JP2003051959A - Image processing apparatus for electrophotography and electrophotographic apparatus utilizing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image processing apparatus that generates CMY image reproduction data for reproducing an image with a reproduction target density from RGB image data, corresponding to the gamma characteristics of a cathode ray tube, and to provide an electrophotographic apparatus utilizing the same. SOLUTION: The image processing apparatus applies color conversion to input gradation data of the RGB color space to obtain gradation data of the CMY color space, and further applies halftone processing, to convert the gradation data of the CMY color pace to image reproduction data. The gamma characteristics A of an output density versus the gradation of the RGB color space are made identical to the gamma characteristics B of an output density versus the gradation of the CMY color space in the halftone processing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus for an electrophotographic apparatus such as a printer or a copier, and an electrophotographic apparatus using the same. For example, RGB image data optimized for the characteristics of a display apparatus is CMYK. The present invention relates to an image processing apparatus for converting the image data into the image data and further generating image reproduction data by halftone processing, and an electrophotographic apparatus using the image processing apparatus. Or optimized for different device characteristics,
Alternatively, the image data of the first color space optimized to the device-independent predetermined characteristic is converted into the image data of the toner color space of the electrophotographic apparatus, and the halftone of the image is expressed by the halftone process. The present invention relates to an image processing apparatus that generates image reproduction data and an electrophotographic apparatus that uses the image processing apparatus.

[0002]

2. Description of the Related Art An image generated using computer graphics is designed on a computer screen and RGB image data is generated. The RGB image data is gradation data for each color pixel, and the image is printed by an electrophotographic device such as a printer.
The electrophotographic apparatus color-converts RGB image data into CMYK image data compatible with a print engine. The CMYK image data obtained by color conversion is converted into image reproduction data for each pixel by halftone processing, and supplied to a print engine that uses a laser beam.

In color electrophotographic devices, K is used for CMY.
CMYK toner with (black) added is used, but K
(Black) is a substitute as a result of mixing CMY.
Therefore, even in the CMY color space, in a specific electrophotographic apparatus, it is general to generate image data in the CMYK color space to which K is added and perform printing. Therefore, in this specification, a more general CMYK color space will be described.
Therefore, the term “CMYK color space” is a concept including the CMY color space that is substantially the same.

In a page printer using a laser beam, the above-mentioned image reproduction data is drive pulse width data for specifying a beam irradiation area within a pixel, and a drive pulse for driving a laser is generated according to this image reproduction data. It

The color conversion process described above converts RGB three-dimensional data into CMYK four-dimensional data (or CMY three-dimensional data). Usually, this color conversion is performed using a color conversion table that discretely shows the relationship between the RGB gradation data and the CMYK gradation data. If each RGB gradation data has 256 gradations, all RGB
To have a combination of CMYK gradation data corresponding to 256 ³ = 16.7 million colors, which is a combination of gradation data, in the color conversion table is an amount of data (64 Mbytes).
Is too large to be realistic. Therefore, normally,
256 ³ = 16.7 million colors of the converted value for the grid points of several hundreds to several tens of thousands of colors imparted to the color conversion table, intermediate their lattice points, seeking a conversion value of CMY by interpolation. Linear interpolation is generally used for this interpolation processing.

On the other hand, a multi-value dither method, which is a binarization method for reproducing gradation of a grayscale image, is widely used for the halftone processing. According to the multi-value dither method, for the gradation data of each color of CMYK which is an input signal, a halftone table (gamma table) having correspondence between gradation data and image reproduction data is referred to, The dot area, which is the toner adhesion area, is determined. This dot area is an area to which the toner is attached by being irradiated with the laser beam, and is specified by the image reproduction data (driving pulse width data). Then, in a cell composed of a plurality of adjacent pixels, a halftone dot in the cell is formed by the plurality of dots, and the halftone of the grayscale image is reproduced by the size of the halftone dot.

A halftone table (gamma table), which is a conversion table for conventional halftone processing, is a CM.
The YK color gradation data is converted into image reproduction data (drive pulse width data) corresponding to the optical density obtained as a result of printing. This conversion characteristic is
Usually, it has a linear characteristic that the drive pulse width simply increases with respect to CMYK gradation data.

[0008]

[Problems to be Solved by the Invention]
The output density of the monitor screen of the computer, for example, the CRT screen is
Has a special gamma characteristic for RGB gradation data
To do. For example, FIG. 9 shows RGB gradation data on a CRT screen.
Is a characteristic diagram showing the relationship between the output x and the output brightness I of the display screen.
It According to this example, the gradation value corresponding to the dark image is low.
In the region, the change rate of the brightness I with respect to the change of the gradation x is small.
On the other hand, in areas with high gradation values that correspond to bright images
Indicates that the change rate of the luminance I with respect to the change of the gradation x is large. Immediately
I = kx ⁿ(N is usually 1.8 to 2.2).

On the other hand, the output of an electrophotographic apparatus such as a printer is a printed matter on paper, and the brightness I when the printed matter is observed is proportional to the reflectance R of the printed matter (I = k'R), and , R = kx ⁿ . The output density (optical density) D of the printed matter is D = − with respect to the reflectance R.
It is defined by the relationship of log ₁₀ R.

FIG. 10 is a characteristic diagram showing a relationship between the gradation value x of RGB on the monitor screen and the output density D of the printed matter according to the above relational expression. According to this characteristic diagram, the change rate of the output density D with respect to the change of the gradation x is large in the region where the input gradation value x is low corresponding to the dark image, while the input gradation value corresponding to the bright image is changed. In areas where x is high, gradation x
The change rate of the output density D with respect to the change of is small.

Therefore, RGB having the characteristic curve of FIG.
Since the gradation data and the linear conversion characteristic in the halftone processing unit are used, the following points have been conventionally problematic. That is, in color conversion, RGB gradation data between grid points is converted into CMYK gradation data by linear interpolation, and as a result, the converted CMYK gradation data is subjected to image reproduction data (drive) by halftone processing. When converted into pulse width data), R shown in FIG.
Output density D for GB gradation data (reproduction target density)
The image reproduction data corresponding to the output density different from that is generated. This means that the output density of the image printed by the electrophotographic apparatus is different from the density corresponding to the brightness of the image designed on the CRT monitor screen of the computer. This is not preferable because the image on the CRT monitor screen is different from the printed image because the image quality is deteriorated.

The above-mentioned problem is caused by using the image data of RGB designed by using a liquid crystal display as an input value and CM.
It also exists in electrophotographic devices that use YK ink. Similarly, sRGB image data specified on Windows, CMYK image data generated by a printing machine, or a device-independent color space (Device Independent Col) using a color management module.
CIELab or CIEXYZ image data or the like, which is an or space), is used as an input value, and an electrophotographic apparatus that converts it into image data of a color space of an ink different from them also has the same problem.

Therefore, an object of the present invention is to reproduce image data of a second color space, which is optimum for an electrophotographic apparatus, without degrading the image quality of image data of a first color space having a predetermined characteristic. (EN) An image processing device capable of generating an image, and an electrophotographic device using the image processing device.

[0014]

In order to achieve the above object, one aspect of the present invention is to convert input gradation data of a first color space into gradation data of a second color space. In the image processing apparatus for converting the gradation data of the second color space into the image reproduction data by the halftone process, the gamma characteristic A of the output density with respect to the gradation of the first color space and the second gamma characteristic of the halftone process. It is characterized in that the gamma characteristic B of the output density with respect to the gradation of the color space is made equal.

According to the above invention, in the color conversion, even if the gradation data of the first color space between the grid points in the color conversion table is obtained by the interpolation process to the gradation data of the second color space. In the halftone processing, since the halftone processing is performed with the same gamma characteristic B, the first
It is possible to generate image reproduction data that reproduces an output density equivalent to the output density assigned to the gradation data of the color space. Therefore, if printing is performed using this image reproduction data, the colors of the designed image can be faithfully reproduced, and high-quality images can be printed.

That is, in the above invention, the gamma characteristic A for color conversion and the gamma characteristic B for halftone processing have a correlation with each other. Alternatively, both gamma characteristics A and B are in a relationship of being related to each other. That is, if the gamma characteristic A has a downward convex or upward convex characteristic, the gamma characteristic B also has the same characteristic depending on it. Thus, the gamma characteristic B has a characteristic that depends on the gamma characteristic A.

In a more preferred embodiment of the above invention, the gamma characteristic A and the gamma characteristic B have equivalent non-linear characteristics. In another preferred embodiment,
In the gamma characteristic A, the magnitude relationship between the change rate of the output density with respect to the input gradation in the bright image area and the change rate of the output density with respect to the input gradation in the dark image area is similar to that of the gamma characteristic B. Will be the same.

In the above embodiment, the gamma characteristic A indicating the relationship between the gradation of the image data in the first color space and the output density, and the gradation and the output of the image data in the second color space in the halftone process. Since the gamma characteristic B indicating the relationship with the image reproduction data corresponding to the density is aligned with an equivalent non-linear characteristic, if the image reproduction data is used for printing, the designed image can be reproduced faithfully. It is possible to print high quality images.

In order to achieve the above object, in another aspect of the present invention, in an image processing apparatus for generating image reproduction data representing an intermediate gradation of an image from input gradation data of a first color space. A color conversion unit for converting the input gradation data of the first color space into gradation data of the second color space by an interpolation process referring to a color conversion table; and gradation data of the second color space. And a halftone processing unit for converting gradation data of the second color space into image reproduction data by referring to a halftone table having correspondence between the first color space and the image reproduction data. Of the input gradation data of
The gamma characteristic A of the output density with respect to the gradation value for each image brightness and the gamma characteristic B of the output density with respect to the gradation value for each image brightness of the halftone table are made equal. .

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. However, such an embodiment does not limit the technical scope of the present invention.
Note that the present invention can be applied to cases where the first color space and the second color space are various color spaces, but in the following description, as an example, the RGB color space based on the characteristics of the CRT monitor is used. On the other hand, an image processing apparatus that generates image reproduction data for the CMYK color space of the page printer will be described.

FIG. 1 is a block diagram of an electrophotographic printing system according to this embodiment. In this example, the host computer 50 generates image data S54 consisting of RGB gradation data (8 bits for each, a total of 24 bits) in the host computer 50, and supplies it to the electrophotographic apparatus 60 such as a page printer. .

Electrophotographic device 60 such as a page printer
Converts the supplied image data S54 into tone data of CMYK which is the second color space, and reproduces a color image using the toner of the color space. The electrophotographic apparatus 60 includes a controller 62 that performs image processing and supplies a laser drive pulse signal S70 to the engine, and a print engine 72 that reproduces an image in accordance with the drive pulse signal S70.

In the host computer 50, character data, graphic data, bitmap data, etc. are generated by an application program 52 such as a word processor or a graphic tool. The respective data generated by these application programs 52 are
The driver 54 for the electrophotographic apparatus installed in the host computer 50 rasterizes and converts the rasterized image data into image data S54 including gradation data of each color of RGB.

The image design by the computer graphic is performed so that the optimum image quality or color is displayed on the monitor screen of a computer such as a CRT. Therefore,
The RGB image data S54 generated by computer graphics follows the gamma characteristic of the output density with respect to the gradation of the CRT monitor. This gamma characteristic is, for example, as shown in FIG. 10, and a region corresponding to a bright image,
That is, in the area where the input gradation value x is high, the change rate of the output density with respect to the change of the input gradation value is small, and in the area where the input gradation value x corresponding to a dark image is low, the output density with respect to the change of the input gradation value is small. Has a large rate of change. Of course, another monitor device may have a different gamma characteristic.

A microprocessor (not shown) is also built in the electrophotographic apparatus 60, and by the microprocessor and the installed control program,
A controller 62 including a color conversion unit 64, a halftone processing unit 66, a pulse width modulation unit 70, etc. is configured. In the print engine 72, for example, the drive pulse signal S7
0 drives the laser diode 74 for image drawing. The photosensitive drum 76 is irradiated with the laser beam to form a latent image, and CMYK toners are sequentially attached to the latent image, and the latent image is overprinted by a transfer drum (not shown).

The color conversion unit 64 in the controller 62 converts the supplied RGB gradation data S54 for each pixel into RG.
It is converted into CMYK gradation data S64 having a complementary color relationship with B. This color conversion process is performed by referring to the color conversion table 65 that indicates the correspondence between the combination of RGB gradation data and the combination of CMYK gradation data.

However, each of the RGB gradation data is 8
In the case of bit and 256 gradations, the combination of RGB gradation data is 256 ³ = 16.7 million (1670)
Color). Therefore, it is not realistic to include all combinations of CMYK gradation data corresponding to these 16.7 million kinds of RGB gradation data in the color conversion table because the data amount (64 Mbytes) becomes too large. . Therefore, the color conversion table 65 has CMYK conversion values for grid points of hundreds to tens of thousands of the 16.7 million colors. Then, when the input RGB gradation data is a value between the grid points, the corresponding C is calculated by an interpolation calculation using adjacent grid points, for example, a linear interpolation calculation.
MYK gradation data S64 is obtained.

The CMYK gradation data S64 is, for example, gradation data of 8 bits for each color, and has a maximum of 256 gradations. The color conversion section 64 uses the RGB gradation data S for each pixel.
54 is converted into gradation data S64 for each pixel in the CMYK color planes and supplied to the halftone processing unit 66.

The halftone processing unit 66 uses C for each dot.
With respect to the MYK gradation data S64, a halftone table (gamma table) 68 having correspondence between gradation data and image reproduction data created in advance is referred to, and image reproduction data S66 for each pixel is generated. The image reproduction data S66 is drive pulse data indicating a beam irradiation region to which the toners of CMYK colors are attached in the pixel.
The drive pulse data S66 that is the image reproduction data is
The driving pulse signal S70 is supplied to the pulse width modulator 70.
Is generated.

In the present embodiment, the halftone processing section 66 uses the multi-valued dither method to reproduce high gradations with high resolution at a low dot density of about 600 dpi in a color printer or the like. can do.

FIG. 2 is a diagram showing an example of a halftone table (gamma table) of the multilevel dither system in the preferred embodiment. In the multi-value dither method, the image reproduction data is output with reference to the halftone table for the input data S64 including the gradation data for each pixel.
For example, the pixel P00 refers to the pattern number “7” of the corresponding pixel in the pattern matrix 21, and the index table 2 showing the correspondence between the gradation and the pulse width of the laser drive.
The table "7" corresponding to the pattern number "7" from 2 is converted to pulse width data corresponding to the gradation data of the pixel P00. In this way, the pattern matrix 21 and the index table 22 constitute a halftone table (gamma table) 68 having correspondence between gradation data and image reproduction data.

When irradiating each pixel with a laser beam, not only binary image reproduction data of not irradiating the laser beam in the pixel but also a multi-valued image of which range in the pixel is irradiated with the laser beam By using the playback data,
It is possible to reproduce a grayscale image with more gradation. The halftone processing unit 66 generates driving pulse width data of the laser beam as the image reproduction data S66 for each pixel by the multi-valued dither method using such a function.

An electrophotographic apparatus such as a page printer uses a laser beam in a direction perpendicular to the paper feeding direction (main scanning direction).
It is possible to control the irradiation of the beam to a partial region in the scanning direction within the pixel by not irradiating the beam while scanning. Therefore, for example, by using 256 kinds of laser drive pulses, 256 kinds of latent images can be formed in the scanning direction within the pixel.

In the pattern matrix 21, eight kinds of patterns are arranged as shown in the figure. Further, in the index table 22, 256 kinds of pulse widths are associated with 256 gradations. Moreover, the correspondence between gradation and pulse width is different in each of the eight types of patterns. In the example of FIG. 2, for patterns 1 and 2 of the index table 22, pulse widths 0 to 255 are associated with gradations 0 to 63, and gradations 64 to 255 are associated.
The pulse widths of 255 (irradiating the beam in all the dots) are all associated with. Therefore, the patterns 1 and 2 are dots that grow even at a relatively low gradation value. Further, in the example of FIG. 2, the index table 22
For patterns 3 and 4, the pulse width 0 is associated with the gradations 0 to 63 and the gradations 64 to 127.
Is associated with 0 to 255 of the pulse width, and 128 to 255 of higher gradation is associated with 2 of the pulse width.
55 is associated. Therefore, this pattern 3, 4
Corresponds to the dots in the halftone dots that grow for the next higher gradation value of patterns 1 and 2.

Similarly, for patterns 5 and 6 of the index table 22, pulse width 0 is associated with gradations 0 to 127, and pulse width 0 is associated with gradations 128 to 191. .. 255 are associated with each other, and a pulse width of 255 is associated with a higher gradation of 192-255.
Are associated with. Further, for the patterns 7 and 8 of the index table 22, the pulse width 0 is associated with the gradations 0 to 191 and the gradations 192 to 25.
5, the pulse widths 0 to 255 are associated.
Therefore, the patterns 5 and 6 correspond to the dots that grow slower with increasing gradation, and the patterns 7 and 8 correspond to the dots that grow the slowest.

The cell area where halftone dots are formed is composed of a plurality of pixels, and toner is attached to the area specified by the image reproduction data in each pixel area to form dots. A halftone dot is formed by a set of dots formed in this pixel.

FIGS. 3 and 4 are diagrams showing conversion tables of the color conversion unit and the halftone processing unit in the conventional example and the present embodiment. The merits of the present embodiment can be understood by comparing the two figures. Both figures are given the same reference number.

3 and 4, an example of the gamma characteristic curve G54 showing the relationship between the RGB gradation data S54 and the output density D on the monitor screen is shown in the first quadrant. In the first quadrant, the horizontal axis x is RGB gradation data S54, and the vertical axis y is the output density D on the monitor screen. The output density D on the monitor screen is obtained by converting the output brightness on the monitor screen into an equivalent density by the method described above. This gamma characteristic curve G54 has a small change rate of the output density D with respect to the change of the input gradation in a relatively bright area in the image, and a change rate of the output density D with respect to the change of the input gradation in a relatively dark area. Has the characteristic of being large. D1, D in the figure
Reference numerals 2 and D3 denote desirable output densities D for the RGB gradation values x1, x2, and x3, that is, reproduction target densities.

In the second quadrant, the characteristic curve of the halftone table (gamma table) 68 of the halftone processing section is shown. According to the halftone table 68, the input CMYK gradation data S64 shown on the horizontal axis y is converted into the image reproduction data S66 corresponding to the output density D shown on the vertical axis y. However, strictly speaking, this characteristic curve 68 is
It also includes the characteristics of the print engine and is not related to the value of the drive pulse width of the laser for producing the output density D.

The third quadrant is a simple folded portion, and the minus direction of the horizontal axis x corresponds to the minus direction of the vertical axis y. Therefore, the ordinate y (minus direction) also corresponds to the CMYK gradation data after color conversion.

In the fourth quadrant, a color conversion table curve 65 of the color conversion section is shown. The horizontal axis x represents the input RGB gradation data S54, and the vertical axis y represents the color-converted CMYK gradation data S64.

The color conversion table 65 is generated as follows. For example, for input value x1 of RGB, CR
The output density D1 is determined by the device characteristics of the T monitor. Therefore, the CM capable of generating the output density D1 by the halftone table characteristic shown in the second quadrant.
The YK value y1 is calculated. Then turn back to the third quadrant,
The intersection of the CMYK value y1 and the input RGB value x1 is the value of the conversion table 65. Similarly, for the input RGB value x2, the intersection with the CMYK value y2 is
It becomes the value of the conversion table 65. In this way, the conversion table 65 is configured by the value y to be converted obtained by the above method with respect to the discretely determined input value x. As described above, the conversion table 65 is used for input RGB
Instead of including the transformed value y for every value x,
It has conversion values for discrete input points such as x1 and x2.

The color conversion table 65 shown in the fourth quadrant of FIG. 3 shows the relationship between RGB and CMYK. In other words, the larger the RGB gradation value, the brighter it becomes, and CMYK
The larger the gradation value, the darker it becomes. Therefore, the RGB high gradation data corresponding to the bright image corresponds to the low CMYK gradation data, while the low RGB gradation data corresponding to the dark image corresponds to the high CMYK gradation data.

The color conversion table is, for example, x1, x2 and y.
It has only the correspondence of discrete grid points such as 1, y2. Therefore, the CMYK gradation data y3 corresponding to the input value x3 in the meantime is obtained by the linear interpolation calculation.

The gamma characteristic of the halftone table 68 of the conventional example is a linear characteristic of simple increase, as shown in FIG. First, RGB which is the grid point of the color conversion table
Image processing for the gradation data x1 and x2 will be described. R
The GB gradation data x1 and x2 are converted into CMYK gradation values y1 and y2 by the color conversion table 65 in the fourth quadrant.
The CMYK gradation values y1 and y2 are folded in the third quadrant,
The halftone table 68 in the second quadrant converts the image reproduction data S66 corresponding to the output densities D1 and D2. On the other hand, the reproduction target densities of x1 and x2 from the first quadrant are D
1, D2, these output densities are
It matches the reproduction target concentration. The reason is that, as described above, the correspondence of the grid points is created so as to have such a relationship when creating the color conversion table.

Next, RG which is the grid point of the color conversion table.
The image processing for the gradation data x3 between the B gradation data x1 and x2 will be described. From the first quadrant, it is understood that the reproduction target density of the RGB gradation data x3 is D3. Then, in the color conversion table 65 of the fourth quadrant, x3
Does not match the grid points x1 and x2, the grid point (x
CMYK gradation value y3 is obtained from the intersection point of x = x3 and the straight line connecting (1, y1) and (x2, y2). That is, it is a linear interpolation calculation. This y3 is folded back in the third quadrant,
The halftone table 68 in the second quadrant converts the image reproduction data S66 corresponding to the output density D4. The output density D4 is different from the reproduction target density D3.

In the example of FIG. 3, the output density D4 is higher than the reproduction target density D3. Therefore, a bright image on the CRT monitor screen is printed as a darker image.

On the other hand, the gamma characteristic of the halftone table 68 in the present embodiment shown in FIG. 4 is the gamma characteristic G5 with respect to the output density of the RGB gradation values.
It has the same characteristics as 4. That is, in the bright area in the image where the CMYK gradation value is low, the change rate of the output density D with respect to the change of the input gradation value is small, while the CMYK gradation value is small.
In a high K gradation value, that is, in a dark region, the change rate of the output density D with respect to the change of the gradation value is large. This is equivalent to the characteristic of the output density with respect to the RGB gradation value shown in the first quadrant.

As a result, the halftone table 6 shown in FIG.
The halftone processing unit 66 of the present embodiment, which performs conversion processing using 8 can generate the image reproduction data S66 corresponding to the ideal output density D3 for the CMYK gradation value y3. That is, the image processing apparatus according to the present embodiment has the RGB gradation data x corresponding to the reproduction target density D3.
3 is subjected to color conversion processing by linear interpolation using the grid points x1 and x2 in accordance with the conversion table 65 of the second quadrant, and CMYK gradation data y3 is generated. Furthermore,
By the halftone process referring to the halftone table 68 in the fourth quadrant, the CMYK gradation data y3 is converted into the image reproduction data S66 corresponding to the reproduction target density D3.

This halftone table 68 is CMY
When the K value has 256 gradations, the output image reproduction data values correspond to all the gradations. Therefore, the halftone processing unit does not perform the linear interpolation calculation.

As shown in FIG. 4, the gamma characteristic G54 of the output density D with respect to the RGB gradation value which is the first color space.
However, since it has a downward convex characteristic, the characteristic of the conversion table (gamma table) 68 of the halftone processing unit also has a downward convex characteristic. On the contrary, R which is the first color space
The gamma characteristic G54 of the output density D with respect to the GB gradation value is
In the case where the conversion table (gamma table) 68 of the halftone processing unit has the upward convex characteristic, it must also have the upward convex characteristic. Furthermore, the first color space R
The gamma characteristic G54 of the output density D with respect to the GB gradation value is
When it has an S-shaped characteristic, the conversion table (gamma table) 68 of the halftone processing unit must also have the S-shaped characteristic.

As described above, in the image processing apparatus of this embodiment, the gamma characteristic of the conversion table (gamma table) 68 of the halftone processing unit is equivalent to the gamma characteristic with respect to the output density of the RGB gradation data. By doing so, the image reproduction data S66 that matches the conventional reproduction target density can be generated. Therefore, such image reproduction data S6
When the image is printed by using 6, the image designed on the CRT monitor screen can be faithfully reproduced and the image quality can be improved.

FIG. 5 is another block diagram of the electrophotographic printing system. This system configuration example is a modification of the system configuration example shown in FIG. In the system of FIG. 5, the driver 5 installed in the host computer 50
4 has a rasterize function 55, a color conversion function 64, and a halftone processing function 66. Each of these functions 5
5, 64 and 66 are the printer driver 5 shown in FIG.
4. The functions of the color conversion unit 64 and the halftone processing unit 66 are the same. Then, image reproduction data (pulse width data) S6 for each color generated by the halftone processing function
6 is supplied to the pulse width modulation section 70 of the controller 62 in the electrophotographic apparatus 60 such as a page printer, converted into a desired drive pulse signal S70, and given to the print engine 72.

In the system example of FIG. 5, the driver 54 installed on the host computer side performs color conversion processing and halftone processing. In the example of FIG.
Although it was performed by the controller in the electrophotographic apparatus, it is performed by the host computer 50 side in the example of FIG. When the price reduction of the electrophotographic apparatus 60 is required, the controller 62
It is required to lower the ability of the company and keep the price down. In that case, it is effective to implement some of the functions performed by the controller of FIG. 1 by the driver program installed in the host computer instead. When halftone processing is realized by the driver 54, a storage medium in which a program for causing the computer to execute the above halftone processing procedure is stored is built in the host computer 50.

[Other Combination of First Color Space and Second Color Space] As the first color space, for example, the RGB color space optimized for the characteristics of the liquid crystal monitor, the characteristics of the scanner or the digital camera are used. The optimized RGB color space, the sRGB color space recommended on Windows, and the device independent CIELab and CIEXYZ color spaces can be applied. In any case, a constant gamma characteristic is provided between the gradation value and the output density. CIE means the International Institute of Illumination, and represents the color that the human eye can perceive in 1931 as an absolute value.

As the second color space, there are CMYK color space which is toner of a laser printer which is a page printer, CMYK color space of ink which is an ink jet printer, and CCMMYK (where C and M include lights). Color space of 6 colors, CCMMYYK (however, C, M, and Y include lights) Color space of 7 colors, CCMMYKK (however, C,
7 color space (M and K include lights), and CM for printing 7 colors by adding RGB ink to normal CMYK
It can be applied to the YKRGB color space.

RGB, CIEXYZ, CIELab L
* And the like are additive color mixture color spaces that are dark at low gradation values and bright at high gradation values. On the other hand, CMYK
It is a subtractive color space in which a low gradation value makes it bright and a high gradation makes it dark.

FIG. 6 shows RGB created on the liquid crystal monitor.
FIG. 6 is a diagram showing a conversion table of a color conversion unit and a halftone processing unit in the case of an image processing apparatus for converting image data in color space into image reproduction data for an electrophotographic apparatus that uses CMYK toners. It is a figure corresponding to FIG.

LCD monitor device characteristics (gamma characteristics)
Is an S-shaped curve as shown in the first quadrant of FIG. That is, the change in the output density is small in the low gradation area of RGB, the change of the output density is large in the intermediate gradation area of RGB, and the change of the output density is small in the high gradation area of RGB again. Corresponding to this, in the present embodiment, the gamma characteristic of the halftone table 68 also has an equivalent S-shaped curve. That is, the change in output density is small in the high CMYK gradation region, the change in output density is large in the RGB intermediate gradation region, and again the change in output density is small in the low RGB gradation region.

However, since RGB is an additive color mixture and CMYK is a subtractive color mixture, the input gradation values have the opposite relationship, but in terms of the relationship between the input gradation value and the output density for each image brightness, both Have the same gamma characteristics.

In the case of FIG. 6 as well, the input x between the sample points (x1, y1) and (x2, y2) of the color conversion table 65 is input.
3, the output density D of the image reproduction data obtained by the color conversion process is the same as the output density D3 on the liquid crystal monitor. Therefore, the color on the liquid crystal monitor is printed as it is.

FIG. 7 is a diagram showing a color conversion table and a halftone table for CMYK image data generated for printing. The CMYK image data generated for printing is data indicating the size (%) of halftone dots.
The gamma characteristic is C as shown in the first quadrant of FIG.
The change in output density is large in the gradation region where MYK is low, and the change in output density is small in the gradation region where MYK is high. That is, it has an upward convex characteristic. Therefore, in the present embodiment, the gamma characteristic of the halftone table 68 also has an upward convex characteristic. That is, it is a characteristic that the change in the output density is large in the gradation region where the CMYK is low and the change in the output density is small in the gradation region where the CMYK is high.

Also in this case, for the input CMYK value x3 between the sample points of the color conversion table 65, the output density D3
Image reproduction data can be generated, and the color of the image data generated for printing can be faithfully reproduced and printed by the electrophotographic apparatus.

FIG. 8 is a diagram showing a color conversion table and a halftone table for L * of CIELab, which is a device-independent color space. CIELab L *
Is data indicating brightness, a * is data in the red-green direction, and b * is data in the blue-yellow direction. Brightness L *
And the CMYK image data of the laser printer can be shown as shown in FIG.

The brightness L * gamma characteristic has a low input tone value, a large change in output density in a dark region of an image,
It has a characteristic that the input gradation value is high, the change of the output density is small in a bright area, and the projection is downward. Accordingly, the gamma characteristic of the halftone table 68 has a high input gradation value,
The dark area has a large change in the output density, and the light area having a low input gradation value has a small change in the output density. This relationship is similar to the relationship shown in FIG.

As shown in FIG. 8, the color conversion table 65
The image reproduction data of the output density D3 can be generated with respect to the input CMYK value x3 between the sample points of, and the image which reproduces the brightness faithful to CIELab's L * which does not depend on the device can be obtained. Can be printed with.

As described above, the scope of protection of the present invention is not limited to the above-mentioned embodiments, but extends to the inventions described in the claims and their equivalents.

[0068]

As described above, according to the present invention, it is possible to print using a computer while maintaining the colors of the designed image on the monitor screen as much as possible.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an electrophotographic printing system according to an exemplary embodiment.

FIG. 2 is a diagram showing an example of a halftone table (gamma table) of a multilevel dither method in the embodiment.

FIG. 3 is a diagram showing a conversion table of a conventional color conversion unit and halftone processing unit.

FIG. 4 is a diagram showing a conversion table of a color conversion unit and a halftone processing unit for image data of a CRT monitor in the present embodiment example.

FIG. 5 is another configuration diagram of the electrophotographic printing system.

FIG. 6 is a diagram showing a conversion table of a color conversion unit and a halftone processing unit for image data of a liquid crystal monitor in the present embodiment example.

FIG. 7 is a diagram showing a conversion table of a color conversion unit and a halftone processing unit for printing CMYK image data in the present embodiment.

FIG. 8 shows L * of CIELab in the present embodiment.
It is a figure which shows the conversion table of the color conversion part and the halftone process part with respect to this image data.

FIG. 9 is a characteristic diagram showing a relationship between RGB gradation data x of a CRT screen and output brightness I of a display screen.

FIG. 10 is a characteristic diagram showing the relationship between RGB gradation values x and output density D of printed matter.

[Description of Reference Signs] 50 host computer 60 electrophotographic device (printer) 62 controller (image processing device) 64 color conversion unit 65 color conversion table 66 halftone processing unit 68 halftone table (gamma table),
Gamma characteristic B S54 RGB gradation data (image data of first color space) S64 YMCK gradation data (image data of second color space) S66 Image reproduction data (driving pulse data) G54 Gamma characteristic A

Front page continuation (51) Int.Cl. ⁷ Identification code FI theme code (reference) H04N 1/405 H04N 1/40 101E 1/407 1/46 Z 1/46 1/40 BF term (reference) 2C262 AA05 AA24 AA26 AA27 AB07 AB11 BA01 BA10 BB03 BC01 BC05 BC10 5B057 AA11 BA02 CA01 CA08 CA12 CA16 CB01 CB08 CB12 CB16 CC01 CE11 CE17 CE18 CH07 CH08 5C077 LL04 MP08 NN17 PP15 PP32 PP33 PP36 PP37 PQ08 PQ12 PQ23 SS06 TT02 5C079 HB01 HB03 HB05 HB08 HB11 HB12 LA12 LB02 MA01 MA04 MA17 NA03 PA03 PA05

Claims (17)

[Claims] 1. An image processing apparatus for generating image reproduction data representing halftone gradation of an image from input gradation data of a first color space, wherein the input gradation data of the first color space is: A color conversion unit that converts the gradation data of the second color space by an interpolation process that refers to a color conversion table, and a halftone table that has correspondence between the gradation data of the second color space and the image reproduction data. With reference to the second
A halftone processing unit for converting the gradation data of the color space of the first color space into the image reproduction data, and the gamma characteristic A of the output density with respect to the gradation value of the input gradation data of the first color space; An image processing apparatus characterized in that the gamma characteristic B of the output density is made equal to the tone value of the tone table. 2. The first color space is an additive color mixture type color space, the second color space is an additive color mixture type color space, and the first gamma characteristic A of the first aspect is the first color space. The change rate of the output density with respect to the change of the input gradation value in the input gradation region of
In the gamma characteristic B, the magnitude relationship with the change rate of the output density with respect to the change of the input gradation value in the second input gradation area including the input gradation value lower than the input gradation area of Change rate of output density with respect to change of input gradation value in input gradation area and input gradation value in fourth input gradation area including input gradation value higher than the third input gradation area The image processing apparatus has the same magnitude relationship with the change rate of the output density with respect to the change of. 3. The gamma characteristic A according to claim 1, wherein the first color space is a subtractive color space, the second color space is a subtractive color space. The change rate of the output density with respect to the change of the input gradation value in the input gradation region of
In the gamma characteristic B, the magnitude relationship with the change rate of the output density with respect to the change of the input gradation value in the second input gradation area including the input gradation value lower than the input gradation area of Change rate of output density with respect to change of input grayscale value in input grayscale area, and input grayscale value in fourth input grayscale area including input grayscale value lower than the third input grayscale area The image processing apparatus has the same magnitude relationship with the change rate of the output density with respect to the change of. 4. The color space of the additive color mixture system is any one of RGB color space, sRGB color space, CIEXYZ color space and CIELab color space, and the color space of the subtractive color mixture system is CMYK. An image processing device characterized by being a color space. 5. The image processing apparatus according to claim 3, wherein the color space of the subtractive color mixture system is a CMYK color space. 6. The first color space according to claim 1, wherein the first color space is RGB, sRGB, or CIELa.
In the color space of b, the second color space is a CMYK color space, and in the gamma characteristic A, the rate of change of the output density with respect to the change of the input gradation value in the first input gradation region is First
Smaller than the rate of change of the output density with respect to the change of the input grayscale value in the second input grayscale area including the input grayscale value lower than the input grayscale area of the third input grayscale. The rate of change of the output density with respect to the change of the input gradation value in the area is with respect to the change of the input gradation value in the fourth input gradation area including the input gradation value lower than that of the third input gradation area. An image processing apparatus characterized by being larger than a change rate of output density. 7. The gamma characteristic A according to claim 1, wherein the first color space is a first CMYK color space, the second color space is a second CMYK color space. The change rate of the output density with respect to the change of the input gradation value in the first input gradation region is
Smaller than the rate of change of the output density with respect to the change of the input grayscale value in the second input grayscale area including the input grayscale value lower than the input grayscale area of the third input grayscale. The rate of change of the output density with respect to the change of the input gradation value in the area is with respect to the change of the input gradation value in the fourth input gradation area including the input gradation value lower than that of the third input gradation area. An image processing apparatus characterized by being smaller than a rate of change in output density. 8. The image processing apparatus according to claim 1, wherein the gamma characteristic A and the gamma characteristic B have equivalent non-linear characteristics. 9. The image processing apparatus according to claim 1, wherein the gamma characteristic A and the gamma characteristic B have equivalent S-shaped characteristics. 10. The first color space is a CIELab color space, the second color space is a CMYK color space, the L * gamma characteristic, and the halftone table of claim 1. An image processing device having the same gamma characteristic. 11. The image processing apparatus according to claim 1, wherein the first color space is a CIELab or CIEXYZ color space, and the second color space is a CMYK color space. 12. The color conversion table according to claim 1, wherein the color conversion table discretizes a relationship between the input gradation data of the first color space and the gradation data of the second color space. And the halftone table continuously has a relationship between the gradation data of the second color space and the image reproduction data. 13. An image processing apparatus for generating image reproduction data representing gradation by halftone dots formed by a plurality of dots from input gradation data of RGB color space, wherein the input gradation data of RGB color space. To a CMYK color space gradation data by an interpolation process referring to a color conversion table, and a halftone table having correspondence between the CMYK color space gradation data and the image reproduction data. And a halftone processing unit for converting the gradation data of the CMYK color space into image reproduction data, and in the gamma characteristic A of the output density with respect to the gradation value of the input gradation data of the RGB color space, The change rate of the output density with respect to the change of the gradation value in the first RGB gradation area, and the change of the gradation value in the second RGB gradation area lower than the first RGB gradation area. And the change rate of the output density, the change rate of the output density with respect to the change of the gradation value in the first CMYK gradation region in the gamma characteristic B of the halftone table, and the first CMYK floor. An image processing apparatus having the same magnitude relationship with a change rate of an output density with respect to a change of a gradation value in a second CMYK gradation area higher than a gradation area. 14. An electrophotographic apparatus comprising: the image processing apparatus according to claim 1; and a print engine that prints the image according to the image reproduction data. 15. The print engine according to claim 13, wherein the print engine irradiates a laser beam in accordance with the image reproduction data to form a latent image, and adheres the toner in the second color space to the latent image. Electrophotographic device. 16. A recording medium recording an image processing program for causing a computer to execute an image processing procedure for generating image reproduction data expressing halftone gradation of an image from input gradation data of a first color space, The image processing procedure includes a color conversion procedure of converting the input tone data of the first color space into tone data of the second color space by an interpolation process referring to a color conversion table, and a second color By referring to a halftone table having correspondence between spatial gradation data and the image reproduction data, the second
A halftone processing procedure for converting gradation data in the color space of the above into image reproduction data, and a gamma characteristic A of output density with respect to a gradation value of the input gradation data in the first color space; A recording medium on which an image processing program is recorded, in which the gamma characteristic B of the output density is made equal to the gradation value of the table. 17. An image processing program for causing a computer to execute an image processing procedure for generating image reproduction data expressing gradation by halftone dots formed by a plurality of dots from input gradation data of RGB color space is recorded. In the recording medium, the image processing procedure includes a color conversion procedure for converting input gradation data in the RGB color space into gradation data in the CMYK color space by an interpolation process that refers to a color conversion table, and the CMYK color space. Halftone processing procedure for converting the gradation data of the CMYK color space into image reproduction data by referring to a halftone table having correspondence between the gradation data of 1. and the image reproduction data, and the RGB color space. In the gamma characteristic A of the output density with respect to the gradation value of the input gradation data, the output density with respect to the change of the gradation value in the first RGB gradation region Of the gamma characteristic of the halftone table, the magnitude relationship between the change rate of the output density and the change rate of the output density with respect to the change of the gradation value in the second RGB gradation region lower than the first RGB gradation region. B, the change rate of the output density with respect to the change of the gradation value in the first CMYK gradation area, and the change of the gradation value in the second CMYK gradation area higher than the first CMYK gradation area. A recording medium having an image processing program recorded therein, which has the same magnitude relationship with the change rate of the output density with respect to.