Method for color correction for multi-color printing plate pictures

A method for color correction in the process for reproducing a picture in which color separation picture signals obtained by photoelectrically scanning an original picture are converted into picture signals which correspond to four printing color inks including a black ink therein to produce respective reproduction pictures for plate making corresponding to each of the color inks. A picture signal which corresponds to a black ink is produced according to gray color components obtained from B, G and R signals of the original picture, and according to chromatic color components obtained by subtracting neutral color components from said B, G and R signals. Color separation picture signals corresponding to other color inks, and deficiency in reproduction density of the black ink to be used in a black print is corrected by said other three color inks.

FIELD OF THE INVENTION 
The present invention relates to a method for color correction in the high 
neutral color density range in a process for reproducing a multi-color 
printing plate picture of all reproducible colors in printings. The method 
is accomplished by using colors, and principally two colored inks among 
the set of three colored inks of yellow(Y), magenta(M), and cyan(C). 
Auxiliarly, black(K) ink has also been used. 
DESCRIPTION OF THE PRIOR ART 
Conventionally, four color reproduction in the field of printing is 
achieved mainly by using three colored inks such as yellow(Y), magenta(M) 
and cyan(C), and in the cases in which the three colored inks are 
deficient in reproducing a picture having a plurality of color density 
areas, auxiliarly black(K) ink has been used for the purpose of expanding 
plural color density areas. 
Such a manner of black plate making mentioned above is termed as "skeleton 
black". On the contrary, there is another form of printing in which the 
quantity of black color ink for printing is increased for fully utilizing 
a neutral color component, together with two of the three colored inks, at 
the most. This manner of printing is termed as "full black manner". 
Between these two printing manners, there may be conceived various kinds of 
printing manners in which, according to the quantity of ink of black 
print, the quantity of the three color inks, Y, M and C, must be reduced. 
This is termed to be "under color removal" (UCR). 
The more black print resembles the full black print, more various 
advantages, such as cheaper cost in inks to be used, easier reproductivity 
in neutral color component and facilitating printing, are expected. 
However, because of difficulty of judgment in the effect of plate making 
or other reasons, manners of black print which are relatively closer to 
the skeleton black plate marking have been prevailingly used, while 
manners of the full black print or those closer thereto scarcely have been 
practically used. 
Recently, the advantage and excellency of the above described manner of the 
full black print have been reconsidered, and a printing method, in which 
neutral color components in the three color inks, Y, M and C are replaced 
with black(K) ink, has been proposed. 
Colors to be reproduced can be reproduced by using from one to three 
colored inks by replacing the quantity of neutral color with that of K ink 
at areas in which Y, M and C inks are overlappedly printed, so that, 
briefly speaking, it means that at all points on a printing each of them 
can be printed by using at the most any of three colored inks among the 
four. 
Regarding each of the quantities of Y, M and C colored inks, if the 
quantity of neutral color is replaced with the very one color ink K, the 
quantity of the ink to be used is reduced, which results in realizing 
large reduction in cost in the printing fee. 
However, in the method for using a conventional color scanner, if all the 
quantity of neutral color in the three colored inks is merely replaced 
with K ink in those areas containing quantities of Y, M and C colored inks 
(i.e., those areas at which density of neutral color is large), density 
reproducible on a paper to be printed by K ink is insufficient in 
comparison with that reproduced by a conventional method of overlapping Y, 
M and C three colored inks with K ink. Thus, no good finished printing 
result could be obtained. 
Accordingly, in conventional color scanners, it has been adapted that 
nearly 100% UCR (under color removal) can be practiced, and merely K print 
has been increased accordingly to a quantity of beng subjected to UCR, so 
that in high density ranges of neutral color, highly qualified 
reproduction could not be obtained. Thus, no remedy for solving the above 
mentioned disadvantages could be found, in that any method for saving 
colored inks in the condition of nearly 100% UCR have not been practiced. 
The applicant has already filed applications regarding techniques for color 
correction and graduation control by separating a color separation signal 
into chromatic components. These applications are disclosed in the 
Japanese Patent Laid-Open Publication Nos. 55-115043, 55-142342, 
55-142343, 55-142344 and 55-142345. 
Particularly, in the Japanese Patent Laid-Open Publication No. 55-115043, 
there is disclosed a technique for freely correcting and controlling hue 
and saturation based on the chromatic color component of 100% UCR ratio 
according to the conventional UCR technique. 
In addition, even in other disclosures, the same as the above mentioned, 
there are some in which chromatic color components are separated from each 
other in a state of the UCR ratio being 100%, and the chromatic color 
component is, as it stands, color corrected through a masking circuit, 
color separation circuit, etc., while the achromatic color component is 
used as a signal for producing a black print. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method for enabling 
printing, nearly the same as that which has been done conventionally under 
the condition of 100% or nearly 100% UCR ratio, by adding such a 
correction that no density areas of a reproduced printing can occur, even 
if 100% or nearly 100% UCR has been practiced. 
That is, in the present invention, the quantity of neutral color of Y, M 
and C three colored inks are replaced with K ink. Also, by the quantity of 
the neutral color according to the quantity of deficiency in density of K 
ink at the high density areas, Y, M and C three colored inks which are 
balanced with neutral color, are added for defining conventional density 
and sharpness areas. In case any inconvenience occurs for 100% UCR, a UCR% 
required quantity corresponding to (100-UCR)% of an ink, in which neutral 
color of Y, M and C three colors are balanced, is added. As mentioned 
above, the present invention aims to realize a cost reduction in the 
printing fee by reducing the quantity of colored inks and at the same time 
composing a simple circuit for achieving UCR. 
Another object of the present invention is to provide a method for 
obtaining a reproduced picture of high fidelity in the original picture 
and to realize beautiful color representation. 
In order to achieve these objects, in the present invention black print is 
not used as an auxiliary factor for three color printing in the skeleton 
manner, but is used as full black manner or those of closer manners 
thereto. In the case of applying black print as a principal factor, if the 
gray component is deficient in black print, deficiency in gray 
reproduction density is corrected by the other three colored inks. 
Further, correction of any turbid component of the colored inks correction 
is performed to each of the color plates. 
The present invention can be particularly easily applied to those 
inventions disclosed in the above described publications and can obtain a 
great effect, and further can be sufficiently applied to those 
conventional color correction methods using UCR.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
FIG. 1 is a graph showing a halftone dot area ratio when a black print is 
made from an achromatic color component obtained by separating color(s) 
into chromatic and achromatic color equivalents to a 100% UCR ratio, and 
gray density of a black ink printed by the black print. FIG. 1 shows 
further a manner of correction. As can be clearly seen from FIG. 1, 
comparison of the density characteristic (2) of an actually used black ink 
to a density characteristic (1) of an ideal black ink, represents the 
color density equivalent to the maximum density in the black portion. This 
has been conventionally obtained by four colors only by one color, and has 
an inclination of reducing density accordingly to advance to the higher 
density portion. 
To understand the characteristic easier, in FIGS. 1, 7, 10 and 11, the axis 
of abscissas is linear, but there are also cases in which the axis of 
abscissas is not linear. 
If the kind of black inks to be used in the black print is specified, and 
also the kind of colored inks to be used in other color plates are 
specified, portions at which density of the above described black ink is 
deficient can be corrected by gray density represented by composition of 
the other three colors: cyan(C), magenta(M) and yellow(Y). 
Gray density characteristic (3) is shown in a dotted line showing a gray 
density which is complementary with that deficient in the black ink. 
The complementary gray density characteristic (3) is a well-balanced gray 
density component by means of three colored inks (C), (M) and (Y), and can 
be previously obtained as cyan ink density characteristic (4), magenta ink 
density characteristic (5) and yellow ink density characteristic (6), in 
accordance with the conditions of printing. 
In addition, in the drawings, each of the ink density characteristics (4), 
(5) and (6) required for obtaining the complementary gray density 
characteristic is represented in the coordinates of the abscissa, which 
shows each of the ink color ink densities and the coordinates of which 
shows density of the original picture. Density of the original picture is 
shown as a characteristic curve initiating at a point (P) from which 
attenuation of black ink density begins. On the axis of the abscissa the 
point (P) is located at a point (M.sub.p). 
However, it is common that, for example, density area of the original 
picture does not coincide with that of printing, so that attention should 
be paid that the drawing is only a diagrammatical one. 
In the drawing (G.sub.p) indicates gray density at the point (P), (G.sub.m) 
indicates the maximum gray density obtained only by a black ink, 
(B.sub.max) shows the maximum gray density (black) required for a 
printing, and (W) is the minimum gray density (white) of the printing. 
A highlight point (white) and a shadow point (black) on the original 
picture in a print plate making are to be reproduced as approximate values 
to the minimum gray density (W) (described above) and the maximum gray 
density (B.sub.max) (described above), so that on operating the color 
scanner, for example, printing halftone dot ratio is previously set up to 
5% and 95%. 
In the above described setup state, when the UCR ratio is 100%, the black 
print signal in neutral color portion becomes complete full black manner 
and according to ideal black ink density characteristic (1), it is formed 
so as to reproduce gray density from W to B.sub.max linearly. 
However, in practice, because of insufficient density reproduction of the 
black ink, at densities higher than that of the original picture (M.sub.p) 
there occurs density deficiency. It is possible to know previously the 
gray density components corresponding to the above described insufficient 
quantity by corresponding them to the printing density of the black print. 
From the printing density, the deficient quantity (components of the three 
colored inks) to density of the ideal ink can be obtained through test 
prints. Accordingly, color components of three colors corresponding to 
complementary three color gray density characteristic are recorded in a 
memory means as a lookup table. By addressing the lookup table with 
achromatic color components, the gray component, which is insufficient 
with the black print, only is read out as color components of three 
colors, and by adding each of the color components to each signal of the 
corresponding color plates, color correction for compensating deficiency 
in density reproductivity of the black ink can be achieved. 
Thus, the method is applied not only to the case of the UCR ratio being 
100%, but can be applied to other cases in which UCR ratio is not 100%. In 
the latter cases, the present method can also be practiced in a similar 
manner as mentioned above, without subtracting UCR signal from the 
conventional Y, M and C signals containing chromatic color eand achromatic 
color components. 
That is, FIG. 4 is a view showing an equivalent characteristic of the 
characteristic of the black ink shown in FIG. 1 in the case of being 
reproduced by Y, M and C three colored inks. 
When the quantity of the black ink is assumed, for example, to K, and 
necessary percentage of UCR to U, the quantity of an ink to be added is 
K.times.(100-U)/100), which corresponds to K.times.(100-U)/100) % of black 
ink. 
The above described fact can be realized by adding quantities of C.sub.c, 
M.sub.c, and Y.sub.c colored inks shown in FIG. 4 to colored inks Y, M and 
C. 
If the above described relationship between K and U is stored in the lookup 
table, a necessary number of lookup tables may also be provided, for 
example, by scaling the value of U (percentage of UCR) by every one %. 
In the present method, the memory means is addressed by an achromatic color 
component signal or a black print signal of full black manner to read out 
densities of each of the color plates corresponding to gray density which 
is deficient in the addressed portion. Each of the gray density signals of 
respective colors is added to each picture signal of the respective color 
plates to increase deficiency in gray density in the black print. Thus, 
according to the present invention, color correction can be achieved. 
FIG. 2 is a block diagram illustrating an actual operational manner of the 
color correction method of the present invention in which deficiency in 
gray density reproduction regarding the above mentioned black print is 
being corrected. 
The reference number (10) designates an input means of a color scanner, and 
it outputs three color separation picture signals (R), (G) and (B) of an 
RGB color system obtained by photoelectrically scanning the original 
picture. 
In addition, each of processing circuits and each of signals which are 
hereinafter explained are digital circuits and digital signals, so that 
hereinafter abbreviation in expressing of the word "digital" is done. 
Each of the picture signals (R), (B) and (B) is input to an achromatic 
color separation circuit (11) in which an achromatic color component for 
producing a black print signal (K) is selectively separated from any of 
the largest in the picture signals (R), (G) and (B) and extracted. 
In the achromatic color separation circuit (11), value (W) of white level 
corresponding to the highlight point of the original picture is previously 
set by a register (12), and the achromatic color separation circuit (11) 
generates the achromatic color component signal or the black print signal 
(K) of full black manner by operating in accordance with the following 
formula: 
EQU K=[W-MAX(R, G, B)] (1) 
Here, MAX(R, G, B) designates a signal selected as having the largest value 
in each of the picture signals (R), (G) and (B) at every pixel being 
scanned in the original picture. 
For example, when each of the picture signals (R), (G) and (B) are in the 
level shown in FIG. 5(a), the picture signal (B) is selected as the MAX(R, 
G, B). 
In the formula (1) the selected picture signal (B) is subtracted from the 
value of white level (W), and value of the difference is set as an 
achromatic color signal (K). 
Each of the picture signals (R, (G) and (B) is input to a color system 
varying circuit (13) which converts RGB system color separation picture 
signals into YMC system color separation picture signals, and the color 
system varying circuit (13) subtracts each of the picture signals (R), (G) 
and (B) from the respective color signals (R), (B) and (B), according to 
complementary relations, Y=W-B, M=W-G, and C=W-R, and converts, as shown 
in FIG. 5(b), them into picture signals (C.sub.1), (M.sub.1) and (Y.sub.1) 
of YMC color system. 
Each of the color signals (C.sub.1), (M.sub.1) and (Y.sub.1) is input to a 
chromatic color separation circuit (14), and the chromatic color 
separation circuit (14) subtracts the achromatic color component signal 
(K) from each of the picture signals (C.sub.1), (M.sub.1) and (Y.sub.1). 
The achromatic color component signal (K), i.e., the black print signal, 
has, as shown in FIG. 5(a) and (b), at least one identical value with the 
picture signals (C.sub.1), (M.sub.1) and (Y.sub.1). As a result, the 
signals output from the chromatic color separation circuit (14) are 
composed of, as shown in FIG. 5(c) and considered as to each of pixels, at 
the most two, among picture signals (C.sub.2), (M.sub.2) and (Y.sub.2). 
The achromatic color component signal (K) is digitalized by an A/D 
converter (not shown), and the digital value is transmitted to a memory 
means (15). The memory means (15) reads out gray corrected data (.DELTA.G) 
according to the value of the achromatic color component signal (K). 
The gray corrected data (.DELTA.G) stored in the memory means (15) are 
stored in a lookup table manner so that each of the colored ink components 
(.DELTA.G.sub.c), (.DELTA.G.sub.M) and (.DELTA.G.sub.Y) of the gray 
corrected data may be obtained from each of the colored ink density 
characteristics which correspond to the complementary gray characteristic 
(3) shown in FIG. 1. 
Each of the color ink components (.DELTA.G.sub.C), (.DELTA.G.sub.M) and 
(.DELTA.G.sub.Y) is transmitted to respective adders (16C), (16M) and 
(16Y) to which each of the chromatic color picture signals C.sub.2), 
(M.sub.2) and (Y.sub.2) is input, respectively. 
The addresses (16C), (16M) and (16Y) add the chromatic color picture 
signals (C.sub.2), (M.sub.2) and (Y.sub.2) to each of the color ink 
components (.DELTA.G.sub.C), (.DELTA.G.sub.M) and (.DELTA.G.sub.Y) for 
gray correction so that each of the corresponding colors may be added, 
respectively. Each of the outputs of the respective adders is transmitted 
as a cyan plate signal (C.sub.3), a magenta plate signal (M.sub.3), and a 
yellow plate signal (Y.sub.3), to each of graduation circuits (17.sub.C), 
(17.sub.M), (17.sub.Y) and (17.sub.K) which are disposed at the after 
stage thereof, together with the achromatic color component signal, i.e., 
the black print signal (K) in the full black manner. 
Further the (K) signal is input to a multiplier (20). The other input of 
the multiplier (20) is given as (100-U)/100. 
The multiplier (20) performs the following multiplication, that is, 
quantity of black ink K.times.((100-U)/100) is carried out, and addresses 
a lookup table (15') by addressing the output thereof. As a result, each 
of quantities of the color inks C.sub.C, C.sub.M and C.sub.Y of the 
respective Y, M and C corresponding to K.times.((100-U)/100) shown in FIG. 
4 is output. 
The output of the above described are input, in the same manner, to the 
addresses (16.sub.C), (16.sub.M) and (16.sub.Y) to correct the quantity to 
be remained as UCR. 
Thus, deficiency in density reproductivity of the black ink is corrected by 
each of the color components gray density of which corresponds to the 
deficiency. With the result, the gray density equivalent to the 
characteristic (1) of the ideal black ink shown in FIG. 1 can be obtained 
on the reproduction picture. 
On the other hand, considering color inks, the corrected color data 
(.DELTA.G.sub.C), (.DELTA.G.sub.M) and (.DELTA.G.sub.Y) are previously 
determined so that gray balance including turbid components of color inks 
being used may be matched. Accordingly, wherever gray density areas to be 
corrected may be, no influence can be given to hue at all. However, 
correction for turbid components of actual inks is necessary for the 
chromatic color picture signals (C.sub.2), (M.sub.2) and (Y.sub.2). For 
this reason, at the after stage of the chromatic color separation circuit 
(14), there is provided a color operation circuit (18) to correct turbid 
components of inks of color plates. 
To the color operation circuit (18), a calculation method for color 
correction, which has been already filed by the present applicant and 
published as the Japanese patent Laid-Open Publication No. 55-115043, a 
masking circuit, a color correcting circuit, etc., having been disclosed 
in the Japanese patent Laid-Open Publication Nos. 55-142342 to 55-142345 
also filed by the Applicant, can be directly applied without any further 
modification. 
Further, as to the color operation circuit (18), it is preferred that such 
which comprises a fundamental masking circuit relying on conventional 
masking equations, a color correction circuit which carries out 
appropriate color correction, an under color removing circuit, etc., may 
be directly applied thereto. 
However, the black ink print signal (K) in the above mentioned full black 
manner produces a black print in a state of an UCR ratio 100% of the 
conventional UCR method, so that if masking treatment is carried out on 
the chromatic color signals (C.sub.2), (M.sub.2) and (Y.sub.2) produced by 
the aforementioned, there occurs some cases in which as quantities of inks 
to be used, negative values may be required of each of resultant color 
plate signals of (C'.sub.2), (M'.sub.2) and (Y'.sub.2). In such cases, a 
subtractor (19) is provided at the preceding stage to the memory means 
(15), the minimum (absolute value is the largest) among the picture 
signals of negative values corresponding to the quantities of the inks is 
selected, then the minimum signal is subtracted from the black print 
signal (K), and the subtracted gray component is changed by each of the 
color ink components in a color correction circuit (18b). Thus, the 
impropriety that the quantity of ink is a negative value can be solved. 
While in a pair of circuits of the achromatic color separation circuit 
(11), and the chromatic color separation circuit (14), the color 
correction circuit (18) is disposed immediately to the rear stage of the 
achromatic color system varying circuit (13), and between the color 
correction circuit (18') and the adder (16c), the chromatic color 
separation circuit (14) is disposed. (In the case of the color correction 
circuit (18) is shifted to the rear stage as above mentioned. Then it is 
referred to the color correcting circuit (18'). Further, inputs of the 
achromatic color separation circuit (11) are outputs obtained from the 
color system varying circuit (13) and the color correction circuit (18'). 
Thus, the output of the achromatic color separation circuit (11) may be a 
signal which corresponds to a value of the formula [W-MAX(R, G, B)]. 
In each of the colored inks there are turbid components. As a main part of 
which cyan (C) contains a turbid component (.DELTA.C.sub.M) of magenta (M) 
and a turbid component (.DELTA.C.sub.Y) of yellow (Y), and as another main 
part of which magenta (M) contains a turbid component (.DELTA.M.sub.Y) of 
yellow (Y). 
The masking circuit outputs each of the color picture signals (C'.sub.2), 
M'.sub.2) and (Y'.sub.2) by subtracting each of these turbid components 
from the corresponding respective colors. 
In this case, for example, as shown in FIG. 5(c), if cyan (C) and magenta 
(M) are determined to be predominant colors, and each of the picture color 
signals (DC'.sub.2), (M'.sub.2) and (Y'.sub.2) corresponds to a UCR ratio 
of 100%, then a quantity of negative value of -(.DELTA.C.sub.Y 
+.DELTA.M.sub.Y) is required for yellow (Y). This inconvenience can be 
eliminated by carrying out masking treatment at the preceding stage of the 
chromatic color separation circuit, but this configuration has no 
adoptability for the conventional circuit construction, and a further 
problem lies in that those color inks Y, M and C actually used as not the 
ideal ones, but each of them has its spectral reflection characteristic 
which results from other color ink components being contained therein, 
respectively. 
This means that in the case of the UCR ratio being 100%, the quantity of 
components of other color inks which are contained in each of the 
quantities to be subtracted from the respective color inks is liable to be 
subtracted excessively in comparison with the case of the ideal ink being 
applied. That is, in any inks actually used, particularly in cyan ink, 
there are contained considerably large quantities of Y ink and M ink 
components, so that, for example, if 100% UCR ratio is carried out to 
reduce C ink as the MIN(minimum) ink quantity to zero, then components of 
Y ink and M ink are exhausted, which results in deficiency of the whole 
quantity of the Y and M inks. 
Hereinafter methods for solving the above described problems will be 
disclosed. 
The present invention provides a method for color correction in which a 
suitable black print signal is produced for each of the color plate 
signals (C'.sub.2), M'.sub.2) and (Y'.sub.2) having been performed the 
above described masking treatment are even for over-subtraction, i.e., for 
subtraction having been performed excessively to color components in 100% 
UCR ratio caused by difference between color inks actually used and the 
ideal color ink, and an appropriate correction to each of the color plate 
signals (C.sub.2 '), (M.sub.2 ') and (Y.sub.2 ') according to the black 
print signal. 
In FIG. 6 there is shown the first embodiment of the present invention in 
which a novel circuit for solving the problems of requiring a negative 
quantity of ink based on turbid components of color inks is added to the 
circuit relating to the black print signal which corrects the deficiency 
of density reproductivity of the black ink shown in FIG. 1. In FIG. 6 
detailed explanation regarding those identical parts shown in FIG. 1 are 
abbreviated. 
The achromatic color component generating means (20) consists of a main 
part of the present invention. The color operation circuit (18) is 
comprised of the fundamental masking circuit (18a) and the color 
correction circuit (18b). 
There are much turbid components of colored inks in cyan(C) ink and 
magenta(M) ink, but extremely little or negligibly little in yellow(Y) 
ink. 
Considering that fact, the predominant color discriminating circuit (21), 
which discriminates a predominant color including more turbid components 
from cyan(C) and magenta(M), is included in the achromatic color component 
generating means (20). 
The predominant color discriminating circuit (21) inputs a red picture 
signal (R) and a green picture signal (G), and both of the picture signals 
(R) and (G) are compared with each other by a comparator (23) after the 
red picture signal (R) has been weighted (n) times by a weighting means 
(22). 
In essence, the red picture signal (R) corresponds, as shown in FIG. 5(a), 
to the quantity of cyan ink (C), and the green picture signal (G) 
corresponds to the quantity of magenta ink. 
The weighted quantity n weighted by the weighting means (22) is represented 
by the rate between turbid components (.DELTA.C.sub.Y), (.DELTA.M.sub.Y), 
of yellow(Y) which are included in the identical quantity of cyan (C) and 
magenta (M) inks, that is n=.DELTA.M.sub.Y /.DELTA.C.sub.Y. 
The comparator (23) outputs, in the condition of nR&lt;G (nC&gt;M), acyan color 
discrimination signal (e.sub.C) which enables a cyan memory means (24), 
and the cyan color discrimination circuit (e.sub.C) is inverted by an 
inverter (25). When nR&gt;G (nC&lt;M), the cyan color discrimination signal 
(e.sub.C) is converted into a magenta color discrimination signal 
(e.sub.M) which enables a magenta memory means (26). 
As can be understood from the above description, the comparator (23) 
compares the required quantity of cyan ink with the effective quantity of 
the turbid components .DELTA.C.sub.Y and .DELTA.C.sub.M contained in the 
required quantity of magenta ink, and discriminates an ink color larger 
thereof. 
Each of the cyan memory means (24) and the magenta memory means (26) is 
addressed by the corresponding respective color picture signals (R) and 
(G), and when either of the color discriminaton signals (e.sub.C) or 
(e.sub.M) enables the memory means (24) or (26), read out data stored 
therein previously (as will be described hereinafter), according to values 
of the picture signals (R) and (G). 
The memory means (24) comprises a gray table (24a) which stores gray 
density data (.DELTA.K) for correcting the black print, and color 
correction table (24b) which stores color correction data 
(.DELTA.C.sub.2), (.DELTA.M.sub.2) and (.DELTA.Y.sub.2) of each gray 
density equivalent to that of the gray density data (.DELTA.K). Quite the 
same as mentioned above, the memory means (26) comprises a gray table 
(26a) which stores gray density data (.DELTA.K) for correcting the black 
print, and a color correction table (26b) which stores color correction 
data (.DELTA.C.sub.2), (.DELTA.M.sub.2) and (.DELTA.Y.sub.2) of each gray 
density equivalent to that of the gray density data (.DELTA.K). 
In FIG. 7 there are shown contents of each of the tables of the cyan memory 
means (24), in which (a) indicates the gray table (24a), (b) indicates a 
cyan color correction table, (c) is a magenta color correction table and 
(d) is a yellow color correction table. 
In the gray table (24a), i.e., in (a), gray density (.DELTA.K), which 
corresponds to a yellow color equivalent neutral density being equal to 
twice value of turbid components (.DELTA.C.sub.Y), is stored corresponding 
to the whole cyan ink area. The reason why .DELTA.C.sub.Y is doubled is 
that, when there is a nearly equivalent quantity of turbid components 
.DELTA.M.sub.Y of magenta ink, the whole turbid components become about 
two times as large. 
In each of the color correction tables (a)-(d), the gray color correction 
data (.DELTA.C.sub.2), (.DELTA.M.sub.2) and (.DELTA.Y.sub.2) corresponding 
to the gray poupard (.DELTA.K) of the gray table (24a) are stored. 
Same as mentioned above, the magenta memory (26) defines the contents of 
the gray table (26a) according to the doubled value of turbid component 
(.DELTA.M.sub.Y) of magenta, and according to the gray density of the gray 
table (26a), the contents of each color correction table (26b) are 
defined. 
The contents of the gray table (24a) or (26a) and the color correction 
table (24b) or 26b) are, if measured in gray density based on results of 
color prints, quite identical with each other. The only difference between 
them is whether a carrier which represents only gray density is a black 
ink on three colored inks. 
The gray density data (.DELTA.K) read out of the gray table (24a) or (26a) 
is subtracted from the full black print signal (K) through a subtracter 
(27). 
Output of the subtracter (27) addresses the memory means (15) as a 
corrected black prinft signal (K.sub.2). 
The memory means (15) reads out the first gray density correction data 
(.DELTA.G) which corresponds to the deficiency in reproduction density of 
the blank ink. 
Each of the color components (.DELTA.C.sub.1), (.DELTA.M.sub.1) and 
(.DELTA.Y.sub.1) of the first gray correction data (G) is added by an 
adder (28) to each of the color correction (.DELTA.C.sub.2), 
(.DELTA.M.sub.2) and data (.DELTA.Y.sub.2) read out of the color 
correction table (24b) or (26b) so that each of the corresponding colors 
may be added with each other. The adder (28) outputs the record gray 
correction data (G.sub.2) and transmits it to each of the adders 
(16.sub.C), (16.sub.M) and (16.sub.Y). 
The corrected black print signal (K.sub.2) is transmitted to a gradation 
circuit (17.sub.K) as a picture signal for the black print. The gray 
density data (.DELTA.K) and the color correction data (.DELTA.C.sub.2), 
(.DELTA.M.sub.2) and (.DELTA.Y.sub.2) read out of the memory means (24) 
and (26) are replaced by their identical gray density components with each 
other between each of the color plates and the black print, then no change 
in color reproductivity can occur, i.e., there cannot be found any 
difference in color reproductivity, if the gray density components of the 
former are replaced with the equivalent gray density of the latter (the 
black print). 
However, to each of the masking treated picture color signals (C.sub.2 '), 
(M.sub.2 ') and (Y.sub.2 '), according to the color ink which is 
predominant in turbid components, a quantity of three colored inks 
equivalent to that of the gray component which corresponds to the turbid 
components included in the color ink is added. Accordingly, no case of 
requiring a quantity of negative value ink occurs. 
In the embodiment shown in FIG. 6, the quantity of three colored inks C, M 
and Y is limited to the possible smallest quantity of the ink so as not to 
produce a negative value thereof. If the aforementioned limited state is 
represented by the UCR ratio of the conventional UCR method, it may be 
said that the UCR ratio can be variably adjusted so that it may be 
maintained at the largest value by always keeping the optimum condition 
therefor accordingly to turbid components of the colored inks. 
In other words, it may be also said that the used quantity of inks is 
automatically variably controlled according to turbid components of the 
colored inks. 
FIG. 8 shows a view showing a simplified achromatic color components 
generating means (20) shown in FIG. 6. 
The achromatic color components generating means (20') shown in FIG. 8 
eliminates the predominant color discriminating circuit (21), and directly 
and simultaneously accesses the cyan memory means (24) and the magenta 
memory (26) by the red picture signal (R) and the green picture signal 
(G), respectively. In this case gray density data (.DELTA.K.sub.C) and 
(.DELTA.K.sub.M) output simultaneously from those both memory means (24) 
and (26) are once added to each other in an adder (29), and the added 
result is transmitted as a gray data (.DELTA.K=.DELTA.K.sub.C 
+.DELTA.K.sub.M) for gray density correction to the subtractor (27). 
The color correction data read out simultaneously from both memory means 
(24) and (26) are added in an adder (30) so that each of those 
corresponding colors may be added, and then transmits them as the color 
correction data (.DELTA.C.sub.2), (.DELTA.M.sub.2) and (.DELTA.Y.sub.2) to 
the adder (28). 
The gray and the color correction data to be stored in the memory means 
(24) and (26) may be a half of the gray density value shown in FIG. 6, 
that is, it may be the turbid components of the inks themselves or may 
somewhat increase the turbid components. As each of cyan and magenta 
shares its turbid components respectively, any extra quantity of turbid 
components for correction in anticipation of the worst condition is 
unnecessary. 
Therefore, according to this embodiment, it is expected that the quantity 
of inks to be used can be further reduced in comparison with the case of 
other embodiments shown in FIG. 6. 
FIG. 9 shows another embodiment of the achromatic color component 
generating means designated by the reference number (20a). 
The full black print signal (K) addresses, as shown in FIG. 1, directly the 
memory means (15), and is transmitted to an input for addition of a 
subtracter (31) and a gradation circuit (17.sub.K ') for the black print. 
An output of the gradation circuit 17.sub.K ') is transmitted to an input 
for subtraction of the subtracter (31) and output as the picture signal 
(K.sub.2) for the black print. The subtracter (31) subtracts the black 
print picture signal (K.sub.2) output from the gradation circuit (17.sub.K 
') from the full black plate signal (K), and outputs density difference 
(K.sub.D) between them. 
Each of the correction data (C.sub.1), (M.sub.1) and (Y.sub.1) output from 
the memory means (15) is added, as described above, to the respective 
three color plate signals (C.sub.2 '), (M.sub.2 ') and 'Y.sub.2 '), and by 
combining each of them with the black print signal (K.sub.2), the ideal 
achromatic color density characteristic shown in FIGS. 1 and 11 can be 
obtained. 
The gradation circuit (17.sub.K ') has a memory table or a random function 
generating means using a linearizer and can produce any desired 
characteristic curve of the relation between input and output. For 
example, as shown in FIG. 1, it is possible to change the black print 
signal (K) as the curve (K.sub.2). 
The density difference (K.sub.D) is a difference between the curve (K) and 
the curve (K.sub.2), and the reproductive characteristic of gray density 
of this density difference (K.sub.D) has, at any density area, always 
linear gray density reproductivity. 
For example, in the case of density components corresponding to the density 
difference (K.sub.D) being replaced with those of three colored inks, an 
appropriate gray density can be obtained in density areas lower than the 
point (P), however, in density areas higher than the point (P), because of 
gray density being deficient as shown in the curve (K'), any appropriate 
correction cannot be carried out, even if gray density components 
corresponding to the density difference (K.sub.D) are added thereto. 
Such being the circumstances, in the present invention it is adapted that 
by a signal of the density difference (K.sub.D), a gray memory means (32) 
is addressed, and from the gray memory means (32) each of the color 
correction signals (.DELTA.C.sub.2), (.DELTA.M.sub.2) and (.DELTA.Y.sub.2) 
having an equivalent neutral color density equal to the density difference 
(K.sub.C) can be obtained. FIG. 10 shows contents of the memory means (32) 
in which (a) indicates total gray density characteristic which is linear 
in the whole address areas (the whole density difference areas). 
FIG. 10(b), (c) and (d) shows each color correction quantity for obtaining 
a gray density characteristic as shown in FIG. 10(a), in which (b) 
indicates cyan components (.DELTA.C.sub.2), (c) indicates magenta 
components and (d) is a yellow component (.DELTA.Y.sub.2). 
Within address space of the memory means (32) having the above described 
characteristics, the required maximum density difference (K.sub.D MAX) is 
appropriately determined in accordance with the variable range of the 
gradation circuit (17.sub.K '), and the memory means (32) can be addressed 
by either positive or negative value of the maximum density difference 
(K.sub.D MAX). 
The correction data (.DELTA.C.sub.2), (.DELTA.M.sub.2) and (.DELTA.Y.sub.2) 
output from the memory means (32) are transmitted to an adder-subtracter 
(33) by attaching a positive and/or negative sign. The adder-subtracter 
(33) carries out addition-subtraction operation between the correction 
data (.DELTA.C.sub.1), (.DELTA.M.sub.1) and (.DELTA.Y.sub.1) output from 
the memory means (15) and the correction data (.DELTA.C.sub.2), 
(.DELTA.M.sub.2) and (.DELTA.Y.sub.2) of the memory means (32) so that 
addition and/or subtraction may be performed between the corresponding ink 
colors. The adder-subtracter (33) transmits the results of addition and/or 
subtraction therebetween to the adders (16.sub.C), (16.sub.M) and 
(16.sub.Y) as the gray correction data (.DELTA.G). 
In the embodiment, by varying the gradation characteristic of the black 
print, the gradation characteristic is shifted appropriately from that of 
the ideal ink, and correction is performed so that gray density of the 
shifted portion may be replaced with gray density of each of the color 
plates. 
Gradation characteristic can be varied freely, so that at any gray density 
areas the conventional UCR ratio can be freely changed. 
This effect makes it possible to control the black print in quite the same 
manner as that of the conventional UCR method in which, for example, as 
shown in FIG. 11, gray density can be represented without using any black 
ink in the lower gray density areas. 
However, quite different from the UCR method, according to the present 
invention, the quantity of gray density to be controlled can be easily 
grasped. 
In FIG. 12 there is shown an example in which the present invention is 
applied to a color scanner having a conventional UCR circuit and being in 
its operating state. 
Circuits having been described heretofore have been already disclosed in 
the Japanese Patent Application Laid-Open Publication No. 55-115043, and 
from Nos. 55-142342 to 55-112345, and/or those of well-known digital 
circuit techniques, so that no detailed explanation is needed. 
The color scanner, having been already installed and operated, comprises 
the input means (10), the color system varying circuit (13a), the 
fundamental masking circuit (18a), the color correction circuit (18b), the 
under color removing circuit (18c) and an output means (34) for 
reproducing the picture. 
Any one of the black print generating means (20), (20'), or (20a) described 
as the embodiments of the present invention is provided to this color 
scanner and each of the picture signals (R), (G) and (B) is input thereto 
from the input means (10). 
In the above mentioned case, the black print signal (K.sub.2) obtained from 
the black print generating means (20) is used as a picture signal(s) for 
black print. Further, even in the case of the conventional color scanner 
being operated in an analog circuit, the present invention can be operated 
in an analog circuit, the present invention can be applied if an AD 
converter (not shown) is connected to the input of the black print 
generating means (20), (20'), or (20a) and a DA converter (not shown) is 
connected to the output of the black print generating means (20), (20') 
and (20a). 
The under color removing circuit (18c) sets the UCR ratio to 100% by an 
under color removal ratio setting means (35), and a signal for black print 
obtained therefrom is not applied. 
The gray density correction signal (G) is added to each of color ink 
picture signals (Y), (M) and (C) through the adders (17.sub.C '), 
(17.sub.M '), and (17.sub.Y '). 
As described above, the color correction method according to the present 
invention can be practiced, even to the color scanner having been actually 
equipped and operated within such things as the adders (17.sub.C '), 
(17.sub.M '), (17.sub.Y '), etc. because it can be provided without making 
any large scale of change and trouble. 
As described in detail, according to the present invention, it is possible 
to reduce unnecessary consumption of expensive colored inks by converting 
always gray components into black print at their maximum and reducing the 
quantity of inks to be used at their minimum, reproduce a picture having 
less turbidity in comparison with that reproduced by the conventional 
method, and it is also possible to easily control color correction of 
other color inks and facilitate correction of colors so as to represent 
desired color effects. In addition, there are further advantages in the 
present invention, for example, by correcting deficiency in density 
reproduction of the black ink according to the density characteristic of 
the black ink. The deficiency in gray density of the black print can be 
easily shared by three color plates, thus, by controlling the shared 
quantity of gray density appropriately, and restraining the quantity of 
colored inks to be used as much as possible, incomplete reproductivity in 
tone which has been a serious disadvantage of using the black print 
according to the conventional full black manner is avoided. Thus, 
according to the present invention appropriate color tone reproductivity 
can be realized.