JP2005252323A - Apparatus and method for forming color image - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve image quality by providing six color decomposing means same as that for an RGB input image to a CMYK input image. <P>SOLUTION: An color image forming apparatus is provided with an image forming means for forming an image using at least one set of color materials of the same color system having a gray level, a first color decomposition means and a second color decomposition means for converting a plurality of image signal components comprising a deep color material into a plurality of image signal components forming materials of the same color system having the gray level. The first and second color decomposition means calculate a plurality of image signal values corresponding to the color materials of the gray level by interpolation operation of a referred look-up table while referring to the look-up table on the basis of the input signal. An input signal into the first color decomposition means is an RGB signal, and an input signal into the second color decomposition means is a CMYK signal. Color spaces of the plurality of image signals are colorimetrically the same including image signals corresponding to the color materials of the gray level to be decomposed by the first and second color decomposition means. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to image processing used for an image forming apparatus that is used in a copying machine, a printer, and a fax machine and forms an image with a plurality of inks or toners having the same hue.

  The market needs for printers are increasing year by year, and image forming apparatuses having an increased number of ink or toner colors have been proposed and realized in comparison with the conventional four-color image forming apparatuses using ink or toner of color. For example, in an inkjet printer, in addition to the conventional four colors of cyan (C), magenta (M), yellow (Y), and black (K), light cyan (LC Alternatively, it is possible to form an image with six colors of ink added with light magenta (denoted as LM or m), and in electrophotographic printers, there are six colors as in the inkjet method. Printers have been proposed (Patent Documents 1 and 2).

  In such a printer, as a matter of course, when RGB signals are input to the printer, RGB → CcMmYK and CMYK signals are input to the printer, CMYK → CcMmYK color separation processing is performed, and CcMmYK color signals are converted. It is configured to send to a printer.

A typical example of color separation from RGB to CcMmYK is LUT interpolation using a color conversion look-up table (LUT) having a configuration as disclosed in Patent Document 3. In RGB → CcMmYK color separation LUT calculation, interpolation grid points of the three-dimensional LUT are addressed with the input RGB values, and the CcMmYK values stored in the addresses are extracted and extracted by tetrahedral interpolation or cubic interpolation. The CcMmYK values are interpolated. As disclosed in Japanese Patent Application Laid-Open No. 2002-33927, using such a three-dimensional LUT, it is possible to control the color mixture of all six color components over the entire RGB space.
(1) The color reproducibility in the low density portion in the portion using the light color component can be improved.
(2) Toner / ink loading limit in all color spaces.
Is achievable.

On the other hand, as an example of color separation of CMYK signals into CcMmYK components, as described in Patent Document 4, a cyan signal is input and a dark cyan signal and a light cyan signal are output, and a magenta signal is input as a dark signal. An example of color separation such as outputting a magenta signal and a light magenta signal is common. That is, as shown in FIG. 7, LUT1 and 801 for outputting dark cyan (C) with C as an input, LUT2 and 802 for outputting light cyan (LC), and Magenta (M) with M as inputs. 4 color components are converted into 6 color components using LUTs 3 and 803 that output, and LUTs 4 and 804 that output light magenta (LM). At this time, the LUT values stored in the LUT1 and LUT3 are typically signal values such as 91 shown in FIG. 8, and the LUT values stored in the LUT2 and LUT4 are Typically, the signal value is 92 as shown in FIG.
JP 2001-290319 A JP 2001-318499 A JP 2002-33927 A Special registration No. 28337550

  As described above, although there are image processing apparatuses that conventionally receive RGB signals and CMYK signals and output CcMmYK signals, the method of performing CMYK → CcMmYK color separation is not sufficient. This is because the color separation method for four colors of RGB → CMYK is performed, and then only the C and M color components are separated independently. Therefore, this is not a color separation method that considers color mixing with other colors. It is. In this case, the advantages (1) and (2) in the RGB → CcMmYK color separation cannot be exhibited, and as a result, the image quality of the CMYK input image is inferior to that of the RGB input image.

  The present invention is made to eliminate such drawbacks, and provides an image forming apparatus that provides six-color separation means similar to that for an RGB input image for a CMYK input image to improve image quality. It is an issue to provide.

To solve the above problem,
Image forming means for forming an image using at least one set of similar color shades;
Comprising a first color separation means and a second color separation means for converting a plurality of image signal components composed of color components of a dark color material into a plurality of image signal components forming a dark and light color material with the same color,
The first and second color separation means refer to the look-up table based on the input signal, and calculate a plurality of image signal values corresponding to the light and shade color materials by interpolation calculation of the look-up table referred to. And
The input signal to the first color separation means is an RGB signal, and the input to the second color separation means is a CMYK signal. A color image forming apparatus characterized in that color spaces of a plurality of image signals including image signals corresponding to a color material are the same colorimetrically;
Moreover,
The look-up table used in the second color separation means describes a profile describing the colorimetric properties when an RGB signal is input to the color image forming apparatus and the colorimetric properties when a CMYK signal is input. A color image forming apparatus, wherein the color image forming apparatus is configured by being calculated from the profile obtained and a look-up table used in the first color separation means,
I will provide a.

In these color image forming apparatuses,
It is also characterized in that the color material of the similar color is a color material of the same color of cyan, magenta or black.

  Further, the image forming means is an electrophotographic printer or an ink jet printer.

  As described above, according to the image processing apparatus of the present invention, high-performance color separation can be performed regardless of whether the input image is RGB or CMYK, and a high-quality image can be obtained.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

<Configuration of image forming apparatus>
FIG. 1 is a schematic cross-sectional view of a full-color image forming apparatus (a multi-function machine having both a copying function, a printer function, and a FAX function) according to an embodiment of the present invention. In this example, the digital color image reader unit 300 is provided in the upper part, and the digital color image printer unit 100 is provided in the lower part.

  In the reader unit 300, the original 30 is placed on the original table glass 31 and exposed and scanned by the exposure lamp 32, and the reflected light image from the original 30 is condensed on the full-color CCD sensor 34 by the lens 33, and the color separation image. Get a signal. The color-separated image signal passes through an amplifier circuit (not shown), is processed by a video processing unit (not shown), and is sent to the printer unit 100 via an image memory (not shown).

  In addition to the signal from the reader unit 300, an image signal from a computer, an image signal from a FAX, and the like are similarly sent to the printer unit 100.

  Here, as an example, the operation of the printer unit 100 will be described based on a signal from the reader unit 300.

  The printer unit 100 is roughly divided into two image forming units, that is, a first image forming unit Sa including the first photosensitive drum 1a and a second image forming unit Sb including the second photosensitive drum 1b. Has been. These image forming portions Sa and Sb have substantially the same configuration (shape) from the viewpoint of cost reduction. For example, the configuration and shape of the developing unit described later are almost the same. As a result, the developing devices 41 to 46 can be interchanged with each other.

  Two drum-shaped photosensitive members (photosensitive drums) as image carriers, that is, a first photosensitive drum 1a and a second photosensitive drum 1b, are each supported rotatably in the direction of the arrow in the figure, and each of the arrows in the figure. Around the photosensitive drums 1a and 1b, pre-exposure lamps 11a and 11b, corona chargers (charging means) 2a and 2b, and a first exposure means 3a which is a laser exposure optical system. , Second exposure means 3b, potential sensors 12a and 12b, movable members (development rotary) 4a and 4b which are rotary developing device holding portions, and three developing devices each containing developer of different colors 41 to 43 and 44 to 46, primary transfer rollers 5a and 5b as primary transfer means, and cleaning devices 6a and 6b are arranged.

  Further, the number of developing units may be five or more in order to improve the image quality. In this embodiment, a configuration using six developing units 41 to 46 will be described.

  The developing unit is loaded with magenta toner 41, cyan toner 42, light magenta 43, yellow toner 44, black toner 45, and light cyan toner 46.

  Here, the dark and light color developers are prepared by changing the amount of the pigment having the same spectral characteristics. Accordingly, the light magenta toner has the same spectral characteristics as the magenta pigment but a low content, and the light cyan toner has the same spectral characteristics as the cyan pigment but a low content.

  In addition to these, it contains toners with different spectral characteristics of pigments from cyan, magenta, yellow and black, such as metallic toners such as gold and silver, and fluorescent toners containing fluorescent agents. It is also possible to mount a developing device (same shape as the above developing device) on the developing rotary.

  In addition, although the two-component developer used by mixing the toner and the carrier is loaded in the developing device, there is no problem even with the one-component developer composed only of the toner.

  Here, the use of dark and light colors for magenta and cyan is aimed at dramatically improving the reproducibility of light images such as human skin (to achieve a reduction in graininess). Is the aim).

  In the laser exposure optical systems 3a and 3b as exposure means, the image signal from the reader unit 300 is converted into an optical signal by a laser output unit (not shown), and the laser beam E converted into the optical signal is converted by the polygon mirror 35. The light is reflected and projected onto the exposure positions 38a and 38b on the surfaces of the photosensitive drums 1a and 1b through the lens 36 and the reflecting mirrors 37.

  At the time of image formation of the printer unit 100, the photosensitive drums 1a and 1b are rotated in the direction of the arrow A, and the photosensitive drums 1a and 1b after being neutralized by the pre-exposure lamps 11a and 11b are uniformly charged by the chargers 2a and 2b. A light image E is irradiated for each separation color to form latent images on the photosensitive drums 1a and 1b.

  Next, the rotary developing device holding section, that is, the first developing rotary 4a and the second developing rotary 4b, which are moving bodies, are rotated, and predetermined developing devices 41 and 44 are moved to the respective developing devices 41 to 41 on the photosensitive drums 1a and 1b. 43, or after moving to the common developing units 40a and 40b between the developing units 44 to 46, the developing units 41 and 44 are operated to invert the electrostatic latent images on the photosensitive drums 1a and 1b. Development is performed to form a developer image (toner image) on the photosensitive drums 1a and 1b using a resin and a pigment as a base. At this time, a developing bias is applied to the developing device.

  Further, as shown in the figure, the toner in the developing devices 41 to 46 is transferred from the toner storage units (hoppers) 61 to 66 for the respective colors disposed between and next to the laser exposure optical systems 3a and 3b. In order to keep the ratio (or toner amount) constant, it is replenished as needed at a desired timing.

  The toner images formed on the respective photosensitive drums 1a and 1b are superimposed on an intermediate transfer member (intermediate transfer belt) 5 as a transfer medium by primary transfer rollers 5a and 5b serving as primary transfer units. The primary transfer is sequentially performed so as to be formed. At this time, a primary transfer bias is applied to the primary transfer rollers 5a and 5b. As a result, the respective toner images are sequentially superimposed on the intermediate transfer belt 5 to form a full color toner image.

  Thereafter, the full-color toner image on the intermediate transfer belt 5 as a transfer medium is secondarily transferred collectively onto a sheet as a recording material. At this time, a secondary transfer bias is applied to the secondary transfer roller 54.

  The intermediate transfer belt 5 is driven by a driving roller 51 so that the transfer cleaning device 50 can be brought into contact with and separated from the driving roller 51 at a position facing the intermediate transfer belt 5.

  The photosensitive drums 1a and 1b are provided on the transfer surface t, which is the same plane portion formed by the intermediate transfer belt 5 stretched by two rollers 51 and 52. The photosensitive drums 1a and 1b are connected to the photosensitive drums 1a and 1b. Primary transfer rollers 5a and 5b, which are primary transfer units, are provided at opposing portions sandwiching the intermediate transfer belt 5.

  In addition, the roller that forms the transfer surface t is located downstream of the intermediate transfer belt 5 in the moving direction B and is opposed to the driven roller 52, and detects the positional deviation and density of the images transferred from the drums 1a and 1b. A sensor 53 for performing image correction is disposed, and control is performed to correct the image density, toner replenishment amount, image write timing, image write start position, and the like at each image forming unit Sa, Sb as needed.

  Further, the transfer cleaning device 50 facing the upstream drive roller 51 finishes superimposing images of necessary colors on the intermediate transfer belt 5, and then is pressed by the facing drive roller 51 and transferred to the recording material. The remaining toner on the intermediate transfer belt 5 is cleaned. After the cleaning is completed, the transfer cleaning device 50 is separated from the intermediate transfer belt 5.

  On the other hand, the recording material is conveyed one by one from the storage units 71, 72, 73 or the manual feed tray 74 by the respective paper feeding means 81, 82, 83, 84, and the skew is corrected by the registration rollers 85, and a desired timing is obtained. Then, the toner image on the intermediate transfer belt 5 is conveyed to a secondary transfer portion between the secondary transfer roller 54 and the intermediate transfer belt 5, which is a secondary transfer means for transferring the toner image onto the recording material.

  The toner image is transferred onto the recording material in the secondary transfer unit, the recording material passes through the conveyance unit 86, and the toner image is fixed by the heat roller fixing device 9, and the paper discharge tray 89 or a paper post-processing device (not shown) ).

  On the other hand, the intermediate transfer belt 5 after the secondary transfer is cleaned with the transfer residual toner by the transfer cleaning device 50 as described above, and again used for the primary transfer process of the image forming portions Sa and Sb.

  When images are formed on both sides of the recording material, the conveyance path switching guide 91 is driven immediately after the recording material passes through the fixing device 9, and the recording material passes through the conveyance vertical path 75 to the reversing path 76. After being guided, the reverse roller 87 is reversely rotated so that the rear end when the paper is fed is used as the head, and the paper is withdrawn in the direction opposite to the fed direction and sent to the double-sided conveyance path 77. After that, the paper passes through the double-sided conveyance path, skew correction and timing are performed by the double-sided conveyance roller 88, conveyed to the registration roller 85 at a desired timing, and an image is again formed on the other surface by the image forming process described above. Transcript.

<Configuration of controller unit and image processing unit>
FIG. 2 is a diagram illustrating a controller unit that controls the image forming apparatus illustrated in FIG. 1. The controller receives an instruction from the user, operates the apparatus and displays the apparatus state, the operation unit 205 for reading the image, the reader unit 206 for reading the image and sending the image signal to the image processing unit 207, and the received input image signal An image processing unit 207 that performs image processing suitable for image processing and output to a printer, sends the image to the printer unit 208, the printer unit 208 that forms an image on a paper medium, and an external host computer The PDL processing unit 202 receives PDL data, operates on the CPU according to a program stored in the RAM, and includes a PDL processing unit 202 that renders the PDL data received by the network I / F and sends it to the image processing unit. C for controlling the image flow according to a program stored in the RAM 204 Consisting of U203. There are a number of components such as a FAX receiving unit in the controller, but the description thereof is omitted here.

  Next, FIG. 3 shows the configuration of the image processing unit. In the reader unit, the luminance image signal obtained by reading the document with the CCD is input to the image processing unit as a digital image signal. In many cases, 8 bits (= 256 gradations) are often used for one pixel. In this way, the input RGB signal is subjected to white reference correction by the shading correction unit 301, and processing called input masking is performed by the input color processing unit 302, so as to remove color turbidity caused by CCD spectral characteristics. It is. Next, the frequency characteristics of the image input by the spatial filter unit 303 are corrected.

  The scanner RGB signal obtained so far and the RGB signal (8 bits for each color) generated by the PDL processing unit 202 are decomposed into CcMmYK signals (10 bits for each color) by the RGB color separation unit 304. Also, the PDL processing unit 202 may generate a CMYK image. When a CMYK image is input, the CMYK color separation unit 307 performs color separation from CMYK to CcMmYK signals. The color processing flow of the PDL image and the processing of the CMYK color separation unit will be described in detail later.

  The output gamma correction unit 305 corrects the output characteristics using the one-dimensional LUT for each color component independently of the color-separated signal in this way, and the halftone processing unit 306 prompts for the tone reproducibility of the printer. Pseudo halftone processing is performed with the number of bits / resolution, and a pseudo halftone signal is sent to the printer unit. Here, the gradation reproducibility of the printer is assumed to be 4 bits / 600 dpi, but the number of bits and the resolution are not limited thereto. The pseudo halftone process is performed by a known screen process or error diffusion process.

<Color processing flow>
FIG. 4 shows a color processing flow diagram of the PDL image. 4A shows an example of a color processing flow for an RGB image, and FIG. 4B shows an example of a color processing flow for a CMYK image.

  FIG. 4A shows an example in which an input RGB image is printed as an image having signal values in the sRGB color space. The RGB image is converted by a CMM (Color Matching Module) 401 built in the PDL processing unit 202 into a DeviceRGB color space depending on the printer using the ICC profile 410 for sRGB and the ICC profile 412 for printer. Here, the sRGB profile is used, but when an RGB image in another RGB color space is output, an ICC profile suitable for the color space may be used. Here, the ICC profile represents a bidirectional correspondence relationship between a device-dependent color space depending on a device such as a monitor or a printer and a device-independent color space such as Lab or XYZ defined in colorimetry, and an arithmetic expression or LUT. The CMM uses the color space information in the ICC profile to perform a three-dimensional LUT interpolation operation, a matrix operation, and a one-dimensional LUT interpolation operation, and performs conversion from the input color space to the output color space. As a specific example of CMM, Canon Information Systems, Inc. There is ColorGear (R) manufactured by the company, and it is assumed that ColorGear (R) is used here, but of course, it does not stick to it. The image data converted into the DeviceRGB color space by the CMM is sent to the image processing unit 207, and converted into a CcMmYK signal of 10 bits for each color component by the LUT interpolation operation using the RGB color separation LUT 413 in the RGB → CcMmYK color separation unit 304. Is done. As described above, the converted CcMmYK signal is appropriately processed by the output gamma correction 305 and the halftone processing unit 306, and is sent to the printer unit.

  FIG. 4B shows an example in which the input CMYK image is a CMYK image that conforms to JapanColor ink that defines the print color. The CMYK image is converted into a CMYK color space signal depending on the printer by using a CMYK simulation profile 414 and a CMYK printer profile 415. After being converted into a CMYK signal, it is converted into CcMmYK by the CMYK → CcMmYK color separation unit 307, and thereafter sent to the printer unit via the output gamma correction unit 305 and the halftone processing unit 306. The description regarding the CMM and ICC profiles is the same as that for RGB printing.

  When receiving a signal from the scanner during copying, the input color processing unit 302 uses a device-independent RGB color space (an RGB color space defined calorimetrically), for example, sRGB. The color separation LUT may be configured by including the conversion from the sRGB color space in the RGB color separation unit.

  FIG. 5 shows a graph of RGB → CcMmYK color separation when RGB is equal, that is, when the input image is gray. When the RGB signal is high, that is, in the region where the density is low, the gray component is expressed only by LC, LM, and Y. When the RGB signal becomes low, that is, when the density becomes high, C, M, and K overlap. You can see that Then, at the point of perfect black, R = G = B = 0, it can be seen that black is 1023 (the maximum value of 10 bits), and C, M, and Y are slightly overlapped.

  Assuming that the maximum toner loading amount of the printer is 205%, CMYK4 color separation with respect to the black point (R = G = B = 0) using the most toner is CMYK = (35%, 35%, 35%, 100) when printing with CMYK four colors. However, when the CMYK → CcMmYK conversion is performed by the conventional method as shown in FIG. 7, CcMmYK = (15%, 70%, 15%, 70%, 35%, 100%) And the amount of applied toner is exceeded. Further, in the low density region, mixing of light color toners or mixing of light color toners with other dark color toners cannot be considered, resulting in a decrease in color reproducibility. In order to eliminate such drawbacks, it is effective to construct a CMYK → CcMmYK color separation LUT having the same characteristics as the RGB → CcMmYK color separation having the characteristics shown in FIG.

  FIG. 6 is a diagram schematically showing a configuration method of the CMYK → CcMmYK color separation LUT. In order to configure the color separation LUT, CMYK grid points are generated, and the CMYK → Lab interpolation unit 601 performs interpolation for each grid point using the CMYK printer profile 415 to calculate a Lab signal. Each grid point is, for example, 16 grid points for each CMYK component, and 16 × 16 × 16 × 16 CMYK values are generated. The Lab signal calculated by the CMYK → Lab interpolation is then converted into DeviceRGB by Lab → DeviceRGB interpolation 602 using the RGB printer profile 412. Finally, the signal is converted into a CcMmYK signal by RGB → CcMmYK color separation 603 using the RGB color separation LUT 413. By storing CcMmYK corresponding to the CMYK lattice points generated in this way, CMYK → CcMmYK color separation LUT 416 is obtained. The interpolation calculation here may use a known interpolation calculation method such as tetrahedral interpolation.

  The color separation LUT is configured in advance using a CMYK printer profile, DeviceRGB → CcMmYK color separation LUT, and RGB printer profile on a program that runs on an external host PC. It is assumed that the data is stored in the decomposition unit 307. In addition, when the RGB → CcMmYK color separation LUT is updated for some reason, each interpolation calculation can be performed using the CMM 401 and the RGB color separation unit 304.

  By combining CMYK → Lab interpolation and Lab → DeviceRGB interpolation, the CMYK printer profile configured for the four-color printer and the RGB printer profile can be colorimetrically matched, and the RGB color separation unit 304 performs color separation. The CcMmYK signal and the CcMmYK signal color-separated by the CMYK color separation unit are the same colorimetrically, and the advantages of the color separation method conventionally used in the RGB input system are compared with the CMYK signal. Even work can be done.

  In the description of the present invention, the color signals to be decomposed into light and dark signals have been described as cyan and magenta. However, black and yellow are also decomposed into dark and light signals, and the same smoothing circuit is applied to the developing device. May be output to an image forming apparatus in which the number is further increased. In the image forming apparatus, when one developing device is added, the number of developing devices is seven. Therefore, it is only necessary to arrange three or four developing devices on the developing rotary, and when two developing devices are added. In this case, four developing devices may be arranged in the developing rotary.

  In the description of the present invention, an electrophotographic printer is used as an example of an image forming apparatus. However, an inkjet printer that performs image formation using dark and light inks similarly receives CMYK inputs and outputs CcMmYK six colors. It goes without saying that is possible.

The figure which shows the example of the schematic sectional drawing of the image forming apparatus used by this invention Drawing showing controller configuration example Drawing showing configuration example of image processing unit Drawing showing an example of color processing flow during printing Drawing showing examples of RGB color separation characteristics Drawing which shows the schematic diagram of the structure method of CMYK-> CcMmYK color separation LUT Drawing showing a conventional CMYK → CcMmYK color separation method Drawing which shows the example of the shading sort LUT in the conventional color separation

Claims (6)

Image forming means for forming an image using at least one set of similar color shades;
Comprising a first color separation means and a second color separation means for converting a plurality of image signal components composed of color components of a dark color material into a plurality of image signal components forming a dark and light color material with the same color,
The first and second color separation means refer to the look-up table based on the input signal, and calculate a plurality of image signal values corresponding to the light and shade color materials by interpolation calculation of the look-up table referred to. And
The input signal to the first color separation means is an RGB signal, and the input to the second color separation means is a CMYK signal. A color image forming apparatus, wherein color spaces of a plurality of image signals including image signals corresponding to color materials are the same colorimetrically. The look-up table used in the second color separation means describes a profile describing the colorimetric properties when RGB signals are input to the color image forming apparatus and the colorimetric properties when CMYK signals are input. Calculated from the profile and the lookup table used in the first color separation means,
The color image forming apparatus according to claim 1.   3. The color image forming apparatus according to claim 1, wherein the dark and light color material of the similar color is a dark and light color material of a similar color of cyan, magenta, or black.   The color image forming apparatus according to claim 1, wherein the image forming unit is an electrophotographic printer.   4. The color image forming apparatus according to claim 1, wherein the image forming unit is an ink jet printer. An image forming step of forming an image using at least one set of similar color shades;
A first color separation means and a second color separation step for converting a plurality of image signal components composed of color components of a dark color material into a plurality of image signal components forming a dark and light color material with the same color;
In the first and second color separation steps, a lookup table is referred to based on an input signal, and a plurality of image signal values corresponding to the shaded color material are calculated by interpolation of the referenced lookup table. And
The input signal to the first color separation means is an RGB signal, and the input to the second color separation means is a CMYK signal. A color image forming method, wherein color spaces of a plurality of image signals including image signals corresponding to color materials are the same colorimetrically.

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