JP2010287996A - Image processor and image forming apparatus including the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform color conversion processing for proper gradation expression, by reading each patch output from a printing device with a maximum density allowed as the case now, and recognizing the present color gamut. <P>SOLUTION: An image processor includes a first color space conversion section which converts a first color space signal to a second color space signal; a second color space conversion part which converts the second color space signal to a color space signal belonging to the printing device, on the basis of color conversion tables; a density measurement section which measures solid densities of C, M, Y, and K output from the printing device; a control section which obtains the coefficient of the density for each patch; and a memory where a plurality of kinds of color conversion tables are stored. The color conversion tables are so determined that the color gamut of an input device lies within a color gamut which the printing device can output, by compressing or expanding the color gamut of the input device in a second color space as a whole, in accordance with combination of coefficients of respective patches, and the second color space conversion section performs color conversion processing by using a color conversion table that agrees with the combination of coefficients of respective patches. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image processing apparatus that performs color space conversion processing and an image forming apparatus equipped with the image processing apparatus.

  For example, the image processing apparatus may be mounted on various image forming apparatuses (for example, forming a toner image), and the image processing apparatus performs image processing on image data for printing, for example. The image processing apparatus mounted on the image forming apparatus performs color conversion processing such as γ conversion on the image data using, for example, a look-up table to improve the image quality of the printed matter. Here, characteristics of members constituting the image forming apparatus change due to an installation environment (humidity or temperature) or a change with time (for example, fatigue of a photosensitive drum or developer in an electrophotographic image forming apparatus). For this reason, the image processing apparatus and the image forming apparatus may be equipped with a calibration function such as checking the density state of the printed material and updating the lookup table.

An example of an image forming apparatus that performs calibration is described in Patent Document 1. Specifically, Patent Document 1 discloses a holding unit that executes calibration relating to printing characteristics and holds calibration information downloaded from a host device, a creating unit that creates calibration information at a predetermined timing, and a creating unit. Compares the value indicated by the calibration information created by and the value indicated by the calibration information held by the holding means, and if the difference exceeds a predetermined value, calibration is performed based on the calibration information created by the creating means. A printing apparatus having execution means for executing is described. Thus, a calibration system that incorporates the advantages of using calibration performed mainly by the host device and calibration performed uniquely by the printing apparatus is provided (Patent Document 1: Claim 1, Paragraph 1). [0058], [0073] etc.).
JP2001-18498

  For example, even if printing is performed based on image data having a pixel value (gradation value) indicating the maximum density on the image data, such as a solid image, the image forming apparatus outputs due to changes over time of the image forming apparatus. (Printing) The maximum possible density may be reduced. When the maximum density that can be output decreases, the color gamut (color reproduction range) becomes narrower. Further, the output characteristics of the image forming apparatus with respect to the pixel values of the input image data may show nonlinearity. Due to these factors, gradation skipping and gradation difference collapse may occur, especially in a high density region. That is, there is a problem that appropriate gradation expression may not be made on the printed matter.

  Note that Patent Document 1 does not describe a case where the maximum density that can be output by the image forming apparatus is reduced by design. In other words, the color conversion table is updated with calibration data on the assumption that the maximum density that can be output by the image forming apparatus does not change. Therefore, the invention described in Patent Document 1 cannot solve the above problem.

  In view of the above-described problems of the prior art, the present invention reads each patch output at the maximum density that the printing device can currently output, and grasps and grasps the current color gamut (color reproduction range) of the printing device. It is an object of the present invention to perform color conversion processing in which an appropriate gradation expression is made in the color gamut.

  The image processing apparatus according to claim 1, based on a color conversion table, a first color space conversion unit that converts a first color space signal dependent on an input device into a second color space signal independent of the device, and the second color space table. A second color space conversion unit that converts the color space signal into a CMYK color space signal subordinate to the printing device; and a density for measuring the density of the C, M, Y, and K solid patches output from the printing device. A measurement unit, a control unit for obtaining a coefficient indicating the density of each patch output from the printing device based on the output of the density measurement unit, a plurality of types, a memory for storing the predetermined color conversion table, Each of the color conversion tables compresses or expands the color gamut of the input device in the second color space as a whole according to the obtained combination of the coefficients of the patches, and in the second color space. Said The color gamut of the force device is determined to fall within the color gamut of the second color space that can be output by the printing device, and the second color space conversion unit is configured to detect the patches measured by the density measurement unit. The color conversion process is performed using the color conversion table that matches the combination of coefficients.

  Conventionally, the color gamut (color reproduction range) of a printing device in a color space that does not depend on the device may be grasped based on a maximum density that is determined in advance by design. A color conversion table for converting a device-independent color space into a CMYK color space signal dependent on a printing device may be created in advance so that gradation representation can be smoothly performed based on the grasped color gamut. is there. Conventionally, the color conversion table is fixedly used even if the maximum density that can be output by the printing device changes. For example, due to changes over time, the maximum density that can be output by the printing device in any or all of C (cyan), M (magenta), Y (yellow), and K (black) decreases. If the color gamut changes, the color conversion table created in advance and used in a fixed manner cannot be said to be optimal, and gradation skipping or gradation difference collapse may occur.

  However, according to this configuration, the second color space conversion unit does not fix the color conversion table to be used, and an appropriate gradation expression is made in the memory according to the maximum density that the printing device can output at present. A plurality of color conversion tables determined so as to be stored are stored. The second color space conversion unit performs color conversion processing by switching the color conversion table to be used according to the density of the patch formed with the maximum density (solid density) that can be output by the printing device at present. Therefore, the image processing apparatus can perform color conversion processing so that appropriate gradation expression is made within a range that the printing device can reproduce at present.

  According to a second aspect of the present invention, in the first aspect of the invention, the color conversion table generation unit generates a new color conversion table by performing an interpolation operation based on the two color conversion tables. When there is no color conversion table that matches the obtained combination of coefficients, the color conversion table generation unit, based on the obtained combinations of all the coefficients of the patches, can currently output the color gamut that the printing device can output. A color conversion table with a wider color gamut and a color conversion table with a narrow color gamut are selected, and the new color conversion table is generated by interpolation using the two selected color conversion tables, and the second color The space conversion unit performs the color conversion process using the color conversion table generated by the color conversion table generation unit.

  There are innumerable combinations of coefficients indicating the density of C (cyan), M (magenta), Y (yellow), and K (black) patches, and the more color conversion tables are prepared, the more the storage unit should store them. The amount of data is enormous. However, according to the configuration, the color conversion table generation unit generates a new color conversion table by interpolation calculation. Therefore, the amount of data stored in the storage unit can be reduced.

  An image forming apparatus according to a third aspect includes the image processing apparatus according to the first or second aspect, wherein the input device reads an original and outputs an RGB color space signal, and a printing device. An image forming unit that forms toner images using cyan, magenta, yellow, and black toners is provided. According to this configuration, printing is performed based on the image data that has been subjected to the color conversion processing so that appropriate gradation expression is performed within a range that the printing device can output at present. Therefore, it is possible to provide an image forming apparatus capable of outputting a printed matter in which gradation expression is appropriately expressed even if the maximum density of a toner image that can be output at present is lower or higher than the ideal maximum density in design. it can.

  According to a fourth aspect of the present invention, in the third aspect of the present invention, the intermediate transfer unit that receives the primary transfer of the toner image formed by the image forming unit and performs the secondary transfer onto the sheet, and the image forming unit A control unit that controls the intermediate transfer unit, and the image forming unit includes a photosensitive drum as an image carrier, a charging device that charges the photosensitive drum, and the charged photosensitive drum. An exposure device that forms an electrostatic latent image; and a developing device that supplies toner to the electrostatic latent image to develop the toner image, and the control unit includes a charging potential of the photosensitive drum by the charging device; Calibration is performed to adjust any one or more of the developing bias applied by the developing device and the transfer bias for transferring the toner image at the intermediate transfer unit, and after the calibration is completed, Image formation Forms the patch having a solid density of C, M, Y, and K, and the density measuring unit measures the density of the formed solid density of C, M, Y, and K, and the second color The space conversion unit changes the color conversion table to be used based on the combination of the patch coefficients of each color measured by the density measurement unit.

  In this configuration, the maximum density that can be output from the image forming apparatus is corrected and recovered by calibration in which various voltages are adjusted so as to return to an ideal state in terms of design. However, even if calibration is performed, it does not always return to the maximum density predetermined in design, and if the color conversion table is fixed, gradation skipping or gradation collapse may occur. However, according to this configuration, the second color space conversion unit uses the patch with the maximum density of each color according to the maximum density that the image forming apparatus can output at present with the color gamut adjusted and recovered. Since the color conversion process is performed by switching the color conversion table, the image forming apparatus can output a printed matter that can be said to have the highest image quality at present.

  As described above, according to the present invention, a printing device (for example, an image forming apparatus that has an image forming unit or an intermediate transfer unit and performs printing) reads each patch output at the maximum density that can be output. Therefore, it is possible to grasp the current color gamut (color reproduction range) of the printing device and perform color conversion processing in which appropriate gradation expression is made in the grasped color gamut.

1 is a schematic front sectional view of a copying machine according to an embodiment. FIG. 3 is a partially enlarged schematic cross-sectional view of an image forming unit according to an embodiment. 1 is a block diagram illustrating an example of a hardware configuration of a copier according to an embodiment. 3 is an example of a chart formed on the intermediate transfer belt according to the embodiment. It is a model front sectional view showing an example of the image reading unit according to the embodiment. It is a block diagram which shows an example of the image reading part which concerns on embodiment. 1 is a block diagram illustrating a portion corresponding to an image processing apparatus in a copying machine according to an embodiment. An example of a color conversion table according to the embodiment is shown, (a) shows an example of a table from RGB to Lab, and (b) shows an example of a color conversion table from Lab to CMYK. 4 is a graph showing an example of a change in the color reproduction range of the image forming unit according to the embodiment, where (a) is a graph of the ab plane and (b) is a graph of the La plane. 6 is a flowchart illustrating an example of color conversion table switching control in the copier according to the embodiment.

  Embodiments of the present invention will be described below with reference to FIGS. However, each element such as configuration and arrangement described in each embodiment does not limit the scope of the invention and is merely an illustrative example.

(Outline of the configuration of the copier 1)
First, an outline of a copying machine 1 (corresponding to an image forming apparatus) according to the embodiment will be described with reference to FIG. FIG. 1 is a schematic front sectional view of a copying machine 1 according to an embodiment of the present invention.

  As shown in FIG. 1, a document cover 1a for pressing a document is provided at the top of the copier 1, and an image reading unit 3 that reads the document as an input device and outputs an RGB color space signal below the document cover 1a. (To be described later). Further, in the copying machine 1, an image forming unit 5a for forming a toner image using cyan, magenta, yellow, and black toners as a printing device, a sheet feeding unit 4a, a sheet conveying path 4b, and a printing device (details will be described later). The intermediate transfer unit 6, the fixing unit 5b, and the like are arranged.

  An operation panel 1b having a start key for instructing the operation start of the copying machine 1 and a touch panel type liquid crystal display unit is provided in front of the copying machine 1 (illustrated by a broken line in FIG. 1). For example, the operation panel 1b receives a setting instruction from the user and displays information transmission such as the status of the copying machine 1. Further, regarding the present invention, a user of the copying machine 1 or a serviceman who performs maintenance can operate the operation panel 1b to instruct execution of switching of the color conversion table T.

  The paper feed unit 4a stores various papers such as plain paper, recycled paper, label paper, and OHP sheets (corresponding to papers of various sizes such as A4 and B4) as recording media, and supplies the paper when forming an image. . The paper transport path 4b transports the supplied paper to the discharge tray 41. Therefore, a plurality of conveyance roller pairs 42 and the like are provided in the sheet conveyance path 4b.

  The intermediate transfer unit 6 is provided above the image forming unit 5a, receives the primary transfer of the toner image formed by the image forming unit 5a, and performs secondary transfer onto the sheet. Then, the intermediate transfer belt 61 (corresponding to the intermediate transfer member) is attached to the driving roller 62, the driven roller 63, the four primary transfer rollers 64, and the like so that the lower outer peripheral surface is in contact with each photosensitive drum 52. It is stretched. Drive means (not shown) such as a motor and gear is connected to the drive roller 62 and rotates. The intermediate transfer belt 61 rotates in the clockwise direction (arrow direction) in FIG. 1 as the driving roller 62 rotates. Here, the primary transfer roller 64 is rotatably arranged one by one so as to face each photosensitive drum 52, and a voltage (transfer bias) of a predetermined magnitude is applied to the primary transfer roller 64. By applying the voltage, the toner images of the respective colors are primarily transferred from the photosensitive drums 52 to the intermediate transfer belt 61. During the primary transfer, the toner images of the respective colors are superimposed without deviation.

  Then, a secondary transfer roller 65 that contacts the intermediate transfer belt 61, faces the driving roller 62, and is rotatably supported is provided in the intermediate transfer unit 6. When the sheet and the toner image enter the nip between the driving roller 62 and the intermediate transfer belt 61, a predetermined voltage (transfer bias) is applied to the secondary transfer roller 65, and the toner image is secondarily transferred to the sheet. The belt cleaning device 66 removes residual toner from the intermediate transfer belt 61 and cleans it. The fixing unit 5b fixes the toner image transferred to the paper. The sheet is pressurized and heated when passing through the fixing unit 5b, and the toner image is fixed on the sheet. Thereafter, the sheet is discharged to the discharge tray 41, and image formation is completed.

(Configuration of the image forming unit 5a)
Next, the image forming unit 5a of the copier 1 according to the embodiment of the present invention will be described with reference to FIGS. FIG. 2 is a partially enlarged schematic cross-sectional view of the image forming unit 5a according to the embodiment of the present invention.

  The image forming unit 5a forms an image (toner image) for printing on a recording medium based on the image data. Then, as shown in FIG. 1, the image forming unit 5a has four image forming units 50K (black), 50Y (yellow), 50C (cyan), and 50M (magenta), and the photosensitive after charging. It comprises an exposure device 51 that scans and exposes the body drum 52 to form an electrostatic latent image.

  As described above, the copying machine 1 according to this embodiment can form a color image using a plurality of colors of toner. Each image forming unit 50 is different in the color of toner to be used, but the basic configuration is the same. In the following description, unless otherwise specified, K, Y, C, and M of the image forming unit 50 are used. Symbols are omitted.

  As shown in FIG. 2, each image forming unit 50 is supported as being rotatable in the direction of the arrow shown in the figure, and is an image carrier that is rotated in a predetermined direction by a motor (not shown) or the like. The photosensitive drum 52 is provided. A charging device 53, a developing device 54, a cleaning device 55, and the like are disposed around the photosensitive drum 52.

  The charging device 53 charges the surface of the photosensitive drum 52. The exposure device 51 scans and exposes the surface of the charged photosensitive drum 52 in accordance with the image data to form an electrostatic latent image. The developing device 54 includes a developing roller 54a to which a predetermined voltage (developing bias) is applied in order to carry the toner and cause the toner to fly to the photosensitive drum 52. The developing device 54 supplies toner to the electrostatic latent image to develop the toner image (make a visible image). The cleaning device 55 cleans the surface of the photosensitive drum 52. With these configurations, a toner image is formed on the peripheral surface of each photosensitive drum 52, and the toner image is primarily transferred to the intermediate transfer unit 6.

  In the copying machine 1 of the present embodiment, a density sensor 7 (corresponding to a density measuring unit) is provided between the image forming unit 50K and the driving roller 62 below the intermediate transfer belt 61. The density sensor 7 is for measuring the density of the C, M, Y, and K solid density patches P output from the copier 1 (printing device), and is the toner first transferred to the intermediate transfer belt 61. Measure the density of the image. For example, the density sensor 7 includes a light emitting unit (for example, an LED) that emits light toward the intermediate transfer belt 61 and a light receiving unit (for example, a voltage) that outputs current (or voltage) according to the amount of reflected light from the measurement target. This is a reflection type photosensor having a phototransistor). For example, the output current of the light receiving unit is converted into a voltage by a resistor (not shown) or the like.

(Hardware configuration of copier 1)
Next, a hardware configuration of the copying machine 1 according to the embodiment of the present invention will be described with reference to FIG. FIG. 3 is a block diagram showing an example of the hardware configuration of the copying machine 1 according to the embodiment of the present invention.

  As shown in FIG. 3, the copying machine 1 according to the present embodiment includes a control unit 9 inside. The control unit 9 controls the image forming unit 5a, the intermediate transfer unit 6 and the like, and controls the entire copying machine 1. Further, the control unit 9 can obtain a coefficient indicating the density of each patch P output from the copying machine 1 based on the output of the density sensor 7. For example, the control unit 9 includes a CPU 9a, a storage unit 9b, and the like. The CPU 9a is a central processing unit, and controls and performs each part of the copier 1 based on a control program stored in the storage unit 9b and developed. The storage unit 9b includes a storage device such as a ROM, a RAM, an HDD, or a flash ROM. The storage unit 9b can store a control program, control data, setting data, image data read by the image reading unit 3 and the like of the copying machine 1. In the present invention, a color conversion table T may be stored in the storage unit 9b.

  The control unit 9 includes an image reading unit 3, a paper feeding unit 4a, a paper conveyance path 4b, an intermediate transfer unit 6, an image forming unit 5a, a fixing unit 5b, an image processing unit 8 (details will be described later), an operation panel 1b, and the like. And controls the operation of each unit based on the control program and data in the storage unit 9b. Further, for example, the control unit 9 grasps the input content to the operation panel 1b such as execution of setting of gradation correction.

  A density sensor 7 is connected to the control unit 9. For example, the output of the density sensor 7 is converted into digital data by an A / D converter (not shown) or the like and then input to the CPU 9a (if the CPU 9a has a port corresponding to analog input, it is directly input to the CPU 9a. May be) Then, the CPU 9 a of the control unit 9 grasps the output voltage of the concentration sensor 7.

(Calibration)
Next, an example of calibration (density adjustment) in the copying machine 1 according to the embodiment of the present invention will be described with reference to FIG. FIG. 4 is an example of a chart C formed on the intermediate transfer belt 61 according to the embodiment of the present invention. Note that the image data of the chart C is stored in the storage unit 9b, and is read out and used as necessary.

  The calibration is a function for adjusting the density of the printed matter output from the copying machine 1. For example, when executing the calibration function, a service person or a user who performs maintenance of the copying machine 1 operates the operation panel 1b to input a calibration execution instruction. Then, the image forming unit 5 a generates a chart C as shown in FIG. 4, and the generated chart C is transferred to the intermediate transfer belt 61.

  As shown in FIG. 4, Chart C is an array of patches P having different densities arranged on the intermediate transfer belt 61. The chart C includes a patch P having the maximum density that can be output (printed) by the copying machine 1. The chart C is formed in order for each toner color (C, M, Y, K) (FIG. 4 shows only the chart C for two colors). The chart C is formed, for example, at a substantially central position in the main scanning direction of the intermediate transfer belt 61 and extending in the rotation direction (sub-scanning direction). The density sensor 7 is disposed at a position where each moving patch P can be read.

  The density sensor 7 reads the chart C formed on the intermediate transfer belt 61 and detects the density of each patch P. For this detection, a density sensor 7 (sensor head) is provided facing the intermediate transfer belt 61 (see FIG. 1). Based on the output voltage value of the density sensor 7 at the time of reading each patch, the control unit 9 grasps the density of each patch P.

  Here, the output voltage of the density sensor 7 is different between when the intermediate transfer belt 61 is read and when the patch P is read. Further, the output voltage of the density sensor 7 differs depending on the ratio (ie, density) of the area where the toner adheres to the surface area of the intermediate transfer belt 61 that irradiates light. Therefore, when the patch P is read, the output voltage value of the density sensor 7 differs depending on the density of the toner image. For example, the higher the density of the toner image (the higher the toner distribution ratio), the smaller the output voltage value of the density sensor 7 as the light diffusion or absorption increases.

  Thus, there is a correspondence between the density of each patch P and the output voltage value of the density sensor 7. For example, the output voltage value of the density sensor 7 when reading each patch P having an ideal density is designed. Obtained in advance through experiments or the like. Then, for example, the correspondence relationship between the density of the toner image and the output voltage for each color is stored as a table in the storage unit 9b. Then, at the time of density measurement (detection) of each patch P, the control unit 9 described later compares and refers to the output voltage value of the density sensor 7 and the table, and measures the density of the currently formed toner image. To detect.

  Then, from the density measurement of each patch P in the chart C, the control unit 9 prints darkly if the density of the currently formed toner image is lower than the target density in terms of design and specifications. In other words, calibration is performed to change and adjust various voltages in the image forming unit 5a and the intermediate transfer unit 6 so that printing is performed thinly if the density is high.

  For example, when the magnitude of the developing bias of the developing device 54 is changed, the amount of toner flying from the developing roller 54a changes. Even if the charging potential of the photosensitive drum 52 by the charging device 53 is adjusted, the density of the formed toner image changes. Further, when the transfer bias to the primary transfer roller 64 and the secondary transfer roller 65 in the intermediate transfer unit 6 is changed, the transfer efficiency is changed and the density of the toner image in the printed matter is changed. Therefore, for example, when the control unit 9 performs adjustment to make the maximum density (printable) that can be output (printed) by the copying machine 1 constant at a predetermined density, a developing bias applied by the developing device 54 or a charging device Calibration is performed by changing any one or more of the charging potential of the photosensitive drum 52 by 53 and the transfer bias for transferring the toner image at the intermediate transfer unit 6.

  In the calibration, a γ correction table for converting the pixel value of each pixel may be generated based on the density deviation in the patch P other than the maximum density. For example, if the maximum density of each patch P is an ideal density in design, but there is a deviation in other density patches P, there is a γ correction table in which pixel values are converted in a direction to eliminate the deviation. Generated. For example, if there is a patch P that is darker than the ideal density, the pixel value is replaced with a thinner one in the density band of the patch P.

(Calculation of coefficient as a guide for maximum concentration)
Next, an example of the calculation of the coefficient indicating the standard of the maximum density that can be printed by the copying machine 1 according to the embodiment of the present invention will be described with reference to FIGS.

  For example, the storage unit 9b stores the output voltage value of the density sensor 7 when the patch P of each color having the target maximum density (to be output) is read in the design of the copying machine 1. When compared with the output voltage value of the density sensor 7 when the patch P having the maximum density that can be output at present is read, the maximum density that can be output by the copier 1 at present is higher than the target maximum density in design. It is possible to grasp that it is dark or light.

  For example, the control unit 9 can calculate a value (hereinafter referred to as “comparison coefficient”) indicating the maximum density that can be output at present for each color based on the comparison of the output values of the density sensor 7. For example, by design, the maximum density of the output toner image is determined in advance for each model of the image forming apparatus in consideration of printing characteristics and the like. Therefore, for example, if the output value of the density sensor 7 is the same as the output voltage value of the density sensor 7 when the maximum density patch P that is a design target is read, the comparison coefficient is 1. 0 is required.

  On the other hand, for example, the output voltage value of the density sensor 7 when reading the current maximum density patch P is the same as the output voltage value when reading the patch P ideally formed with a density of about 90%. If there is, the current maximum density patch P that can be output has a density reduced by about 10%. On the other hand, if the density of the patch P having the maximum density that can be output is higher than the target density by design, the comparison coefficient is 1.0 or more (for example, 1.2 or the like). ).

  Further, based on the ratio between the output voltage value of the density sensor 7 at the time of reading the current maximum density patch P and the output voltage value of the density sensor 7 at the time of reading the design maximum target density patch P, a comparison is made. A use factor may be determined. Note that the coefficient for comparison does not necessarily need to be 1.0 when the maximum density that is the target for design is obtained, and may be 1.0 or more and the following.

(Configuration of Image Reading Unit 3)
Next, based on FIG. 5, the structure of the image reading part 3 which concerns on embodiment of this invention is demonstrated. FIG. 5 is a schematic front sectional view showing an example of the image reading unit 3 according to the embodiment of the present invention.

  The image reading unit 3 is unitized as a scanner, irradiates the original with light, reads the original based on the reflected light, and generates image data. For example, the image reading unit 3 includes a first moving frame 321, a second moving frame 322, a lens 33, an image sensor 34, and the like.

  The first moving frame 321 includes a lamp 35 and a first mirror 361 that extend in the main scanning direction and irradiate the reading target with light. The second moving frame 322 includes a second mirror 362 and a third mirror 363. Each mirror guides the reflected light of the document to the lens 33, and the lens 33 forms an image of the reflected light of the document. The image sensor 34 (for example, CCD) is for reading an object to be read, and a plurality of light receiving elements are arranged in a plurality of lines. The image sensor 34 strikes the original from the lamp 35 and performs photoelectric conversion according to the amount of reflected light when the reflected light imaged by the lens 33 is incident. That is, the image sensor 34 receives reflected light from the reading target and reads the reading target.

  When reading a document on the contact glass 31, reading is performed by moving each moving frame in the horizontal direction by rotating the winding drum 37 (each moving frame and the winding drum 37 are connected by a wire 38). . The winding drum 37 is rotated by a winding motor 37M that rotates forward and backward (see FIG. 6).

  Here, a white reference plate 39 for performing shading correction is provided below the left end of the contact glass 31. The white reference plate 39 is a plate extending in the main scanning direction of the image reading apparatus 1 (a direction perpendicular to the document conveyance direction, a direction perpendicular to the paper surface in FIG. 5). The white reference plate 39 is read and a white reference in shading correction is obtained. Determined.

(Hardware configuration of image reading unit 3 and generation of image data)
Next, based on FIG. 6, the hardware configuration of the image reading unit 3 according to the embodiment of the present invention and the generation of document image data will be described. FIG. 6 is a block diagram illustrating an example of the image reading unit 3 according to the embodiment of the present invention. Note that the flow of image data is illustrated by white arrows.

  The image reading unit 3 is provided with a reading control unit 30. The reading control unit 30 is a board on which various electronic components such as a CPU, a memory, and a chip are mounted. The reading control unit 30 is communicably connected to the control unit 9 of the copying machine 1 main body. The reading control unit 30 receives an instruction from the control unit 9 when reading various documents such as when the start key of the operation panel 1b is pressed. The reading control unit 30 is connected to a take-up motor 37M that rotates the take-up drum 37, a lamp 35, an image sensor 34, and the like. For example, the operation of the take-up motor 37M and the lamp 35 that irradiates light on a document. Controls turning on / off, driving of the image sensor 34, and the like.

  When generating image data of a document in the image reading unit 3, the image sensor 34 outputs a current (voltage) corresponding to the intensity of reflected light for each pixel. Note that the image sensor 34 of the copying machine 1 according to the present embodiment is a color-compatible line sensor, and outputs R, G, and B signals. Each output current (voltage) of the image sensor 34 is input to the A / D conversion unit 30a. An amplifier may be provided before the A / D converter 30a. The A / D conversion unit 30a converts each analog output current (voltage) of each pixel of the image sensor 34 into digital data and outputs the digital data to the shading correction unit 30b.

  In the image sensor 34, variation (individual characteristic difference) of each light receiving element corresponding to each pixel arranged in a line shape, a difference in condensing degree between the central portion and the peripheral portion of the lens 33, and a light emission amount depending on a position of the lamp 35. Even if each reference plate having the same density in the main scanning direction or a document is read, a difference in current (voltage) output from each light receiving element of the image sensor 34 is generated depending on the pixel position. Therefore, the shading correction unit 30b corrects distortion depending on the position of the pixel.

  For example, the shading correction unit 30b uses the digital data of each pixel when the white reference plate 39 is read before reading the document as the white reference, and uses the digital data of each pixel of the image sensor 34 in the off state of the lamp 35 as the black reference. get. Thereafter, the shading correction unit 30b performs gradation (quantization) for each pixel according to the size of the output (digital data after A / D conversion) of the image sensor 34 between the white reference and the black reference. Do. For example, the shading correction unit 30b performs quantization to a total of 24 bits per pixel in RGB (for example, Red = 8 bits, Green = 8 bits, Blue = 8 bits, each taking a value of 0 to 255, 256 tone). The shading correction unit 30b may perform γ correction in consideration of the reading characteristics of the image reading unit 3 (a γ correction unit may be provided separately).

  The image data of the document image output from the shading correction unit 30 b is input to the image processing unit 8. The image processing unit 8 according to the present embodiment is a circuit configured by combining calculation related to image data, a work RAM as a work area, an ASIC as a dedicated circuit, and the like (details will be described later). Note that an image processing program can be stored in the CPU 9a or the storage unit 9b of the control unit 9 so that the control unit 9 functions as the image processing unit 8 in terms of software.

(Image processing apparatus 2)
Next, based on FIGS. 7 and 8, functions and operations of portions corresponding to the image processing apparatus 2 in the copying machine 1 according to the embodiment of the present invention will be described. FIG. 7 is a block diagram showing a portion corresponding to the image processing apparatus 2 in the copying machine 1 according to the embodiment of the present invention. FIG. 8 shows an example of a table according to the embodiment of the present invention, (a) shows an example of a table from RGB to Lab, and (b) shows an example of a color conversion table T from Lab to CMYK.

  In the present embodiment, the copying machine 1 is provided with a density sensor 7, a control unit 9, a storage unit 9b, an image processing unit 8, and the like. In the image processing unit 8, a color conversion table that generates a new color conversion table T by performing interpolation based on the first color space conversion unit 81, the second color space conversion unit 82, and the two color conversion tables T. A generation unit 83 and the like are provided. These configurations correspond to the image processing apparatus 2 in the copying machine 1. In other words, the copier 1 of this embodiment includes the image processing apparatus 2 (the control unit 9 controls the copier 1 and also functions as the control unit 9 of the image processing apparatus 2). The image processing unit 8 can perform various types of image processing such as enlargement / reduction processing, density conversion processing, and spatial filter processing. However, since the image processing that can be performed is various, the following description will explain the color conversion image processing according to the embodiment of the present invention, and for other image processing, the image processing unit 8 performs known image processing. The detailed explanation is omitted as it can be done.

  A signal (image data) input to the first color space conversion unit 81 of the image processing unit 8 is an image signal in a color space depending on the image reading unit 3, and specifically, output from the image sensor 34. RGB signal. The first color space conversion unit 81 converts the RGB signal (corresponding to the first color space signal) subordinate to the image reading unit 3 into a Lab color space signal (corresponding to the second color space signal) independent of the device (note that , Lab should be officially written as L * a * b *, but is abbreviated as Lab below). The first color space conversion unit 81 performs color conversion processing using, for example, a table T0 stored in the memory 84 or the storage unit 9b.

  As shown in FIG. 8A, the table T0 for conversion from RGB to Lab includes RGB values depending on the RGB color space of the image reading unit 3 (for example, R, G, and B are 8-bit 256th floors, respectively). 3D) is a table of three-dimensional input-three-dimensional output showing the correspondence between Lab and Lab values. The table T0 in FIG. 8A shows the correspondence between 8-bit RGB values and Lab values. In this table T0, Lab values for representative RGB values (in FIG. 8A, representative points are selected in increments of 51 pixel values) are stored. If the RGB value of each pixel of the image data input to the first color space conversion unit 81 does not match the representative point, the first color space conversion unit 81 calculates the Lab value near the input RGB value. The Lab value corresponding to the input RGB value is obtained by taking out from the table T0 and performing an interpolation operation.

  Here, the color gamut of the RGB signal output from the image reading unit 3 has less fluctuation than the color gamut of the toner image output from the copying machine 1. For this reason, in the copying machine 1 of the present embodiment, the table T0 for conversion from RGB to Lab is fixed. That is, the color gamut that can be captured by the image reading unit 3 in the Lab color space is handled as being fixed.

  The second color space conversion unit 82 converts the Lab color space signal (corresponding to the second color space signal) to the copying machine 1 based on the color conversion table T for conversion from Lab to CMYK as shown in FIG. Conversion to CMYK color space signals depending on (printing device). Based on the image data converted from the CMYK color space signal output from the second color space conversion unit 82, a toner image is formed by the exposure device 51 and the like.

  For example, the color conversion table T for Lab → CMYK conversion is created as follows. When the color conversion table T is created or when the color conversion table T described later is switched, for example, image data for generating a solid image (maximum density, density 100%) of each color toner is read from the storage unit 9b. Based on the image data, a patch P having the maximum density of C (cyan), M (magenta), Y (yellow), and K (black) is generated by the image forming unit 5a. Note that the chart C used in the calibration includes the patch P having the maximum density, so the image data of the chart C may be used.

  The color of each of the obtained patches P having the maximum density is measured by a colorimeter (not shown) to obtain the Lab value of each patch P. Based on the relationship between the obtained Lab value and the CMYK value of each printed patch P, the positions (coordinates) of C, M, Y, and K that can be formed by the image forming unit 5a in the Lab color space are grasped. Also, for example, the coordinates of the positions of R (red), G (green), and B (blue) generated by mixing each toner can be grasped based on the contribution ratio (mixing ratio) of the toner of each color. (The same applies to colors other than R, G, and B). As a result, the outermost contour of the color gamut that can be output by the copying machine 1 in the Lab color space is grasped.

  Then, for example, if each Lab range is carved in a predetermined step and a Lab grid is formed, a color conversion table T for Lab → CMYK conversion can be obtained (for example, after conversion, C, M, Y, K 8 bits each, 256 gradations). The color conversion table T for Lab → CMYK conversion is stored in, for example, the storage unit 9b or the memory 84.

  Here, when the color gamut that can be read (captured) by the image reading unit 3 and the color gamut (color reproduction gamut) that can be output by printing in the copying machine 1 are compared, the image reading unit 3 may be wider. Many. The copying machine 1 according to the present embodiment performs printing (copying) based on the image data obtained by the image reading unit 3. For this reason, matching must be performed between the color gamut of the image reading unit 3 and the color reproduction range of the image forming unit 5a.

  Therefore, in the color conversion table T for conversion from Lab to CMYK, the color gamut on the Lab space of the image reading unit 3 output from the first color space conversion unit 81 can be output by the copying machine 1. The entire color gamut of the image reading unit 3 is compressed or expanded so as to be within the range, and mapping (rendering intent) is performed so that it falls within the color gamut that the copying machine 1 can output, and conversion from Lab to CMYK A color conversion table T is generated.

  Here, as described above, the color gamut in reading by the image reading unit 3 is fixed with little fluctuation, and the color gamut in the Lab color space that can be output by the first color space conversion unit 81 is fixed. . On the other hand, for example, the color gamut (color reproduction range) in the Lab color space that can be output by printing by the copying machine 1 by forming the patch P of each color of the maximum density is also grasped. Therefore, first, a color conversion table T for converting the color gamut in the Lab color space that can be output by the copying machine 1 into the CMYK color space is prepared. In the actual color conversion process, the second color space conversion unit 82 temporarily performs processing for compressing or expanding the color gamut of the image reading unit 3 into a color gamut that can be output by the copying machine 1 in the Lab color space. After that, the color conversion process from Lab to CMYK may be performed using the color conversion table T.

(Color gamut variation)
Next, a change in the color reproduction range of the image forming unit 5a according to the embodiment of the present invention will be described with reference to FIG. FIG. 9 is a graph showing an example of a change in the color reproduction range of the image forming unit 5a according to the embodiment of the present invention, where (a) is a graph of the ab plane and (b) is a graph of the La plane.

  Usually, the maximum density that can be output (printed) by the copying machine 1 is predetermined for each model so that the color reproduction range in printing is constant. Therefore, the maximum density that can be output (printed) by the copying machine 1 is adjusted at the time of factory shipment or inspection at the time of manufacture. Even after the copying machine 1 is installed, the maximum density that can be output from the copying machine 1 can be adjusted by calibration. For example, in the graph of FIG. 9, an area surrounded by a circle is an example of a color gamut on the Lab space (target) to be output by the copying machine 1 of this embodiment in design.

  However, the density of the toner image formed by the image forming portion 5a changes (for example, the maximum density decreases) with time, such as temperature, humidity, wear of the photosensitive drum 52, and deterioration of the developer. For example, in the graph of FIG. 9, an area surrounded by a triangle is an example of a color gamut in the Lab space that the copying machine 1 of the present embodiment can output at present due to a change with time. The alphabets attached to the ● and ▲ marks in each graph of FIG. 9 are Y = yellow, G = green, C = cyan, B = blue, M = magenta, R = red, W = white, respectively. , K = black.

  For example, as shown in the upper graph of FIG. 9, the image forming unit 5a of the present embodiment forms a toner image using toners of C, M, Y, and K. The maximum density that can be output with toner of each color Decreases, the maximum density of G, B, and R formed by mixing the toners also decreases, and the color gamut becomes narrower as shown by the region surrounded by ▲.

  In general, the color conversion table T from the Lab color space to the CMYK color space is generated so that the gradation is appropriately expressed in accordance with the state of being output (printed) at the target maximum density by design. The However, if the maximum density of the toner image formed by the image forming unit 5a changes with time, the color gamut becomes narrower, so that gradation continuity is lost and gradation differences are lost. Crushing may occur.

(Generation of multiple types of color conversion tables T and color conversion tables T)
First, in the copying machine 1 of this embodiment, a plurality of types of color conversion tables T from predetermined Lab color spaces to CMYK color spaces are stored in the memory 84 and the storage unit 9b. One of them is a color conversion table T that is generated so that gradation is appropriately expressed in accordance with a state in which it is output (printed) at a target maximum density by design. In other words, the color conversion table T is used when all the comparison coefficients are 1.0.

  However, in the copying machine 1, a change in the maximum density that can be output occurs with a change with time or calibration. In order to cope with this change in the maximum density that can be output, the copier 1 of this embodiment stores a plurality of types of color conversion tables T. Therefore, the relationship among the multiple types of color conversion tables T will be described. The memory 84 (or the storage unit 9b) of the image processing unit 8 of the copier 1 according to the embodiment of the present invention stores a color conversion table T for conversion from Lab to CMYK corresponding to the combination of comparison coefficients. Is done.

  In the following description, a case where the comparative coefficient is 0.7 or less is used as a guideline for parts replacement or inspection. That is, if the lower limit is 0.7 and printing is not permitted below 0.7, the Lab → CMYK color conversion table T when the C, M, Y, and K comparison coefficients are all 0.7 is obtained. Prepared in advance.

  Here, when both of the comparison coefficients are 0.7, the color gamut in the Lab color space at the maximum density that can be output (printed) by the copying machine 1 in this state can be grasped in advance. For example, C, M, and Y when a patch P of each color toner is printed at a density corresponding to 0.7 for each of the comparison coefficients and each of the comparison coefficients is 0.7 by the colorimeter. , K may be grasped on the Lab space. If the coordinates of C, M, Y, and K when the coefficient for comparison is 0.7 are known, the coordinates of R, G, and B are also obtained based on the blending ratio (contribution rate) of C, M, and Y.

  Further, when the comparison coefficient is 1.0, the length of the straight line connecting the origin of the Lab color space and the coordinates of each point of C, M, Y, and K (distance between the origin and each coordinate) is 1. . The coordinates when the straight line connecting the origin and the coordinates of each point is 0.7 (when shortened by 30%) can be obtained by calculation. For R, G, and B, the coordinates when the distance between the origin and each coordinate when the comparison coefficient is 1.0 are shortened by 30% can also be obtained by calculation.

  As described above, if the coordinates of C, M, Y, K, R, G, and B can be grasped by the use and calculation of the colorimeter, in the Lab color space when the comparison coefficient is 0.7 for each color. The current color gamut that can be output can be grasped. Note that the color gamut when the total comparison coefficient is 0.7 falls within the color gamut when the total comparison coefficient is 1.0.

  As described above, the color gamut that can be output at present is often narrowed due to changes over time or the like, but it may be wider than a color gamut that is predetermined in design. For example, after calibration, the maximum density that can be output may be increased by voltage control such as development bias. Also, if the temperature and humidity are different, the maximum concentration that can be output may increase. When the chart C is read and adjusted, the density may be excessively higher than the maximum density predetermined in design (for example, about 1.15 to 1.2 in terms of comparison coefficient).

  Accordingly, considering that the color gamut that can be output by the copying machine 1 is expanded at present, the comparison coefficients are (C, M, Y, K) = (1.2, 1.2, 1.2, 1). It is also possible to prepare a color conversion table T in the case of .2). As a result, a smooth gradation expression can be achieved in response to a case where the color gamut that can be output is wider than the designed color gamut.

  The color conversion table T when the color gamut that can be output is wider than the designed color gamut is, for example, for comparison of C, M, Y, and K by a colorimeter or calculation as in the case where the color gamut is narrow. C, M, Y, K and R, G, B coordinates in the Lab space when the coefficient is 1.2 are grasped. Then, the current color gamut that can be output is grasped. Then, with the coordinates of C, M, Y, and K as vertices, the color gamut of the image reading unit 3 is enlarged or reduced as a whole, and mapping is performed so as to drop into the output gamut, while performing color conversion. The table T may be generated.

  In this way, each color conversion table T compresses or expands the color gamut of the image reading unit 3 in the second color space (Lab color space) as a whole in accordance with the obtained combination of coefficients of each patch P, The color gamut of the image reading unit 3 in the second color space is determined to be within the color gamut in the second color space that can be output by the copying machine 1 (printing device). If the color conversion table T includes a combination of comparison coefficients having the lowest output density at present and a combination of comparison coefficients having the highest density, the colors of the other combinations of comparison coefficients can be obtained. The conversion table T can be generated by interpolation calculation. In the interpolation calculation, a color conversion table T having all comparison coefficients of 1.0 can be used.

  For example, the maximum density C, M, Y, K patch P is read and the comparison coefficients are (C, M, Y, K) = (0.85, 0.85, 0.85, 0.85). Take the case as an example. The color gamut of this combination is an ideal color gamut in the design of the comparison coefficients (C, M, Y, K) = (1.0, 1.0, 1.0, 1.0). Then, the comparison coefficients fall within the color gamut range in the combination of (C, M, Y, K) = (0.7, 0.7, 0.7, 0.7). Therefore, the color conversion table T in the combination of the comparison coefficients (C, M, Y, K) = (1.0, 1.0, 1.0, 1.0) and the comparison coefficients (C, M, Y, K) = (0.7, 0.7, 0.7, 0.7) based on the color conversion table T, the comparison coefficient is (0.85, 0. 85, 0.85, and 0.85) can be generated.

  For example, the comparison coefficient 0.85 is an intermediate value between the comparison coefficient 1.0 and the comparison coefficient 0.7. For the same CMYK value, each Lab value of the color conversion table T whose comparison coefficients are all 1.0 and each Lab value of the color conversion table T when the comparison coefficients are all 0.7 are added to 2 respectively. By dividing by, each value of Lab with respect to the CMYK value when the coefficient for comparison is 0.85 is obtained. If this calculation is repeated, the color conversion table T from Lab to CMYK when the comparison coefficient is 0.85 is completed.

  In the copying machine 1 according to the present embodiment, in addition to the color conversion table T (total comparison coefficient is 1.0) which is a standard in the case where the density that can be output is ideal in design, the minimum allowable (for all comparison) If there is a color conversion table T in a state where the coefficient is 0.7) and the highest (all comparison coefficients are 1.2), a color conversion table T of any combination of comparison coefficients can be generated by interpolation calculation. . However, for example, the comparison coefficient is (C, M, Y, K) = (0.7, 1.0, 1.0, 1.0) in the memory 84 or the storage unit 9b of the image processing unit 8, for example. ), (1.2, 1.0, 1.0, 1.0), (0.9, 0.9, 0.9, 0.9), (0.8, 0.8, 0.8) , 0.8), a color conversion table T of various combinations of comparison coefficients may be stored. If there are more color conversion tables T, the color conversion table T can be determined according to the combination of the comparison coefficients without interpolation calculation, and can also be used for interpolation calculation.

(Switching and generating color conversion table T)
Next, an example of switching and generation control of the color conversion table T in the copying machine 1 according to the embodiment of the present invention will be described with reference to FIG. FIG. 10 is a flowchart C showing an example of switching control of the color conversion table T in the copying machine 1 according to the embodiment of the present invention.

  First, the start in FIG. 10 is when the switching control of the color conversion table T from the Lab color space to the CMYK color space is started. For example, the switching control of the color conversion table T is performed, for example, when a predetermined number (several hundred to several thousand) is printed from the previous switching of the color conversion table T, or when the user or service person performs color conversion on the operation panel 1b. This may be performed when switching execution of the table T is instructed. Further, the color conversion table T may be switched when the calibration of the copying machine 1 is completed so that it is output at the maximum density determined by design. That is, after the calibration is completed, the image forming unit 5a forms a patch P having a solid density of C, M, Y, and K, and the density sensor 7 (density measuring unit) forms the formed C, M, Y, and K. The second color space conversion unit 82 measures the density of the patch P having a solid density, and uses the color conversion table T to be used based on the combination of the coefficients of the patches P of each color measured by the density sensor 7 (density measurement unit). It may be changed.

  When the switching control of the color conversion table T is started, the control unit 9 causes the image forming unit 5a to form the solid density patch P of each color (C, M, Y, K) (step # 1). Transfer is performed on the transfer belt 61 (step # 2). Then, the density sensor 7 reads the solid density patch P (step # 3). At this time, a calibration chart C may be generated and read by the density sensor 7.

  Next, from the output value obtained when the density sensor 7 reads the solid density patch P of each toner, the control unit 9 or the image processing unit 8 compares the C, M, Y, and K colors for comparison. Is obtained (step # 4). Then, the second color space conversion unit 82 confirms whether the Lab to CMYK color conversion table T that matches the combination of the comparison coefficients is stored in advance in the memory 84 or the like (step # 5). The color conversion table T may be stored in the memory 84 or the storage unit 9b, and the color conversion table T may be read from the memory 84 or the storage unit 9b. A case where the conversion table T is stored will be described.

  If the Lab-to-CMYK color conversion table T that matches the combination of comparison coefficients is stored in advance in the memory 84 or the like (Yes in step # 5), the second color space conversion unit 82 determines the comparison coefficient. A color conversion table T that matches the combination is selected (step # 6 → end). When actually performing color conversion processing, such as when copying, the second color space conversion unit 82 performs conversion processing from the Lab color space to the CMYK color space using the selected color conversion table T. . For example, when the main power is turned on, the color conversion table T selected from the memory 84 or the like is read and set. That is, the second color space conversion unit 82 performs color conversion processing using the color conversion table T that matches the combination of coefficients of each patch P measured by the density sensor 7 (density measurement unit).

  On the other hand, if the Lab to CMYK color conversion table T that matches the comparison coefficient combination is not stored in advance in the memory 84 or the like (No in step # 5), the color conversion table generation unit 83 performs color calculation by interpolation. A conversion table T is generated (step # 7). That is, when there is no color conversion table T that matches the obtained combination of comparison coefficients, the color conversion table generation unit 83 outputs the copy machine 1 at present based on the obtained combinations of coefficients of all patches P. A color conversion table T having a wider color gamut than a possible color gamut and a color conversion table T having a narrow color gamut are selected, and a new color conversion table T is generated by interpolation using the two color conversion tables T.

  For example, for the comparison coefficient of the patch P of each color of the maximum density that can be output at present, the color conversion table T of the comparison coefficient that is dark in all colors C, M, Y, and K and the light combination in all colors The color conversion table generation unit 83 generates a new color conversion table T by interpolation using the color conversion table T of the comparison coefficients (step # 7). When actually performing color conversion processing such as copying, the second color space conversion unit 82 uses the newly generated color conversion table T to perform conversion processing from the Lab color space to the CMYK color space. Do. That is, the second color space conversion unit 82 performs color conversion processing using the color conversion table T generated by the color conversion table generation unit 83. The newly generated color conversion table T is stored in, for example, the memory 84 (step # 8 → end). For example, when the main power is turned on, the selected color conversion table T is read from the memory 84 and set.

  In this way, the second color space conversion unit 82 of the present embodiment does not fix the color conversion table T to be used, and the storage unit 9b appropriately prints according to the maximum density that the printing device can output at present. A plurality of color conversion tables T determined so as to express various gradations are stored. The second color space conversion unit 82 performs color conversion processing by switching the color conversion table T to be used according to the density of the patch P formed with the maximum density (solid density) that the printing device can output at present. Do. Therefore, the image processing apparatus 2 can perform color conversion processing so that appropriate gradation expression is made within a range that the printing device can reproduce at present.

  The number of combinations of coefficients indicating the density of C (cyan), M (magenta), Y (yellow), and K (black) patches is innumerable, and the more color conversion tables T are prepared, the storage unit 9b. The amount of data to be stored becomes enormous. Therefore, the color conversion table generation unit 83 generates a new color conversion table T by interpolation calculation. Therefore, the amount of data stored in the storage unit 9b can be reduced. In addition, printing is performed based on image data that has been subjected to color conversion processing so that appropriate gradation expression can be made within a range that the printing device can output at present. Therefore, even if the maximum density of the toner image that can be output at present is thinner or darker than the ideal maximum density in design, an image forming apparatus (for example, a copying machine) that can output a printed matter in which gradation representation is appropriately performed 1) can be provided.

  In addition, the maximum density that can be output from the image forming apparatus is corrected and recovered by calibration in which various voltages are adjusted so as to return to an ideal state in terms of design. However, even if calibration is performed, it does not always return to the maximum density predetermined in design, and if the color conversion table T is fixed, gradation skipping or gradation collapse may occur. However, according to this configuration, with the color gamut adjusted and recovered, the second color space conversion unit 82 is based on the maximum density patch P of each color according to the maximum density that the image forming apparatus can output at present. Since color conversion processing is performed by switching the color conversion table T to be used, the image forming apparatus can output a printed matter that can be said to have the highest image quality at present.

  Although the embodiments of the present invention have been described above, the scope of the present invention is not limited to these embodiments, and various modifications can be made without departing from the spirit of the invention.

  The present invention is applicable to an image processing apparatus and an image forming apparatus that perform color space conversion processing.

1 Copier (printing device, image forming apparatus) 3 Image reading unit (input device)
2 Image processing apparatus 5a Image forming unit (printing device)
DESCRIPTION OF SYMBOLS 51 Exposure apparatus 52 Photosensitive drum 53 Charging apparatus 54 Developing apparatus 6 Intermediate transfer part 7 Density sensor (density measurement part)
8 Image processing unit 81 First color space conversion unit 82 Second color space conversion unit 83 Color conversion table generation unit 84 Memory 9 Control unit 9b Storage unit P Patch T Color conversion table

Claims (4)

A first color space conversion unit that converts a first color space signal dependent on an input device into a second color space signal independent of the device;
A second color space conversion unit for converting the second color space signal into a CMYK color space signal subordinate to the printing device based on a color conversion table;
A density measuring unit for measuring the density of patches of solid density of C, M, Y, K output from the printing device;
Based on the output of the density measurement unit, a control unit for obtaining a coefficient indicating the density of each patch output from the printing device;
A plurality of types, a memory for storing the predetermined color conversion table;
Each of the color conversion tables compresses or expands the color gamut of the input device in the second color space as a whole according to the obtained combination of the coefficients of the patches, and in the second color space. A color gamut of the input device is determined to fall within a color gamut in the second color space that can be output by the printing device;
The image processing apparatus according to claim 2, wherein the second color space conversion unit performs a color conversion process using the color conversion table that matches a combination of coefficients of the patches measured by the density measurement unit. Based on the two color conversion tables, a color conversion table generation unit that generates a new color conversion table by performing an interpolation operation;
When there is no color conversion table that matches the obtained combination of coefficients,
The color conversion table generator generates a color conversion table having a color gamut wider than a color gamut that can be output by the printing device and a color conversion of a narrow color gamut based on a combination of all the obtained coefficients of the patches. Select a table, generate a new color conversion table by interpolation using the two selected color conversion tables,
The image processing apparatus according to claim 1, wherein the second color space conversion unit performs a color conversion process using the color conversion table generated by the color conversion table generation unit. Including the image processing apparatus according to claim 1,
An image reading unit that reads a document and outputs an RGB color space signal as the input device, and an image forming unit that forms a toner image using cyan, magenta, yellow, and black toners as the printing device. Image forming apparatus. An intermediate transfer unit that receives a primary transfer of a toner image formed by the image forming unit and performs a secondary transfer on a sheet;
A control unit that controls the image forming unit and the intermediate transfer unit;
The image forming unit includes a photosensitive drum as an image carrier, a charging device that charges the photosensitive drum, an exposure device that forms an electrostatic latent image on the charged photosensitive drum, and an electrostatic latent image. A developing device for supplying toner to the toner and developing the toner image;
The control unit is any one of a charging potential of the photosensitive drum by the charging device, a developing bias applied by the developing device, and a transfer bias for transferring a toner image at the intermediate transfer unit. Or calibration to adjust more than one,
After completion of the calibration,
The image forming unit forms the patch having a solid density of C, M, Y, and K,
The density measuring unit measures the density of the formed solid patches of C, M, Y, and K,
The image forming apparatus according to claim 3, wherein the second color space conversion unit changes the color conversion table to be used based on a combination of the coefficients of the patches of the respective colors measured by the density measurement unit. .

Images