Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
1999-09-22
2003-03-04
Nguyen, Madeleine (Department: 2622)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C358S523000
Reexamination Certificate
active
06529291
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the digital color image processing arts. It finds particular application in conjunction with rendering of an image including use of a variable under-color correction system, and will be described with particular reference thereto. However, it is to be appreciated that the invention is applicable to other image rendering applications.
Color in printed digital images results from the combination of a limited set of colors over a small area in densities selected to integrate the desired color response. This is accomplished in many printing devices by reproducing so called “separations” of the image, where each separation provides varying gray values of a single primary color. When the separations are combined together, the result is a full color image.
The particular color of each separation depends on the “color-space” being implemented. Examples of color space models include, RGB, CMY, CMYK, Lab, Yes, YIQ, HSV, HLS.
In practice, color images are commonly printed in a cyan-magenta-yellow-black (CMYK) color-space. This color-space is based upon the CMY color-space, but attempts to improve the quality of “black” in the image and reduce use of color inks. In theory, images can be printed using the CMY color space, with a mixture of the three colors producing black. In practice, however, printing with only cyan, magenta, and yellow inks often does not produce the highest quality black, but instead results in a muddy brownish output due to impurities in the inks, the particular paper or other image recording media used, and the partial reflection of light instead of its complete absorption into the inks. Furthermore, select use of black ink in place of the primary colors reduces expense and minimizes the total amount of ink used which is often desirable in ink-jet and other printing applications where the ability of the recording substrate to absorb ink is limited.
Methods for converting to the CMYK color space include those referred to as “under-color removal” (UCR) and “gray-component replacement” (GCR). UCR/GCR methods vary, but commonly involve examining the individual pixels of an image using the lowest or “darkest” of the three cyan-magenta-yellow colors to determine an amount of black to be added (Under-color Removal). One or more of the CMY colors are then adjusted to account for the addition of black ink (Grey Component Replacement). For example, if a given pixel of an image is represented in the CMY color space by C=0.5, M=0.4, and Y=0.25, then the black or K value would be based upon the lowest or Y value. In a 50% under-color removal (UCR) method, K=50% of Y=0.125. In a typical gray component replacement (GCR) step, the remaining CMY values would then each be reduced by 0.125 so that the resulting UCR/GCR pixel is represented by C=0.375, M=0.275, Y=0.125, and K=0.125. Of course, other UCR/GCR methods are known, but each seeks to determine the level of black for a given pixel, and to thereafter adjust the other colors accordingly to account for the addition of black ink.
In the digital processing of color images, the individual color separations are conveniently represented as monochromatic bitmaps, which may be described as an electronic image with a plurality of discrete elements (i.e. “pixels”) defined by position and gray value. In such a system, gray value is described as one level in a number of possible states or levels. When more than two different levels are used in the description of an image, the levels are termed “gray” (without regard to the actual color) to indicate that the pixel value is between some maximum and minimum gray level. Most printing systems have the ability to reproduce an image with only a small number of gray values per pixel, most commonly two, although other numbers are possible. A printing system that is able to reproduce only two gray values for each pixel is said to produce binary output, i.e., the pixel is either “on” or “off.”
On the other hand, image input devices, including digital cameras, scanners, and the like, are capable of describing each pixel of an image with many gray levels, for example 256 gray levels. Such input data is commonly called “continuous” or “contone” data. Accordingly, it is necessary that the input contone image (with many “gray” levels) be describable with the smaller set of gray levels reproducible by the output device in a manner that captures the intent of the user. In the digital reproduction of color images, this means that each of the color separations of the color-space must be reduced from the large number of continuous gray levels as input, to the smaller number of levels suitable for output. The multiple color separations are then combined together for printing to yield the final color print.
Given that common image output devices are “binary”—i.e., produce either “on” or “off” pixels for each color separation, it is necessary to employ half-toning techniques for each color separation to achieve the desired color within each separation before the color separations are combined for printing. Through half-toning, gray value variation within a color separation is represented by controlling the number of pixels that are “on” within a discrete area or cell of the separation. In such cases, the human eye and brain interpret the controlled number of “on” pixels in a halftone cell as a “gray level,” with greater numbers of “on” pixels in a given cell or area being interpreted as more color. In theory, a human observer does not see the individual “on” and “off” pixels within a halftone cell, but instead sees an average amount of ink on paper. In practice, the effectiveness of half-toning methods varies.
Existing binary imaging systems do not allow for precise color classification and therefore a pixel will be either “color” or “neutral.” In turn, transition from “color” to “neutral” in rendering of an image results in an and/or switching environment.
The above concept is illustrated in
FIG. 1
which is intended to represent a full color sweep strip
10
from a saturated color (e.g. a chromaticity of 1.0) to a full neutral (e.g. a chromaticity of 0.0). It is appreciated that no actual color sweep is shown in the figure. Rather, the values 1.0 to 0.0 represent the amount of saturation which would exist at a particular color, as a color transitions from full saturation to full neutral. When in a color area
12
(between values 1.0 to 0.5), the CMYK contributions determine the output and, when in a black area
14
(less than 0.5) the output is strictly K or black. The transition from CMYK contribution to strictly K occurs at a specific switch-point
16
, which results in a dramatic transition from the use of a four color mixture to black toner only. This transition point can be seen by the human eye and therefore results in an undesirable discontinuity of the color sweep.
In image rendering systems, which provide for black replacement, as shown in
FIG. 2
, there are two paths for converting the input, such as Lab to CMYK. It is to be noted that while the present discussion will focus on Lab and CMYK, the concepts may be extended to other color space models.
System
18
uses a black detection controller
20
to determine if input pixel information
22
is to be classified as neutral or color. For color pixels, an output from a sophisticated and expensive, non-linear conversion table
24
, which has multiple inputs and outputs, is used for conversion to CMYK. When pixels are determined to be neutral, an output of a single TRC look-up table
26
is used which converts the L-channel of an input pixel information
22
to the K (black) channel, with CMY set to zero. The selection of which output to use in image formation is accomplished by switching unit
28
, which is controlled by controller
20
. Thus, system
18
is a switching type system wherein upon a determination of pixel information
22
as being one of color or neutral, a selection of the output from non-linear conversion table
24
Fan Zhigang
Schweid Stuart A.
Shiau Jeng-nan
Fay Sharpe Fagan Minnich & McKee LLP
Nguyen Madeleine
Xerox Corporation
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