Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
2002-03-28
2004-08-10
Garcia, Gabriel (Department: 2624)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C358S003240
Reexamination Certificate
active
06775030
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to the field of electronic reproduction technology and pertains to a method of adapting color values that have been produced for a first printing process to a second printing process so that the visual impression of the colors in both printing processes is the same.
In reproduction technology, printing originals for printed pages are produced that contain all the elements to be printed, such as texts, graphics and images. In the case of the electronic production of printing originals, these elements are present in the form of digital data. For an image, the data are produced, for example, by the image being scanned point-by-point and line-by-line in a scanner, each image point being broken down into color components and the color components being digitized. Images are usually broken down in a scanner into the color components red, green, and blue [R,G,B], that is to say, into the components of a three-dimensional color space. For the colored print, however, other color components are needed. In the case of four-color printing, these are the printing colors cyan, magenta, yellow, and black [C,M,Y,K], that is to say, the components of a four-dimensional color space. For such a purpose, the image data from the RGB color space of the scanner must be transformed into the CMYK color space of the printing process to be used.
Such color space transformations are needed in reproduction technology because all the devices and processes have their restrictions and special features in the representation and reproduction of the colors, and all the devices and processes have different such characteristics. For this reason, for various devices and processes such as scanners, monitors, proof output devices, printing processes, and so on, there are different color spaces that respectively describe the color characteristics of the device or process in an optimum way and that are referred to as device dependent color spaces.
In addition to the device dependent color spaces there are also device independent color spaces, which are based on the human visual characteristics of a standard observer, as referred to in the prior art. Such color spaces are, for example, the XYZ color space defined by the Commission Internationale d'Éclairage (CIE) standardization commission or the LAB color space that is derived therefrom, the LAB color space having made more progress in the technology. If one wishes to know whether two colors will be sensed by the human eye as the same or different, then the measurement of the XYZ or LAB color components is sufficient for such a purpose. The LAB color components form a color space with a lightness axis [L] and two color axes [A,B], which can be imagined in the plane of a color circle through whose center the lightness axis runs. The LAB color components are related to the XYZ color components through nonlinear conversion equations.
A device or process can be characterized in terms of its color characteristics by all the possible value combinations of the associated device dependent color space being assigned the LAB color components that a human sees in the case of the colors produced with these value combinations. For a printing process, the various CMYK value combinations respectively produce a different printed color. Using a color measurement instrument, the LAB components of the printed colors may be determined and assigned to the CMYK value combinations. Such an assignment, which sets the device dependent colors produced with a device or process in a relationship with a device independent color space (XYZ or LAB), is also referred to as a color profile, as an output color profile in the case of a printing process. The definition and data formats for color profiles have been standardized by the International Color Consortium (ICC) Specification ICC.1:1998-09. In an ICC color profile, the association between the color spaces in both directions is stored, for example, the association LAB=f1 (CMYK) and the inverted association CMYK=f2 (LAB).
The association defined by a color profile can be implemented by a look-up table. If, for example, the CMYK color components of a printing process are to be assigned the LAB color components, the look-up table must have a storage location for each possible value combination of the CMYK color components, in which location the associated LAB color components are stored. The simple association method has the disadvantage, however, that the look-up table can become very large. If each of the color components [C,M,Y,K] has been digitized with 8 bits, that is to say has 2
8
=256 density steps, there are 256
4
=4,294,967,296 possible value combinations of the CMYK color components. The look-up table must, therefore, have 4,294,967,296 storage cells each with a word length of 3 bytes (one byte each for L,A,B). The look-up table therefore reaches a size of 12.3 gigabytes.
To reduce the size of the look-up table, a combination of look-up table and interpolation method is, therefore, used to describe a color profile and to implement a corresponding color space transformation. The associations for all the possible value combinations of the CMYK color components are not stored in the look-up table; only those for a relatively coarse, regular grid of reference points in the CMYK color space. The grid is formed by only each kth value being taken as a grid point in each component direction. For k=16, therefore, in each component each 16th value from the 256 possible values is taken as a grid point. Accordingly, in each component direction, the grid has 256/16=16 grid points, that is to say, 16×16×16×16=65,536 grid points for the entire CMYK color space. For each grid point, the associated components of the LAB color space are stored as reference points in the look-up table. For CMYK value combinations that lie between the grid points, the LAB values to be assigned are interpolated from the adjacent reference points. For the inverted assignment CMYK=f2 (LAB), a grid of 16×16×16=4096 grid points, for example, is formed in the LAB color space, and the associated CMYK values are stored as reference points in the look-up table.
The assignments given in the color profiles between device dependent color spaces and a device independent color space (e.g., LAB) can be used for the color space transformation between the device dependent color spaces, so that, for example, the color values [C1,M1,Y1,K1] of a first printing process can be converted into the color values [C2,M2,Y2,K2] of a second printing process such that, according to the visual impression, the second print has the same colors as the first print.
FIGS. 1
a
and
1
b
show a color space transformation for such a printing process adaptation according to the prior art in a block diagram. In
FIG. 1
a
, a first color space transformation (1) from the color values [C1,M1,Y1,K1] of the first printing process into LAB color values, and a second color space transformation (2) from the LAB color values into the color values [C2,M2,Y2,K2] of the second printing process are carried out one after another. The two color space transformations (1) and (2) can also be combined into an equivalent color space transformation (3), which assigns the color values [C1,M1,Y1,K1] and the color values [C2,M2,Y2,K2] directly to one another (
FIG. 1
b
). Because in each case, through the device independent LAB intermediate color space, the color values [C1,M1,Y1,K1] and [C2,M2,Y2,K2] that result in the same LAB color values are assigned to one another, the associated printing colors in the two printing processes are sensed as visually the same within the printing color gamut. However, one disadvantage of such a method is that the black build-up (as it is referred to in the art) of the first printing process is lost. Black
Bestmann Günter
Krabbenhöft Uwe-Jens
Greenberg Laurence A.
Heidelberger Druckmaschinen AG
Locher Ralph E.
Stemer Werner H.
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