, y) represents the darkest printable color black, which is produced by printing a dot at location (x, y).
Shades of gray other than white or black cannot be produced at the pixel location by this printing process since at each location a dot is either printed or not printed. Therefore, the shades of gray must be simulated by printing a pattern of dots over a wider area than just one pixel.
Accordingly, an input shade of gray having a value of I is produced over a selected area by printing a dot at each location of the selected area with a probability of I/255. On average, I dots out of every 255 locations are printed. If the selected area is too small, it may be impossible to place exactly on average I dots out of 255 locations over this area. Therefore, the shade of gray is not accurately reproduced but only approximated. This accounts for much of the loss of detail of a full-color image when printed on binary devices.
It should be noted that the threshold values in Table 1 are uniformly distributed with the threshold values spaced every four units from 0 to 252. The threshold values of Table 1 can be used to govern the probability of a dot being printed.
For example, if a gray level value of I=33 is to be produced over an entire 8×8 square pixel area with the threshold values given by Table 1, nine dots will be printed at the positions having threshold values of 0, 4, 8, 12, 16, 20, 24, 28, and 32. Therefore, the gray level value of I=33 is approximated by 9 dots out of 64 where {fraction (9/64)} locations is approximately equal to {fraction (33/255)}. The nine dots are placed at the locations marked by X in Table 2. It is assumed that Table 2 was constructed so that this pattern of nine dots is a desirable arrangement of nine dots for a gray level value of I=33.
TABLE 2
X
X
X
X
 

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