Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
1998-10-14
2001-12-18
Williams, Kimberly A. (Department: 2622)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C358S518000, C358S523000, C358S524000
Reexamination Certificate
active
06331899
ABSTRACT:
BACKGROUND OF THE INVENTION
TECHNICAL FIELD
The invention relates to color printing. More particularly, the invention relates to the automatic generation of single-channel color transformations from one printing device to another that allow output from one printer to closely resemble, or simulate, the output from another printer.
DESCRIPTION OF THE PRIOR ART
In most cases, the primary colorants of a simulated printer differ from the primary colorants of an output printer in both hue and density. The most general approach to simulating a printer using a color transformation from one printing device to another, i.e. the approach that is thought to provide the best color fidelity, involves the transformation
c
o
=f
c
(
c
s
, m
s
, y
s
, k
s
),
m
o
=f
m
(
c
s
, m
s
, y
s
, k
s
),
y
o
=f
y
(
c
s
, m
s
, y
s
, k
s
),
k
o
=f
k
(
c
s
, m
s
, y
s
, k
s
), (1)
where c
o
represents cyan of the output printer for cyan; c
s
, m
s
, y
s
, and k
s
represent the colorants of the simulated printer; and f
c
represents a function that generates the output printer cyan colorant from the input colorants of the simulated printer.
A similar notation is used for the other colorants.
This transformation has c
s
, m
s
, y
s
, and k
s
values as inputs from an image separated for the simulated device (e.g. DIC inks) and performs the transformations f
c
, f
m
, f
y
, and f
k
, respectively, to output device colorant values for each of the output c
o
, m
o
, y
o
, and k
o
values.
A shorthand description of the foregoing transformation is
v
o
=f
(
v
s
), (2)
where v
o
is the four-dimensional vector with components c
o
, m
o
, y
o
and k
o
; and v
s
is the four-dimensional vector with components c
s
, m
s
, y
s
, and k
s
.
The function f represents the vector function with component functions f
c
, f
m
, f
y
, and f
k
.
For purposes of the foregoing, it is assumed that the transformation t
s
from colorant v
s
to device-independent coordinates T
s
(e.g. CIELAB D50 two-degree observer) for the simulated device is available and is given by
T
s
=t
s
(
v
s
). (3)
The transformation t
o
from output device colorant v
o
to device-independent coordinates T
o
is governed by
T
o
=t
o
(
v
o
). (4)
If the function f of Equation 2 above is used for the device simulation, the following tristimulus values are obtained on the output device
{circumflex over (T)}
s
=t
o
(
f
(
v
s
)). (5)
Minimizing over possible functions f,
{circumflex over (f)}=argf; min d
(
T
s
,{circumflex over (T)}
s
). (6)
an optimal solution {circumflex over (f)} is obtained. In Equation 6, d is a distance function, such as CIELAB &Dgr;E. In the ideal situation, where T
s
={circumflex over (T)}
s
for all colors,
t
s
(
v
s
)=
o
(
f
(
v
s
)). (7)
In this case, the optimal solution is
f
^
(
v
s
)
=
t
o
†
(
t
s
(
v
s
)
)
.
(
8
)
This equation reflects in an abstract manner the commonly used approach to simulation that first transforms from the colorant of the simulated printer to device independent coordinates, and that then transforms from device independent coordinates to the output printer colorants. The function
t
o
†
represents the conversion from device independent coordinates to the output colorant, and (in a loose sense) inverts the function t
o
, even though the function t
o
is not invertible in the strict, mathematical sense (because it is a continuous mapping from four input dimensions to three output dimensions).
An additional complication is the mismatch of gamuts between the simulated device and the output device. Further details of gamut mapping are found in H. Kang,
Color Technology For Electronic Imaging Systems
, SPIE press, Bellingham, Wash. (1997).
Even though the approach of Equation 2 above gives the most accurate color fidelity, the class of single-channel color transformations given by
c
o
=f
c
(
c
s
)
m
o
=f
m
(
m
s
)
y
o
=f
y
(
y
s
)
k
o
=f
k
(
k
s
) (9)
is also very useful.
Given the limitations of the single-channel transformations of Equation 9 above, it is impossible to simulate devices without errors unless all the single colorant hues and the color mixing properties are the same on both the output and the simulated device. For example, if the hue of the magenta colorant of the simulated device differs from the hue of the magenta colorant of the output device, then the single-channel transformation is not able to match the magenta colors precisely.
Nevertheless, in practice, a single-channel transformation often performs well enough in terms of color fidelity and, in addition, offers several advantages. First, a practical implementation of this approach involves a lookup table having only 256 elements for the cyan transformation, and involves similar transformations for the other colorants. These lookup tables are simple, small and fast, both in hardware and in software implementations.
There are, in addition, potential image quality advantages to the single-channel transformations. For example, it is often desired to map the yellow colorant of the simulated device to the yellow colorant of the output device. Even though this mapping may not be as accurate colorimetrically, it can be a subjectively preferred mapping because it minimizes printing engine artifacts that are seen when yellow and magenta toners are mixed on electrophotographic printers; or it minimizes the visibility of halftone dots on inkjet printers.
For the black channel of the simulated device, it is often also preferred to map only to the black channel of the output device. This approach has the advantage of minimizing sensitivity to shifts in gray balance, as well as advantages in situations where the cost of black only printing is lower than mixed colorant printing due to accounting, e.g. the number of black only prints counted vs . . . the number of color prints counted.
One standard practice that is used to generate single-channel transformations involves measuring single ink densities on the simulated device and, from this, generating targets that consist of (x, d
s
) pairs of input ink percent, x, and densities, d
s
. This target, (x, d
s
), is then used on the output device, together with measurements of single toner density response on the output device of (x, d
o
) pairs that describe the current behavior of the output device. These two quantities are combined to generate lookup tables for each of the four color channels that compensate for the density differences.
This approach has certain disadvantages, among which is a metamerism problem in that a densitometer may give the same readings even though the colors are different when observed by a viewer and, conversely, the densitometer may give different readings even though the colors are the same when observed by a viewer. This metamerism problem is due to the differences in the spectral response of the densitometer and the spectral response of the average human eye, and the differences in the spectral properties of the inks and the toners. Another disadvantage of this approach is that it does not accurately simulate critical colors, such as flesh tones, that might be more important than the single ink colors.
A second common practice is to start from a given (x, d
s
) target and tune the target by iterative printing of pages that consist of images and patches separated to the simulated device, thereby improving the target by trial and error. This approach is time consuming, requires experienced operators, and can be sensitive to the images chosen for the iterations. In other words, one might overlook problems in the targets if certain colorant values are not contained in the images used during the iterations.
A third approach that is used to generate single-channel transformations involves measuring colorimetric data for single inks for the simulated device and matching these measurements to measurements for the output device. This is a very simple approach that is easy to i
Electronics For Imaging, Inc.
Glenn Michael A.
Williams Kimberly A.
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