JPH01284074A - Color recording system - Google Patents

Abstract

PURPOSE:To attain accurate color reproduction through simple arithmetic operation by obtaining a value corresponding to the saturation from a 3-primary color signal from an input device, converting the 3-primary color signal into an equivalent neutral density signal of each color member, deciding the quantity of addition of a black color in response to the said equivalent neutral density signal and the saturation so as to decide a background color eliminating quantity. CONSTITUTION:A maximum/minimum detection means 8 uses a comparator of the pre-stage in the 2-stage of comparators so as to compare the quantity of two signals at first and selects 1 larger and a smaller output. The quantity of black color addition ak is decided from equation, where amin is a minimum value of the equivalent neutral density signal. A coefficient deciding device 9 applies the operation of h=amin<2> to generate a coefficient signal (h) for eliminating background color and the result is outputted to a background color elimination circuit 11. A minimum signal amin from a maximum/minimum detection means 8 and coefficient signals alpha, beta from the coefficient decision device 9 are given to a black color giving circuit 10, which consists of a subtractor, an adder and a multiplier (they are not shown), and applies the arithmetic operation as shown in equation and outputs a black color addition signal ak to the background color elimination circuit 11.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to (1) a color recording system that reproduces colors using a plurality of color materials including black, such as ink, and in particular, printing after removing the undercolor; The present invention relates to a color image processing method for determining the amount of ink overprinting.

[Conventional technology]

Conventionally, in printing technology, four-color printing is usually used when reproducing color original images. i.e. yellow, magenta,
Color separations are created for each color of printing ink, black and white. This is because, for example, in the case of three-color printing of yellow, magenta, and cyan, the ink does not have perfect color characteristics and cannot meet the ideal printing conditions, resulting in a reproduced image with poor contrast. This is because there is no

Furthermore, during four-color printing, so-called undercolor removal may be performed on other printing inks, ie, yellow, magenta, and cyan printing inks. When this undercolor removal is performed, the achromatic portion is printed with only black ink, so the color reproduction range in the low brightness portion is widened, and gray stability can be maintained relatively high. Further, there is an effect that printing becomes reliable and the amount of ink used is reduced.

In this way, there are various advantages when performing four-color printing by removing the undercolor.

However, when performing four-color printing, there is a problem in that it is difficult to determine the amount of undercolor removal and the amount of black addition, that is, the amount of black overprinting.

In order to solve this problem, a method for determining the amount of undercolor removal and the amount of blackening in a color scanner for printing has been proposed in Japanese Patent Laid-Open No. 57-173838 and Japanese Patent Laid-Open No. 58-190951.
This method is disclosed in Japanese Patent Application Laid-Open No. 58-211757, etc.

The method disclosed in Japanese Unexamined Patent Publication No. 57-173838 is characterized in that undercolor removal is carried out separately for an achromatic color area and a chromatic color area. This method reproduces an achromatic color area with only black, and further changes the amount of black printing with a gradient in a transition area from a chromatic color area to an achromatic color area.

Also, JP-A-58-190951, JP-A-58-190951,
No. 211757 discloses that the amount of black printing depends on the gradation value.

A method for determining the amount of undercolor removal is shown. This method achieves a completely achromatic structure up to the gray value corresponding to the main color of the black ink being used, and increases the color components of the chromatic ink in the shadows above this gray value. In other words, up to a certain level of density that can be reproduced with black ink, black is reproduced only with black ink, and for black densities higher than that, high-density black is reproduced by adding equal amounts of inks of the other three colors. There is.

In addition, methods for determining the amount of blackening and undercolor removal in digital color recording methods such as inkjet, thermal transfer recording, and laser color xerography are described in, for example, Japanese Patent Laid-Open No.
It is disclosed in JP-A No. 161981, JP-A-59-163973, and the like.

The method disclosed in Japanese Patent Application Laid-open No. 59-16198i is to obtain the black overprint amount by multiplying the minimum value of cyan, magenta, and yellow color data by a certain constant, and calculate this black overprint amount from each color data. Performs undercolor removal.

Furthermore, the method disclosed in Japanese Patent Application Laid-Open No. 59-163973 determines two chromatic inks to be combined with black based on the reflectance of multiple color inks, and prevents the chromatic inks and black ink from overlapping. By recording as follows,
Amount of black printing using a simple calculation method.

The amount of undercolor removal is determined.

[Problem to be solved by the invention]

However, the method disclosed in JP-A-57-173838 that performs undercolor removal by distinguishing between gray areas and colored areas requires a large number of coefficients when determining the amount of black overprinting and the amount of undercolor removal. do. Determination of these coefficients can still only be done empirically, and the amount of inking described above,
The difficulty of determining the amount of undercolor removal cannot be resolved.

Also, JP-A-58-190951, JP-A-58=
No. 211757 discloses that the amount of black printing depends on the gradation value.

Although it is shown that the under color removal amount is determined, these publications only describe the processing method in the black reproduction section.In the case where there is a transition from the chromatic color area to the black reproduction area, that is, In an image where the saturation changes gradually, such as a general picture, there is a risk that a false contour of color, that is, a saturation gap will occur.

Furthermore, the method disclosed in Japanese Patent Application Laid-Open No. 59-161981 is generally called fixed rate undercolor removal and undercolor addition.
In this case, there is a problem that accurate color reproduction cannot be performed. The reason why accurate color reproduction cannot be achieved can be found in, for example, ``Considerations of Inkling in Printing'' (IN, Proceedings of the 1st Color Engineering Conference).

It is described in the 4th Society of Optics, 1984.1-7, etc.

Furthermore, in the method disclosed in Japanese Patent Laid-Open No. 59'-163973, calculations are performed based on the principle of average additive color mixture, so there is a problem that accurate color reproduction cannot be achieved during actual recording. It is known that this is caused by light penetration and diffusion efficiency in the white part of the paper.
A. C. Yule, "Theory of Color Reproduction," Printing Society Publishing Department, February 1971, P2
47-248.

In view of the shortcomings of the prior art described above, the present invention provides a black ink that does not require empirical adjustment, can perform accurate color reproduction through simple calculations, and does not create a saturation gap between the chromatic color area and the black area. The purpose is to provide a color recording method that performs reprinting.

[Means to solve the problem]

In order to achieve the above object, the color recording method of the present invention has the following features:
A value corresponding to the saturation is obtained from the three color signals from the input device, and the three color signals are converted into an equivalent neutral density signal of each color material, and black is calculated according to the equivalent neutral density signal and the saturation. The method is characterized in that the amount of reprinting is determined and the amount of undercolor removal is determined.

When the black overprint amount is ak and the minimum value of the equivalent neutral density signal is amlh, the black overprint amount ak is α,
It can be determined by a cubic function of almIn with β as a coefficient.

Further, the coefficients α and β are the differences between the maximum value and the minimum value of the equivalent neutral concentration signal, respectively.
It can be expressed as a monotone function of ln.

Further, the relationship between the blackening amount ak and the minimum value amIn is set to ak≦ak1. , can be done.

Further, the black overprinting amount is set to ak, and the maximum value, intermediate value, and minimum value of the equivalent neutral density signal are set to a3. O+”sld+
When amin, the undercolor removal amount can be determined by the following equation.

1 3slh J'(1slh-amlh)" (1h2) ' (1
slh-aJallIId2 1 8slh almIn-〇However, h,, h2 is a function of a□i, a l +a a
*+alMld+a1min is the amount of three color ink of maximum, middle, and minimum dots after black printing, and h2 is the monotonous increase in the value related to the saturation of the equivalent neutral density signal. It is desirable that the following conditions are satisfied.

h. Based on the calculated value and the equivalent neutral density signal of each color material obtained by converting the three color signals, the blacking amount is determined by, for example, calculating an approximate formula. At this time, the amount of blackening changes not only according to the equivalent neutral density signal but also according to the saturation. Therefore, even if the image changes continuously from a highly chromatic area to an achromatic area, the amount of ink printing for optimal color reproduction is continuously determined, without creating a saturation gap. Smooth color reproduction is possible. Further, since the amount of undercolor removal also changes depending on the saturation, the optimum amount of undercolor removal is determined in a continuous area from the colored area to the black area.

Here, the theoretical background on which the present invention is based will be explained using an example in which ink is used as a coloring material.

Now, consider the case where a color image is reproduced by combining a plurality of chromatic inks as shown in FIG.

FIG. 2(a) shows the cyan ink portion 1. Magenta ink section 2. The area ratio of is 100%, and only yellow ink fiv) 3 is 1 (not 10%), indicating a form that expresses a highly saturated color.Hereinafter, this will be referred to as a form.

Further, FIG. 2(b) shows the cyan ink portion 1. Magenta ink 82. This is a case where the yellow ink portions 3 have the same area ratio, and shows a form expressing an achromatic color. Hereinafter, this will be referred to as form B. In addition, in FIG. 2 et al., 4 indicates a blank section.

The conditions for performing ink overprinting on these image shapes HA and B and making them match the form using three-color ink are shown below.

In the form shown in FIG. 1(a), the smallest dot portion of the three colors, that is, the yellow ink portion 3 is provided with a fan. Since magenta is superimposed, the area of the minimum dot part is a.

The part is black. Therefore, the ink printing 'Jak only needs to replace the yellow ink amount ak.

Note that for the sake of simplicity, it is assumed here that the ink amount and area correspond, and are given the same reference numerals.

In addition, the amount of undercolor removal is determined by the Neugebauer (Neu
DemlChel (DemlChel) using the formula of
), the area remains the same except for the smallest dot part.

Formulating the above results in equation (1).

ax+=ax-”I J Note that +ar in aC and a represents the area of the cyan, magenta, and yellow ink portions 1.2.3 before black printing in Figure 2, and ab+ aC1+ all all all is black Black after reprinting.

Represents the area of cyan, magenta, and occupancy.

In order to generalize this formula, the size relationship of the three color dots is expressed as maximum area am4M+intermediate area a11. minimum area awl,
When expressed as Equation (2).

a k ”” a + a I n
1 Note that a+sax+a+m+d+a+llIn is a
lllAII+ a Pass d+ This is the area after ink printing corresponding to all.

Next, consider the shape 3B shown in FIG. 2(b). When considering this shape, the fold B, the Neugebauer equation is valid, and the average reflectance R of the image shown in Figure 2(b)
is obtained by equation (3).

; a(1-a)2・RX where, Rc+ R11,RV, Rak Rr,,
Rm, Rw, Rcxy are cyan, magenta, yellow,
It represents the reflectance of the cyan/magenta overprinted area, the magenta/yellow overprinted area, the yellow/cyan overprinted area, the white paper, and the cyan/magenta/yellow overprinted area, respectively. Further, a represents the area ratio of cyan, magenta, and yellow.

Here, let us consider the case where inking is performed in the form shown in Fig. 2 et al. Since this shape gB is an achromatic color, if only one color of yellow in the yellow ink portion 3 is replaced with black, the other two colors, cyan and magenta, must be completely removed under the color. That is, it is necessary to set the amount of undercolor removal to 100%. Therefore, the formula obtained by blackening the form ffB becomes the formula [4].

Rm(1-ah)・Rw+ak'Rk”・(4)
However, since R5 matches the color before and after black printing in terms of black reflectance colorimetry: well, the formula (3
), the right-hand sides of (4) must be equal. Here, by formulating equations (3) and (4), equation (5) is obtained.

a II= (Rcxy-Rw)-' X (a3・(Rcxv Ra RG RR+R
C+RX+RV R11)+ a2・(3Rw -2
Rc -2R)l -2L+L+L+Ri)+a・(
-3Rw+Rc+Rx+Ry) )・・・・・・(5
) Therefore, blacking Nak has a cubic relationship with the three-color area ratio a before blacking. Let each coefficient be α, β, T
Then, Rc, lv Re Rc R11+Rc,
-+-R1I+Ry RwR6□-R.

RCNY RW 3RW+RC+R11+Ry γ =□ RCXY Rw ・ ・ ・ ・ ・ (6) From the calculation of the above equation, the following equation (7) is derived for each coefficient.

In addition, in the inequality part that includes an equal sign, each equality is cyan,
This only holds true if the magenta and yellow coloring materials are rectangular dyes with ideal rectangular spectral density distributions, and is an inequality sign in the case of general coloring materials.

Therefore, from equations (5), (6), and (7),
The amounts of black, cyan, magenta, and yellow ink when form B is overprinted with black are expressed by the following equations.

Equation (8) will be further considered.

The area ak of black printing is amtn=0 or amt,,=1
When , a k ” a sin.

Now, if a k a * i 1 is set as P, the formula (7
), so ak-akI, , ≧0 (however, 0≦ake≦1) holds true.

The amount of black overprinting has been described above, and next, the amount of undercolor removal will be described.

If we organize the amount of undercolor removal for the above forms A and B,
In the case of highly chromatic color, the under color removal amount is 0 from equation (2), and in the case of achromatic color, form BB
In this case, it can be seen from equation [8] that the undercolor removal amount must be 100%.

In form A, the area a*ln of the minimum dot color is the area al1M of the other two colors. 3.6. This is the case when it is completely included in Analysis of such a case has already been carried out, and the maximum color area akmaX after removing the undercolor is as follows:
It is said that the following relationship holds true.

(For example, see "First Color Engineering Conference 1-7 Consideration of ink marks in printing (■)").

ak In the previous explanation, in the case of form A, ak, , = a , , Il, == 1 In other words, it was assumed that the undercolor removal amount was 0, but the same result can be obtained from equation (9). .

However, when considering the case of form B1, that is, a116Il: alIId: alIIll-a, as mentioned above, in general other than rectangular dyes, since the following is true, according to the formula (9), a+aai+<0.

However, in reality, such a thing is impossible.

The reason why such a problem occurs in equation (9) is explained as follows.

Equation (9) holds true under the condition that area amin is completely included in area a718, but in form B, area aal is not necessarily included. is not included in the area alla, but rather there are many areas where the area amin exists alone. The magnitude of such an inclusion relationship is generally known as the Demichel relationship. Such a difference in the inclusion relationship causes a problem in equation (9).

Therefore, if we consider undercolor removal from the perspective of preserving saturation before and after ink printing, the following can be considered regarding the form.

Before ink printing, the area where the area aslh exhibits saturation is amIn(1asa
++' a+++id), and the area ak68
The part that exhibits saturation is akIa++(1asld'amen).

After black printing, the minimum area is ASL. Since is replaced by the black area ak, there is no area where the area ak%Il exhibits saturation, and the area where all after black printing exhibits saturation is a I+aax (18h).

Therefore, in order to equalize the colors before and after ink printing, the saturation must also be preserved, which can be formulated as the following equation.

a +max(18k) =a*ax(1asld'amen)"@in(1a
*am' asld) =amax~a1o...α mouth, that is, 1 3.

Formula (10a> has form B1, that is, a yr a
When w = a + a 1d = a m In = a is substituted, a13.8zO is obtained, and the same result as the above formula (8) is obtained.

Therefore, when determining the undercolor removal amount, the undercolor removal algorithm must be incorporated in consideration of the inclusion relationship of the three color ink amounts not including black: all #ll ald+a*Iゎ.

In the above description, the maximum area aaax has been described, but the same results can be obtained for the intermediate area a1m.

From the above-mentioned analysis of the highly chromatic form A and the achromatic form gB, it can be seen that the following conditions are required for the blackening amount ak and the undercolor removal amount in order to achieve accurate color reproduction.

Condition (1): The black printing amount a1 is the minimum value a1 of the three color ink amounts for achromatic colors. ．． For, always ak≧amin,
As the color increases, it is necessary to use aM-81.

Condition (ii): For highly chromatic colors, the under color removal amount must be 0, and for achromatic colors, the under color removal amount must be 100%.

Using color printing to confirm this, Schiff 7,?
We created samples of three-color printing of zenta and yellow, and two of cyan, magenta, yellow, and black, and measured their color characteristics with Munsell Hue, Munsell Baligny, and Munsell Chroma. Ink dot area ratio a. ,a
X, a + ak were measured. 3 obtained in this way
About matching color pairs of color samples and ink printing samples 3
The relationship between the minimum ink amount aalh of a color sample and inking iak was actually measured.

Figure 3 is a three-dimensional graph showing the actual measurement results, with the X-axis showing the Munsell chroma of the matching pair, the Y-axis showing the Munsell value of the matching pair, and the Z-axis showing the ink printing sample of the matching pair. The difference between the black printing amount ak and the minimum value a1ゎ of the three-color sample is calculated as ak-alIIIl.

As can be seen from this graph, aka + al++ is always positive, and when the Munsell values are the same, Munsell chroma, that is, as the saturation increases, ak - aki,
It can be seen that decreases monotonically. Therefore, the above-mentioned condition (i) was confirmed to be correct also from the results of actual measurements.

However, as it is, in color recording devices,
Munsell hue, Munsell value, and Munsell chroma must always be determined, which is not practical.

Therefore, three color ink signals ac+am+a that do not include black
A method for determining the blackening amount ak directly from k will be described below with reference to FIG.

In this figure, the minimum value amIn of the three color ink signals is plotted on the horizontal axis, and the minimum value a1 of the three color signals is plotted on the vertical axis from the black printing amount ak. ．．
This is the result obtained by subtracting akastn and regressing the measured data shown in FIG. 3 using equation (8). The correlation coefficient of the regression equation for the measured data at this time was 99.8%. From FIG. 4, it can be seen that ak≧a1°, and as the Munsell chroma increases, ak=amIn.

Next, am
Coefficient α of In3, ak, , coefficient β of `` and a m a
The relationship between w a a I n is shown in FIG. As can be seen from the figure, the coefficients α and β are monotonous functions for a□Eak, , and both are a w* a ll a
As m I n approaches 1, it converges to 0. On the other hand, when 364w-al,=0, α>0. β<0. Note that these coefficients α.

β can be approximated by, for example, a quadratic expression.

Since the black overprint 11ab was determined as described above, the removal of the undercolor was next confirmed.

The confirmation method is to use the same sample that was used to check the amount of ink printing, and check that the a + aax +
A method was used to compare ai+ and the color-matched sample amaw+ amen.

The comparison results are shown in FIG. In the figure, the horizontal axis is the maximum value signal a11. obtained from the three color signals R, G, and B when determining the amount of black overprinting. It is. Also, on the vertical axis, equation (9),
Maximum ink ffi after black reprinting calculated in (10a)
The difference between a1MaM and the maximum ink amount obtained by actually measuring a black printing sample is taken. In Figure 6, the formula (
9) The difference between the calculated value and the measured value is represented by a solid line, and the equation (10a
) The difference between the calculated value and the measured value is shown by a broken line.

From FIG. 6, it can be seen that when the maximum value signal a1.8 is small, that is, when it is close to an achromatic color, Equation (10a) applies well, and when the maximum value signal a1.8 is large, that is, when it becomes highly chromatic, Equation (9) applies well. It turns out that it applies well. This is considered to be due to the inclusion relationship between the amounts of ink of the three colors as described above.

Therefore, in this embodiment, equation (9) and equation (10a) are used depending on the maximum value signal a1.8.

This can be formulated as the following equation.

allllll "'Fl+'(amax-ak)"(1-Eft)' (aa
ax amIn) ak ......αυ However, hl (0) = O, hl (H = 1. hl is a monotone function of ak.).

That is, when the maximum value signal allllll11 is large, (am,, -ak)/(1-ak) becomes dominant, and when it is small, (a---a-1,)/(1-ak
) becomes dominant. For example, hI-ak#
, 2 can be approximated.

The difference between the actual measured value and the old formula a is shown by the dashed dotted line in FIG. When formula 0υ was used, good agreement with actual measurements was obtained with a correlation coefficient of 99.6%.

In this way, regarding undercolor removal, the actual measurement results a□X-
It has been found that it is necessary to perform 100% undercolor removal when am+h is below a certain value. Note that this constant value changes depending on the recording method.

Although the area a1° of the maximum color is used above to determine the undercolor removal amount, it goes without saying that the same applies to the area a mid of the other color. In addition, in this case as well, the function h
A function h2 similar to 1 is used.

From the above, in a color recording system that reproduces colors using color materials such as inks of multiple colors including black, values corresponding to saturation are calculated from three color signals from an input device, and
It is relatively simple to convert these three color signals into color material signals of multiple chromatic colors, and to determine the amount of ink printing and the amount of undercolor removal according to these color material signals and saturation. Accurate colorimetric color reproduction can be performed using only simple calculations and without the need for empirical adjustments. Furthermore, continuous changes in saturation are possible even in the changing area between the colored area and the black area.

The theoretical background on which the present invention is based has been explained above based on the halftone dot area 1 signal, and in this case, the form B1 shown in FIG. 2(b), that is, cyan and magenta.

It is assumed that when yellow has the same area ratio, it becomes an achromatic color.

However, since this method relies on the printing apparatus to adjust the maximum density of cyan, magenta, and yellow, there is a concern that the quality of the reproduction image quality will be determined by the quality of the adjustment on the printing apparatus side.

i.e. cyan, magenta.

Even if yellow has the same area ratio, it will not be an achromatic color if the maximum density of each color in the recording device is different.

In order to avoid this problem, it is effective to reconstruct the above explanation by replacing the halftone area ratio signal with an equivalent neutral density signal.

The neutral density signal is a signal obtained by normalizing the dot area ratio signals of fan, magenta, and yellow based on the black signal that is the same color in combination of these three colors, and is expressed by the following formula. Ru.

Ct = M −= Y When E, C, = M, =
Y,=B.

Here, C=, Mak Y= are three-color liquor neutral concentration signals, and B is a signal representing a color matching thereto.

By definition, if the three color signals take the same value, the neutral density signal will always be achromatic, so it is not possible to separate the adjustment of the maximum density of coloring material on the recording device side from the processing of the present invention. It has the effect of being able to.

Therefore, the explanation based on the above-mentioned halftone dot surface [1] will be replaced with the neutral density signal and explained in detail.

The only difference between the two signals is the relative relationship between the three color signal amounts and the black amount. When based on the halftone area ratio signal, the second
Area ak of achromatic ink printing of shape IB shown in figure (b)
is determined by equation (8), and as mentioned above, in general coloring materials, ak≧a□. becomes.

On the other hand, based on the alcohol neutral concentration signal, in the achromatic color of form B shown in Fig. 2, etc.), B t
+ “” C@i in. Here, C1o means the minimum signal among the alcohol neutral concentration signals C, ME, YE. After that, do the same for the maximum signal and intermediate signal: □8°C□
, is written.

The difference in the method of determining the black signal in the case of such an achromatic color is as follows:
It also affects chromatic colors.

As explained earlier, FIG. 4 shows the relationship between the black signal, that is, the black addition@a5, and the minimum value amIn of the three-color halftone area ratio when based on the halftone area ratio signal, It is expressed in the form of equation (8). Also, the coefficient α of equation (8)
, β change monotonically with respect to the signal corresponding to each saturation of the target color, as shown in FIG.

This relationship is the coefficient α when expressing achromatic colors.

β to α. ,β. Let α be the value used to express a certain saturation C. , β0, then αc×C0, β0〉β.

It is.

In addition, the amount of black printing in the halftone area ratio signal is set to a in the case of achromatic color.
k. , in the case of saturation C, let it be ake, then akc ako
” (αC-αG)'amin'+(βC-β0)・
a□,,' can be expressed as 10(α.-α.+β.-βC)・a 1lin.

The ink printing amount BkC in a chromatic color of saturation C when using the isozake neutral concentration signal is B ko in the definition of isozake neutral concentration.
``a ko'''' a m1n-c#+*, so the above formula % formula % From the above, the amount of inking Bk based on the alcohol neutral concentration signal, including achromatic colors and chromatic colors, is: Bk-α,・C+a
l n' + β1・Cwa l,, 2 + (1-C1-β1) °C... C□w When Cmth=O, α, = β1=OC□X
When Cm1h-1, α<0. β>0, and always B, ≦0□0.

When this is diagrammed, when an equal liquor neutral concentration signal is used, it becomes as shown in FIG. 7 corresponding to FIG. 4, and FIG. 8 corresponding to FIG. 5.

Next, consider undercolor removal.

In the undercolor removal based on the halftone surface @black signal mentioned above, a
k≧a11. It is formulated using Equation (5) under the following conditions.

This is because Equation (10a) is well applied in areas that are close to achromatic colors, and Equation (9) is well applied in areas that are highly chromatic.
This is a result of changing the weight of a). That is, a.

From the relationship ≧a1e, the undercolor removal rate is set to be lower in areas that are close to achromatic than in areas that are highly chromatic.

When performing similar undercolor removal using the equal alcohol neutral concentration signal, from the relationship B5≦C□, replace ak in formula αD with C1, and replace ak, , with B, , 2αυ is rewritten as C1aax”'h,・(C,ak−C,+,,)”(1−h+)・(C
-a--BJl-(1 knee+,.

If so, the same effect as described above can be achieved.

In the above explanation, in order to distinguish between the dot area ratio signal and the isoblast density signal, the isoblast density signal is referred to as B.
k+ C1111111C@l , , as ak
+akak,.

To avoid confusion with aMI, , from now on, a C+ a
X+ a Y+ak+ alldM+ aald+
The following explanation assumes that all amlh are isochroic concentration signals.

In the present invention, each of the three color signals from the input device is
Isochleic density signals ac, al normalized by black color
Convert to l aY and (i) its maximum value am. and the minimum value a a l h
(11) Find the coefficients α and β of the formula indicating the black overprint amount ak from the differences ak, , −ak, , and (iii> From the coefficients α, β and the minimum value amIn, Determine the black overprint amount ak by the formula, ak=α・akI,,3+β・ak1..2+(l−α
-β) 'aalr+(1v) Coefficient h necessary to calculate the amount of undercolor removal from the maximum value allaM (however, h
: Determine h,, h2), and (v) determine the coefficient, the black printing amount ak, and the three-color ink signal a.
From llall+@id+aalh, the undercolor removal amount is determined based on the following equation.

1-allL, 1 1 a*ll.

alalh"'0 Here, if we obtain an approximate formula from the measurement results of color-matching the reproduced image by ink printing and the reproduced image by three-color ink,
Accurate colorimetric color reproduction can be performed. Also, a
+a a * a s l n corresponds to the saturation, so even in the changing area between the colored area and the black area, the coefficients α and β change according to the saturation, so that the optimal inking amount ak is continuous. This allows continuous changes in saturation.

Furthermore, since the amount of undercolor removal is controlled based on a coefficient corresponding to the saturation, it is possible to determine the optimum amount of undercolor removal in a continuous area from the colored area to the black area. For example, when the saturation is low, the undercolor removal amount is set to 100%, and when the saturation is high, the undercolor removal amount is set to 0, and the undercolor removal amount is continuously changed according to the saturation. , a black-printed image that is color-matched to the reproduced image using three-color ink is obtained.

〔Example〕

DETAILED DESCRIPTION OF THE INVENTION Hereinafter, features of the present invention will be specifically described based on examples with reference to the drawings.

FIG. 1 shows an example of a color recording apparatus for implementing the color recording method of the present invention.

In the figure, 5 is an input device such as a reading device that separates and reads manuscript information into three colors, and the read manuscript information is
For example, it is converted into a digital data signal by an A/D converter (not shown) provided in the input device 5, and outputted in parallel as three color signals R1G and B of red, green, and blue. Color signals R, G.

B is supplied to a masking circuit 7 via an equivalent neutral concentration conversion circuit (indicated by an END conversion circuit in the figure) 6. Color signals R1, G,, B after equal alcohol neutral concentration conversion.

are cyan and magenta by the masking circuit 7.

It is converted into a yellow isotropic concentration signal Ce, MI:, Yt.

These isochrigenic concentration signals Ce, Me, Ya are:
The signal is input to the maximum/minimum detection means 8 and also to the undercolor removal circuit 11 .

The maximum/minimum detection means 8 is composed of, for example, two stages of comparators (not shown) each for detecting the maximum value and the minimum value, and first two of the three signals are detected by the previous stage comparator. The magnitude of the signals is compared and the larger and smaller outputs are selected. Then, by comparing the output of the previous comparator with the remaining signal in the subsequent comparator, the isochroic concentration signals CE, M, .
The maximum signal ak of Y. 8 and outputs the minimum signal ak1. These maximum and minimum signals a@@wHakle are input to the coefficient determiner 9.

The coefficient determiner 9 is composed of a subtracter 1 and a multiplier (not shown), and calculates the following formula to obtain a coefficient signal α.

β is output to the inking circuit 10.

Note that the equation surface is a quadratic equation approximating the changes in the coefficients α and β, and the constants p and q are determined depending on the coloring materials used.

Further, the coefficient determiner 9 performs the following calculation to generate a coefficient signal for undercolor removal, and outputs this to the undercolor removal circuit 11. Note that the coefficient signal is determined by the above-mentioned function hl+h2
are shown together.

h=amam2 The minimum signal amlh from the maximum/minimum detection means 8 and the coefficient signals α and β from the coefficient determiner 9 are input to the inking circuit 10. The blacking circuit 10 is composed of a subtracter, an adder, 8 multipliers (not shown), calculates the following equation, and outputs a blacking signal ak to the undercolor removing circuit 11.

ak=α・aIIl113+β・amth2+ (1a
-β) 'aml+ In the under color removal circuit 11, isochroic density signals CE+ME from the masking circuit 7,
Using YE, the output amIn from the maximum/minimum detection means 8, the coefficient signal from the coefficient determiner 9, and the output ak from the inking circuit 11, undercolor removal is performed in the following procedure, and the positive Only the color signal a + +12 is output.

That is, first, the following calculation is performed to obtain the signal alc+a1m+aIY. However, all + ai's Sarah 4-/cusl is C
, M, or Y. Next, only the positive signal al+a2 among the signals a+c+ a+x+ alv is output.

Through the above operations, achromatic signals, cyan and magenta, are generated from the three color signals R, G, and B.

Two of the yellow colors and black isotropic concentration signal a1. a2゜a
5, and the output thereof is converted into a halftone signal by the gradation converter 12, and then sent to the printer section.
Color recording is done.

As described above, in this example, the maximum signal a1.8 and the minimum signal a0. ．． Since the amount of blacking is determined based on the difference between the two, the arithmetic circuit for obtaining the signal for controlling the amount of blacking can be simplified.

Although the above embodiments have been explained by taking a printer using ink as an example, the present invention is not limited to this, and the present invention is also applicable to digital color recording methods such as inkjet, thermal transfer recording, and laser color xerography. be able to.

〔Effect of the invention〕

As described above, according to the present invention, a plurality of chromatic isochrivolous density signals obtained by converting the three-color color signal and a saturation corresponding to the saturation detected based on the isochiblic density signals are obtained. The amount of blackening is determined according to the value. for example,
By calculating the coefficients of the formula that approximates the amount of ink overprinting from the isochorigenic density signal of each color, and calculating the ink overprinting signal based on the minimum value of the isochiblic density signal of each color, the above-mentioned The coefficient changes depending on the saturation. Therefore, the amount of inking also changes continuously, and the saturation can change continuously between the colored area and the black area, and no saturation gap occurs. Furthermore, since the amount of undercolor removal is also controlled according to the saturation, the optimal amount of undercolor removal can be continuously obtained from the colored area to the black area. Furthermore, according to the present invention, all the i-series numbers necessary for the calculation are generated within the circuit, so no complicated adjustment work is required, and the calculation can be performed using a relatively simple circuit.

[Brief explanation of the drawing]

Fig. 1 is a configuration diagram showing an embodiment of the present invention, Fig. 2 is a diagram showing a recording format for explaining the theoretical background of the present invention, and Fig. 3 is a diagram showing a recording format for explaining the theoretical background of the present invention. Graph showing the experimental results. Figure 4 is a graph showing the minimum value of the ink signal, the difference between the inking signal and the minimum value of the ink signal, and the relationship between saturation. Figure 5 shows the change in the coefficients of the approximation formula. Graph, Figure 6 is a graph showing the relationship between the maximum value of the ink signal and the amount of color ink after blacking, and Figure 7 is the relationship between the amount of blacking and the difference between the minimum value of the equivalent neutral density signal and the saturation. FIG. 8 is a graph showing changes in the coefficients of the approximation equation when an isoblast neutral concentration signal is used. 1: Introduction to ink 2: Magenta ink section 3: Yellow ink section 4: Blank page Patent applicant Fuji Xerox Co., Ltd. Agent
People Masu Kobori (and 2 others) Figure 3 Manserk Oma Figure 4 OkOm+n (%) Figure 6

Claims (1)

[Claims] 1. Obtain values corresponding to saturation from three color signals from an input device, convert the three color signals into equivalent neutral density signals of each color material, and convert the three color signals into equivalent neutral density signals. and a color recording method, characterized in that an amount of inking is determined according to the saturation, and an amount of undercolor removal is determined. 2. When the black overprint amount is a_k and the minimum value of the equivalent neutral density signal is a_m_i_n, the black overprint amount a
2. The color recording method according to claim 1, wherein _k is determined by the following formula. a_k=α・a_m_i_n^3+β・a_m_i_n
^2+(1-α-β)・a_m_i_nHowever, α, β:
Coefficient 3, the coefficients α and β are the difference a_m_a_x−a_m_i_ between the maximum value and minimum value of the equivalent neutral concentration signal, respectively.
4. The color recording method according to claim 3, wherein the color recording method is a monotonic function of n. However, when a_m_a_x-a_m_i_n=0, α=0,
When β=0a_m_a_x−a_m_i_n=1, α
<0, β>01≧a_m_a_x−a_m_i_n≧0 4. The color according to claim 2, wherein the relationship between the black printing amount a_k and the minimum value a_m_i_n is a_k≦a_m_i_n. Recording method. 5. The black overprint amount is a_k, and the maximum value, intermediate value, and minimum value of the equivalent neutral density signal are a_m_a_x, a_m_
Claim 1, characterized in that when i_d and a_m_i_n, the undercolor removal amount is determined by the following formula.
Color recording method described in section. a_i_m_a_x={h_1・(a_m_a_x-a
_m_i_n)+(I-h_1)・(a_m_a_x-
a_k)}/1-a_m_i_na_i_m_i_d=
{h_2・(a_m_a_x−a_m_i_n)+(I
-h_2)・(a_m_a_x-a_k)}/1-a_
m_i_na_i_m_i_n=0 However, h_1 and h_2 are functions of a_m_a_x, a_i
_m_a_x, a_i_m_i_d, a_i_m_i_
n is the amount of three-color ink of the maximum, middle, and minimum dots after black reprinting 6, the functions h_1 and h_2 are monotonically increasing functions of values related to the saturation of the color material signals of the plurality of chromatic colors, Claim 5, characterized in that it satisfies the following conditions:
Color recording method described in section. h_1(0)=0h_2(0)=0 h_1(1)=1h_2(1)=1