Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
1998-12-09
2001-12-11
Grant, II, Jerome (Department: 2627)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C358S501000
Reexamination Certificate
active
06330078
ABSTRACT:
The present invention relates to the processing of image data in a digital color printing or reproduction system. More specifically, the present invention relates to the calibrating of a signal transformation between respective color spaces by including a feedback step in the calibration algorithm for adjusting colorimetric data signals and to provide improved overall accuracy in the print output.
BACKGROUND OF THE INVENTION
The use of color in the digital environment presents continual problems with regard to accuracy and matching of colors that are intended to appear the same when presented through different devices or on different mediums. Specifically, it is hoped that a color can be perceived as the same even though viewed on a photograph and then scanned into the digital environment, displayed on a CRT monitor or printed on a color printer. Since each of these elements involve a different form of color definition in a distinct color space, the transformation of color through the different color spaces while maintaining a perceptually accurate matching, is a difficult problem to solve. More particularly, how a color appears on a color photograph is typically defined by one form of colorimetric data, i.e., a device independent color space, such as CIE XYZ or CIE Lab (for general background discussion of different color spaces see Billmeyer and Saltzman,
Principles of Color Technology
, 2nd Ed., Wiley and Sons, 1981; Russ,
The Image Processing Handbook
. 2nd Ed., CRC Press, 1995 ). The data signals for colors for a printer are defined in a device dependent color space, such as CMY or CMYK. Most calibration of a transformation between a device dependent color space and a device independent color space occurs through a lookup table (LUT) comprised of a limited number of predetermined matches between a standard target and the device output. For example, a common industry target is a Kodak Q
60
target comprised of 264 different color patches which can be used to calibrate a device such as a printer for those limited 264 colors. For the literally millions of colors that could be printed by a printer in addition to the exemplary 264 patches from the target, some form of interpolation about the predetermined values is employed.
For detailed discussions of color printing and calibrating systems, the following commonly-assigned patents should be referenced:
U.S. Pat. No. 5,787,193
U.S. Pat. No. 5,528,386
U.S. Pat. No. 5,739,927
U.S. Pat. No. 5,483,360
U.S. Pat. No. 5,689,350
U.S. Pat. No. 5,471,324
U.S. Pat. No. 5,649,072
U.S. Pat. No. 5,416,613
U.S. Pat. No. 5,594,557
U.S. Pat. No. 5,307,182
U.S. Pat. No. 5,592,591
U.S. Pat. No. 5,305,119
U.S. Pat. No. 5,581,376
All of which patents are herein incorporated by reference.
There are many ways to implement a calibrated lookup table in a digital signal processing system. The present invention is applicable to any method implemented either by software-based or hardware-based algorithms. A demonstration of a particular application, which is described later for the present invention, is a software implemented algorithm for effecting the lookup table by a three-layer feedforward neural network. Such neural networks are common and well known and can be referenced in Timothy Masters,
Practical Neural Networks in C++
, Academic Press, Inc. 1993.
The transformation process by which a colorimetric data signal is converted from one color space to another is conventionally referred to as a transform. For printer calibration, the transformation from a device dependent color space, i.e., CMYK, to a device independent color space, i.e., CIE Lab, is often referred to as a forward transform [T(CMYK)→Lab], while the transformation from the device independent color space to the device dependent color space is referred to an inverse transform [T
−1
(Lab)→CMYK].
It is a fact of the transforming process that the forward transform, T(CMYK)→Lab, provides a more accurate color match than an inverse form of the transform, T
−1
(Lab)→CMYK.
Keeping in mind that the printer calibration of the transform processing is merely a mathematical modeling, reasons for the disparity in accuracy between forward and inverse transforms can be better appreciated. First, the respective color spaces can be of different dimensions, such as when transforming from a three-dimensional space, Lab, to a four-dimensional space, CMYK. Interpolation relative to the fourth dimension for calibrating a color match presents a particular problem for accuracy. Secondly, there is no problem with the forward transform that the transformed data signal will be out-of-gamut, because the input color space is device dependent and the color gamut is defined by the limit of colors which could be generated by the corresponding printer. On the other hand, the input color space of an inverse transform is device independent and the output colors could be outside the gamut defined by the calibrated printer. Another reason for higher accuracy for the forward transform is that the mathematical modeling is evaluated in the Lab space, which is defined as a uniform color space in terms of human visual perception, and usually yields better optimization results.
The present invention takes advantage of the disparity in accuracy between the forward and inverse forms of the transform to calibrate or adjust an input color data signal during the transformation process itself to compensate for the detected inaccuracy of the inverse transform and thereby provide improved results.
BRIEF SUMMARY OF THE INVENTION
One aspect of the present invention is a method and apparatus for improved calibrating of a digital color printer. In converting a selected color from one color space, such as the CIE Lab space, to a different space, such as a printer-dependent CMYK color space, a mathematical model functions as linear or non-linear interpolation about a lookup table for the data signal transformation process. A selected color to be printed is first transformed to the printer color space and then transformed back to the original input color space to determine if there is a difference between the original color and the then transformed color in the same color space. Such a difference will almost always be detected. The difference is then used to adjust the original color data signal to a modified color signal which when it is transformed to the printer color space provides a printer color which is more accurate relative to the result without adjustment.
In accordance with another aspect of the present invention, the transforming includes a forward and inverse transform comprising mathematical models. The difference is identified between the original data item and the transformed data item, which difference can be applied as a feedback signal for adjusting the original item of color data to a data item which is transformed to a printer color that more accurately matches the original color data.
In accordance with another aspect of the present invention, the original data item comprises a device independent data signal such as Lab and the printer color signal is a device dependent color space such as CMYK space.
One benefit obtained by the present invention is improved accuracy in calibration of digital color printers.
Another benefit is a calibrating technique which relies solely upon feedback generating in the transforming of a single data item to adjust the data item to compensate for errors in the transformation process.
Another advantage of the subject invention is a calibration technique which effects a mathematical model that is easily implementable in different software or hardware calibration techniques.
Further objects and advantages of the present invention will become apparent from the following descriptions of the various embodiments and characteristics of the present invention.
REFERENCES:
patent: 3590142 (1971-06-01), Keller
patent: 4679072 (1987-07-01), Takayama
patent: 5241373 (1993-08-01), Kanamori
patent: 5305119 (1994-04-01), Rolleston et al.
pate
Fay Sharpe Fagan Minnich & McKee LLP
Grant II Jerome
Xerox Corporation
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