Image processing apparatus with separate color conversion for CMY signals and K signal

The CiMiYi signals of input CiMiYiKi signals are converted into CoMoYo signals of CoMoYoKo signals by using three-dimensional LUT color converting devices. The Ki signal of the input CiMiYiKi signals is subjected to gradation conversion to obtain the Ko signal of the CoMoYoKo signals by using a one-dimensional LUT.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image processing apparatus for 
converting particular CMYK (cyan, magenta, yellow, and black) signals into 
other CMYK signals. 
2. Description of the Related Art 
In a color copying machine and a color printer, color conversion processing 
is generally carried out by limiting or assuming an input color so as to 
match the color reproduction characteristics of an image output device 
(image recording device) being used. 
In a color copying machine, for instance, particular RGB (red, green, and 
blue) signals are obtained as input color signals when a color image on an 
original is optically read by an image reading device, i.e., an input unit 
of the color copying machine. Meanwhile, an image output device which is 
an output unit of the color copying machine generally uses particular 
colorants, C, M, Y, and K, and has particular color reproduction 
characteristics. Hence, in the image processing apparatus, the RGB signals 
are converted into CMYK signals in such a manner that the color of a print 
matches the color of the original. 
In a color printer, it is assumed that particular color signals are 
inputted from an external computer or the like, and that NTSC RGB signals 
which are generally used in television are inputted to the color printer. 
Meanwhile, an image output device (printer unit) which is an output unit 
of the color printer generally uses particular colorants, C, M, Y, and K, 
and has particular color reproduction characteristics. Hence, in the image 
processing apparatus (image processing unit), input color signals such as 
the NTSC RGB signals are converted into the CMYK signals in such a manner 
that the color of a print matches the color displayed on an external 
display. 
Recently, color-image input/output media have become diversified, and color 
image information in one color-image input/output medium has come to be 
converted into that for another through a network. Therefore, color 
signals have also come to be diversified, and, among pieces of application 
software for editing color image information, there have appeared those 
which are provided with specifications which permit the handling of many 
color signals. 
FIG. 14 shows color representation specifications of typical pieces of the 
aforementioned application software. As color spaces, it is possible to 
use (1) the RGB space, (2) HSL and HSB spaces defined by a modification 
calculation of the RGB space, and (3) the CMYK space which is the recorded 
color itself. In general, however, the CMYK signals are used in the form 
of use in which a printed output is used as a final output, and the CMYK 
signals are transmitted to a printing-plate scanner. 
Even if the color space may be the same, actual color signals can be 
different. For instance, even in the case of the RGB signals, the NTSC RGB 
signals differ from RGB signals obtained by a general color scanner. 
Further, even among a plurality of kinds of color scanners, differences 
arise in the output color signals due to the difference in the respective 
spectral responses and the like. In the CMYK space, if the colorant sets 
differ, the printed color differs even if the same CMYK signals are used. 
That is, apart from the difference in the color space, the index as to 
whether or not the color signal is device-dependent exits as an index of 
the color signal. Device-independent signals are signals which can be 
converted into colorimetric color coordinate spaces (XYZ, Lab, Luv, etc. 
recommended by CIE (Commission Internationale de l'Eclairage)) by using 
known defining formulae, and the NTSC RGB signals fall under this 
category. On the other hand, device-dependent signals are signals which 
are set by assuming various characteristics of particular devices, and 
CMYK signals and RGB signals which are obtained by color scanners fall 
under this category. 
In a case where device-dependent signals are processed by a device 
different from an assumed one, the relationship of correspondence between 
the device-dependent signals and the device-independent signals must be 
described in some form. This relationship of correspondence is the color 
matching shown in FIG. 14, and in application software for illustration 
"Illustrator 3.2," for example, color coordinate values with respect to a 
plurality of CMYK signals are shown in correspondence with the types of 
printing ink and the like. 
In order to allow the CMYK signals thus generated by assuming a particular 
image output device to be outputted by another image output device 
different from the assumption, it is conceivable to use the matrix 
operation which has long been used for the color conversion in order to 
convert the CMYK signals into those for other image output devices on the 
basis of the aforementioned relationship of correspondence. 
However, as is well known, the color conversion based on the matrix 
operation has the drawback that the color reproducibility is not 
sufficient. For instance, in the so-called digital color proofing in which 
CMYK signals generated by assuming the printing use are transmitted to a 
particular digital printer, and are printed out, and the finish of the 
color in printing is checked in printing, it is particularly required to 
simulate with high accuracy the finish of the color in printing. In the 
color conversion based on the matrix operation, however, it is utterly 
impossible to satisfy such a requirement. 
Accordingly, it is conceivable to use the color conversion based on the 
reference to a conversion table, such as the one shown in Japanese 
Examined Patent Publication No. Sho. 58-16180, in the case of converting 
particular CMYK signals into other CMYK signals, as described above. 
FIG. 15 illustrates a conceivable example of the image processing apparatus 
for converting particular CMYK signals into other CMYK signals by 
referring to such a conversion table. To distinguish the CMYK signals 
prior to conversion from the CMYK signals subsequent to conversion, the 
CMYK signals prior to conversion will be referred to as CiMiYiKi signals, 
and the CMYK signals subsequent to conversion as CoMoYoKo signals. 
In this example, the color conversion based on reference to a 
three-dimensional conversion table used for converting Lab signals 
(although the Lab signals should be written as L*a*b* signals, they are 
written as the Lab signals for convenience sake) is replaced by the color 
conversion based on reference to a four-dimensional conversion table. 
CiMiYiKi signals, each consisting of 8-bit data, are commonly inputted to 
four-dimensional LUT (lookup table) color converting devices 1 to 4, in 
order to obtain CoMoYoKo signals, each consisting of 8-bit data, from the 
four-dimensional LUT color converting devices 1 to 4. 
In this case, if conversion tables of the four-dimensional LUT color 
converting devices 1 to 4 are given values corresponding to the values of 
the CiMiYiKi values on a one-to-one correspondence basis, and the values 
themselves in the conversion tables are fetched as the CoMoYoKo signals, 
the table size of the four-dimensional LUT color converting devices 1 to 4 
becomes enormously large. 
In addition, if the conversion tables of the four-dimensional LUT color 
converting devices 1 to 4 are respectively given values corresponding to 
divided areas of the values of the CiMiYiKi signals, and the values 
themselves in the conversion tables are fetched as the CoMoYoKo signals, 
the conversion accuracy declines remarkably. 
Therefore, it is practical to provide an arrangement in which an 
interpolation method in the case of the three-dimensional input address 
space such as the one shown in the aforementioned publication No. Sho. 
58-16180 is extended to a four-dimensional input address space as 
suggested in Japanese Unexamined Patent Publication No. Hei. 2-87192. 
However, the image processing apparatus shown in FIG. 15, which is 
conceivable as the one for converting particular CMYK signals into other 
CMYK signals, requires four four-dimensional LUT color converting devices. 
Hence, even if an interpolation configuration is provided, there is a 
drawback in that the table size becomes remarkably large. 
That is, in a case where, for example, the Lab signals are converted into 
the CMYK signals, it suffices if four three-dimensional LUT color 
converting devices are provided. In this case, in a case where, as the 
interpolation configuration, the conversion table of each 
three-dimensional LUT color converting device is divided into 16 parts, 
and one piece of lattice-point data is assumed to be 8 bits, the table 
size of one three-dimensional LUT color converting device becomes 
17.times.17.times.17.varies.4.8 Kbytes, and the total table size of the 
four three-dimensional LUT color converting devices amounts to a 
reasonably large size of about 19.2 Kbytes. 
In contrast, in the image processing apparatus shown in FIG. 15, if, as the 
interpolation configuration, each of the conversion tables of the 
four-dimensional LUT color converting devices 1 to 4 is divided into 16 
parts, and one piece of the lattice-point data is assumed to be 8 bits, 
the table size of one four-dimensional LUT color converting device becomes 
about 4.8 Kbytes.times.17, i.e., about 82 Kbytes, and the total table size 
of the four four-dimensional LUT color converting devices 1 to 4 amounts 
to a very large size of about 326 Kbytes. 
Moreover, in the image processing apparatus shown in FIG. 15, since the 
input address space is four-dimensional as described above, there is a 
drawback in that the interpolation calculating units of the respective 
four four-dimensional LUT color converting devices 1 to 4 become complex 
in an exponential manner. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to make it possible to 
convert particular CMYK signals into other CMYK signals with a simple 
configuration equivalent to that in a case which is adapted for three 
inputs in terms of the table size and the interpolation configuration of a 
color converting means of a conversion-table referencing type, and with 
high accuracy equivalent to that in a case which is adapted for four 
inputs in terms of the conversion accuracy. 
According to a first aspect of the invention (correspondence is made with 
reference numerals used in the embodiment of FIG. 1 or 6 that will be 
described later), there is provided an image processing apparatus for 
converting CiMiYiKi signals which are particular CMYK signals into 
CoMoYoKo signals which are other CMYK signals, comprising CMY-signal 
converting means (11) for converting the CiMiYi signals into the CoMoYo 
signals by using a color converting device (12 to 14, or 18) of a 
three-dimensional-conversion-table referencing type; and K-signal 
converting means (21) for subjecting the Ki signal to gradation conversion 
to obtain the Ko signal. 
According to a second aspect of the invention (correspondence is made with 
reference numerals used in the embodiment of FIG. 5 that will be described 
later), there is provided an image processing apparatus for converting 
CiMiYiKi signals which are particular CMYK signals into CoMoYoKo signals 
which are other CMYK signals, comprising CMY-signal converting means (11) 
for converting the CiMiYi signals into the CoMoYo signals by using a color 
converting device (12 to 14, or 18) of a 
three-dimensional-conversion-table referencing type; and K-signal 
non-converting output means (22) for outputting the Ki signal as the Ko 
signal without converting the Ki signal. 
According to a third aspect of the invention (correspondence is made with 
reference numerals used in the embodiment of FIG. 6 that will be described 
later), in the image processing apparatus according to the first or second 
aspect of the invention, the CMY-signal converting means (11) includes 
only one color converting device (18) of a 
three-dimensional-conversion-table referencing type, a conversion table of 
the color converting device (18) of a three-dimensional-conversion-table 
referencing type is changed in a frame-sequential manner, and the CoMoYo 
signals are obtained from the color converting device (18) of a 
three-dimensional-conversion-table referencing type in a frame-sequential 
manner. 
According to a fourth aspect of the invention (correspondence is made with 
reference numerals used in the embodiment of FIGS. 8 and 9 that will be 
described later), there is provided an image processing apparatus for 
converting plural kinds of input color signals, each kind having four or 
three signals, including CiMiYiKi signals which are particular CMYK 
signals, into CoMoYoKo signals which are other CMYK signals, comprising 
input-color-signal identifying means (320) for identifying a kind of 
actually inputted color signals on the basis of information added to the 
actually inputted color signals; signal converting means (19) for 
converting three signals of the actually inputted color signals, excluding 
the Ki signal when the actually inputted color signals are the CiMiYiKi 
signals, into the CoMoYo signals or CoMoYoKo signals by using a color 
converting device of a three-dimensional-conversion-table referencing 
type; signal output means (64, 23) for subjecting one of the actually 
inputted color signals, to gradation conversion to obtain the Ko signal, 
or for outputting the one of the input color signals as the Ko signal 
without converting the one of the input color signals, the one of the 
actually inputted color signals being the Ki signal when the actually 
inputted color signals are the CiMiYiKi signals; and selecting means (80) 
for selecting, on the basis of the kind of the actually inputted color 
signals identified by the input-color-signal identifying means (320), both 
of the signal converting means (19) and the signal output means (64, 23) 
when the actually inputted signals are the CiMiYiKi signals, and only the 
signal converting means (19) when the actually inputted signals are not 
the CiMiYiKi signals. 
According to a fifth aspect of the invention (correspondence is made with 
reference numerals used in the embodiment of FIGS. 8 and 9 or 12 that will 
be described later), there is provided an image processing apparatus for 
converting CiMiYiKi signals which are CMYK signals that are generated by 
color signal generating means (212, 222) connected to a network (100), 
into CoMoYoKo signals which are other CMYK signals, comprising color 
signal input means (310, 330) for receiving the CiMiYiKi signals from the 
color signal generating means (212, 222); conversion information 
recognizing means (320) for recognizing information to be used in 
converting the CiMiYiKi signals with mapping to a physical color space; 
CMY-signal converting means (19) for converting the CiMiYi signals that 
are supplied from the color signal input means (310, 330) into the CoMoYo 
signals by using a color converting device of a 
three-dimensional-conversion-table referencing type; conversion table 
setting means (90) for setting a conversion table of the CMY-signal 
converting means (19) on the basis of a recognition result of the 
conversion information recognizing means (320); and signal output Means 
(64, 23) for subjecting the Ki signal that is supplied from the color 
signal input means (310, 330) to gradation conversion to obtain the Ko 
signal, or outputting the Ki signal as the Ko signal without converting 
the Ki signal. 
According to a sixth aspect of the invention (correspondence is made with 
reference numerals used in the embodiment of FIGS. 8 and 9 or 12 that will 
be described later), there is provided an image processing apparatus for 
converting CiMiYiKi signals which are CMYK signals stored in an external 
storage medium (243), into CoMoYoKo signals which are other CMYK signals, 
comprising color signal input means (310, 330) for receiving the CiMiYiKi 
signals from the external storage medium (243); conversion information 
recognizing means (320) for recognizing information to be used in 
converting the CiMiYiKi signals with mapping to a physical color space; 
CMY-signal converting means (19) for converting the CiMiYi signals that 
are supplied from the color signal input means (310, 330) into the CoMoYo 
signals by using a color converting device of a 
three-dimensional-conversion-table referencing type; conversion-table 
setting means (90) for setting a conversion table of the CMY-signal 
converting means (19) on the basis of a recognition of the conversion 
information recognizing means (320); and signal output means (64, 23) for 
subjecting the Ki signal that is supplied from the color signal input 
means (310, 330) to gradation conversion to obtain the Ko signal, or 
outputting the Ki signal as the Ko signal without converting the Ki 
signal. 
In normal printing, a gray component replacement (GCR) method which is 
called "skeleton black method" and uses a black signal (K signal) only in 
relatively high-density portions is generally adopted. A gray component 
replacement method similar to the above is also carried out in the 
standard color image data (SCID) which is a standard digital chart in 
printing in Japan and the United States. 
In general, gray component replacement can be expressed by the following 
Formulae (1) to (4) by using C', M', and Y' signals at the time of 
reproducing a particular color by the three colors, cyan, magenta, and 
yellow: 
EQU K=min{C', M', Y'}.times..alpha. (1) 
EQU C=C'-K.multidot..beta. (2) 
EQU M=M'-K.multidot..beta. (3) 
EQU Y=Y'-K.multidot..beta. (4) 
Here, the coefficient .alpha. is generally limited to 1/2 or less, while 
the coefficient .beta. is set to 1, for example. 
In this case, a reproduced color P{C', M', Y'} using three colors, cyan, 
magenta, and yellow, is substantially equivalent to a reproduced color 
P{C, M, Y, K} using four colors, cyan, magenta, yellow, and black, so that 
EQU P{C', M', Y'}.varies.P{C, M, Y, K} (5) 
Accordingly, if the gray component replacement method is used as a premise, 
the following formula holds from Formulae (1) to (5): 
EQU P{C, M, Y, K}.varies.P{C+K.multidot..beta., M+K.multidot..beta., 
Y+K.multidot..beta., 0} (6) 
Therefore, in the case where CiMiYiKi signals which are certain CMYK 
signals are converted into CoMoYoKo signals which are other CMYK signals, 
if it is assumed that Formulae (1) to (5) hold with respect to both the 
CiMiYiKi signals and the CoMoYoKo signals, the conversion from the 
CiMiYiKi signals into the CoMoYoKo signals becomes possible by the steps 
shown in the following Formula (7): 
EQU {Ci, Mi, Yi, Ki}.fwdarw.{Ci+Ki.multidot..beta., Mi+Ki.multidot..beta., 
Yi+Ki.multidot..beta.}.fwdarw..LAMBDA.{Ci+Ki.multidot..beta., 
Mi+Ki.multidot..beta., Yi+Ki.multidot..beta.}.fwdarw.{Co', Mo', 
Yo'}.fwdarw.{Co, Mo, Yo, Ko} (7) 
Namely, in the general gray component replacement method which is called 
"skeleton black method" and used for printing and the like, and for which 
Formulae (1) to (5) hold, the conversion of the CiMiYiKi signals which are 
particular CMYK signals into CoMoYoKo signals which are other CMYK signals 
can be realized by the conversion from the CiMiYi signals into the CoMoYo 
signals by using an input/output function .LAMBDA. with respect to the 
trichromatic representation and by the conversion from the Ki signal into 
the Ko signal. 
Then, the conversion from the CiMiYi signals into the CoMoYo signals by 
using an input/output function .LAMBDA. with respect to the trichromatic 
representation can be realized by a color converting device of a 
three-dimensional-conversion-table referencing type. Meanwhile, the 
conversion from the Ki signal into the Ko signal can be realized by the 
one-dimensional gradation conversion. Depending on the CiMiYiKi signals or 
the CoMoYoKo signals, the Ki signal can be outputted directly as the Ko 
signal. 
Accordingly, in the image processing apparatus in accordance with claim 1 
or 2 arranged as described above, the CiMiYi signals are converted into 
the CoMoYo signals by the CMY-signal converting means 11, the Ki signal is 
converted into the Ko signal by the K-signal converting means 21, and the 
Ki signal is outputted as the Ko signal by the K-signal non-converting 
output means 22, thereby converting the CiMiYiKi signals into the CoMoYoKo 
signals. 
The CMY-signal converting means 11 can be realized with a small table size 
and a simple interpolation configuration as the color converting device 12 
to 14 or 18 of a three-dimensional-conversion-table referencing type. 
Also, the K-signal converting means 21 can be realized with a small table 
size as a one-dimensional gradation converting device. Therefore, in the 
image processing apparatus according to the first or second aspect of the 
invention, the CiMiYiKi signals can be converted into the CoMoYoKo signals 
with a simple configuration equivalent to that in a case where, for 
instance, the Lab signals are converted into the CMYK signals. 
In the image processing apparatus according to the third aspect of the 
invention, the CMY-signal converting means 11 is constituted by a single 
color converting device 18 of a three-dimensional-conversion-table 
referencing type. Since the CiMiYi signals are converted into the CoMoYo 
signals by the color converting device 18 of a 
three-dimensional-conversion-table referencing type in a frame-sequential 
manner, the CiMiYiKi signals can be converted into the CoMoYoKo signals 
with a simpler configuration. 
In the image processing apparatus according to the fourth aspect of the 
invention, when the input color signals are the CiMiYiKi signals, both the 
signal converting means 19 and the signal output means 64, 23 are selected 
by selecting means 80, the CiMiYi signals are converted into the CoMoYo 
signals by the signal converting means 19. At the same time, the Ki signal 
is converted into the Ko signal by the signal output means 64, 23, or the 
Ki signal is outputted as the Ko signal. Meanwhile, in a case where the 
input color signals are such as the Lab signals or the RGB signals, only 
the signal converting means 19 is selected by the selecting means 80, and 
the signals such as the Lab signals or the RGB signals are converted into 
the CoMoYoKo signals by the signal converting means 19. Thus, not only the 
CiMiYiKi signals but also such signals as the Lab signals and the RGB 
signals can be converted into the CoMoYoKo signals by common color 
converting units. 
In the image processing apparatus according to the fifth or sixth aspect of 
the invention, information to be used in converting device-dependent 
CiMiYiKi signals, which are generated by the color-signal generating means 
212, 222 connected to the network 100 or are stored in the external 
storage medium 243, with mapping to a physical color space is recognized 
by the conversion-information recognizing means 320. Since a conversion 
table in the CMY-signal converting means 19 is set in correspondence with 
the result of that recognition, the device-dependent CiMiYiKi signals can 
positively be converted into the CoMoYoKo signals such that the color 
printed when the CiMiYiKi signals are outputted by an image output device 
connected to the relevant image processing apparatus matches the color 
printed when the CiMiYiKi signals are outputted by an image output device 
for which the CiMiYiKi signals are assumed to be used.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates an image processing apparatus according to an embodiment 
of the present invention, in which case CMYK signals for printing are 
outputted to an image output device such as a digital color xerography 
printer. 
CiMiYiKi signals, which are CMYK signals for printing, are obtained by 
quantizing the dot area ratio into 8 bits, respectively. The CiMiYi 
signals among these signals are converted into CoMoYo signals, which are 
8-bit dot-area-ratio signals respectively matched to the color 
reproduction characteristics of an image output device 350, by 
three-dimensional LUT color converting devices 12, 13, and 14 which 
constitute a CMY-signal converting means 11, as will be described later. 
Meanwhile, the Ki signal is subjected to gradation conversion by a 
one-dimensional LUT 21 constituting a K-signal converting means, as will 
be described later, and is thereby converted into a Ko signal, which is an 
8-bit dot-area-ratio signal similarly matched to the color reproduction 
characteristics of the image output device 350. In this example, the 
CoMoYoKo signals thus obtained are simultaneously inputted to the image 
output device 350 so as to form a visible image by the image output device 
350. 
The three-dimensional LUT color converting devices 12, 13, and 14 are 
respectively configured to effect interpolation by a cubic interpolation, 
for example. Specifically, a CMY space for representing the CiMiYi signals 
is divided into 16 segments in the respective axial directions, and is 
hence divided into a total of 4096 basic cubes to effect interpolation. As 
shown in FIG. 2, each of the three-dimensional LUT color converting 
devices 12, 13, and 14 is comprised of a near-lattice-point address 
generating unit 15, a LUT lattice-point data storage unit 16, and an 
interpolation calculating unit 17. 
Addresses of near lattice points P1 to P8, which are vertices of a basic 
cube having therein a point (coordinate) O determined by CiMiYi signals 
respectively having 8 bits in the CMY space as shown in FIG. 3, are 
generated by the near-lattice-point address generating unit 15 by using 
the respective four high-order bits of the respective 8-bit CiMiYi 
signals. The LUT lattice-point data storage unit 16 is accessed by the 
near-lattice-point addresses which are represented by 12 bits in total. 
Stored in advance in the LUT lattice-point data storage unit 16 are 
lattice-point data, respectively having 8 bits and described later, which 
are for the Co signal in the case of the three-dimensional LUT color 
converting device 12, for the Mo signal in the case of the 
three-dimensional LUT color converting device 13, and for the Yo signal in 
the case of the three-dimensional LUT color converting device 14 with 
respect to the eight near lattice points P1 to P8 of the basic cube 
designated by the near-lattice-point addresses from the near-lattice-point 
address generating unit 15. The lattice-point data respectively having 8 
bits with respect to the eight near lattice points P1 to P8 are 
consecutively read from the LUT lattice-point data storage unit 16 by 
means of the near-lattice-point addresses from the near-lattice-point 
address generating unit 15, and are supplied to the interpolation 
calculating unit 17. 
In addition, the four low-order bits of the CiMiYi signals respectively 
having 8 bits are supplied to the interpolation calculating unit 17, and 
an interpolation calculation such as the one described later is effected 
by the interpolation calculating unit 17. Consequently, the Co signal, the 
Mo signal, and the Yo signal are respectively obtained as 8-bit data by 
the respective interpolation calculating units 17 of the three-dimensional 
LUT color converting devices 12, 13, and 14. 
The relationships between the near lattice points P1 to P8 of the basic 
cube which are respectively determined by the four high-order bits of the 
CiMiYi signals and the point O in the basic cube are determined by the 
respective four low-order bits of the CiMiYi signals. 
As shown in FIG. 3, in the interpolation calculating unit 17, the basic 
cube is divided into eight rectangular parallelepipeds by planes which 
pass through the point O and are parallel to a CM plane, an MY plane, and 
a YC plane, respectively. If it is assumed that the volumes of the 
rectangular parallelepipeds in which segments connecting the respective 
near lattice points P1 to P8 and the point O are set as diagonal lines are 
V1 to V8, that the total sum of the volumes V1 to V8 is V, and that 
lattice-point data concerning the near lattice points P1 to P8 from the 
LUT lattice-point data storage unit 16 are D1 to D8, then it is possible 
to obtain the following interpolated data value Ans corresponding to the 
coordinate of the point O: 
EQU Ans=(D1.multidot.V7+D2.multidot.V8+D3.multidot.V5+D4.multidot.V6+D5.multido 
t.V3+D6.multidot.V4+D7.multidot.V1+D8.multidot.V2)/V (8) 
Here, the lattice-point data D1 to D8 are determined in advance such that 
the color reproduced when an image is printed by the CiMiYi signals 
matches the reproduced color of an output image obtained by the image 
output device 350 by converting the CiMiYi signals into CoMoYo signals. 
For this reason, an examination is made in advance as to how the CiMiYi 
signals inputted to the relevant image processing apparatus are mapped to 
a colorimetric and physical color space, e.g., the Lab space, and how the 
CoMoYo signals outputted from the relevant image processing apparatus to 
the image output device 350 are mapped to the same Lab space. In addition, 
also determined in advance are functions for conversion from the CiMiYi 
signals into the Lab signals, and functions for conversion from the CoMoYo 
signals into the Lab signals or functions for conversion from the Lab 
signals into the CoMoYo signals, which are inverse functions thereof. 
The conversion from the CiMiYi signals into the Lab signals is expressed by 
EQU L=fe(Ci, Mi, Yi) a=fa(Ci, Mi, Yi) b=fb(Ci, Mi, Yi) (9) 
Meanwhile, the conversion from the Lab signals into the CoMoYo signals is 
expressed by 
EQU Co=ge(L, a, b) Mo=ga(L, a, b) Yo=gb(L, a, b) (10) 
Their functions fe, fa, and fb and ge, ga, and gb can be determined by the 
Neugebauer's formula described on page 234 of "Theory of Color 
Reproduction" published by the Printing Bureau of the Printing Society of 
Japan, or by a method based on the least-squares approximation as 
polynomials of higher degree on the basis of a plurality of color samples 
and their colorimetric values using a combination of known dot area 
ratios, or learning by a neural network. Polynomials of higher degree 
cannot be generally solved analytically, but can be determined by the 
technique of computer color matching (CCM). 
Accordingly, the conversion from the CiMiYi signals into the CoMoYo signals 
can be expressed by composite functions of the functions fe, fa, and fb 
and the functions ge, ga, and gb as follows: 
EQU Co=ge.multidot.fe(Ci, Mi, Yi) Mo=ga.multidot.fa(Ci, Mi, Yi) 
Yo=gb.multidot.fb(Ci, Mi, Yi) (11) 
The lattice-point data based on the formula for conversion from the CiMiYi 
signals into the CoMoYo signals thus determined are stored in advance in 
the respective LUT lattice-point data storage units 16 of the 
three-dimensional LUT color converting devices 12, 13, and 14. As a 
result, CoMoYo signals are obtained from the respective interpolation 
calculating units 17 of the three-dimensional LUT color converting devices 
12, 13, and 14, such that the color reproduced when the CoMoYo signals are 
outputted by the image output device 350 matches the color reproduced when 
the CiMiYi signals are printed out as assumed. 
As for the conversion from the Ki signal into the Ko signal, it suffices if 
gradation samples of the monochromatic black color when the Ki signal is 
printed and when the Ko signal is outputted by the image output device 350 
are collected in advance for the respective cases, and after the 
relationship between the dot area ratio and lightness L, i.e., the 
relationship between the value of the Ki signal or the Ko signal and the 
lightness L: 
EQU L(printing)=s(Ki) (12) 
EQU L(image output device)=t(Ko) (13) 
is determined for the respective cases, and after the relationship between 
the Ki signal and the Ko signal is set such that the lightnesses L 
(printing) and L (image output device) of the two signals become equal, 
the data are stored in advance in the one-dimensional LUT 21 such that the 
input/output characteristic of the one-dimensional LUT 21 assumes that 
relationship. 
Since it is assumed that 
EQU s(Ki)=t(Ko) (14) 
if an inverse function of t is t.sup.-1, then Ko can be expressed as 
EQU Ko=t.sup.-1 .multidot.s(Ki) (15) 
Therefore, it suffices if the one-dimensional LUT 21 is configured as a 
gradation converting device having an input/output characteristic such as 
the one shown in Formula (15). Hence, the Ko signal is obtained from the 
one-dimensional LUT 21 such that the lightness at a time when the Ko 
signal is outputted by the image output device 350 is equal to the 
lightness at a time when the Ki signal is printed out as assumed. 
The example shown in FIG. 2 is a case in which the lattice-point data 
concerning the eight near lattice points P1 to P8 of the basic cube are 
transmitted serially from the LUT lattice-point data storage unit 16 to 
the interpolation calculating unit 17. However, an arrangement may be 
provided such that the lattice-point data concerning the respective near 
lattice points are separately stored in advance in eight LUT lattice-point 
data storage units, and are read therefrom in parallel to the 
interpolation calculating unit 17. According to this arrangement, 
processing can be carried out at a higher speed. 
In the case where the cubic interpolation is used, instead of being divided 
into 16 segments in the respective axial directions, the CMY space for 
representing the CiMiYi signals may be divided into, for instance, 8 or 32 
segments in the respective axial directions within the range of the number 
of divisions which can be shown by 8 bits or less. Still alternatively, 
the CMY space may not be divided into the same number of divisions in the 
respective axial directions, and may be divided into 16 segments in the 
C-axis direction, for example, and into 8 segments in the M- and Y-axis 
directions, respectively. Thus, the configuration of interpolation may be 
modified, as necessary. 
In addition, instead of the cubic interpolation, it is possible to use 
other interpolation methods such as a triangular prism interpolation and 
an oblique triangular prism interpolation which are described in 
Transactions of 1993 24th Image Engineering Conference, pp. 347 to 350, or 
a tetrahedral interpolation disclosed in aforementioned publication No. 
Hei. 2-87192. 
Further, the respective components of the CiMiYi signals and CoMoYo signals 
may not necessarily be 8 bits. 
Depending on the CiMiYi signals which are CMYK signals for printing or the 
color reproduction characteristics of the image output device 350, there 
are cases where the functions s and t in Formula (14) are equal, so that 
the Ki signal can be outputted as it is as the Ko signal without being 
converted. 
In that case, it is sufficient to use a K-signal non-converting output 
means in which data having an input/output characteristic of using input 
values directly as output values, as shown in FIG. 4, are stored in 
advance in the one-dimensional LUT 21 in the example shown in FIG. 1, and 
the one-dimensional LUT 21 outputs the Ki signal as the Ko signal without 
substantially converting the Ki signal. Alternatively, as shown in FIG. 5, 
it is possible to use a K-signal non-converting output means which is not 
provided with the one-dimensional LUT 21, and in which a line 22 for 
inputting the Ki signal is used as a line for outputting the Ko signal, 
and the line 22 outputs the Ki signal as the Ko signal without 
substantially or formally converting the Ki signal. 
According to the embodiment shown in FIG. 1 or 5, CiMiYi signals which are 
particular CMYK signals can be converted into CoMoYo signals which are 
other CMYK signals by using a simple configuration equivalent to that in 
the case adapted for three inputs in terms of the table size and the 
interpolation configuration of the color converting means of a 
conversion-table referencing type, and with high accuracy equivalent to 
that in the case adapted for four inputs in terms of the conversion 
accuracy. 
FIG. 6 shows an image processing apparatus according to another embodiment 
of the present invention, in which case CoMoYoKo signals are obtained in a 
frame-sequential manner and are outputted to the image output device in a 
frame-sequential manner. 
In this embodiment, a one-page portion of CiMiYiKi signals is written in 
advance in buffer memories 31, 32, 33, and 34, respectively. The 
CMY-signal converting means 11 is constituted by a single 
three-dimensional LUT color converting device 18, and converts CiMiYi 
signals into CoMoYo signals in a frame-sequential manner, as will be 
described later. The one-dimensional LUT 21 constitutes the K-signal 
converting means in the same way as in the embodiment shown in FIG. 1. 
In this example, however, conversion tables are set in the 
three-dimensional LUT color converting device 18 and the one-dimensional 
LUT 21 from a ROM 40, as will be described later. In addition, outputs 
from the three-dimensional LUT color converting device 18 and the 
one-dimensional LUT 21 are added by an adder circuit 50, and an output 
from the adder circuit 50 is supplied to the image output device 350. 
A one-page portion of CiMiYiKi signals is stored in the buffer memories 31 
to 34 until a timing T0 shown in FIG. 7. Immediately after the timing T0, 
a conversion table for subjecting the Ki signals to gradation conversion 
into the Ko signals is set in the one-dimensional LUT 21 from the ROM 40. 
During a subsequent period Q1, a one-page portion of the Ki signals is 
read from the buffer memory 34, and is inputted to the one-dimensional LUT 
21, which, in turn, converts the Ki signals into the Ko signals. 
Since as the aforementioned lattice-point data 0s are set in the 
three-dimensional LUT color converting device 18 from the ROM 40 
immediately after the timing T0, or since the outputs from the buffer 
memories 31 to 33 are respectively set at 0s during the period Q1, the 
output from the three-dimensional LUT color converting device 18 becomes 0 
during the period Q1. Accordingly, the Ko signals are outputted from the 
adder circuit 50 to the image output device 350 during the period Q1. 
Immediately after the period Q1, a conversion table, which is similar to 
the one set in the three-dimensional LUT color converting device 12 in the 
embodiment shown in FIG. 1, is set in the three-dimensional LUT color 
converting device 18 from the ROM 40 so as to convert the CiMiYi signals 
into Co signals. During a subsequent period Q2, a one-page portion of the 
CiMiYi signals is read from the buffer memories 31 to 33, and is inputted 
to the three-dimensional LUT color converting device 18, which, in turn, 
converts the CiMiYi signals into the Co signals. 
Since as the conversion table 0s are set in the one-dimensional LUT 21 from 
the ROM 40 immediately after the period Q1, or since the output from the 
buffer memory 34 is set at 0 during the period Q2, the output from the 
one-dimensional LUT 21 becomes 0 during the period Q2. Accordingly, the Co 
signals are outputted from the adder circuit 50 to the image output device 
350 during the period Q2. 
Similarly, immediately after the period Q2, a conversion table for 
converting the CiMiYi signals into Mo signals is set in the 
three-dimensional LUT color converting device 18 from the ROM 40. During a 
subsequent period Q3, a one-page portion of the CiMiYi signals is read 
from the buffer memories 31 to 33, and is converted into the Mo signals by 
the three-dimensional LUT color converting device 18, and the Mo signals 
are supplied to the image output device 350 through the adder circuit 50. 
Further, immediately after the period Q3, a conversion table for converting 
the CiMiYi signals into Yo signals is set in the three-dimensional LUT 
color converting device 18 from the ROM 40. During a subsequent period Q4, 
a one-page portion of the CiMiYi signals is read from the buffer memories 
31 to 33, and is converted into the Yo signals by the three-dimensional 
LUT color converting device 18, and the Yo signals are supplied to the 
image output device 350 through the adder circuit 50. 
In the above-described manner, the CoMoYoKo signals are outputted from the 
image processing apparatus to the image output device 350 in a 
frame-sequential manner. 
In this embodiment, since the CMY-signal converting means 11 can be 
constituted by one three-dimensional LUT color converting device 18, the 
CiMiYiKi signals can be converted into the CoMoYoKo signals with a simpler 
configuration than that in the embodiment shown in FIG. 1. 
Incidentally, the sequence of the signals outputted to the image output 
device 350 can be arbitrarily set, in conformity to the sequence of 
printing by the image output device 350, by a control unit for controlling 
the setting of conversion tables in the three-dimensional LUT color 
converting device 18 and the one-dimensional LUT 21 from the ROM 40 as 
well as the reading of signals from the buffer memories 31 to 34. 
In addition, the buffer memories 31 to 34 are a mere example of a means for 
repeatedly obtaining the CiMiYi signals, in particular, and the buffer 
memories 31 to 34 are not necessarily required in a case where CiMiYiKi 
signals are stored in a storage means such as an external storage medium. 
Furthermore, a selector or a switching circuit may be used instead of the 
adder circuit 50. 
In this embodiment as well, in a case where the Ki signals can be outputted 
directly as the Ko signals without being converted, it suffices if data 
having the input/output characteristic such as the one shown in FIG. 4 are 
set in the one-dimensional LUT 21 from the ROM 40, or if the input line 
for the Ki signals on such as the output side of the buffer memory 34 is 
used as it is as the output line for the Ko signals. 
FIG. 8 illustrates an example of a network image output system which uses 
an image processing apparatus according to still another embodiment of the 
present invention, in which case color signals, which are CMYK signals, 
RGB signals, or Lab signals generated by a color-signal generating means 
connected to a network or transmitted from an external storage medium, are 
converted into other CMYK signals or monochromatic K, C, M, or Y signals, 
and are printed out by a printer connected to the network. 
That is, a printing scanner 212, a printing scanner 222, a color scanner 
232, a reading device 242, and a reading device 252 are connected to a 
network 100 via, for example, controlling computers 211, 221, 231, 241, 
and 251, respectively. 
The printing scanners 212 and 222 are for obtaining RGB signals by reading 
reversal films 213 and 223, respectively, and for converting the RGB 
signals into particular CMYK signals for printing. As for the printing 
scanner 212 and the printing scanner 222, however, particular CMYK signals 
from them differ from each other because their manufacturers are different 
or for other reasons. 
For this reason, the CMYK signals as well as information to be used in 
converting the CMYK signals with mapping to a physical color space, e.g., 
the Lab space are transmitted from the controlling computers 211 and 221 
to the network 100. 
The color scanner 232 is for obtaining RGB signals by reading an original 
233 and for outputting the RGB signals as they are. The RGB signals from 
the color scanner 232 as well as information to be used in converting the 
RGB signals with mapping to a physical color space, e.g., the Lab space 
are transmitted from the controlling computer 231 to the network 100. 
The reading devices 242 and 252 are, for example, CD-ROM readers for 
reading color signals recorded in external storage media 243 and 253 such 
as CD-ROMs. However, particular CMYK signals are read from the external 
storage medium 243, while Lab signals are read from the external storage 
medium 253. 
For this reason, particular CMYK signals from the external storage medium 
243, as well as information to be used in converting the CMYK signals with 
mapping to a physical color space, e.g., the Lab space are transmitted 
from the controlling computer 241 to the network 100. In addition, the Lab 
signals from the external storage medium 253, as well as information 
indicating that the color signals are the Lab signals, are transmitted 
from the controlling computer 251 to the network 100. 
Incidentally, the aforementioned conversion information which is added to 
the particular CMYK signals or RGB signals from the printing scanners 212, 
222, the color scanner 232, and the reading device 242 also naturally 
indicates the type of the respective color signals, i.e., that the 
respective color signals are the CMYK signals or the RGB signals. 
A printer 300 is connected to the network 100. This printer 300 is provided 
with, for instance, an interface unit 310, a control unit 320, a buffer 
memory unit 330, a color converting unit 340, the image output device 350, 
an operating unit 360, and a display unit 370. 
The control unit 320 controls the overall printer 300, and is comprised of 
a CPU 321, a ROM 322 in which a control program and the like to be 
executed by the CPU 321 is written, and a RAM 323 which is used as a work 
area for the CPU 321. 
This control unit 320 also serves as an input-color-signal identifying 
means and a conversion-information recognizing means, and is adapted to 
identify, from the aforementioned input-color-signal identification 
information, the type of the input color signals, i.e., whether the input 
color signals are CMYK signals, RGB signals, or Lab signals, and is also 
adapted to recognize the aforementioned conversion information so as to 
generate control signals CT1, CT2, CT3, and CT4 which will be described 
later. 
The buffer memory unit 330 is for temporarily storing a one-page portion of 
input color signals so that the CMYK signals will be outputted to the 
image output device 350 in a frame-sequential manner, as will be described 
later. The buffer memory unit 330 has four buffer memories 331 to 334. The 
control signal CT1 for controlling the writing and reading of the input 
color signals is supplied to the buffer memory unit 330. 
The color converting unit 340 forms an essential portion of the image 
processing apparatus of the printer 300, and has a configuration such as 
the one shown in FIG. 9 or 12, as will be described later. The color 
signals read from the buffer memory unit 330, as well as the control 
signals CT2, CT3, and CT4, are supplied to the color converting unit 340. 
The image output device 350 is, for instance, a xerographic printer for 
printing out a CMYK color image by CMYK signals which are outputted from 
the color converting unit 340 in a frame-sequential manner. 
The operating unit 360 and the display unit 370 constitute user interfaces 
of the printer 300. This example is a case in which if the input color 
signals are RGB signals or Lab signals, the output image can be printed 
out in a monochromatic color, as will be described later. However, whether 
a full-color image is to be outputted or a monochromatic color image is to 
be outputted is determined by an instruction from the controlling 
computers 231, 251 or the operation of the operating unit 360. 
FIG. 9 shows an example of the color converting unit 340. The color 
converting unit 340 is provided with a switching circuit 60, 
one-dimensional LUTs 71 to 73, a three-dimensional LUT color converting 
device 19, a selector 80, a one-dimensional LUT 23, and a ROM 90. 
The switching circuit 60 assigns and outputs the signals read from the 
buffer memories 331 to 334 of the buffer memory unit 330 to output lines 
61 to 64 by means of the control signal CT2. 
The one-dimensional LUTs 71 to 73 outputs the signals sent to the 
respective lines 61 to 63 without substantially converting them or by 
subjecting them to gradation conversion by using a conversion table set 
from the ROM 90. 
The three-dimensional LUT color converting device 19 converts the signals 
from the one-dimensional LUTs 71 to 73 into CoMoYo signals for the image 
output device 350 in a frame-sequential manner by using a conversion table 
set from the ROM 90. 
The selector 80 selects either the output by the three-dimensional LUT 
color converting device 19 or the output by the line 64 on the basis of 
the control signal CT4. Meanwhile, the one-dimensional LUT 23 outputs to 
the image output device 350 the signals from the selector 80 by subjecting 
them to gradation conversion by means of a conversion table set from the 
ROM 90 or without substantially converting them. 
Various kinds of conversion tables, which will be described later, are 
stored in advance in the ROM 90, and those tables which are selected among 
them, as will be described later, are set in the one-dimensional LUTs 71 
to 73, the three-dimensional LUT color converting device 19, and the 
one-dimensional LUT 23. 
In a case where CiMiYiKi signals, which are particular CMYK signals from 
the printing scanners 212, 222 or the external storage medium 243 shown in 
FIG. 8, are transmitted to the printer 300 as input color signals, a 
one-page portion of Ci, Mi, Yi, and Ki signals are stored in the buffer 
memories 331, 332, 333, and 334, as shown in FIG. 9, by means of the 
control signal CT1 from the control unit 320 which also serves as the 
input-color-signal identifying means and the conversion-information 
recognizing means. 
If CiMiYiKi signals are stored in the buffer memories 331 to 334 until the 
timing T0 shown in FIG. 10, a one-page portion of CiMiYiKi signals is 
repeatedly read from the buffer memories 331 to 334 during periods Q1, Q2, 
Q3, and Q4, respectively, subsequent to the timing T0 by means of the 
control signal CT1. 
In this case, the switching circuit 60 is changed over in such a manner as 
to direct the Ci, Mi, Yi, and Ki signals from the respective buffer 
memories 331, 332, 333, and 334 to the lines 61, 62, 63, and 64 by means 
of the control signal CT2, as shown in FIG. 10. 
If it is assumed that the printing by the image output device 350 is 
effected in the order of K, C, M, and Y, a conversion table for subjecting 
the Ki signals to gradation conversion into the Ko signals for the image 
output device 350 is set in the one-dimensional LUT 23 from the ROM 90 by 
means of the control signal CT3 immediately after the timing T0. In 
addition, during the period Q1, the selector 80 is set in a state for 
selecting the signals on the line 64 by means of the control signal CT4. 
However, since the CiMiYiKi signals from the printing scanners 212, 222 and 
the external storage medium 243 shown in FIG. 8 are respectively 
device-dependent signals, and are mutually different, an appropriate 
conversion table is selected as the conversion table set in the 
one-dimensional LUT 23 depending on the scanner or external storage medium 
from which the CiMiYiKi signals, i.e., the input color signals, are 
inputted. 
Accordingly, during the period Q1, the Ki signals on the line 64 are 
subjected to gradation conversion by the one-dimensional LUT 23, which, in 
turn, outputs the Ko signals to the image output device 350. 
Immediately after the period Q1, conversion tables respectively having 
input/output characteristics of using input values directly as output 
values, as shown in FIG. 4, are set in the one-dimensional LUTs 71 to 73 
and 23 from the ROM 90. In and after the period Q2, the selector 80 is set 
in a state for selecting the output from the three-dimensional LUT color 
converting device 19. 
Then, immediately after the period Q1, i.e., immediately before the period 
Q2, a conversion table for converting the CiMiYi signals into Co signals 
for the image output device 350, such as the one set in the 
three-dimensional LUT color converting device 12 in the embodiment shown 
in FIG. 1, is set in the three-dimensional LUT color converting device 19 
from the ROM 90. However, an appropriate conversion table is selected as 
that conversion table depending on the scanner or external storage medium 
from which the CiMiYiKi signals, i.e., the input color signals, are 
inputted. 
Accordingly, during the period Q2, the CiMiYi signals on the lines 61 to 63 
are inputted from the one-dimensional LUTs 71 to 73 to the 
three-dimensional LUT color converting device 19 without being 
substantially converted, and are converted into the Co signals by the 
three-dimensional LUT color converting device 19. The Co signals are 
outputted from the one-dimensional LUT 23 to the image output device 350 
without being substantially converted. 
Immediately before the period Q3, a conversion table for converting the 
CiMiYi signals into the Mo signals for the image output device 350, such 
as the one set in the three-dimensional LUT color converting device 13 in 
the embodiment shown in FIG. 1, is set in the three-dimensional LUT color 
converting device 19 from the ROM 90. However, an appropriate conversion 
table is selected as that conversion table depending on the scanner or 
external storage medium from which the CiMiYiKi signals are inputted. 
Accordingly, during the period Q3, the CiMiYi signals on the lines 61 to 
63, i.e., the signals from the one-dimensional LUTs 71 to 73, are 
converted into the Mo signals by the three-dimensional LUT color 
converting device 19, and the Mo signals are outputted from the 
one-dimensional LUT 23 to the image output device 350. 
Similarly, immediately before the period Q4, a conversion table for 
converting the CiMiYi signals into the Yo signals for the image output 
device 350 is set in the three-dimensional LUT color converting device 19. 
During the period Q4, the CiMiYi signals are converted into the Yo signals 
by the three-dimensional LUT color converting device 19, and the Yo 
signals are outputted from the one-dimensional LUT 23 to the image output 
device 350. 
In the above-described manner, the CoMoYoKo signals, which are obtained by 
converting the CiMiYiKi signals, i.e., the input color signals, are 
transmitted from the image processing apparatus to the image output device 
350 in a frame-sequential manner in the order of the Ko, Co, Mo, and Yo 
signals. Each plate is printed by the image output device 350 in that 
order, thereby forming a CMYK color image. It goes without saying that the 
order of outputting the CoMoYoKo signals from the image processing 
apparatus can be arbitrarily set by the control unit 320 in correspondence 
with the order of printing by the image output device 350. 
In a case where the Lab signals from the external storage medium 253 shown 
in FIG. 8 are transmitted as input color signals to the printer 300, a 
one-page portion of L, a, and b signals is stored in the buffer memories 
331, 332, and 333 by means of the control signal CT1 from the control unit 
320. 
In the case where the input color signals are the Lab signals in the 
above-described manner, the switching circuit 60 is changed over in such a 
manner as to direct the L, a, and b signals from the respective buffer 
memories 331, 332, and 333 to the lines 61, 62, and 63 and the L signal 
from the buffer memory 331 to the line 64 as well. 
In a case where a full-color image is outputted, a one-page portion of the 
Lab signals is stored in the buffer memories 331 to 333 until the timing 
T0 shown in FIG. 11, the one-page portion of the Lab signals is repeatedly 
read from the buffer memories 331 to 333 by means of the control signal 
CT1 during the periods Q1 to Q4 subsequent to the timing T0. Accordingly, 
as shown in the drawing, during the periods Q1 to Q4, the L, a, and b 
signals are repeatedly outputted to the lines 61, 62, and 63, and the L 
signals are repeatedly outputted to the line 64. 
However, in a case where a full-color image is outputted, the selector 80 
is constantly set in a state for selecting the signals from the 
three-dimensional LUT color converting device 19 by means of the control 
signal CT4, so that the L signals on the line 64 are not involved in 
outputting an image. 
Until immediately after the timing T0 at the latest, conversion tables for 
correcting the difference in size between the color reproduction space of 
input color signals, i.e., the Lab signals, and the color reproduction 
space in the image output device 350 are set in the one-dimensional LUTs 
71 to 73 from the ROM 90 by means of the control signal CT3. 
Accordingly, the Lab signals on the lines 61 to 63 are subjected to range 
conversion by the one-dimensional LUTs 71 to 73 in correspondence with the 
color reproduction space in the image output device 350. Thus, as shown in 
FIG. 11, during the periods Q1 to Q4, L', a', and b' signals subjected to 
range conversion are obtained from the one-dimensional LUTs 71, 72, and 
73. 
If it is assumed that the printing by the image output device 350 is 
effected in the order of K, C, M, and Y, until immediately after the 
timing T0 at the latest, a conversion table for converting the L'a'b' 
signals into the K signals is set in the three-dimensional LUT color 
converting device 19 from the ROM 90 by means of the control signal CT3, 
while a conversion table for gradation correction of the K signals is set 
in the one-dimensional LUT 23 from the ROM 90. 
Therefore, during the period Q1, the L'a'b' signals from the 
one-dimensional LUTs 71 to 73 are converted into the K signals by the 
three-dimensional LUT color converting device 19. Further, the K signals 
are subjected to gradation correction by the one-dimensional LUT 23, 
thereby making it possible to obtain the Ko signals for the image output 
device 350. However, the gradation correction characteristic of the 
one-dimensional LUT 23 is provided such that at the time when the image on 
a K plate is outputted by the image output device 350 by means of the Ko 
signals from the one-dimensional LUT 23, the dot area ratio and lightness 
of the K-plate image assume a linear relationship. 
Immediately after the period Q1, a conversion table for converting the 
L'a'b' signals into the C signals is set in the three-dimensional LUT 
color converting device 19 from the ROM 90, and a conversion table for the 
gradation correction of the C signals is set in the one-dimensional LUT 23 
from the ROM 90. 
Accordingly, during the period Q2, the L'a'b' signals from the 
one-dimensional LUTs 71 to 73 are converted into the C signals by the 
three-dimensional LUT color converting device 19, and the C signals are 
further subjected to gradation correction by the one-dimensional LUT 23, 
thereby making it possible to obtain the Co signals for the image output 
device 350. The gradation correction characteristic of the one-dimensional 
LUT 23 at this time is also provided such that at the time when the image 
on a C plate is outputted by the image output device 350 by means of the 
Co signals from the one-dimensional LUT 23, the dot area ratio and 
lightness of the C-plate image assume a linear relationship. 
Immediately after the period Q2, a conversion table for converting the 
L'a'b' signals into the M signals is set in the three-dimensional LUT 
color converting device 19 from the ROM 90, and a conversion table for the 
gradation correction of the M signals is set in the one-dimensional LUT 23 
from the ROM 90. 
Accordingly, during the period Q3, the L'a'b' signals from the 
one-dimensional LUTs 71 to 73 are converted into the M signals by the 
three-dimensional LUT color converting device 19, and the M signals are 
further subjected to gradation correction by the one-dimensional LUT 23, 
thereby making it possible to obtain the Mo signals for the image output 
device 350. The gradation correction characteristic of the one-dimensional 
LUT 23 at this time is also provided such that at the time when the image 
on an M plate is outputted by the image output device 350 by means of the 
Mo signals from the one-dimensional LUT 23, the dot area ratio and 
lightness of the M-plate image assume a linear relationship. 
Immediately after the period Q3, a conversion table for converting the 
L'a'b' signals into the Y signals is set in the three-dimensional LUT 
color converting device 19 from the ROM 90, and a conversion table for the 
gradation correction of the Y signals is set in the one-dimensional LUT 23 
from the ROM 90. 
Accordingly, during the period Q4, the L'a'b' signals from the 
one-dimensional LUTs 71 to 73 are converted into the Y signals by the 
three-dimensional LUT color converting device 19, and the Y signals are 
further subjected to gradation correction by the one-dimensional LUT 23, 
thereby making it possible to obtain the Yo signals for the image output 
device 350. However, since the Y signals consist of only a saturation 
component, the gradation correction characteristic of the one-dimensional 
LUT 23 at this time is provided such that at the time when the image on a 
Y plate is outputted by the image output device 350 by means of the Yo 
signals from the one-dimensional LUT 23, the dot area ratio and saturation 
(the saturation can be expressed by a square root of the sum of the square 
of an a-axis component and the square of a b-axis component in the Lab 
space) of the Y-plate image assume a linear relationship. 
In the above-described manner, the CoMoYoKo signals obtained by converting 
the Lab signals are outputted from the one-dimensional LUT 23 to the image 
output device 350 in a frame-sequential manner in the order of the Ko, Co, 
Mo, and Yo signals, thereby allowing a CMYK full-color image to be 
outputted from the image output device 350. In this case as well, the 
order of outputting the CoMoYoKo signals from the image processing 
apparatus can be arbitrarily set by the control unit 320 in correspondence 
with the order of printing by the image output device 350. 
In a case where a monochrome image is outputted, in the same way as the 
case of outputting a full-color image, the switching circuit 60 is changed 
over in such a manner as to direct the L, a, and b signals from the 
respective buffer memories 331, 332, and 333 to the lines 61, 62, and 63 
and the L signal from the buffer memory 331 to the line 64 as well. 
However, the selector 80 is set in a state for selecting the L signals on 
the line 64 oppositely to the case of outputting a full-color image. 
Then, the Lab signals are read from the buffer memories 331 to 333, and 
before the L signals are outputted to the line 64 a conversion table for 
subjecting the L signals to gradation conversion for monochrome use is set 
in advance in the one-dimensional LUT 23 from the ROM 90 by means of the 
control signal CT3. 
Accordingly, in the case where a monochrome image is outputted, the L 
signals obtained on the line 64 are subjected to gradation conversion by 
the one-dimensional LUT 23, and L" signals subjected to gradation 
conversion by the one-dimensional LUT 23 are supplied to the image output 
device 350, thereby outputting a monochrome image by the image output 
device 350. 
In this case, for the same reason as that described above for the case 
where a full-color image is outputted, in a case where a monochrome image 
formed on a K plate, a C plate, or an M plate is outputted, the gradation 
conversion characteristic of the one-dimensional LUT 23 is provided such 
that the dot area ratio and lightness of the output image of the K plate, 
C plate, or M plate assume a linear relationship. Meanwhile, in a case 
where a monochrome image formed on a Y plate is outputted, the gradation 
conversion characteristic of the one-dimensional LUT 23 is provided such 
that the dot area ratio and saturation of the output image of the Y plate 
assume a linear relationship. 
Also, the above-described operation basically applies to the case where RGB 
signals from the color scanner 232 shown in FIG. 8 are transmitted to the 
printer 300 as input color signals. 
That is, in this case, as shown in FIG. 11 which illustrates the case where 
the R, G, and B signals are stored in the buffer memories 331, 332, and 
333 as shown in FIG. 9, and a monochrome image is outputted, the switching 
circuit 60 is changed over in such a manner as to direct the R, G, and B 
signals from the buffer memories 331, 332, and 333 to the lines 61, 62, 
and 63 and direct the G signal from the buffer memory 332 to the line 64 
as well. The G signal corresponds to the L signal, i.e., the lightness 
signal among the Lab signals, and most contains lightness information 
among the RGB signals. 
Alternatively, the G, R, and B signals are respectively stored in the 
buffer memories 331, 332, and 333, and the switching circuit 60 is changed 
over in such a manner as to direct the G, R, and B signals from the buffer 
memories 331, 332, and 333 to the lines 61, 62, and 63 and direct the G 
signal from the buffer memory 331 to the line 64 as well. 
Then, in a case where a full-color image is outputted, in the same way as 
the case where the input color signals are the Lab signals, as shown in 
FIG. 11, the RGB signals obtained repeatedly on the lines 61 to 63 after 
being repeatedly read from the buffer memories 331 to 333 are converted 
into CoMoYoKo signals in a frame-sequential manner in the order of, for 
instance, Ko, Co, Mo, and Yo signals, thereby outputting a CMYK full-color 
image by the image output device 350. The gradation correction 
characteristic of the one-dimensional LUT 23 is also set in the same way 
as the case where the input color signals are the Lab signals. 
Also in the case where a monochrome image is outputted, in the same way as 
the case where the input color signals are the Lab signals, the G signals 
obtained on the line 64 are subjected to gradation conversion by the 
one-dimensional LUT 23, and G" signals subjected to gradation conversion 
by the one-dimensional LUT 23 are supplied to the image output device 350, 
thereby outputting a monochrome image by the image output device 350. The 
gradation conversion characteristic of the one-dimensional LUT 23 is also 
set in the same way as the case where the input color signals are the Lab 
signals. 
Although, in FIG. 8, only the Lab signals and the RGB signals are shown as 
input color signals consisting of three signals, such input color signals 
as XYZ signals and YCbCr signals can also be converted into the CoMoYoKo 
signals in a similar manner so as to output a CMYK full-color image. Also, 
a monochrome image of the C, M, Y, or K plate can be outputted by means of 
the lightness signal or the signal most containing the lightness 
information among the input color signals. 
In accordance with the above-described embodiment shown in FIG. 9, the 
CiMiYiKi signals which are particular CMYK signals can be converted into 
the CoMoYoKo signals which are other CMYK signals by the simple converting 
means comprising one three-dimensional LUT color converting device 19 and 
four one-dimensional LUTs 71 to 73 and 23. In addition, such color signals 
as the Lab signals and the RGB signals can also be converted into the 
CoMoYoKo signals by the same converting means so as to output a CMYK 
full-color image. Further, a monochrome image of the C, M, Y, or K plate 
can be outputted by means of color signals such as the Lab signals and the 
RGB signals. 
FIG. 12 shows an image processing apparatus according to a further 
embodiment of the invention which apparatus is used in the image output 
system shown in FIG. 8. The color converting unit 340 in this embodiment 
is arranged such that the switching circuit 60 assigns and transmits the 
signals read from the buffer memories 331 to 334 to the three output lines 
61 to 63 by means of the control signal CT2. In this example, the selector 
80 shown in FIG. 9 is not provided, and the output from the 
three-dimensional LUT color converting device 19 is inputted directly to 
the one-dimensional LUT 23. Accordingly, the control signal CT4 in the 
embodiment shown in FIG. 9 is not required in this embodiment. 
In a case where CiMiYiKi signals, which are particular CMYK signals from 
the printing scanners 212, 222 or the external storage medium 243 shown in 
FIG. 8, are transmitted to the printer 300 as input color signals, a 
one-page portion of Ci, Mi, Yi, and Ki signals are stored in advance in 
the buffer memories 331, 332, 333, and 334 until the timing T0, as shown 
in FIG. 13, in the same way as in the embodiment of FIG. 9. Then, during 
the periods Q1, Q2, Q3, and Q4 subsequent to the timing T0, a one-page 
portion of CiMiYiKi signals is repeatedly read from the buffer memories 
331 to 334. 
In this case, if it is assumed that the printing by the image output device 
350 is effected in the order of K, C, M, and Y, a conversion table having 
the characteristic of using input values directly as output values, as 
shown in FIG. 4, is set in a one-dimensional LUT 71 from the ROM 90 by 
means of the control signal CT3 immediately after the timing T0. 
Meanwhile, conversion tables for setting the respective output values to 
zeros irrespective of input values are set in the one-dimensional LUTs 72 
and 73. Further, a conversion table for setting the input values only with 
respect to the axial direction of the signals from the one-dimensional LUT 
71 directly as output values is set in the three-dimensional LUT color 
converting device 19. Furthermore, a conversion table for subjecting the 
Ki signals to gradation conversion into Ko signals for the image output 
device 350 is set in the one-dimensional LUT 23. 
As shown in FIG. 13, the switching circuit 60 is changed over in such a 
manner as to direct the Ki signals from the buffer memory 334 to the line 
61 during the period Q1 by means of the control signal CT2. Outputs from 
the lines 62 and 63 are forcibly set at zeros, respectively, or the Ki 
signals are also directed from the buffer memory 334 to the lines 62 and 
63 in an overlapping manner. Alternatively, the Ci, Mi, or Yi signals from 
the buffer memories 331, 332 or 333 are directed thereto as a dummy. 
Accordingly, during the period Q1, the Ki signals on the line 61 are 
inputted to the one-dimensional LUT 23 without being substantially 
converted by the one-dimensional LUT 71 and the three-dimensional LUT 
color converting device 19, and are subjected to gradation conversion into 
Ko signals by the one-dimensional LUT 23. Thus, the Ko signals are 
outputted from the one-dimensional LUT 23 to the image output device 350. 
After the period Q1, the operation is the same as that for the embodiment 
shown in FIG. 9. That is, immediately before the period Q2, a conversion 
table having the input/output characteristic of using input values 
directly as output values, as shown in FIG. 4, is set in the 
one-dimensional LUTs 71 to 73 and 23 from the ROM 90. At the same time, a 
conversion table for converting the CiMiYi signals into Co signals for the 
image output device 350 is set in the three-dimensional LUT color 
converting device 19. 
Accordingly, during the period Q2, the CiMiYi signals on the lines 61 to 63 
are inputted from the one-dimensional LUTs 71 to 73 to the 
three-dimensional LUT color converting device 19 without being 
substantially converted, and are converted into the Co signals by the 
three-dimensional LUT color converting device 19. The Co signals are 
outputted from the one-dimensional LUT 23 to the image output device 350 
without being substantially converted. 
Immediately before the period Q3, a conversion table for converting the 
CiMiYi signals into Mo signals for the image output device 350 are set in 
the three-dimensional LUT color converting device 19 from the ROM 90. 
Hence, during the period Q3, the CiMiYi signals are converted into the Mo 
signals by the three-dimensional LUT color converting device 19, and the 
Mo signals are outputted from the one-dimensional LUT 23 to the image 
output device 350. 
Similarly, immediately before the period Q4, a conversion table for 
converting the CiMiYi signals into Yo signals for the image output device 
350 is set in the three-dimensional LUT color converting device 19. During 
the period Q4, the CiMiYi signals are converted into the Yo signals by the 
three-dimensional LUT color converting device 19, and the Yo signals are 
outputted from the one-dimensional LUT 23 to the image output device 350. 
In a case where the Lab signals from the external storage medium 253 or the 
RGB signals from the color scanner 232 are transmitted to the printer 300 
as input color signals (see FIG. 8), a one-page portion of the L, a, and b 
signals or the R, G, and B signals is stored in the buffer memories 331, 
332, and 333, and the switching circuit 60 is changed over in such a 
manner as to direct the L, a, and b signals or the R, G, and B signals 
from the buffer memories 331, 332, and 333 to the lines 61, 62, and 63. 
Meanwhile, in a case where a full-color image is outputted, in the same way 
as in the embodiment shown in FIG. 9, during the periods Q1 to Q4, a 
one-page portion of the Lab signals or RGB signals is repeatedly read from 
the buffer memories 331 to 333, and conversion tables similar to those of 
the embodiment shown in FIG. 9 are set in the one-dimensional LUTs 71 to 
73 and 23 from the ROM 90. Hence, as is apparent if it is considered that 
the output from the line 64 and the output from the selector 80 are 
omitted in FIG. 11 that relates to the embodiment shown in FIG. 9, in the 
same way as in the embodiment shown in FIG. 9, the Lab signals or the RGB 
signals are converted into the CoMoYoKo signals in a frame-sequential 
manner, and a CMYK full-color image is outputted by the image output 
device 350. 
Also in a case where a monochrome image is outputted, the L signals from 
the buffer memory 331 if, for example, the input color images are the Lab 
signals, and the G signals from the buffer memory 332 if the input color 
signals are the RGB signals, are respectively directed to the line 61. In 
addition, the outputs from the lines 62 and 63 are forcibly set at zeros, 
respectively, and conversion tables similar to those set immediately 
before the period Q1 in the case of outputting a full-color image is set 
in advance in the one-dimensional LUTs 71 to 73 and the three-dimensional 
LUT color converting device 19 from the ROM 90. Also, a conversion table 
similar to that set immediately before the period Q1 in the case of 
outputting a monochrome image in the embodiment shown in FIG. 9 is set in 
advance in the one-dimensional LUT 23, Thus, in the same way as the 
embodiment shown in FIG. 9, a monochrome image of the K, C, M, or Y plate 
is outputted from the image output device 350. 
In the embodiment shown in FIG. 12 as well, it is possible to obtain 
advantages similar to those of the embodiment shown in FIG. 9. However, if 
a further comparison is made between the two embodiments, the embodiment 
in FIG. 9 has the advantage of shortening the processing time since 
wasteful reference to the conversion tables and interpolation calculation 
in the one-dimensional LUTs 71 to 73 and the three-dimensional LUT color 
converting device 19 are eliminated with respect to the gradation 
conversion of the Ki signals in the conversion from the CiMiYiKi signals 
into the CoMoYoKo signals and the gradation conversion of the L signals 
and the G signals in outputting a monochrome image from input color 
signals such as the Lab signals and the RGB signals. 
In contrast, the embodiment shown in FIG. 12 has the advantage that the 
circuit configuration can be made simple since the operation of selecting 
signals by the selector 80 in the embodiment in FIG. 9 is omitted, and 
conversion can be effected merely by the setting of conversion tables in 
the one-dimensional LUTs 71 to 73 and 23 and the three-dimensional LUT 
color converting device 19 from the ROM 90 except for the assigning of 
signals by the switching circuit 60. 
It should be noted that, if only the case of converting the CiMiYiKi 
signals into the CoMoYoKo signals is considered, the embodiments in FIGS. 
9 and 12 may be arranged such that the signals on the lines 61 to 63 are 
directly inputted to the three-dimensional LUT color converting device 19. 
In that case, in the embodiment shown in FIG. 12, it suffices if the 
following settings are provided. That is, during the period in which the 
Ki signals are converted into the Ko signals as during the period Q1 shown 
in FIG. 13, the Ki signals from the buffer memory 334 are directed to the 
line 61, and the outputs from the lines 62 and 63 are forcibly set at 
zeros, respectively. Immediately before then, a conversion table for using 
input values directly as output values only with respect to the axial 
direction of the signals on the line 61 is set in the three-dimensional 
LUT color converting device 19. Alternatively, during the period during 
which the Ki signals are converted into the Ko signals, the Ki signals 
from the buffer memory 334 are directed to all of the lines 61 to 63, and 
immediately before then a conversion table for using input values directly 
as output values with respect to all the axial directions is set in the 
three-dimensional LUT color converting device 19 immediately before then. 
Also in the embodiments shown in FIGS. 9 and 12, the buffer memory unit 330 
is merely an example of a means which is capable of repeating the input 
color signals. The buffer memory unit 330 is not necessarily required if 
consideration is given to only those input color signals which are stored 
in an external storage medium, and in a case where the input color signals 
can be repeatedly obtained from the external storage medium. 
As described above, in accordance with the present invention, particular 
CMYK signals can be converted into other CMYK signals with a simple 
configuration equivalent to that in the case adapted for three inputs in 
terms of the table size and the interpolation configuration of the color 
converting means of the conversion-table referencing type, and with high 
accuracy equivalent to that in the case adapted for four inputs in terms 
of the conversion accuracy.