JPH1169188A - Color image processor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a device that has high color correction accuracy with the addition of a small scale of a circuit by selecting a set of coefficient data of a linear connection operation that is held by a coefficient holding means and a conversion characteristic that is used by a gradation characteristic converting means in accordance with a decision result of a hue deciding means at the time of generating output image signals of C, M and Y. SOLUTION: A set of coefficient data of a linear connection operation that is held by a coefficient holding means and a conversion characteristic that is used by a gradation characteristic converting means are selected in accordance with a decision result of a hue deciding means at the time of generating output image signals of C, M and Y. In this device, a hue deciding part evaluates a large and small relation of RGB input image signals and outputs a hue number as a hue area signal. A masking operation circuit consists of registers REG1 to 3 which hold a coefficient and multipliers MUL1 to 3, and each register REG1 to 3 outputs a coefficient in response to the hue area signal. A gradation conversion circuit consists of a conversion table and a multiplexer that selects data that corresponds to the hue area signal in conversion data according to a conversion table.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image processing apparatus for forming a full-color image such as a color copying machine and a color facsimile.

[0002]

2. Description of the Related Art Generally, for example, in a digital color copying machine, an original image is read after being separated into basic colors of R (red), G (green), and B (blue), and an image signal obtained by the color separation is read. Are C (cyan), M (magenta),
Y (yellow) is converted into three primary color components, and the C, M, and Y recording systems are energized based on the image signals of the respective colors.
A desired color image is reproduced by superimposing the M and Y color materials. Ideally, the original color and the reproduced color should match by using the R, G, and B color separation data as their complementary colors, C, M, and Y data, but from the color separation characteristics of the reading system, Therefore, a desired reproduced color cannot be obtained as it is because of the deviation of the color material and the unnecessary absorption component of the coloring material. Therefore, in the conversion from RGB to CMY, R, G, B
Is used for calculating C, M, and Y by the linear combination of. Equation 1 expresses them in the form of a matrix operation equation.

[0003]

(Equation 1)

However, generally, color separation characteristics and color materials have nonlinear distortion, and sufficient color reproducibility cannot be obtained only by linear conversion as shown in Equation 1. On the right side of the matrix operation expression, not only the first-order terms of R, G, and B, but also R * R, G *
G, B * B, R * G, G * B, B * R quadratic terms, and further R * R * R, R * R * G, R * R * B, G * G * R, G
* G * G, G * G * B, B * B * R, B * B * G, B *
Precise color reproducibility can be obtained by adding cubic terms such as B * B and R * G * B and their coefficients.

[0005]

However, the accuracy increases with the addition of higher-order terms of second or higher order, but on the other hand, a large number of multipliers are required, resulting in high cost and large equipment. . For example, Japanese Patent Application Laid-Open No. 5-29
According to the technique described in Japanese Patent Publication No. 2302, a plurality of hue spaces are divided based on the magnitude relation of R, G, B (or C, M, Y after complementary color conversion), and a primary matrix coefficient optimized for each hue is used. By performing the color correction calculation, the color reproduction accuracy is improved by adding a relatively small-scale circuit. With this method, the correction of nonlinear distortion due to the difference in hue is improved, but it is not possible to correct nonlinear distortion in the density (lightness) direction in the same hue space. Accuracy was poor.

Accordingly, a first object of the present invention is to provide a color image processing apparatus having good color correction accuracy by adding a circuit of a small scale. The second object of the present invention is to
An object of the present invention is to provide a color image processing apparatus which has good color correction accuracy and can obtain good color reproducibility. A third object of the present invention is to provide a color image processing device capable of preventing deterioration of gradation characteristics without increasing the circuit scale.

[0007]

According to the first aspect of the present invention, there is provided a color image processing for processing an input image signal separated into R, G, and B to generate at least C, M, and Y output image signals. In the apparatus, a hue judging means for judging a hue indicated by the input image signal to be any one of three or more hue regions set in advance, a linear combination operation means for performing a linear combination operation of R, G and B, and the linear combination Coefficient holding means for holding at least a set of the number of hue regions for the coefficient data of the linear combination operation in the operation means; and converting the tone characteristics of the signal after the linear combination operation in the linear combination operation means into at least the number of hue regions. And a printer γ correction unit for correcting the gradation characteristics of the output system to predetermined characteristics after the gradation characteristic conversion by the gradation conversion unit. ,
When generating C, M, and Y output image signals, a set of coefficient data of a linear combination operation held by the coefficient holding unit and a gradation characteristic conversion unit are used in accordance with the determination result of the hue determination unit. The first object is achieved by selecting a gradation conversion characteristic.

According to a second aspect of the present invention, in the color image processing apparatus according to the first aspect, the hue areas determined by the hue determination means are RY, YG, GC, CB, and B.
The second object is achieved by the six regions of -M and MR.

According to a third aspect of the present invention, in the color image processing apparatus according to the first aspect, the hue areas determined by the hue determination means are RY, YG, GC, CB, and B.
The second object is achieved by the seven regions of -M, MR and gray.

According to a fourth aspect of the present invention, in the color image processing apparatus according to the first, second, or third aspect, the gradation characteristic conversion unit and the printer γ correction unit are configured as one unit. The third object is achieved.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to FIGS. FIG. 1 is a diagram showing a basic configuration of an image processing unit used in a color image forming apparatus. In this figure, processing blocks that are not directly related to color reproducibility, such as scaling processing, spatial filtering, and processing / editing processing, are omitted. The scanner (image reading section) separates the original into R, G, and B basic colors and reads the image. The image processing section receives the RGB input from the scanner as an image signal, converts the RGB input into a CMY output signal, and converts the original into a CMY output signal. ) To obtain a copy image. The image processing unit first performs scanner γ conversion and performs complementary color conversion or characteristic conversion.

The conversion circuit is an L-type memory using a ROM (read only memory, read only memory) or a RAM (read / write memory, random access memory).
Generally, conversion means using a UT (look-up table) is used. FIG. 9 shows an example of the conversion characteristics of the scanner γ conversion unit. However, it is assumed that each of the RGB data is quantized to 8 bits in linear reflectance, and 0 represents a dark portion and 255 represents a bright portion. The conversion characteristic (1) in this figure is a characteristic for simply performing a complementary color conversion, and simply reverses the relationship between the numerical value and the brightness while keeping the reflectance linear. The conversion characteristic (2) reverses the relationship between a numerical value and light and dark and performs characteristic conversion to linear lightness. Here, simple complementary color conversion and conversion to lightness linear have been described as examples, but linear density or other non-linear conversion may be performed.

Next, a color correction process is performed to correct the reading characteristics of the scanner (image reading unit) and the deviation of the color material used in the printer (image forming unit) from the ideal, and calculate a CMY output image signal. In the subsequent printer γ correction, the raw γ characteristic of the printer including the subsequent gradation processing (processing for gradation reproduction such as dither processing and error diffusion processing) is corrected to have a predetermined characteristic. Here, the relationship between the image signal before the printer γ correction and the copy image is corrected to have the same brightness and linear characteristics as the input characteristics. FIG. 10 is a diagram showing an example of a conversion characteristic for printer γ correction. This data depends on the output color. It is convenient to use an LUT method with a RAM for the printer γ correction circuit so that it can be rewritten between colors or optimized in accordance with the aging of the γ characteristic of the printer and environmental changes.

FIG. 2 is a block diagram of the color correction processing section according to the present embodiment. The color correction processing unit includes a masking operation unit that performs a masking operation represented by the above equation 1 on the RGB input image signal, a hue determination unit that determines a hue region and generates a hue region signal, and a lightness (density) direction. It is composed of a gradation converter for correcting non-linear distortion.
The processed image signal is sent to the printer through the printer γ correction unit and the gradation processing unit. Here is CM
The configuration is such that the Y color signals are processed in a frame-sequential manner, and the masking coefficient data, gradation conversion data, and printer γ correction data can be switched between those for the processing target color each time the target color changes.

As shown in FIG. 7, the hue judging section evaluates the magnitude relation of the RGB input image signals, judges six hues or seven hues including gray, and outputs a hue number as a hue area signal. For this reason, the hue determination section is composed of a comparator for comparing the magnitude of data and a decoder for converting the comparison result into a hue number. When determining a gray area, R
In order to calculate the maximum value and minimum value of GB and their difference, a selector and a subtractor and a comparator for comparing the difference with a predetermined threshold thr are added to the configuration. In the simplest case, the number of hue divisions is determined based on only the maximum values of R, G, and B.
It may be divided.

Here, R, G, and B are divided into six parts based on the magnitude relation of the three persons. However, it is also possible to improve the color correction accuracy by further dividing the parts into 12 parts or the like. However, in consideration of the circuit scale and the correction accuracy, the cost performance of the hue six division is relatively good. In addition, as shown in FIG. 7B, not only the original hue but also the gray area is determined as one area, and by optimizing the processing, particularly the gray balance can be improved.

FIG. 3 is a diagram showing an example of a masking operation circuit. The register includes registers REG1 to REG3 for holding the coefficients, multipliers MUL1 to MUL1 for calculating the product of the coefficients and the RGB image signals, and adders ADD1 and ADD2 for calculating the sum thereof. Each register holds coefficients by the number of hue divisions, and outputs coefficients corresponding to hue area signals. The coefficient is configured to be rewritten by a CPU (Central Processing Unit) (not shown) according to the output target color. FIG.
Is an example of a masking coefficient, which is data for the case of dividing into 6 or 7 hues.

FIG. 4 is a diagram showing an example of the gradation conversion circuit. Here, an example when the hue is divided into six is shown.
In order to perform gradation conversion corresponding to each hue, RAM or R
The conversion tables LUT1 to LUT6 by the OM and a multiplexer MPX for selecting data corresponding to the hue area signal from the conversion data of the tables. By using a memory having the capacity of the conversion table corresponding to the number of areas, a circuit configuration is simplified by inputting a hue area signal as an address signal of the memory instead of the multiplexer as shown in FIG. be able to.

In the example shown in FIG. 5, since a signal for selecting a color to be processed is input as an address signal, a ROM table is used, and even in the case of a RAM, data for all colors is written at once. By doing so, there is no need to rewrite data between colors, and the device can respond to high-speed processing.

FIG. 11 is a diagram showing an example of gradation conversion characteristics optimized for each hue region for each of C, M, and Y colors.
In this case, three types of conversion tables are selected according to the content of the color component of the output target color of the document color. In this case, since the conversion table may have three types of hues for gradation conversion even in 6 to 7 hue divisions,
The configuration may be such that the number of conversion tables is reduced. Of course, it is also possible to configure so that a table optimized for each of 6 to 7 hues can be selected to further improve the color reproduction accuracy.

FIG. 6 is a diagram showing an example in which the gradation conversion table and the printer γ correction table are combined into one, and are used in competition with the gradation conversion unit. In this case, the printer γ correction circuit becomes unnecessary, and the output data of the gradation conversion unit is input to the gradation processing unit. FIG. 12 is a tone conversion table obtained by combining the printer γ correction table shown in FIG. 10 and the tone conversion table for Y in FIG. By combining them into one by synthesis, not only can the circuit be simplified, but also by performing a smoothing process on the synthesis table,
It is also possible to prevent gradation jumps caused by double table conversion.

[0022]

According to the first aspect of the present invention, when the C, M, and Y output image signals are generated, the linear combination operation held in the coefficient holding unit is performed according to the determination result of the hue determination unit. Is selected, and a conversion characteristic used by the gradation characteristic conversion means is selected, so that a color image with high color correction accuracy can be obtained by adding a small-scale circuit.

According to the second and third aspects of the present invention,
A color image having good color reproducibility can be obtained.

According to the fourth aspect of the present invention, since the tone characteristic converting means and the printer γ correcting means are integrated into one means, it is possible to prevent the tone characteristic of a color image from deteriorating without increasing the circuit scale.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a basic configuration of an image processing unit used in a color image forming apparatus according to an embodiment.

FIG. 2 is a block diagram of a color correction processing unit according to the embodiment.

FIG. 3 is a diagram illustrating an example of a masking operation circuit.

FIG. 4 is a diagram illustrating an example of a gradation conversion circuit.

FIG. 5 is a diagram showing an example of a gradation conversion circuit whose configuration is simplified.

FIG. 6 is a diagram illustrating an example in which a gradation conversion table and a printer γ correction table are combined and configured into one.

FIG. 7 is a table showing hue determination conditions.

FIG. 8 is a table showing examples of masking coefficients.

FIG. 9 is a diagram illustrating an example of conversion characteristics of a scanner γ conversion unit.

FIG. 10 is a diagram illustrating an example of conversion characteristics for printer γ correction.

FIG. 11 is a diagram illustrating an example of a gradation conversion characteristic.

12 is a diagram illustrating a printer γ correction table illustrated in FIG. 10 and FIG.
1 is a gradation conversion table obtained by combining the gradation conversion tables for Y.

[Explanation of symbols]

 REG register MUL multiplier ADD adder LUT Look-up table MPX multiplexer

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Claims (4)

[Claims] 1. A color image processing apparatus for processing an input image signal separated into R, G, and B to generate at least C, M, and Y output image signals. Hue determination means for determining any of the three or more set hue regions; linear combination operation means for performing a linear combination operation of R, G, and B; Coefficient holding means for holding a set of the number of hue regions; and a tone having a tone conversion characteristic of at least the set of the number of the hue regions by converting a tone characteristic of the signal after the linear combination operation in the linear combination operation means. Conversion means, and printer γ correction means for correcting the gradation characteristics of the output system to predetermined characteristics after the gradation characteristic conversion by the gradation conversion means, and generating C, M, Y output image signals. When performing the hue determination The determination result in accordance with the color image processing apparatus characterized by selecting a gradation conversion characteristic used in pairs and gradation characteristic conversion means of the coefficient data of the linear combination operation held in the coefficient holding means. 2. The hue area determined by the hue determination means includes RY, YG, GC, CB, BM, and MR.
2. The color image processing apparatus according to claim 1, wherein the six areas are: 3. The hue area determined by the hue determination means includes RY, YG, GC, CB, BM, and MR.
And seven gray areas.
The color image processing apparatus as described in the above. 4. A color image processing apparatus according to claim 1, wherein said tone characteristic conversion means and printer γ correction means are constituted as one means.