JPH09186891A - Color gradation control method - Google Patents

Abstract

(57) Abstract: In a digital color copying machine or the like, the color balance of an image output section is maintained and the output density characteristic is optimized. SOLUTION: CMYK read by a color scanner
After averaging the patch data (1101), the inverse function of the characteristic curve of the C toner, which is the characteristic of the scanner, is obtained by the extended Bezier (MB) curve (1102). On the other hand, CMY
Using the K patch data, the mask coefficients a _m and a _y for the C toner reference M and Y toners are calculated (1103), and the a
_m, with a _y, M for C toner, gray balance coefficient Y toner b _m, obtaining a b _y (1104). C above
The inverse function of the toner characteristic curve is multiplied by b _m and b _y respectively to obtain M and Y.
Obtain the color balance correction curve of the toner (110
6). The density correction curve is obtained for all four colors (CMYK) independently of the color balance correction curve.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color density control method for independently correcting color balance and density characteristics of an image output device (printer) in a color image processing system such as a digital color copying machine.

[0002]

2. Description of the Related Art As a conventional technique, for example, Japanese Patent Application Laid-Open No. 2-7
As described in Japanese Patent No. 6760, a grayscale image is output from a printer by using a grayscale signal built in the device, the scanner reads the grayscale image, and a deviation from an initial value of a characteristic of the printer is detected to obtain a color image signal. There is a method of correcting the characteristics (table) of the output tone conversion circuit which gives the correction to the. Since this is a method that uses only the grayscale image, it is difficult to detect and correct the variation of the characteristic peculiar to each color toner or the monochromatic image forming characteristic. In addition, as described in Japanese Patent Laid-Open No. 4-277974, the patterns of white (medium itself) and black ink are scanned to correct the dynamic range and non-linearity of the scanning system, and the pattern of ink used is determined. There is a method of scanning and obtaining the gray balance condition from the color component ratio of each ink and setting the correction parameter. Since the balance condition is set by paying attention only to the ink peculiar to the ink, the characteristic peculiar to the output device (that is, the output characteristic for each color or the output characteristic of the composite color) is not reflected.

[0003]

The problem to be solved by the present invention is to automatically set the color balance according to the characteristics peculiar to the toner (ink) of a digital color copying machine and the characteristics peculiar to the output device which change over time ( This is to simultaneously realize the generation of a correction curve for guaranteeing color balance and the correction of density characteristics (generation of a density correction curve).

[0004]

According to the present invention, a test color patch having a plurality of density levels is output from a printer as a dither pattern using each color toner to be used, and the color patch is separated into three colors by a scanner. It is read as R, G, B signals, the characteristics peculiar to the toner and the output characteristics of the printer at the time of color patch output are detected to calculate the color balance condition, and the correction curve (parameter) for the image reading signal is set to output the printer output. To correct the image signal for.

Specifically, according to the first aspect of the invention, a color patch is printed out with a density pattern that specifies a predetermined gradation used when outputting a color gradation image, and the image is read by an image scanner. , The color balance and density characteristic optimization conditions are calculated and set in a correction circuit or the like. According to the second aspect of the present invention, the condition for maintaining the gray balance is calculated from the read value of the color patch, the correction curve is set for maintaining the gray balance, and the tone of the gradation is adjusted. The γ correction curve (density correction curve) is set independently. According to the third aspect of the present invention, the characteristic unique to the toner and the monochromatic output characteristic are obtained from the measurement of the color patch with the monochromatic toner, and the gray balance condition is set by using the measured values of the composite color patch together. In the invention according to claim 4,
The density reproduction range is detected from the measured values of a plurality of color patches having different density levels, and the input / output characteristics are optimized. According to the fifth aspect of the invention, the balance condition is determined by associating the toner with the lowest maximum density with a ratio. According to the sixth aspect of the invention, the setting is performed using only the color patch generated by the two specified density values. According to the invention described in claim 7, interpolation is performed by a table set in advance. According to the eighth aspect of the invention, the toner density instruction value is obtained by the parameter determined from the gray balance condition for one density level, and correction (fine adjustment) is performed using the previously obtained variation characteristic of the balance coefficient. In the invention according to claim 9, the gray balance is set by using only the single color and mixed color patches with a single toner density instruction value.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

<Color Patch> The data for color patch output incorporated in the system is a toner density instruction value, and the density of the output color patch (corresponding to the amount of adhered toner) varies depending on the printer condition. It can be detected from the reading value of the scanner.

1 and 2 show examples of the structure of a test sheet composed of a plurality of color patches output from a printer.

The test sheet 100 shown in FIG. 1 is a patch of three colors of C, M, Y (3K) and each patch 111, 112, 113, 114 of C, M, Y, K toner single color having a density level of n + 1. ) This is an example of 120 patches.
Appropriate density indication values are given to the density levels 0 to n, and the patches in the horizontal direction are unified by the same density indication value.

The test sheet 200 of FIG.
In this example, the set patches 210 and 220 are output.
Since the output characteristics of the printer differ depending on the tether pattern used when outputting a color patch, it is necessary to switch the setting conditions for each dither pattern used in order to set the color balance exactly. The test sheet 200
"Type 1" and "Type 2" in the table correspond to the types of dither patterns. Further, on the test sheet 200, an explanation 230 is provided so that the operator can easily operate.
Is printed, and two "+" marks 24 are printed.
Reference numerals 1 and 242 are designed so that position adjustment (soft skew correction) can be performed when reading with a scanner described later.

<Conversion of color system> The color patch read signal by the scanner uses an additive color mixture system of three primary colors R, G and B, but the printer side has three primary colors of toner color C,
Since the subtractive color mixture system using M and Y is used, it is necessary to first convert the color system.

In the scanner, R = G = B for standard white
= 1 and normalized to R = G = B = 0 with respect to the reference black, and R = G = B is guaranteed with respect to no color and it is assumed that the gray balance is maintained. .

The red (R) signal has a red complementary color (pure cyan; C _p ) density, and the green (G) signal has a green complementary color (pure magenta; M _p ) density and a blue (B) density. from the signal complementary color (pure yellow; Y _p) of the blue Motomari concentration, it can be expressed by the following equation.

[0014]

[Equation 1]

On the other hand, the toner colors C, M, and Y are not pure colors, and each toner contains a color absorption component (sub-absorption color) to be shared by the other toners. 3 to 5
The concrete measurement results of the C, M and Y toner characteristics are shown in FIG. Further, FIG. 6 shows specific measurement results of C, M, and Y three-color overlapping (3K).

Now, the pure color components C _{p of} the C, M and Y toners,
The composition ratios of M _p and Y _p are {c _c , m _c , y _c },
_{{C m, m m, y} m}, {c y, m y, y y} When is associated with, any color patch readings and patch generation when the toner density readings by scanner (test sheet) The following equation holds for (represented by C, M, and Y).

[0017]

[Equation 2]

By the inverse matrix of this, the scanner reading value can be converted into the toner color system.

[0019]

(Equation 3)

The present invention basically finds the parameters of the matrix which is this conversion formula.

<Dynamic Range Setting> Since the color balance in the saturated region cannot be guaranteed, it is necessary to check the output characteristics of the printer before setting the color balance, and map the toner concentration instruction value to a range within the non-saturated region. .

The output characteristics of the printer can be measured by reading a single-color color patch of plural densities for each toner from a scanner as shown in FIGS. 3 to 5, and the non-saturated area is maximized based on the measured values. Set the dynamic range to use.

<Color Balance Setting> The basic color balance is gray balance, and in the standard state, when an uncolored (gray) image is read by a scanner and a reproduced image is output from a printer, it may be reproduced without coloring. Most important.

For that purpose, when R = G = B holds, C, M, Y (C ≠ M ≠ Y ≠) obtained by the equation (5).
It is necessary that the C) value is corrected according to the output characteristics of the printer at that time, and that the reproduction of no color is guaranteed.

In order to simplify the mutual relation, the relative balance will be considered based on the cyan toner.

The values of Cp, Mp, and Yp of an arbitrary composite color (FIG. 6: three-color overlapping patch) read by the scanner are {c ₀ , m ₀ , y ₀ } and the toner density instruction when the patch is output The values are C ₀ , M ₀ , and Y ₀ (where C ₀ = M ₀ = Y ₀ ), and C _{p of} each single-color patch with the same toner density instruction value,
The values of M _p and Y _p are {c _c0 , m _c0 , y _c0 } for a cyan toner patch, {c _m0 , m _m0 , y _m0 } for a magenta toner patch, and a yellow toner patch. On the other hand, it is assumed that they are {c _y0 , m _y0 , y _y0 }.

Here, mask coefficients a _m and a _y for magenta toner and yellow toner based on cyan toner are introduced and are related to the following equation. The mask coefficient may be regarded as an effective coefficient of the toner component when outputting a composite color with reference to the output of a single color.

[0028]

(Equation 4)

The parameters of the matrix are corrected based on the mask coefficients a _m and a _y obtained from this equation, and here, the gray balance coefficients b _m and b _{y based} on cyan toner are newly _added.
Is introduced and the value is calculated from the following two equations. It is not necessary to base on cyan, but it is convenient to base on the toner with the lowest maximum density, which in this example is cyan only. FIG. 7 shows a specific example of the CMY component ratio of cyan toner.

The grage balance coefficient corresponds to the ratio of the toner density instruction values for reproducing the gray in consideration of the mask coefficients a _m and a _y .

[0031]

(Equation 5)

Based on the gray balance coefficient obtained here, the parameters of the above conversion equations (4) and (5) are related to the following equation.

[0033]

(Equation 6)

It should be noted that the toner density instruction value need not necessarily be calculated by the parameters of the equation (9), but can be calculated in two steps as follows. That is,

[0035]

(Equation 7)

The toner density instruction obtained as is C ',
Assuming M'and Y ', the corrected indicated values C, M and Y are related to the following equations.

[0037]

(Equation 8)

For the sake of simplicity, all the coefficients for achieving the gray balance with respect to the representative designated value in the non-saturated area may be obtained and applied to the entire area. It is preferable to set it as (function form). Note that it is convenient to use an MB curve (extended Bezier curve) to set the function type (for example, Japanese Patent Laid-Open No. 2-2
22264 gazette). The MB curve is expressed by the following equation.

[0039]

[Equation 9]

FIG. 8 shows an example of the MB curve. From FIG.
If the swelling control parameters c and d are increased while the control points P _{0 to} P ₃ are left unchanged, the bulge of the curve becomes large, and conversely, if the bulge control parameters c and d are made small, the curve bulge becomes small. Moreover, the value can be arbitrarily set, and when the value is negative, the direction of the bulge is the opposite direction. An arbitrary curve can be generated by using this.

On the other hand, there is also a simple correction method that is neither too simple nor too strict. This is a coefficient for establishing a gray balance with respect to the representative indication value in the non-saturated area, that is, nine parameters on the left side of the equation (9) are obtained, and as parameters of a constant applied to the entire area, (4), ( Substituting into the formula 5), the toner density instruction value is obtained as C ″, M ″, and Y ″, and the instruction value is corrected by the preset variation coefficient β _m β _y of the balance coefficient.

This variation rate β _m β _y is 1 / b _m , 1 assuming that only the gray balance coefficients b _m and b _y among the coefficients for establishing the gray balance change depending on the density.
/ B corresponds to a percentage of the value obtained for representative indication of _y, the final toner concentration readings becomes the following equation.

[0043]

(Equation 10)

<Tone γ setting> Different characteristics are required for the tone (tone) of the gradation image or depending on the target image data, but basically the output characteristics of the printer should be maximized. Set.

It is preferable to set the basic γ characteristic (density characteristic) in combination with the dynamic range setting described above.
It is common to have linear characteristics. Also, it is necessary to set a separate γ curve to realize a special tone, but it is preferable to use an MB curve. Normally, the same correction curve may be applied to all toner colors, but when changing the color tone, it is set for each toner color.

<UCR and BG> Although not directly related to color balance, UCR (UnderCol
or Removal) and (Black Generation) to prevent the density of the black portion from decreasing and avoid the use of unstable output areas due to high density color mixing.

UCR is performed on the basis of the minimum value U of the above C _p , M _p , and Y _{p. When} the UCR parameter is n, the target of color balance is C _p -nU, M _p -nU. ，
Y _p nU next, C corresponding to these, M, thereby obtaining the Y toner concentration readings. Further, BG is given a black toner density instruction value with K = nU, but normally γ correction dedicated to black toner is performed. The value of n is set empirically.

<Color Image Processing System> FIG. 9 shows a functional block diagram of an embodiment of a color image processing system to which the present invention is applied. This system is an example of a digital color copying machine, and includes an image scanner 900, a scanner γ (density) setting unit 901, a color system conversion unit 902,
Gray γ setting unit 903, tone γ setting unit 9
04, changeover switch 905, dither processing unit 90
6, a laser printer 920, an arithmetic processing unit 930, and a color patch generator 940. Also, FIG.
The system is also capable of interfacing an image signal in the R, G, B system or the C, M, Y, (K) system with the outside. In the configuration of FIG. 9, 901 to 906 are collectively referred to as an image processing unit (IPU) 910. The arrangement relationship between the gray γ setting unit 903 and the tone setting unit 904 may be reversed.

In the normal copy mode, the changeover switch 9
05 is set to the state shown in the figure. The original set on the image scanner 900 is read as an image signal color-separated into three primary colors of R, G, and B. First, the scanner γ setting unit 901 ensures gray balance guarantee and proper output characteristics. As described above, γ correction (density correction) is performed. In the next color system conversion unit 902, conversion to R, G, and B complementary color densities (C _p , M _p , Y _p ) is performed, and the arithmetic processing unit 930 performs the printer 920 in advance.
It is adjusted to the dynamic range set according to the state of. In the next gray γ setting unit 903, the γ correction ensuring the gray balance is performed by the parameters set by the arithmetic processing unit 930 in advance according to the state of the printer 920, and the toner color (toner density instruction value) is changed. Is converted. The next tone γ setting unit 904 sets the tone γ for adjusting the tone.
The main purpose is to make corrections, but in some cases,
Gamma correction for changing the color tone is also performed here. After that, in the dither processing unit 906, as the final stage of the image data processing, dither processing according to a designated dither table is performed, and the processing result (this corresponds to the modulation level of the laser beam) is input to the laser printer 920. To be done. However, depending on the specification, the data may be output without performing dither processing.

On the other hand, in the color balance setting / adjustment mode, the changeover switch 905 is connected to the lower side (color patch generator side). The color balance setting process is
The color patch generator 940 specifies the toner of each color of C, M, Y, and K and the mixture of three colors (3K), and specifies the toner density (for example, 8, 16, 32, 48, 64, 96,
It is started by repeatedly outputting data for color patch output of 128, 198, and 255 gray scales). The dither processing unit 906 performs dither processing according to the data, and the laser printer 920 passes the test sheet 10 to the test sheet 10.
Output as 0. The color patch of the test sheet 100 is read by the image scanner 900. The read data of the color patch is stored in the scanner γ setting unit 9
01, R prepared under the same conditions as in the copy mode,
It is taken into the arithmetic processing unit 930 as G and B signals. In the arithmetic processing unit 930, the C, M, and Y toner density instruction values when the color patch data is output are fetched from the color patch generator 940, and the corresponding patch R, G, B from the scanner γ setting unit 901 is fetched. The characteristic value of the toner and the output characteristic of the printer 920 are obtained from the relationship with the value, and each parameter (density correction) of the color system conversion unit 902, the gray γ setting unit 903, and the tone γ setting unit 904 is set.

<Application Example to Other Systems> FIG. 10 shows a functional block diagram of another embodiment of the color image processing system to which the present invention is applied. The difference from FIG. 9 is that it has a filter unit 907 and has a more general configuration. Further, it has an undercolor removal and black plate generation unit 908, and is configured to be compatible with 4-color printing. In addition, γ of the scanner γ setting unit 901
The system is also capable of changing set values. When setting (changing) the scanner γ, the gray balance of the scanner is set using the black toner patch in the test sheet.

<Flow of color balance setting> FIG.
The processing flow for color balance setting is shown. This is the arithmetic processing unit 93 in the system of FIG. 9 or 10.
0 is executed.

After averaging the color patch data of each of the C, M, Y, and K single color toners (step 110).
1) The C patch data is complemented with the MB curve, and the inverse function of the characteristic curve is obtained (step 110).
2). On the other hand, using the C, M, Y, and K patch data, first, the mask coefficients a _m and a _y for the C toner reference M toner and Y toner are obtained by the equation (6) (step 11).
03). Next, based on the obtained mask coefficients a _m and a _y , the gray balance coefficients b _m and b _y for the C toner reference M toner and the Y toner are calculated by the equations (7) and (8) (steps). 1104). This step 1103
1104 is a toner density instruction value (for example, 8, 16, 3).
It is repeated for each gradation of 2, 48, 64, 96, 128, 196, 255 (step 1105). As a simple method, if only a certain representative indicator value (for example, indicator value 196) in the unsaturated region is used, step 1105
Is not required. Next, in step 1102, the inverse function of the C toner characteristic curve is calculated as b
_m times, and b _y times, M toner, and Y toner correction curve (color balance curve) (step 1106). In addition,
The black toner is the same as the C toner.

The color balance curve obtained in step 1106 is almost linear in density, but the highlight, middle,
When changing the density of the shadow or the like, a density correction curve for that purpose is necessary, and this is also possible with the MB curve. The density correction curve is common to the four colors (CMYK).

[0055]

According to the first aspect of the present invention, even if the characteristics of the toner to be used are changed or the output characteristics of the printer are changed (with time), no special knowledge or experience is required. Both can easily (almost automatically) set the color balance. According to the second aspect of the invention, since the density and tone can be set independently of the color balance setting, it is possible to adjust the density and change the tone without worrying about the hue deviation. According to the invention of claim 3, the color characteristics of the toner itself, the output characteristics of the monochromatic toner and the output characteristics of the composite color can be measured at the same time, so that the color balance condition can be immediately obtained by a simple operation. According to the invention as set forth in claim 4, since the correction can be performed by making the most of the output performance of the printer at that time, the characteristic setting can be optimized. According to the fifth aspect of the present invention, the reference toner can be set to 1 and the others can be set to 1 or less, so that the balance can be ensured and the handling becomes easy. According to the invention of claim 6, the processing amount can be reduced and the configuration can be simplified. According to the invention described in claim 7, the processing speed can be increased. According to the invention described in claim 8, since it can be realized by relatively simple arithmetic processing, the configuration can be simplified while maintaining accuracy. According to the invention described in claim 9, even simpler processing can be further simplified. In addition, the accuracy of color patches can be made rough.

[Brief description of the drawings]

FIG. 1 is a configuration example of a color patch test sheet.

FIG. 2 is another configuration example of a color patch test sheet.

FIG. 3 is a specific example of CMY characteristics of cyan toner.

FIG. 4 is a specific example of CMY characteristics of magenta toner.

FIG. 5 is a specific example of CMY characteristics of yellow toner.

FIG. 6 is a concrete example of CMY characteristics of three-color superposition.

FIG. 7 is a specific example of the CMY component ratio of cyan toner.

FIG. 8 is an example of an extended Bezier curve (MB curve).

FIG. 9 is a diagram showing a configuration example of a color image processing system to which the present invention is applied.

FIG. 10 is a diagram showing another configuration example of a color image processing system to which the present invention is applied.

FIG. 11 is an example of a processing flow for color balance setting according to the present invention.

[Explanation of symbols]

 100, 200 Test sheets 111 to 114, 120 Patch 900 Image scanner 901 Scanner γ setting unit 902 Color system conversion unit 903 Gray γ setting unit 904 Tone γ setting unit 905 Changeover switch 906 Dither processing unit 910 Image processing unit 920 Laser printer 930 Arithmetic processing unit 940 color patch generator

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kiyoshi Suzuki 1-3-6 Nakamagome, Ota-ku, Tokyo Inside Ricoh Co., Ltd.

Claims (9)

[Claims] 1. A color gradation control method in a color image processing system including a color image scanner and a full color printer, wherein a color patch using toner peculiar to the printer is output from the printer as a density pattern designating a predetermined gradation. Color gradation control characterized by reading the color patch from a color image scanner, measuring the output characteristics peculiar to the toner used, and automatically setting the color balance and density characteristics optimized for the printer used. Method. 2. The color gradation control method according to claim 1, wherein a correction curve for correcting a toner concentration instruction value of each color or a first step of setting a parameter thereof for maintaining a gray balance, and a hue are set. A color gradation control method comprising a second step of setting a density correction curve or parameters thereof for defining density characteristics regardless of the above. 3. The color gradation control method according to claim 2, wherein in the first step, a characteristic of each toner is obtained as a ratio of a main absorption component and a sub-absorption component from a scanner reading value of a color patch with a single color toner. In addition, the output characteristics of each toner are obtained, and the gray balance condition is obtained by using the scanner reading value of the composite color patch generated by a plurality of toners together, and the correction curve of the toner density instruction value or its parameter is set. And color gradation control method. 4. The color gradation control method according to claim 2, wherein in the second step, the density reproduction range is obtained based on the output characteristic peculiar to the printer obtained from the scanner reading value of the color patch, and the input / output characteristic is obtained. A color gradation control method characterized by setting a density correction curve or a parameter thereof for optimizing the color gradation. 5. The color gradation control method according to claim 3, wherein in the first step, a gray balance is obtained by associating with a measurement value only by a ratio based on the toner of the color having the lowest maximum density. A characteristic color gradation control method. 6. The color gradation control method according to claim 3, wherein in the first step, a color patch based on two specific density instruction values is used, and a curve fitted to a toner characteristic prepared in advance is fitted. A color gradation control method characterized by performing interpolation to obtain a gray balance. 7. The color gradation control method according to claim 6, wherein in the interpolation of the toner characteristic, the correspondence between the measured value and the parameter of the curve after the interpolation is tabulated in advance. Method. 8. The color gradation control method according to claim 3, wherein in the first step, each toner density instruction value is applied by applying a parameter determined from a gray balance condition in one specific density instruction value to the entire area. A color gradation control method, characterized in that the indicated value is corrected by the variation rate of the balance coefficient that has been obtained and set over the entire area in advance. 9. The color tone control method according to claim 3, wherein the second step uses only a single color and a mixed color color patch according to one specific density instruction value, and determines from the gray balance condition obtained from it. A color gradation control method characterized in that each toner density instruction value is obtained by applying the above-mentioned parameter to the entire area.