JPH04335771A - Color correcting device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color correction method for color image signals, and more particularly to a color correction and color conversion apparatus for use in correction of captured images, display color correction, print color correction, and the like.

0002

2. Description of the Related Art When a color image is read in by a scanner, displayed on a display, and recorded by a printer or printing machine, the color expression space differs for each medium, so it is necessary to correct the colors to match each medium. For example, the printer's target color image color may include the original color, display color,
This is the color of the printing press and requires color correction to suit each printer.

A conventionally known general color correction method involves inputting a target color signal into a masking equation, calculating this equation to generate a color correction signal, and
This is a method that attempts to express the target color. for example,
Generally, a linear masking equation as shown below is used to convert an RGB image for display on a display into a CMY image for hard copying with a printer.

0004

[Math 1]

[0005] Here, R, G, B, C, M, and Y are variables each indicating the density value of each primary color, and a11 to a33 are called primary masking coefficients and are parameters that influence conversion. These nine parameters are determined from the ratio of the highest density values of the three primary colors. For example, if several pixels are selected as sampling pixels, the error between the RGB density value and the CMY density value for the same sampling pixel is determined. These nine parameters are set by performing calculations using the method of least squares so that . Color adjustment is performed by adjusting nine parameters or by adjusting gamma correction curves for each of Y, M, and C.

The above is an example of color correction using a linear masking equation, but the quadratic terms RR, GG, BB, R of RGB
A method of improving color reproducibility by establishing a quadratic masking equation including G, GB, and BR is also known. Further, Japanese Patent Application Laid-open No. 60-220660 proposes a method in which a color space with three axes of RGB is divided into a plurality of regions, a color correction matrix is prepared for each region, and color correction is performed. Color adjustment is
The coefficients of the color correction matrix will be adjusted in each region.

Furthermore, in Japanese Patent Application Laid-Open No. 1-234252, the difference between the largest density value and the second largest density value is used as an attribute of the color of the largest value for RGB color signals.
The defined primary color component, and the secondary color component defined as the difference between the second largest value and the smallest density value as an attribute of the mixed color of the color with the largest value and the color with the second largest value. , the smallest density value is separated into tertiary color components defined as attributes of a mixed color of three colors, Y, M, and C are adjusted from each component using a lookup table, and then they are combined. We propose a color correction method based on

[0008] Color conversion is usually called color sign, and W
, Y, M, C, R, G, B, Bk colors to other W, Y,
This function converts to M, C, R, G, B, and Bk colors. Conventional printers use Y, M, and C recorded information that has been binarized using the dither method or area gradation method to determine, for example, 1, 1, 1, which color is a black dot, and then convert each dot to another color. is converting.

[0009]

However, color correction using masking equations cannot sufficiently correct the nonlinear color correction space, resulting in a large average color difference. Also, color adjustment is difficult because it uses a gamma correction curve.

[0010] In Japanese Patent Application Laid-open No. 60-220660, the color space is divided and optimized for each to reduce color differences, but when a specific color area is adjusted, color conversion occurs at the boundaries of surrounding color areas. There is a problem that errors occur.

[0011] In Japanese Patent Laid-Open No. 1-234252, for example, when adjusting the R system, it is not easy to know how much each of Y, M, and C can be increased or decreased. Setting the color adjustment LUT is complicated.

In the conventional color assignment method, Y, M,
The dither pattern cannot be changed for each C ink color, and color shift reduction processing such as screen angle processing cannot be performed.

An object of the present invention is to provide a color correction method for efficiently adjusting or converting a specific color system.

[0014]

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a means for obtaining the maximum value, intermediate value, minimum value and respective color information of the three primary color data of pixels input sequentially, and a means for obtaining the respective color information. The primary color component obtained by subtracting the intermediate value from the maximum value,
Means for separating the secondary color component obtained by subtracting the minimum value from the intermediate value and the tertiary color component having the minimum value, and a conversion formula or conversion table set for each area divided into multiple areas based on color information for each separated color component. and a means for synthesizing the converted primary, secondary, and tertiary color component values into R, G, and B display display data or Y, M, and C recording print data according to the color information. Features a correction device.

With this device, gray balance can be achieved by preparing tables for converting tertiary color components, which are achromatic color components, for each of R, G, and B, or Y, M, and C of the ink colors used for recording and printing. can.

[0016] In order to handle the four-color inks of Y, M, C, and Bk, it is necessary to remove the undercolor and ink, but these can be done using a conversion formula or conversion table from the tertiary color components of this device. Generate undercolor removed and blackened values.

Color assignment (color conversion) converts primary, secondary, and tertiary colors YMCRGBBk to other WYM colors using color assignment data set on the panel or computer.
CRGBBk and synthesized to generate R, G, B display display data or Y, M, C recording print data.

[0018] Furthermore, means for obtaining the maximum value, intermediate value, minimum value and respective color information of the three primary color data of pixels input sequentially, and a primary color component obtained by subtracting the intermediate value from the maximum value based on this color information; Means for separating the secondary color component obtained by subtracting the minimum value from the intermediate value and the tertiary color component having the minimum value, and the 0th order color component obtained by subtracting the maximum value of input data from the upper limit of data based on this color information, and the maximum value. Means for separating the primary color component obtained by subtracting the intermediate value from the intermediate value, the secondary color component obtained by subtracting the minimum value from the intermediate value, and the tertiary color component having the minimum value; This is achieved by a color correction device comprising means for converting WYMCRGBBk into other WYMCRGBBK according to assignment data and synthesizing these to generate display data R, G, B or recording print data Y, M, C.

[0019]

[Operation] The color correction device decomposes the R, G, B values or Y, M, C values of an input pixel into primary, secondary, and tertiary color components, and divides each into multiple color regions based on color information. It is easy to adjust the color because it is adjustable.

Furthermore, since the color correction device can set a conversion table for each ink color to be recorded in the tertiary color component, gray balance can be easily achieved.

By adjusting the tertiary color component values up and down for all color information, the brightness and darkness of the recorded image can be adjusted.

Furthermore, since a conversion table for inking/undercolor removal can be set in the tertiary color conversion table, four colors can be supported.

[0023] The color correction device is capable of converting primary, secondary, and tertiary colors or 0, 1, 2, and tertiary colors into other colors and compositing them in the means for compositing each color component, so color conversion is easy. It can be performed.

[0024]

Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In this embodiment, RG from the computer
An example in which B data is converted to CMY density data and color correction processing is performed is shown. Note that the data is 8 bits, from 0 to 2.
It is assumed that the value is 55.

FIG. 1 is a block diagram of a color correction circuit 1 that performs color correction processing according to the present invention. YMC, which is sequentially input to an input terminal 2, is obtained by converting RGB data for display display into density data. If all YMC values are 0, it represents white, and when all have the maximum value, it represents black. The input YMC data is latched into the latch circuit 3 as color density data of one pixel. The comparator 4 compares the magnitudes of YM, MC, and CY, and outputs a signal αβγ representing the magnitude relationship. Specifically, as shown in FIG. 2, the comparator 4a compares the magnitudes of Y and M, and if Y is greater than or equal to the value of M, the magnitude determination signal α is set to 1. Similarly, the comparators 4b and 4c compare MC and CY, and output a magnitude determination signal βγ. Table 1 shows the magnitude relationship between these signals and YMC.

[0026]

[Table 1]

For example, when αβγ is (110), Y≧
M≧C. Note that the input data for αβγ is YMCRG.
It shows which color region of the BBk system it is included in, and represents the color information of the input data.

The selection signal generator 5 generates a large/medium/small selection signal 6 and a color adjustment LUT selection signal 7 from the input data based on the color information αβγ.
, a YMC composite signal 8, a color correction LUT selection signal 9, and a color component selection signal 10.

The color component separation circuit 11 includes a selection signal generator 5
According to the large, medium, and small selection signal 6 from the YMC input data, primary, secondary, and tertiary color components are determined and output. The primary color is
It is composed of only one color, and the colors Y, M, and C have the maximum values. Secondary colors are composed of two colors: R for Y and M, G and M for Y and C.
and C become B. The tertiary color is Bk, which is a combination of three colors. The value of the color component is, for example, when C>M>Y, the primary color is C and its component A is (CM), the secondary color is B and its component B is (
MY), the tertiary color is Bk and its component C is (Y). Table 2 shows the relationship between color information, primary, secondary, and tertiary color component values, and color.

[0030]

[Table 2]

FIG. 3 shows an example of the configuration of the color component separation circuit 11. The selector circuit 12 selects the input data YMC in the order of large, medium and small based on the large, medium and small selection signal 6 from the selection signal generator 5 and outputs it to the latch circuit 13 . Table 3 shows the contents of the large, medium, and small selection signal 6. 01 selects Y, 10 selects M, and 11 selects C. The large, medium, and small selection signal 6 is output in the order of large, medium, and small according to the value of αβγ. Note that when αβγ is (111), Y
Since the MC data takes the same value, the order cannot be assigned, but
Here, it is assumed to be YMC.

[0032]

[Table 3]

As a result, the latch circuit 13 latches the input data in the order of large, medium, and small. The subtracter 14a subtracts the intermediate value from the maximum value and outputs the primary color component A. The subtracter 14b subtracts the minimum value from the intermediate value and outputs the secondary color component B. The latch circuit 13 outputs the minimum value of the third-order component C.

The color adjustment circuit 15 inputs the primary color component A, secondary color component B, and tertiary color component C from the color component separation circuit 11, and performs color adjustment processing on each component. FIG. 4 shows an example of the color adjustment circuit 15, in which the primary color component A, the secondary color component B, and the tertiary color component C are input to the primary adjustment LUT1.
6. Conversion is performed using the secondary color adjustment LUT 17 and the tertiary color adjustment LUT 18. Each of the color adjustment LUTs 16, 17, and 18 has a color conversion curve for each of the seven color information, and the first, second, and third color conversion curves are selected by the color adjustment LUT selection signal 7. As shown in FIG. 5, several types of color conversion curves may be prepared and selected. For example, if the skin color is yellowish, (αβγ) is the primary color adjustment L at (110).
When the a curve in FIG. 5 is selected in the UT 16 and color conversion is performed, the yellowish tinge of the skin color is reduced. Also, tertiary color adjustment LUT1
8, when the b curve in FIG. 5 is selected, the entire recorded image becomes brighter. Note that the adjustment process for each component may be performed using a conversion formula.

When obtaining gray balance, prepare a conversion curve for the tertiary color component C in gray for each ink color,
Measure and set the printer's gray characteristics so that grays are recorded as gray. In Figure 4, the gray balance/inking LUT 19 is used to perform gray balance independently.
added to. A conversion curve for each ink color is selected by an ink color selection signal 20. The ink color selection signal 20 is Y
00 when recording M, 01 when recording C, 10 when recording Bk, and 11 when recording Bk.

[0036] Also, this gray balance/inking LU
T19 performs undercolor removal and inking when four colors are supported. For example, when using YMC ink, the tertiary color component C is set as shown in the c curve in Figure 6, and the black component of the input pixel is subtracted to remove the undercolor, and when using Bk ink, the undercolor is compensated for and the color density is In order to increase the value, set it like the d curve in Fig. 6. In addition to the curve shown in FIG. 6, other similar curves may be set.

For example, the YMC synthesizer 21 holds primary, secondary, and tertiary color components in a latch circuit 22, a latch circuit 23, and a latch circuit 24 as shown in FIG. are selected and added by the adder 25. The selection signal generator 5 outputs a YMC composite signal 8 as shown in Table 4 according to the color information.

[0038]

[Table 4]

The YMC synthesizer 21 outputs data in YMC order according to the YMC composite signal 8. For example, if the color information (αβγ) is (001), the YMC composite signal 8
are (001), (011), (111), and YM
In the output of the C synthesizer 21, Y is the tertiary color component value, M is the sum of the secondary color component value and the tertiary color component value, and C is the one, two,
It is the sum of the tertiary color component values.

The color correction circuit 26 sets color correction coefficients to correct the color characteristics of the printer, and the color adjustment circuit 15
to make it easier to adjust the colors. FIG. 8 is a block diagram of the color correction circuit 26. The color correction LUT 27 has YMC input data multiplied by the correction coefficient as Y, M, C, R, G, B.
, set for each Bk system. The color correction method of this example is as follows:
This is a color masking method of X9, and the following formula is executed.

[0041]

[Math 2]

[0042] This equation is transformed to obtain the following equation.

[0043]

[Math 3]

The product obtained by multiplying the color correction coefficient by the input data from this equation is stored in the LUT, and six terms are added to perform color correction processing. For example, as shown in FIG. 9, the color correction coefficient is
The recording density space is divided into six regions centered on Y, M, C, R, G, and B, and a plurality of color patches are extracted from each region, and color correction coefficients are determined using the least squares method. Furthermore, for achromatic colors, the color correction coefficient is determined using only the gray color chart. The dividing plane shown in Figure 9 takes the midpoint of each side,
This is the plane created by connecting this, the white point, and the black point. Note that it may be slightly shifted from the midpoint. YM in this color correction LUT27
The YMC data from the C synthesizer 21 and the color correction LUT are inputted to the selection signal 9 and the ink color selection signal 20 to obtain color correction data. Table 5 shows the contents of the color correction LUT selection signal 9.

[0045]

[Table 5]

The color correction selection signal 9 is 001 and Y,010.
Select the color correction curve of M at 011, C at 011, R at 100, G at 101, B at 110, and Bk at 111. for example,
If the primary, secondary, and tertiary colors are C, B, and Bk, the color correction LUT
The selection signal 9 first becomes 011, and C color correction is performed. Then, at step 110, B color correction is performed. Finally, at step 111, Bk color correction is performed. The order of color correction is not particularly determined, but it is the order of primary, secondary, and tertiary colors.

Here, the color correction matrix of each area is MC
, MB, MBk, the respective color correction calculations are as follows.

[0048]

[Math 4]

[0049]

[Math 5]

[0050]

[Math 6]

By multiplying these by the component ratio of each color component as a weighting coefficient and adding them, it is possible to perform color correction that matches the color of the input data.

FIG. 10 is a block diagram of the weighting coefficient calculation circuit 28, which calculates the component ratio for each color component. The component ratio is
A value obtained by dividing the value of each color component by the maximum value of the YMC data after color adjustment is used. When the maximum value is 0, all component ratios are set to 0. In addition, after separating the input data into 0th color (W), 1st, 2nd, and 3rd colors, and adjusting the 1st, 2nd, and 3rd colors,
The adjusted values of the first, second, and third color components may be added and subtracted from 255 to obtain the W component value, and the value of each color component may be divided by 255 to obtain the component ratio. In this case, the color correction circuit uses W system (
Prepare a color correction coefficient for (light color).

Selector circuit 29 of weighting coefficient calculation circuit 28
outputs component values in the order of first, second, and tertiary colors in accordance with the color component selection signal 10, and an adder 30 adds and outputs the first, second, and tertiary color component values. The divider 31 divides the first, second, and third color component values by the sum of the first, second, and third color component values, and outputs each component ratio.

The multiplier 32 multiplies the calculation result of the primary color from the color correction LUT 27 by the component ratio of the primary color. Next, the calculation result of the secondary color in the color correction LUT 27 is multiplied by the component ratio of the secondary color, and finally the calculation result of the tertiary color in the color correction LUT 27 is multiplied by the component ratio of the tertiary color. Adder 33 and latch circuit 34 add the three results of multiplier 32 and output the result to limiter circuit 35 . Limiter circuit 35
is limited to 0 if the calculated result value is negative, and to 255 if it exceeds the maximum value of the data.

If the color correction circuit 26 is not provided, the color adjustment circuit 15 may adjust the color conversion curve to perform color correction.

According to this embodiment, it is possible to perform color correction using a more suitable color correction coefficient for each input data based on the color component information of the input data, and to perform color adjustment for each color area divided by color information. Color difference can be reduced and color adjustment can be performed efficiently.

In the above circuit, by changing the YMC composite signal 8 in various ways, it is possible to assign colors of the YMCRGBBk system to other WYMCRGBBk systems. For example, when color assigning Y to R and R to Y, the color conversion for each color information is shown in Figure 11.
Shown below. For color conversion, αβγ is (010), (100),
At (110), the Y component changes to the R component, and the R component changes to the Y component. Other things remain unchanged. The YMC composite signal 8 in color assignment is as shown in Table 6.

[0058]

[Table 6]

Furthermore, if the 0th order color (W) component is separated from the YMC data, W can be converted to another color. Figure 12 shows how the WYMCRGBBk colors can be compared to other WYMCR colors.
This is a circuit that assigns colors to the GBBk system. The adder 36 sums the first, second, and third color components, and the subtracter 37 sums the first, second, and third color components.
The output of the adder is subtracted from 55 to obtain the W component. The latch circuit 38 holds this W component. The adder 29 is YMC
The 0, 1, 2, and 3 dimensional color components are added using the composite signal 8. Table 7 shows the contents of the YMC composite signal 8 when W is color assigned to Bk, Bk to W, Y to M, and R to Y.

[0060]

[Table 7]

Note that if color adjustment is not performed, color assignment can be performed using a circuit configuration as shown in FIG. The subtracter 39 of the color component separation circuit 11 subtracts the maximum value of the input data from 255 and outputs the zero-order color (W) component. Adder 29
adds the 0, 1, 2, and 3 color components using the YMC composite signal 8. The user can easily issue color assignment instructions from the printer panel or host computer.

In this embodiment, color conversion can be easily performed by changing the YMC composite signal.

FIG. 14 shows a flowchart when color correction processing is performed by software. In step 100, an adjustment curve is set in the color adjustment LUT for each color area. When performing gray balance, undercolor removal, and inking, adjustment curves are set for each YMC. Also, color correction coefficients are set for each YMCRGBBk system. YMC at step 110
Input data, determine the size, medium, and small relationships of Y, M, and C, and obtain color information. In step 120, from this color information, one, two,
Find the tertiary component. In step 130, each component is transformed using a color adjustment LUT of primary, secondary, and tertiary colors specified by the color information. In step 140, each of the converted components is synthesized into Y, M, and C based on the color information. Here, if a color assignment has been set, Y, M, and C are combined according to the setting. In step 150, the first, second, and third color components are divided by the sum of the first, second, and third color components to determine the component ratio of each component. Note that when the sum of the primary, secondary, and tertiary color components is 0, the component ratios are all 0. In step 160, the color correction coefficients of the primary, secondary, and tertiary colors selected based on the color information are multiplied by the YMC data to obtain color correction results of the primary, secondary, and tertiary colors. At step 170,
The component ratio of the first, second, and tertiary colors is multiplied by the color correction results of the first, second, and tertiary colors, and then added. In step 180, if the added result is less than or equal to 0, it is set to 0, and if it is greater than or equal to 255, it is set to 255. Note that when color correction processing is not performed, a color adjustment curve is selected and color matching is performed.

According to the above embodiment, color correction can be performed using a color correction coefficient more suitable for each input data based on the color component information of the input data, and color adjustment can be performed for each color area divided by the color information. Color differences can be reduced and color adjustment can be performed efficiently.

FIG. 15 is an example in which the color correction processing of the present invention is applied to a color printer, in which RGB data of computer graphics created in the computer 40 is color corrected and output to the color printer 41. The color correction matrix is determined by the least squares method so that the color displayed on the display 42 is the true value and the color recorded by the color printer 41 is matched to the true value. First, the gamma correction LUT 43 is set with conversion data for correcting the density characteristics of the recording system 44 to produce as smooth a gradation as possible. Further, a plurality of colors in each region are recorded for each of Y, M, C, R, G, B, and Bk systems, and color correction coefficients are determined for each and set in the color correction LUT 27 of the color correction circuit 1.

In the recording operation, RGB data from the computer 40 is converted into CMY density data by the density conversion LUT 45, and this is input to the color correction circuit 1 shown in FIG. After gamma correction is performed in the gamma correction LUT 43, the image is recorded in the recording system 44 to obtain a duplicate image 46. If the color tone does not match, another adjustment curve of the color adjustment circuit 15 is selected to adjust the color tone.

FIG. 16 is an example in which the color correction processing of the present invention is applied to a color scanner, in which color correction is performed in the color scanner 48 so that the colors of a document 47 are displayed on the display 42 as faithfully as possible. For the true value data, use the color of original 47 reprinted into RGB luminance data,
A color correction matrix is determined by the least squares method so as to reduce the difference between this and the read RGB reflection data. The region division is performed by dividing the RGB reflection data space into R as shown in FIG.
, G, B, Y, M, and C axes, a plurality of color patches are extracted from each region, and color correction coefficients are determined using the least squares method. Furthermore, for achromatic colors, the color correction coefficient is determined using only the gray color chart. Color correction using an optimal color correction matrix for each area can reduce color reproduction errors. In order to display RGB reflection data on the display 42 with linear luminance, the gamma correction LUT 49 has
Conversion data is set.

The reading system 50 inputs the reflected light from the original 47, and converts it into RGB reflection data at the A/D converter 51. This is color corrected by the color correction circuit 1 of this method, gamma corrected by the gamma correction LUT 49, and then output to the computer 40. If the color tone does not match, another adjustment curve of the color adjustment circuit 15 is selected to adjust the color tone.

The main color correction method can also be applied to color correction of an input signal from a camera, color correction of a display, color correction between a printer and a printing press, etc.

[0070]

According to the present invention, since color adjustment can be performed for each color area divided by color information, color differences can be reduced and color adjustment can be performed efficiently.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a color correction circuit implementing the color correction method of the present invention.

FIG. 2 is a block diagram of a comparator of the color correction circuit.

FIG. 3 is a block diagram of a color component separation circuit of the color correction circuit.

FIG. 4 is a block diagram of a color adjustment circuit.

FIG. 5 is a characteristic diagram showing an example of a conversion curve of a color adjustment LUT.

FIG. 6 is a characteristic diagram showing an example of a conversion curve of an LUT for undercolor removal/inking.

FIG. 7 is a block diagram of a YMC synthesizer.

FIG. 8 is a block diagram of a color correction circuit.

FIG. 9 is an explanatory diagram showing an example of recording density space division.

FIG. 10 is a block diagram of a weighting coefficient calculation circuit.

FIG. 11 is an explanatory diagram showing an example of color conversion in color assignment.

FIG. 12 is a block diagram of a color assign circuit including a zero-order color component.

FIG. 13 is a block diagram of a color assignment circuit without color adjustment.

FIG. 14 is a flowchart of color correction processing.

FIG. 15 is a block diagram in which a color correction circuit is applied to a color printer.

FIG. 16 is a block diagram in which a color correction circuit is applied to a color scanner.

FIG. 17 is an explanatory diagram showing an example of dividing an RGB reflection data space.

[Explanation of symbols]

2... Input terminal, 3... Latch circuit, 4... Comparator, 5... Selection signal generator, 6... Large, medium and small selection signal, 7... Color adjustment LUT selection signal, 8... YMC composite signal, 9... Color correction LUT selection signal , 10... Color component selection signal, 11... Color component separation circuit, 1
5... Color adjustment circuit, 21... YMC synthesizer, 26... Color correction circuit, 27... Color correction LUT, 28... Weighting coefficient calculation circuit, 3
2... Multiplier, 33... Adder, 34... Latch circuit, 35...
limiter circuit.

Claims (1)

[Claims] Claim 1: R, G, B display display data or Y, M,
In a color correction device that generates C recording print data, there is a means for determining the maximum value, intermediate value, minimum value and respective color information of the three primary color data of pixels input sequentially, and a means for determining the maximum value, intermediate value, and intermediate value based on this color information. Means for separating the primary color component obtained by subtracting the value, the secondary color component obtained by subtracting the minimum value from the intermediate value, and the tertiary color component having the minimum value, and for each region divided into a plurality of regions based on the separated color information. A means for converting using a set conversion formula or conversion table, and converting the converted primary, secondary, and tertiary color component values into R, G, and B display display data or Y, M, and C recording print data according to color information. A color correction device comprising: means for compositing.