Color corrector in an apparatus for producing color image

A color corrector for correcting chromaticity of a color image to be reproduced is provided in a color printer or a CRT display device for reproducing a color image from respective signals corresponding to predetermined primary colors of a plurality of primary colors constituting the color image. This color corrector comprises: a corrective operation circuit for executing an operation between a color correction coefficient matrix and signals corresponding to the primary colors to correct relative levels of the signals corresponding to the primary colors; a gray component extraction circuit for making level comparison between the signals corresponding to the primary colors to thereby extract a gray component; a correction signal output circuit for executing an operation between the extracted gray component and correction coefficients determined by the matrix to output gray balance correction signals every primary colors; and an adding circuit for adding signals of which levels are corrected in correspondence with respective primary colors outputted from the corrective operation circuit to outputs from the correction signal output circuit.

BACKGROUND OF THE INVENTION 
This invention relates to a color corrector in a color image reproducing 
apparatus for displaying/printing a color image on the basis of color 
signals or color data (hereinafter referred to as video signals) 
corresponding to primary colors such as Red (R), Green (G) and Blue (B), 
etc., and more particularly to a color corrector in a color image 
reproducing apparatus for converting video signals of R, G and B to 
recording density signals or data (hereinafter referred to as recording 
density signals) of Cyan (C), Magenta (M) and Yellow (Y) which are 
complementary colors thereof, respectively. 
For example, in color Televisions (TV), an approach is employed to 
represent a color image as sum of respective components of the three 
primary colors respectively having predetermined chromaticities to further 
convert electro-optic conversion characteristics of emitting bodies of 
respective primary colors on the reproducing side to electric signals to 
which correction (which is called a .gamma.-correction) is implemented. 
Accordingly, in reproducing such electric signals, three primary colors 
having the same chromaticities and conversion characteristics as those 
used at the conversion to electric signals are used in principle. 
In practice, for the reason why it is difficult to increase the brightness 
of a display image, there are instances where primary colors of which 
chromaticities deviate from the above-mentioned predetermined 
chromaticities are used as a light source for regenerative display. 
Further, in the case of providing a hard copy from the above-described 
electric signals by using a printer, an approach is employed to convert 
signals corresponding to the above three primary colors to signals 
corresponding to Cyan, Magenta and Yellow (the subtractive primaries) 
which are respectively complementary colors thereof to convert 
.gamma.-corrected signals to original ones (inverse .gamma.-correction) so 
that density signals corresponding to respective complementary colors are 
provided to carry out printing by using these density signals thus to 
reproduce or reconstruct an image. Also in this case, it frequently takes 
place that inks used have the relationship that their colors are not 
completely complementary to the original three primary colors (R, G, B). 
In the case where the chromaticities of primary colors used at the time of 
reproduction or reconstruction of an image deviate from desired 
chromaticities as in the above-mentioned example, colors of a display 
image or hard copy are reconstructed or reproduced in a manner that their 
chromaticities also deviate from those of the original image. In order to 
correct such a deviation, a color corrector is used. 
A conventional example of a color corrector for printer used for the above 
purpose is shown in FIG. 1. 
This color corrector circuit 50 comprises a frame memory 51 for storing 
color video signals R, G and B corresponding to respective primary colors 
(Red, Green, Blue), an inverse .gamma.-correction/luminance-density 
conversion circuit 52 for implementing an inverse .gamma.-correction to 
color video signals R, G and B read out from the frame memory 51 to 
provide signals of R.gamma., G.gamma. and B.gamma., and to convert these 
.gamma.-corrected color video signals R.gamma., G.gamma. and B.gamma. to 
optical density signals DR, DG and DB of Cyan, Magenta and Yellow which 
are respectively complementary colors thereof, and a color masking circuit 
53 provided with the optical density signals DR, DG and DB to output 
printing or recording density signals C, M and Y. 
As previously described above, since, in most cases, inks (dye stuffs) of 
respective colors of Cyan, Magenta and Yellow used in a printer have not 
desired optical characteristics as the primary colors, for example, a 
component of a primary color is mixed with other colors, when printing is 
simply carried out in accordance with the optical density signals DR, DG 
and DB, a hard copy having a chromaticity deviating from that of the 
original image may be provided. 
The color masking circuit 53 carries out a signal processing based on the 
color masking matrix expressed as the following equation (1) in order to 
lessen the influence of the above-mentioned contamination color, thus to 
output recording density signals C, M and Y such that a reproduced color 
on a hard copy has a density close to the density corresponding to optical 
density signals DR, DG and DB. 
##EQU1## 
Generally, the color correction coefficient Aij shown in FIG. (1) is set by 
using various least square methods. These least square methods are 
described in detail in, e.g., "Journal of the Society of Image 
Electronics" Vol. 18, No. 1, 1989, pp. 20 to 28. 
However, with a method of setting the correction coefficient Aij so that 
each reproducibility of intermediate colors becomes good by the least 
square method as shown in the above example, there are instances where the 
gray balance of a reproduced color on a hard copy or a display image is 
collapsed or destroyed. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a novel color corrector capable 
of preventing collapse or loss of gray balance followed by color 
correction in a display or printing of a color image, thus to carry out a 
clear color correction free from such a collapse. 
To attain the above-described object, a color corrector according to this 
invention comprises an element for performing a predetermined operation on 
the basis of a correction coefficient matrix obtained by the least square 
method and signals corresponding to inputted respective primary colors, an 
element for correcting intermediate colors reproduced by the above 
operation, an element for separating a gray component from signals 
corresponding to respective primary colors delivered to the element for 
correcting intermediate colors, an element for performing an operation to 
determine correction signals of the gray component on the basis of the 
separated gray component and color correction coefficients every primary 
colors, and an element for adding the correction signals of the gray 
component outputted from the last-mentioned operation element to outputs 
from the element for correcting intermediate colors, thus to carry out 
color correction free from collapse of the gray balance by the action of 
these elements. 
In accordance with one aspect of this invention, the color corrector 
comprises a maximum level primary color signal selection circuit for 
selecting a signal of which signal level is maximum from three primary 
color signals of a color image inputted to a color masking circuit to 
output the selected signal, multiplying circuits for implementing a 
predetermined signal processing to an output signal from the maximum level 
primary color signal selection circuit on the basis of color correction 
coefficients in respective row directions of the color masking matrix, the 
multiplying circuits being correspondingly provided every respective rows 
of the color masking matrix, and adding circuits for adding output signals 
from the multiplying circuits correspondingly provided every respective 
rows to respective recording density signals of three colors outputted 
from the color masking circuit. 
The maximum level primary color signal selection circuit selects a signal 
of the maximum level from the three primary color signals of a color image 
to output that signal. 
Respective multiplying circuits implements a predetermined operational 
processing to an output signal from the maximum level primary color signal 
selection circuit on the basis of coefficients every rows of the color 
masking matrix to output the result thereof. 
Respective adding circuits respectively add outputs from the respective 
multiplying circuits to recording density signals of respective colors 
serving as outputs from the color masking circuit to output gray component 
correction recording density signals. 
By carrying out printing on the basis of these gray component correction 
recording density signals, it is possible to provide a recording 
(printing) result having an excellent reproducibility of an intermediate 
color without allowing the gray balance to be destroyed. 
As described above, the color corrector for color printer according to this 
invention is of a structure to select a signal of the maximum level from 
respective primary color luminance signals to multiply it by a 
predetermined coefficients to add them to respective outputs from the 
color masking circuit. Thus, a print out excellent in reproducibility of 
an intermediate color and free from collapse of an achromatic component 
can be provided by a simple configuration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred embodiments of a color corrector for a color printer according to 
this invention will now be described in detail with reference to the 
attached drawings. In this invention, there are presented a first 
embodiment most suitable for a hard copy, etc. to carry out color 
correction by the subtractive mixture of colors and a second embodiment 
most suitable for a CRT display to carry out color correction by the 
additive mixture of colors. Explanation will be given in succession in 
connection with these embodiments. 
FIG. 2 is a block diagram showing the configuration of a color corrector 
according to the first embodiment using the subtractive mixture of colors, 
and FIGS. 3(a) to (i) are characteristic diagrams when color correction is 
carried out by using this color corrector. In the block diagram of FIG. 2, 
this color corrector 1 for a color printer comprises an inverse 
.gamma.-correction/logarithmic conversion circuit 2 for implementing 
inverse .gamma.-correction and logarithmic conversion to three primary 
color video signals R, G and B of a color image applied to respective 
input terminals 2R, 2G and 2B, wide-band amplifying circuits (AMP) 3R, 3G 
and 3B for amplifying output signals RO, GO and BO from the inverse 
.gamma.-correction/logarithmic conversion circuit 2, a color masking 
circuit 4 for implementing a signal processing based on the matrix 
represented by the above equation (1) to output signals RA, GA and BA from 
the respective wide-band AMPs 3R, 3G and 3B to output the results thereof, 
clamping circuits 5R, 5G and 5B for allowing the pedestal levels of output 
signals RA, GA and BA from the respective wide-band AMPs 3R, 3G and 3B to 
match with each other, a maximum level primary color signal selection 
circuit 6 for making an operation of comparison between signal levels of 
output signals RC, GC and BC from the respective clamping circuits 5R, 5G 
and 5B, or similar operation to select a signal of the maximum level to 
output that signal, multiplying circuits 7C, 7M and 7M for implementing a 
predetermined signal processing to an output signal 6a from the maximum 
level primary color signal selection circuit 6 to output the results 
thereof, adding circuits 8C, 8M and 8Y for adding output signals HC, HM 
and HM from corresponding multiplying circuits 7C, 7M and 7Y to respective 
output signals 4c, 4m and 4y from the color masking circuit 4, and output 
terminals 9C, 9M and 9Y for recording signals connected to the output 
terminals of the respective adding circuits 8C, 8M and 8Y. 
More particularly, the respective multiplying circuits 7C, 7M and 7Y are of 
a structure to carry out a signal processing so as to satisfy the 
relationship represented by the following equation (2). Practically, these 
circuits are comprised of one or plural operational amplifiers and the 
peripheral circuit parts thereof, etc. 
##EQU2## 
It is to be noted that while explanation has been given in connection with 
an analog signal in the first embodiment, in the case where a digital 
image signal is dealt, these multipliers 7C, 7M and 7Y may be comprised of 
a digital signal processing circuit and a conversion Table constituted 
with ROM, etc., or a CPU and programs for the computational processing, 
etc. 
Further, the maximum level signal selection circuit 6, the respective 
multipliers 7C, 7M and 7Y, and the respective adders 8C, 8M and 8Y may be 
comprised of a single digital signal processing circuit or a processor for 
digital signal processing. In addition, the color masking circuit 4 may be 
united in such a digital signal processing circuit or digital signal 
processor. 
In principle, color corrective operation is performed to color signals 
extracted their gray components beforehand and thereafter the gray 
components are added to corrected color signals for maintaining the gray 
balance of a reproduced image from them. 
In above-mentioned configuration, the color corrective operation is 
performed to color signals with their gray components and the gray 
components are extracted from the color signals corresponding to the 
portions where inks such as C, M and Y overlap with each other. Then 
compensation signals for recovering the gray balance calculated from the 
gray components so extracted and the coefficients of the color corrective 
operation, are added to the color corrected signals. These two methods 
provide the same result. 
The principle thereof will now be described with reference to the following 
equations (3) to (8). 
First, conversion from respective primary color video signals R, G and B to 
density signals DR, DG and DB is carried out. When the maximum values of 
the respective primary color luminance signals R, G and B are represented 
by RM, GM and BM, respectively, respective density signals DR, DG and DB 
are expressed by the following equation (3): 
##EQU3## 
It is assumed that, for keeping the gray balance unchanged during color 
corrective operation, the gray component is extracted previously. When 
respective density signals in the case where the gray component is added 
thereafter are represented by DRd, DGd and DBd, respectively, these 
density signals are expressed by the following equation (4): 
##EQU4## 
When substitution of the equation (3) into the equation (4) is made to 
carry out arrangement of the equation with respect to the density signal 
DRd, the density signal DRd is expressed by the following equation (5). 
##EQU5## 
Conversion from the density signal DRd to a recording signal C for print is 
expressed by the following equation (6). 
##EQU6## 
Here, if the maximum luminance values RM, GM and BM that the respective 
primary color video signals R, G and B can take satisfy the relationship 
represented by the following equation (7), the recording signal C is 
expressed by the following equation (8): 
EQU RM=GM=BM=W110 (7) 
where W110 indicates a value when the maximum level is assumed as 110IRE. 
##EQU7## 
When similar calculations are carried out also in connection with Magenta 
and Yellow to make arrangement of the equation (8), recording signals C, M 
and Y are expressed by the following equation (9): 
##EQU8## 
The first term of this equation (A11RA+A12GA+A13BA) is realized in the 
color masking circuit 4. 
The term (W110 -GRAY) in the above equation (9) will now be described with 
reference to FIG. 3. 
FIG. 3(a) to (c) show waveforms of respective primary color video signals 
RA, GA and BA wherein the abscissa and the ordinate represent time and the 
signal level, respectively. According as the signal level becomes high, 
the luminance becomes high. 
FIG. 3(d) is a signal waveform in which only the maximum level of each of 
the primary color video signals RA, GA and BA shown in FIGS. 3(a) to (c) 
is selected and represented by the solid line. This corresponds to an 
output signal 6a from the maximum level primary color selection circuit 6. 
FIGS. 3(e) to (g) show respective density signals DR, DG and DB (the right 
side in the figure) obtained by subtracting signals corresponding to 
primary color video signals RA, GA and BA of FIG. 2 (the center in the 
figure) from signals (the left side in the figure) showing the maximum 
luminance level W110. 
FIG. 3(h) shows by the solid line the minimum level of each of the density 
signals DR, DG and DB shown in FIGS. 3(e) to (g). The solid line 
corresponds to the gray component. 
FIG. 3(i) shows a difference between the signal indicating the maximum 
luminance level W110 and the gray component, and the waveform on the right 
side of this figure is the same as that shown in FIG. 3(d). 
As stated above, differences between signals obtained by extracting, as the 
gray component, the minimum levels of density signals DR, DG and DB 
converted from the primary color video signals R, G and B and a signal 
indicating the maximum luminance level W110 are provided by selecting the 
maximum levels of the respective three primary color video signals R, G 
and B. Thus, the second term of the equation (9) 
[(W110-GRAY)(1A11-A12-A13)]is realized in the multiplying circuit 7C. 
It is to be noted while it has been described that the color corrector of 
the first embodiment is constructed to carry out a color correction by the 
subtractive mixture of colors in order to apply this color corrector to a 
color printer, etc., this invention is not limited to such an embodiment. 
For example, in the case where this invention is applied to a CRT display 
device, etc., also by employing a second embodiment where color correction 
by the additive mixture of colors which will be described below is carried 
out, effects and/or advantages similar to the above are provided. 
FIG. 4 is a block diagram showing a color corrector according to the second 
embodiment of this invention. In this second embodiment, the same 
technique as that of the first embodiment is applied to color correction 
in a regenerative display of a device such as CRT, etc. 
In the case of CRT display, since a color image is reproduced by the 
additive mixture of colors, the conversion to complementary colors and the 
inverse .gamma.-correction required in providing a hard copy to reproduce 
a color image by the subtractive mixture of colors become unnecessary. 
Accordingly, color video signals corresponding to respective primary 
colors and color correction coefficient A*ij are subjected to matrix 
operation at a matrix operation circuit 40. 
On the other hand, respective color video signals R, G and B undergo a 
processing such that the d.c. levels are caused to match with each other 
at clamping circuits 5R, 5G and 5B. Thereafter, a gray component is 
extracted at a minimum level primary color signal selection circuit 60. 
Then, the gray component thus extracted is multiplied by respective 
correction coefficients K1, K2 and K3 obtained from the color correction 
coefficients A*ij at multiplying circuits 7R, 7G and 7B. Thus, correction 
signals of the gray component for the respective color signals are 
determined. Then, respective correction signals are added to color 
corrected signals at adding circuits or adders 8R, 8G and 8B. The signal 
thus obtained are outputted as gray balance corrected color signals R*, G* 
and B* from output terminals 9R, 9G and 9B. 
In this embodiment, since conversion to complementary colors is not carried 
out, the gray component becomes equal to the minimum value among the color 
video signals R, G and B corresponding to respective colors. The principle 
of the second embodiment is the same as that of the first embodiment 
except for this point. 
Namely, color corrective operation is applied to color signals without 
considering the gray balance of them and thereafter gray components 
obtained in harmony with that color corrective operation are added to 
color corrected signals for compensation. Where the gray component of the 
color video signal which has not yet undergone correction and the color 
correction coefficient are represented by GRAY and A*ij, respectively, 
color video signals R, G and B corresponding to respective primary colors 
finally obtained are expressed by the following equation (10): 
##EQU9## 
Arrangement of the above equation gives: 
##EQU10## 
The first to third terms of the right side of the above equation (11) 
correspond to a matrix operation, and the fourth term thereof indicates 
correction signals of the gray component. Accordingly, the correction 
coefficients K1, K2 and K3 are expressed by the following equation (12): 
##EQU11## 
As seen from the above-described two embodiments, the feature of this 
invention resides in that the color correction carried out in display or 
printing of a color image is such that the correction of the chromaticity 
of a reproduced color and the correction of deviation of the gray balance 
produced as the result of the former correction are carried out by 
separate systems. For this reason, color correction can be advantageously 
made to much degree without damaging the quality of am image displayed or 
a printed hard copy. Thus, the color corrector according to this invention 
is extremely advantageous to a printer for which a color correction is 
generally required to much degree.