JP4935689B2 - Image forming apparatus and image forming apparatus calibration method - Google Patents

Description

  The present invention relates to an image forming apparatus such as a printer and a calibration method for the image forming apparatus.

  Conventionally, as a technique for calibrating the output characteristics of an image forming apparatus such as a printer, a patch pattern represented by a predetermined gradation value is printed on a printing medium and then read by a scanner or a line-type colorimeter. The print density based on the density difference between the density of the patch pattern and the predetermined density (printer density at the time of shipment) that is the target that is the gradation value on which the patch pattern is printed. A method for correcting the output characteristics is widely known (see, for example, Patent Document 1).

  In the apparatuses described in Patent Document 1 and Patent Document 2, a position detection mark is printed on a print medium together with a patch pattern, and calibration is performed based on the position where the position detection mark is detected. That is, in the read image obtained by reading the patch sheet, the position where the position detection mark is read is detected, and the image area of the patch pattern in the read image is accurately recognized, so that the calibration is performed accurately. . Note that the position detection mark described in Patent Document 2 is printed with toner of each color of C (cyan), M (magenta), Y (yellow), and K (black).

JP 2006-293213 A JP 2007-47744 A (refer to page 13, paragraphs (0074) to (0077) and FIGS. 10 and 11).

  However, for example, when electrophotographic image formation is performed, if the position detection mark is printed as described in Patent Document 1, the position detection mark partially lacks density due to density unevenness caused by transfer failure or the like. It sometimes happened. For this reason, in the read image obtained by reading the patch sheet, there is a problem that the position of the position detection mark cannot be accurately detected, calibration cannot be performed, or calibration cannot be performed appropriately.

  The technique described in Patent Document 2 exposes a patch pattern with a stable exposure apparatus by exposing a position detection mark that also serves as a dummy pattern before exposing the patch pattern. It is not intended to improve the position detection accuracy of the position detection mark by printing the position detection mark with toner of each color.

  SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms or application examples.

  Application Example 1 An image forming apparatus for forming an image using a black color material, a cyan color material, a magenta color material, and a yellow color material, and an image for calibration based on a predetermined gradation value And an image forming means for forming a position detection image arranged at a predetermined position with respect to the calibration image using the color material, and recording the calibration image and the position detection image on a print medium; and A calibration unit that calibrates a density output characteristic of the image forming unit based on a density difference between a density of the calibration image recorded on the print medium and a density determined in advance for the calibration image; The image forming apparatus is characterized in that the position detection image having a substantially maximum density is formed for each of the plurality of color materials, and the position detection images of the respective colors are superimposed and recorded on the print medium.

  According to this configuration, the position detection image printed on the printing medium is recorded by overlapping the images having the substantially maximum density for each of the black color material, the cyan color material, the magenta color material, and the yellow color material. Therefore, a decrease in the density of the position detection image due to uneven density is suppressed. Therefore, in the image of the read image data, when searching for an image area having a density corresponding to the position detection image, the possibility that the search of the image area corresponding to the position detection image will fail due to insufficient density of the position detection image is reduced. The image region corresponding to the position detection image can be detected with high accuracy. In addition, since the position detection accuracy of the position detection image is improved, it is possible to more accurately detect the density of the image area corresponding to the calibration image in the image of the read image data and perform calibration more appropriately. .

  Application Example 2 In the image forming apparatus, the calibration image including a plurality of patches each having a predetermined color and gradation value, and a combination of colors and gradation values different from the plurality of patches. A storage unit for storing calibration image data expressing the position detection image with gradation values of red, green, and blue; and gradation values of the calibration image data, gradation values of cyan, magenta, yellow, and black And the first correspondence table in which the gradation value of the position detection image is associated with the substantially maximum gradation value for each color of cyan, magenta, yellow, and black, and the first correspondence table. A conversion unit that performs a conversion process on the calibration image data, and the image forming unit includes the calibration image based on the calibration image data after the conversion process. Image forming apparatus and forming the pre said position detection image.

  According to this configuration, the gradation value of the calibration image data is converted into gradation values corresponding to the black color material, the cyan color material, the magenta color material, and the yellow color material by a single conversion process according to the correspondence table. In addition to the conversion, the gradation value of the position detection image can be converted to a substantially maximum gradation value for each color of cyan, magenta, yellow, and black. Therefore, the detection accuracy when searching for an image region corresponding to the position detection image can be improved by simple processing according to the correspondence table.

  [Application Example 3] In the image forming apparatus, the first correspondence table is the calibration image data, and the position detection image is a predetermined floor excluding gradation values having the same value for each color of red, green, and blue. In the first correspondence table, gradation values having the same value for each color of red, green, and blue are associated with gradation values corresponding to a single black color material, and the position detection image is represented by the first correspondence table. The predetermined gradation value to be represented is associated with a gradation value representing a composite black by a cyan color material, a magenta color material, and a yellow color material.

  According to this configuration, when the position detection image formed by the black color material has insufficient density due to uneven density, the position detection of the composite black using the cyan color material, the magenta color material, and the yellow color material is performed. By overlapping the images, the insufficient black density is compensated for, so that the influence of density unevenness on the search for the image area corresponding to the position detection image is reduced. Therefore, even if the density reduction due to density unevenness occurs in the position detection image formed by the black color material, the image area corresponding to the position detection image can be detected with high accuracy.

  Application Example 4 The image forming apparatus further includes a second correspondence table in which gradation values of red, green, and blue are associated with gradation values of cyan, magenta, yellow, and black, and the conversion The image forming apparatus is characterized in that the conversion processing according to the second correspondence table is performed during normal printing, and the conversion processing according to the first correspondence table is performed during calibration.

  According to this configuration, by switching the correspondence table used for the conversion process between normal printing and calibration execution, printing that records the position detection image by superimposing the images of each color at the time of calibration execution is performed. Can do. For example, even if the image forming apparatus has a configuration in which it is difficult to change the processing contents by implementing functions such as conversion processing in hardware, the first correspondence table used during normal printing is used as the first correspondence table when performing calibration. By switching to this correspondence table, it is possible to realize calibration printing and normal printing in which position detection images of cyan, magenta, yellow, and black are overlaid and recorded.

  Application Example 5 In the image forming apparatus, the image forming apparatus includes reading means that generates read image data by reading the calibration image and the position detection image recorded on the print medium. The read image data is bitmap data having a gradation value for each pixel of a large number of pixels arranged in a matrix, and the calibration unit pays attention to the pixels of the read image data in order. By determining whether or not the gradation value corresponding to the position detection image is based on the gradation value of the target pixel and the gradation value of the pixel adjacent to the attention pixel, the position detection image The corresponding image area is searched, and the position of the read image data to be acquired as the density of the calibration image is detected from the relative position with the searched position detection image to the print medium. Image forming apparatus and acquires a concentration of the recording has been calibration image.

  According to this configuration, in addition to the gradation value of the target pixel, whether or not the gradation value corresponds to the position detection image is determined based on the gradation value of the pixel adjacent to the target pixel. In the position detection image, the influence of partial density reduction due to density unevenness is reduced, and an image region corresponding to the position detection image can be detected with higher accuracy.

  Application Example 6 In the image forming apparatus, the calibrating unit is configured such that an average value of a gradation value of the target pixel and a gradation value of a pixel adjacent to the target pixel is a gradation value corresponding to the position detection image. An image forming apparatus that searches for an image area corresponding to the position detection image by determining whether or not

  According to this configuration, by using the average value of the pixel of interest and the pixels located around the pixel of interest for determining whether the pixel of interest is included in the position detection image, a portion due to uneven density in the position detection image Even if there is a significant decrease in density, the amount of decrease in density used for determination is reduced, so that the image area corresponding to the position detection image can be detected with higher accuracy.

  [Application Example 7] A calibration method for an image forming apparatus that forms an image using a black color material, a cyan color material, a magenta color material, and a yellow color material. An image formation for forming a calibration image based thereon and a position detection image arranged at a predetermined position with respect to the calibration image using the color material, and recording the calibration image and the position detection image on a print medium A calibration step for calibrating density output characteristics of image formation based on a density difference between a density of the calibration image recorded on the print medium and a density predetermined for the calibration image, and In the image forming step, the position detection image having a substantially maximum density is formed for each of the plurality of color materials, and the position detection images of the respective colors are superimposed and recorded on the print medium. Calibration method for an image forming apparatus.

  According to this configuration, when searching for an image area having a density corresponding to the position detection image, the possibility of failure in searching for the image area corresponding to the position detection image due to insufficient density of the position detection image is reduced. The image area corresponding to the detected image can be detected with high accuracy. In addition, since the position detection accuracy of the position detection image is improved, it is possible to more accurately detect the density of the image area corresponding to the calibration image in the image of the read image data and perform calibration more appropriately. .

  Hereinafter, embodiments of the present invention will be described based on examples.

(First embodiment)
FIG. 1 is a diagram illustrating a hardware configuration of the copying apparatus according to the first embodiment. As shown in FIG. 1, the copying apparatus 1 includes a print engine (image forming unit) 10, a scanner unit (reading unit) 20, an operation panel 30 having a display unit such as an LCD and various switches, a controller 40, It has. The copying apparatus 1 prints a patch sheet S including a plurality of patches by the print function of the print engine 10, and the print engine 10 has the read density of each patch read by the scanner function of the scanner unit 20. The density output characteristic is calibrated. Hereinafter, each configuration of the copying apparatus 1 will be described.

  The print engine 10 is a part that performs electrophotographic image formation. As shown in FIG. 2, the photoconductor drum 11, the charging unit 12, the exposure unit 13, the developing unit 14, and the primary transfer unit 15. The intermediate transfer member 16, the secondary transfer unit 17, and the fixing unit 18 are provided.

  The photosensitive drum 11 is an image carrier for carrying a latent image, and has a cylindrical conductive substrate and a photosensitive layer formed on the outer peripheral surface thereof, and is rotatable with respect to the central axis. It is configured. At the time of printing, the charging unit 12 charges the surface of the photosensitive drum 11, and the exposure unit 13 irradiates the charged photosensitive drum 11 with a beam from a light source such as a built-in laser or LED array. Beam irradiation from the exposure unit 13 is controlled in accordance with an image signal input from the controller 40, whereby an electrostatic latent image is formed on the surface of the photosensitive drum 11.

  The developing unit 14 is a developing rotary that has mounting portions 14a to 14d to which cartridges 19 that store toner that is a color material are detachably mounted, and that can rotate about a rotating shaft 14e. Cartridges 19a to 19d that contain black (K), yellow (Y), magenta (M), and cyan (C) toners are mounted on the mounting portions 14a to 14d of the developing unit 14, respectively. When the developing unit 14 rotates, the necessary cartridge 19 comes close to the photosensitive drum 11 and the latent image formed on the surface of the photosensitive drum 11 is developed with toner, whereby a single color image of the toner color is formed.

  The primary transfer unit 15 transfers the monochrome image formed on the photosensitive drum 11 to the intermediate transfer member 16 that is rotationally driven at the same peripheral speed as the photosensitive drum 11. At the time of color printing, the primary transfer unit 15 transfers the CMYK single-color images developed by the developing unit 14 on the surface of the intermediate transfer body 16 so that the color images on which the CMYK single-color images are superimposed are intermediate transferred. Formed on the body 16. Then, the secondary transfer unit 17 transfers the color image formed on the intermediate transfer body 16 to a printing medium such as paper, and the fixing unit 18 welds the color image transferred onto the printing medium to the medium and becomes permanent. Printing is performed by recording as an image.

  Next, the configuration of the scanner unit 20 will be described. As shown in FIG. 1, the scanner unit 20 includes an original table 21 on which an original is placed, a carriage 22 on which an image receiving element such as a CCD is mounted, a scanning mechanism 23 that scans the carriage 22 along the original table 21, and the like. It has the composition of. The scanning mechanism 23 scans the carriage 22 over the entire document, and the CCD of the carriage 22 reads the document placed on the document table 21, so that the RGB gradation value (hereinafter also referred to as “RGB value”) is used. The represented bitmap data (hereinafter referred to as “read image data”) is generated. In this embodiment, the read image data is gradation data of 256 gradations (“0” to “255”) of each RGB color, and the gradation value of each RGB color “255” is white with the minimum density, It is assumed that each RGB color gradation value “0” corresponds to black having the maximum density. The read image data generated in this way is transferred from the scanner unit 20 to the controller 40.

  The controller 40 is a part that controls the print engine 10, the scanner unit 20, and the operation panel 30, and includes a CPU 41, a ROM 42, a RAM 43, an EEPROM (storage unit) 44, a communication I / F 45, and an engine I / F 46. , A scanner I / F 47 and a panel I / F 48.

  The ROM 42 stores in advance a control program for controlling the copying apparatus 1. The RAM 43 is used as a reception buffer, a working memory, and the like. The CPU 41 is a main control device of the controller 40, and executes various control programs stored in the ROM 42 by using the RAM 43 to perform various operations performed in the copying apparatus 1. Control processing. In the EEPROM 44, patch image data (calibration image data) PD, target data TD, and a color conversion table TT are stored in advance. The target data TD is data indicating correspondence between a predetermined density of each color of CMYK and an initial gradation value set in advance in the printer for reproducing the density with respect to the predetermined density (color value). Yes, it is used for the calibration process described later. The EEPROM 44 also stores a color correction table CT generated by the calibration process.

  The communication I / F 45 is an interface connected to the host computer 2 so that data communication is possible. The print data transmitted from the printer driver of the host computer 2 is received by the communication I / F 45, and the received print data is stored in the reception buffer of the RAM 43. The scanner I / F 47 is an interface connected to the scanner unit 20, and the panel I / F 48 is an interface connected to the operation panel 30.

  The engine I / F 46 is an interface part to the print engine 10, and performs processing for converting image data into data in a format that can be processed by the print engine 10 and outputting the data to the print engine 10. Specifically, the engine I / F 46 performs processing such as color conversion processing, halftone processing, and pulse width modulation on the input of RGB format image data, and outputs the data after pulse width modulation for each CMYK color. It consists of an output specific integrated circuit (ASIC).

  Next, the functional configuration of the controller 40 will be described. FIG. 3 is a diagram illustrating a functional configuration of the controller 40. As shown in FIG. 3, the controller 40 includes a drawing processing unit 100, a color conversion unit (conversion means) 110, a color correction unit 120, a halftone processing unit 130, and a color correction table generation unit 140. is doing.

  The drawing processing unit 100 reads the print data stored in the reception buffer, interprets a command described in the print data, and develops the interpretation result into a bitmap image to generate RGB format image data. It is a part to do.

  The color conversion unit 110 converts RGB format image data into CMYK format image data by performing color conversion processing based on the color conversion table TT stored in the EEPROM 44. The EEPROM 44 stores in advance a color conversion table (second correspondence table) TTa for normal printing and a color conversion table (first correspondence table) TTb for calibration. The color conversion unit 110 selectively refers to the color conversion tables TTa and TTb stored in the EEPROM 44 depending on whether the job to be executed is normal printing or calibration patch sheet S printing. Thus, color conversion for normal printing or color conversion for calibration is performed. One of the features of the copying apparatus 1 is that it includes a color conversion table TTb for calibration in addition to the color conversion table TTa for normal printing processing, and the reason will be described later.

  The color correction unit 120 performs color correction processing on the image data after color conversion processing according to the color correction table CT stored in the EEPROM 44, thereby correcting the gradation value according to the density output characteristics of the print engine 10. This is a part for adjusting the output density of each color of CMYK by the print engine 10.

  The halftone processing unit 130 performs processing to convert image data corresponding to dots formed by the print engine 10 by performing halftone processing on the image data in the CMYK format. The image data after the halftone process is subjected to pulse width modulation by the engine I / F 46, and the image signal after the pulse width modulation is output to the print engine 10.

  The controller 40 having the above-described functional configuration controls the printing of the patch sheet S performed at the time of calibration in addition to the normal printing performed on the print data. When the print data is received from the host computer 2, the controller 40 instructs the color conversion unit 110 to perform color conversion according to the normal color conversion table TTa for printing, and the print data stored in the reception buffer is processed. The normal printing process is controlled by causing the drawing processing unit 100, the color conversion unit 110, the color correction unit 120, and the halftone processing unit 130 to sequentially perform processing.

  Here, a color conversion table TTa used for color conversion in normal printing processing will be described. Since the RGB value and the CMY value are in a complementary color relationship, the CMY value can theoretically be obtained according to the following formulas: C = 255-R, M = 255-G, and Y = 255-B. However, since the γ characteristics of the print engine 10 are different depending on the engine, a color conversion table that does not necessarily follow the above theoretical formula is designed according to the characteristics of the print engine 10, and the color conversion table is copied in a state where the color conversion table is stored in the EEPROM 44. The device 1 is shipped. That is, in the color conversion table designed according to the engine characteristics, even if (R, G, B) = (0, 255, 255), (C, M, Y) = (255, 0, 0) is not necessarily required. For example, there are cases where the tone value is converted into a gradation value including a high density C value and a low density M value or Y value. Further, when 100% gradation value or high density gradation value is specified for all the CMY toners, the color of the image forming apparatus is suppressed in order to suppress the occurrence of blurring in which a large amount of toner is scattered. Black is added as a material, and the total toner amount is suppressed by performing color conversion processing such as under color removal and black color generation. For this reason, a color conversion table TTa taking into account undercolor removal and black color generation is designed.

  Here, in the case of (R, G, B) = (0, 0, 0), for example, a color such as (C, M, Y, K) = (250, 250, 250, 0) which is a composite black. Whether the color is converted to black (K) single color (C, M, Y, K) = (0, 0, 0, 255) or not is the type of image to be printed, such as an image or text And a color conversion table designed according to engine characteristics. Therefore, in the color conversion table TTa used for normal printing processing, (R, G, B) = (0, 0, 0) is not necessarily printed in black (K) single color. Also, (R, G, B) = (α, 255, 255) is cyan (C) monochrome printing, and (R, G, B) = (255, β, 255) is magenta (M) monochrome printing. (R, G, B) = (255, 255, γ) does not designate yellow (Y) single color printing.

  On the other hand, when executing calibration, it is desirable that the actual density on printing can be measured from a calibration image printed by designating a predetermined gradation value for each single color of CMYK. Here, for example, patch image data such as (C, M, Y, K) = (X, 0, 0, 0) is prepared as a calibration image for cyan (C), and halftone processing is performed. Printing is desirable. However, as described above, when RGB data is input, the engine I / F 46 executes processing from color conversion processing to halftone processing by the ASIC, so in order to support input of CMYK data, Although it is necessary to add another halftone processing unit that performs halftone processing on data, this makes it impossible to use existing hardware and increases the size of the hardware. In order to print each CMYK single color with an existing hardware configuration without increasing the hardware scale, a patch image is used so that each single color of CMYK after color conversion has a predetermined gradation value with a single color toner. It is necessary to design in advance the RGB value and color conversion table corresponding to the calibration image portion of the data PD. However, according to this color conversion table, the RGB values corresponding to the image portion for calibration are printed in a single color, so that the color output characteristics that should be output may be impaired in normal printing. .

  Therefore, the color conversion table TTa for normal printing is designed in consideration of undercolor removal, black color generation, engine characteristics, and the like, while the color conversion table TTb for calibration uses RGB for each of CMY. In the portion to be converted into a single color gradation value, each theoretical RGB value corresponding to CMY is converted into a CMY value, that is, the RGB value corresponding to each patch P in the calibration image portion is a single color. Designed to output with toner. Further, as will be described in detail later, the color conversion table TTb converts the RGB value satisfying R = G = B into a gradation value corresponding to printing with K single color. Then, when executing calibration, the controller 40 instructs the color conversion unit 110 to perform color conversion according to the color conversion table TTb for calibration instead of the normal color conversion table TTa for printing, and causes the EEPROM 44 to perform the calibration. Printing of the patch sheet S is controlled by causing the color conversion unit 110, the color correction unit 120, and the halftone processing unit 130 to sequentially process the stored patch image data PD.

  Here, the patch sheet S printed at the time of executing calibration will be described. As shown in FIG. 4, the patch sheet S includes a patch image (calibration image) PG including a large number of patches P arranged in a matrix, and a position detection mark arranged at a predetermined position with respect to the patch image PG. (Position detection image) M. Each patch P of the patch image PG has a predetermined color and density, and yellow (Y), magenta (M), cyan (C), black (in the row direction of the matrix (left-right direction in FIG. 4)). The patches P are arranged in 8 rows so that the order of K) is repeated twice. For the four columns on one side in the column direction (vertical direction in FIG. 4), the density gradually increases by a predetermined gradation value from the density corresponding to the gradation value “0” to the density corresponding to the gradation value “255”. In the other four columns, the patches P are arranged so that the density gradually decreases from the density corresponding to the gradation value “255” to the density corresponding to the gradation value “0”.

  The patch image data PD stored in the EEPROM 44 is RGB format bitmap data representing the patch image PG and the position detection mark M of the patch sheet S described above. In the patch image data PD, the cyan (C) patch P is (R, G, B) = (α, 255, 255), and the magenta (M) patch P is (R, G, B) = (255, β). 255), yellow (Y) patch P is (R, G, B) = (255, 255, γ), and black (K) patch P is (R, G, B) = (δ, δ, δ). ) RGB values. In the present embodiment, the patch image data PD is gradation data expressed in 256 gradations (0 to 255) for each color of RGB, and “α”, “β”, and “γ” are “0 to 254”, respectively. And “δ” is an integer of “0 to 255”.

  On the other hand, the RGB value of the position detection mark M is a predetermined value that is not used as any patch P. That is, the RGB values of the position detection mark M are (R, G, B) = (α, 255, 255), (255, β, 255), (255, 255, γ), (δ, δ, δ). (R, G, B) = (5, 3, 0) as an example. The RGB value of the position detection mark M is associated with the CMYK value having the substantially maximum gradation value for each color of CMYK. This is one of the features of this embodiment. As will be described later, the position detection mark M has a maximum gradation value or a maximum gradation value for each single color so that the position detection mark M can be accurately detected even if the position detection mark M has uneven density. A position detection mark M is obtained by printing the images by using close data.

  In the color conversion table TTb, since the RGB value (= (0, 0, 0)) corresponding to pure black is used for generating a patch of K single color, the RGB value is close to 0, and the value of each RGB color. RGB values that are not all the same are set as gradation values for position detection, and the RGB values are designed to be converted into gradation values that output a high density for each color in all colors of CMYK. What can be done in this way is a great effect of separating the color conversion table TTa for normal printing and the color conversion table TTb for calibration. In other words, in the color conversion table TTa for normal printing, even if (R, G, B) = (0, 0, 0), printing is performed with a single K color, or the processing of under color removal and black color generation is considered. This is because gradation values that are printed in composite black and that are close to the maximum density for all CMYK colors cannot be specified.

  That is, as shown in FIG. 5 as an example, in the color conversion table TTb for calibration, the RGB values (= (α, 255, 255)) corresponding to the cyan (C) patch P represent cyan (C). The CMYK value (= (255−α, 0, 0, 0)) and the RGB value (= (255, β, 255)) corresponding to the patch P of magenta (M) are CMYK values (= RGB values (= (255, 255, γ)) corresponding to the patch P of (0, 255-β, 0, 0)) and yellow (Y) are CMYK values (= (0, 0) representing yellow (Y). , 255-γ, 0)), and the RGB values (= (δ, δ, δ)) corresponding to the patch P of black (K) are CMYK values (= (0, 0, 0, δ) representing black (K). )). On the other hand, the RGB value (= (5, 3, 0)) corresponding to the position detection mark M is a CMYK value (in the example of FIG. 5, (C, M, Y, K)) that is substantially the maximum for each color of CMYK. = (253, 250, 251, 255)). The CMYK value is gradation data expressed with 256 gradations (0 to 255) for each color of CMYK.

  Therefore, when the color conversion unit 110 performs color conversion processing according to the color conversion table TTb for calibration, a single color image having a substantially maximum density for each color of CMYK is formed on the patch sheet S as the position detection mark M. Then, an image obtained by superimposing the respective monochrome images having the maximum density is recorded as the position detection mark M.

  The patch sheet S printed in this way is set on the document table 21 of the scanner unit 20 by the user, and when the scanner unit 20 reads the paper set on the document table, the patch sheet S is read by the gradation values of each RGB color. Read image data in a bitmap format representing an image is generated. The color correction table generation unit 140 acquires read image data from the scanner unit 20 via the scanner I / F 47 and performs a process of searching for an image region representing the position detection mark M in the image of the read image data. Then, the position of each patch P is specified from the position of the searched position detection mark M, and the density of each patch P is detected.

  Next, the color correction table generation unit 140 sets the actual density of each patch and each patch P reflecting the print engine characteristics when each patch is printed, with respect to the gradation value on which each patch P is printed. The color correction table CT is generated by comparing the density of the target data TD and obtaining a correction value so as to eliminate the density difference, and the generated color correction table CT is stored in the EEPROM 44. In normal printing performed thereafter, the color correction unit 120 performs color correction processing using the color correction table CT stored in the EEPROM 44, so that the output characteristics of the color and density of the print engine 10 become target characteristics. It is corrected to approach. That is, the color correction table generation unit 140 generates a color correction table CT and stores the color correction table CT in the EEPROM 44, and the color correction unit 120 performs color correction according to the stored color correction table CT. Calibration is performed. The color correction unit 120 and the color correction table generation unit 140 correspond to the calibration unit described in the claims.

  Next, processing at the time of executing calibration will be described in detail with reference to the flowchart of FIG. 6 is a process performed by the CPU 41 executing a control program stored in the ROM 42, and steps S120 to S150 are processes performed by the CPU 41 as the color correction table generation unit 140.

  For example, when an instruction to execute calibration is received from the printer driver of the host computer 2 or the operation panel 30, the process of FIG. 6 is started. When the process is started, first, the CPU 41 causes the print engine 10 to output the patch sheet S (step S100). Here, by instructing the color conversion unit 110 to perform color conversion on the patch image data PD stored in the EEEROM 44 using the color conversion table TTb for calibration, the patch image data PD is converted into CMYK image data. To convert the color. Then, by causing the halftone processing unit 130 to perform halftone processing on the CMYK image data after color conversion, and outputting the image signal subjected to pulse width modulation after the halftone processing to the print engine 10, the patch sheet S Printed. In order to accurately reflect the density output characteristics of the print engine 10 on the patch sheet S, the color correction process by the color correction unit 120 is not performed in step S100.

  Thus, when color conversion is performed according to the color conversion table TTb for calibration, the position detection mark M forms an image according to the CMYK value (= (253, 250, 251, 255)). As a result, in electrophotographic image formation, the monochrome image TG (K) (see FIG. 7 (a)) and cyan (C) gradation value with K toner according to the gradation value (= 255) of black (K). Monochromatic image TG (C) with C toner according to (= 253) (see FIG. 7B), Monochromatic image TG (M) with M toner according to magenta (M) gradation value (= 250) (FIG. 7C )), And a monochromatic image TG (Y) (see FIG. 7D) using Y toner in accordance with the gradation value (= 251) of yellow (Y) is primary-transferred at the same position on the intermediate transfer body 16. Is done. A color image CG (CMYK) (see FIG. 7E) obtained by superimposing the monochrome images TG (K), TG (C), TG (M), and TG (Y) of the position detection mark M is the intermediate transfer body 16. The position detection mark M is printed by forming a color image CG (CMYK) on the print medium and secondarily transferring it onto the print medium.

  Here, as shown in FIG. 7A, when the density unevenness UNI occurs in the monochromatic image TG (K) of the position detection mark M due to a transfer failure or the like in the primary transfer, the density decreases due to the density unevenness UNI. The single color image TG (C), the single color image TG (M), and the single color image TG (Y) are overlapped with each other, so that a partial density reduction of black (K) is caused by each single color image TG (C), It is supplemented by TG (M) and TG (Y). As described above, even if density unevenness occurs in each of the monochrome images TG (K), TG (C), TG (M), and TG (Y) of the position detection mark M, the color image CG (CMYK) is obtained. A partial decrease in density due to density unevenness is suppressed, and a sufficient density is secured over the entire area of the position detection mark M.

  Next, the CPU 41 causes the scanner unit 20 to read the patch sheet S and generate read image data of the patch sheet S (step S110). Here, for example, the CPU 41 causes the display unit of the operation panel 30 to display a message prompting the user to perform a document reading operation after setting the printed patch sheet S on the document table of the scanner unit 20. When the user sets the patch sheet S on the document table of the scanner unit 20 according to the displayed message and performs a reading operation of the document on the operation panel 30, the patch unit S is read by the scanner unit 20. Let Then, the scanner unit 20 outputs the read image data generated by reading the patch sheet S to the controller 40.

  Next, the CPU 41 performs a position detection mark search process for searching for an image area corresponding to the position detection mark M of the patch sheet S in the read image of the read image data (step S120).

  FIG. 8 is a flowchart showing the procedure of the position detection mark search process. When the position detection mark search process is started, first, the CPU 41 sets a pixel at a predetermined position (for example, the pixel at the upper left corner of the image) as a target pixel in the read image of the read image data (step S200), and sets the count value C. The initial setting is “C = 0” (step S210).

  Next, the CPU 41 determines whether or not each gradation value of the RGB value of the target pixel has a density less than a predetermined threshold corresponding to the color (black) of the position detection mark M (step S220). If each gradation value of the RGB values is less than the threshold value (step S220: Yes), the count value is incremented by 1 (step S230), and then the target pixel is scanned by one pixel in the raster direction, thereby creating a new pixel. (Step S240), the determination in Step S220 is performed again. That is, if there is a pixel to the right of the target pixel, the right adjacent pixel is set as the new target pixel, and if the target pixel is the right end pixel, the left end pixel of the next lower line is set as the new target pixel. After setting, the determination in step S220 is performed for a new target pixel.

  Here, as described in FIG. 7, the black (K) monochrome image TG (K), the cyan (C) monochrome image TG (C), the magenta (M) monochrome image TG (M), and the yellow (Y ) Of the single color image TG (Y) of the color detection CG overlap each other to form the color image CG (CMYK) of the position detection mark M, and thus uneven density such as uneven transfer occurs in any single color image TG. Even in this case, the insufficient density due to the uneven density is compensated by the other single-color image TG, and a sufficient density is secured. For this reason, for example, even when the density unevenness occurs in the monochrome image TG (K) of the position detection mark M due to the black (K) toner, the density of the read image data read from the patch sheet S is Each value of the RGB value of the target pixel (see FIG. 9C) in the region UNIG corresponding to the unevenness is sufficiently small, and it is determined in step S220 that the gradation value is less than the threshold value.

  In step S220, if any one of the RGB values is not less than the threshold value (step S220: No), the CPU 41 determines whether the count value C is “0” (step S250). . If the count value C is “0” (step S250: Yes), the target pixel is advanced by one in the raster direction and the next pixel is focused (step S240), and then the process proceeds to step S220. Thus, by repeating the processing of steps S220 to S250, the upper left pixel in the image region MG corresponding to the position detection mark M in the read image is first set as the target pixel (see FIG. 9A), and the raster When the target pixel advances in the direction, the number of pixels from the first target pixel is set to the count value C (see FIG. 9B).

  On the other hand, if the count value C is not “0” (step S250: No), the CPU 41 determines whether or not the count value C is a value corresponding to the width of the position detection mark M (step S260). If the count value C is a value corresponding to the width of the position detection mark M (step S260: Yes), the pixel one more right than the pixel at the right end of the image region MG corresponding to the position detection mark M is noticed. Since it can be determined that the pixel is in a state (see FIG. 9D), a pixel area corresponding to the count value C is detected as a pixel corresponding to the position detection mark M to the left of the target pixel (step S270). . If the count value C does not correspond to the width of the position detection mark M (step S260: No), the width of the region having RGB values less than the threshold does not correspond to the position detection mark M. It is determined that the region to the left of the target pixel is not a region corresponding to the position detection mark M, the target pixel is scanned by one pixel (step S290), and then the search is continued by returning to step S210.

  When the pixel corresponding to the position detection mark M is detected in step S270, the CPU 41 determines whether or not all the position detection marks M included in the patch sheet S have been detected (step S280). If all the position detection marks M have not been detected (step S280: No), the pixel of interest is scanned for one pixel (step S290), and the process returns to step S210 to search for the pixel corresponding to the position detection mark M. to continue. If all the position detection marks M included in the patch sheet S shown in FIG. 4 have been detected (step S280: Yes), the position detection mark search process is terminated, and the process returns to the process of FIG.

  Returning to the processing of FIG. 6, the CPU 41 determines whether or not the positions of the plurality of detected position detection marks M are the predetermined positions determined on the patch sheet S, so It is determined whether or not the setting position and orientation of the patch sheet S are correct (step S130). If the position of each detected position detection mark M is not a predetermined position, there is a possibility that the patch sheet S has been set upside down or left and right or tilted with respect to the document table. Therefore, it is determined that the position or orientation of the patch sheet S set on the document table 21 is not correct (step S130: No). In this case, the CPU 41 instructs the user to reset the patch sheet S by displaying a predetermined screen instructing the resetting of the patch sheet S on the display unit of the operation panel 30 (step S160). Returning to step S110, the patch sheet S whose set position has been corrected is read again.

  On the other hand, if the CPU 41 determines that the position and orientation of the patch sheet S are correct (step S130: Yes), the patch image PG in which a plurality of patches are arranged is also at a predetermined position. The density for each patch P is detected by acquiring the density at a predetermined position predetermined for each P from the read image data (step S140).

  Next, the CPU 41 performs a process for generating the color correction table CT (step S150). Here, the correction value for the density of each patch P is calculated by comparing the detected density with the density of the target value for each patch P indicated by the target data TD. The calculated correction values are stored in the EEPROM 44 as a color correction table CT. In the subsequent print processing, the CPU 41 performs calibration for the density output characteristics of the print engine 10 by controlling the color correction processing using the color correction table CT stored in the EEPROM 44.

  The process of step S100 corresponds to the image forming step described in the claims, and the processes of steps S120 to S150 correspond to the calibration step described in the claims.

  According to the first embodiment, the following effects can be obtained.

  (1) Since the position detection mark M printed on the patch sheet S is formed by overlapping the single color images TG of the CMYK color toners, either one of the single color images TG has a partial density shortage due to density unevenness, or Even in the case where the overall density is insufficient, the insufficient density is compensated for by the single-color image TG of the other color toner, and the printed position detection mark M has a sufficient density. Since the position detection mark M is printed with more toner by overlapping the single color images TG of the respective color toners, the present invention is not limited to the case where density unevenness occurs in the image forming process before the primary transfer. Even when the density unevenness occurs in the image forming process after the secondary transfer, the density of the position detection mark M is sufficiently secured. Accordingly, as shown in FIG. 9, in the search in the case where the region UNIG corresponding to the density unevenness is included in the image region MG corresponding to the position detection mark M, the state (C = 7) in FIG. ), The RGB value is less than the threshold value, and the count value C is insufficient to correspond to the width of the position detection mark M, so that it is erroneously determined not to be a pixel corresponding to the position detection mark M (step S260: No) can be prevented. As a result, the image region MG corresponding to the position detection mark M in the read image of the patch sheet S can be detected more reliably, and the position detection accuracy of the position detection mark M is improved and set to the correct position. Based on the read image data of the patch sheet S, the density output characteristics can be appropriately calibrated.

  (2) During normal printing, color conversion processing is performed according to the color conversion table TTa, and when performing calibration, color conversion processing is performed according to the color conversion table TTb for calibration. The position detection mark M in which the CMYK color toner images overlap can be printed on the patch sheet S without being changed. Therefore, even if it is difficult to change the processing contents because the functions such as the color conversion processing are implemented in the engine I / F 46, the position detection mark formed by overlapping the single color images TG of the CMYK color toners. By printing M, the detection accuracy of the position detection mark M can be improved.

  (3) At the time of executing calibration, the position detection mark M formed by overlapping the single color images TG of the CMYK toners is printed by a single conversion process using the calibration color conversion table TTb. For example, after the color conversion process using the color conversion table TTa, the same process is performed by adjusting the CMYK value corresponding to the position detection mark M to the CMYK value at which the density of each color of CMYK is substantially maximum for the image data after color conversion. However, in the first embodiment, since the conversion process is performed once using the color conversion table TTb, the detection accuracy of the position detection mark M can be improved with a simpler configuration. Can do.

(Second embodiment)
In the first embodiment, in the position detection mark search process, the determination is made using the pixel value of the target pixel. However, in the second embodiment, in addition to the pixel value of the target pixel, a pixel that is further adjacent to the target pixel. By searching for the position detection mark M with reference to the pixel value, the influence of density unevenness in the search for the position detection mark M is further reduced.

  Specifically, in the process of step S220 in the flowchart of FIG. 8, the average value of the RGB value of the target pixel and the RGB value of the pixel adjacent to the target pixel is calculated, and each value of the average RGB value is less than the threshold value. It is determined whether or not. At this time, the average value of the RGB values may be calculated for the adjacent pixel on the left side of the target pixel and the target pixel (see FIG. 10A). You may make it calculate the average value of RGB value about an attention pixel (refer FIG.10 (b)). Alternatively, the pixel of interest may be determined based on an average value of weighted averages in which weights larger than those of adjacent pixels are set.

  According to the second embodiment, the following effects can be obtained in addition to the effects (1) to (3) of the first embodiment.

  (4) Even when a partial density drop occurs in the position detection mark M due to the influence of density unevenness, the RGB values used for the determination in step S220 are averaged, so that the density unevenness is determined in the determination in step S220. The effect of density reduction is reduced. Therefore, the position detection mark M can be detected more reliably, and the detection accuracy of the position detection mark M can be further increased.

(Third embodiment)
In the first embodiment, an example of a copying apparatus having a printing function and a scanner function has been described. However, a system in which a printer having a printing function and a scanner having a scanner function are separately configured may be used.

  That is, as shown in FIG. 11, the calibration system 3 according to the third embodiment includes a printer (image forming apparatus) 4 having a print engine 10, an operation panel 30, and a controller 40 ', a document table 21, and the like. A scanner (reading device) 5 and a host computer 6 having a printer driver 7 and a scanner driver 8 are provided.

  The controller 40 'includes a CPU 41, a ROM 42, a RAM 43, an EEPROM 44, a communication I / F 45, an engine I / F 46, and a panel I / F 48. The functions thereof are shown in FIG. It has a functional configuration. In the third embodiment, the same reference numerals are given to the same configurations as those in the first embodiment.

  A process for calibrating the printer 4 in the calibration system 3 will be described with reference to the flowchart of FIG. Upon receiving the user instruction from the printer driver of the host computer 6 and starting the processing of FIG. 6, first, the printer 4 outputs a patch sheet S by performing printing according to the patch image data (step S100). The user sets the patch sheet S on the document table of the scanner 5, and the scanner 5 reads the patch sheet S according to the control of the scanner driver 8 of the host computer 6 (step S110). The printer driver 7 receives the read image data of the patch sheet S from the scanner driver 8 and outputs it to the printer 4. Next, the controller 40 ′ of the printer 4 performs a position detection mark search process using the read image data received from the printer driver 7 (step S120), and if the sheet setting position is normal (step S130: Yes). Then, the density for each patch P is detected (step S140), and calibration is performed by generating a color correction table from the read image data (step S150). If the sheet setting position is not normal (step S130: No), a screen for instructing resetting is displayed on the display unit of the operation panel 30 or the display (not shown) of the host computer 6 (step S160). The processes after S110 are performed on the reset patch sheet S.

  In step S120, the controller 40 ′ of the printer 4 performs the position detection mark search process, but the printer driver 7 performs the position detection mark search process and outputs the density of the patch P to the printer 4. The controller 40 ′ of the printer 4 may be configured to perform calibration.

  Alternatively, the printer driver 7 may generate the color correction table CT using the detected altitude of the patch P and output it to the printer 4 so that the EEPROM 44 stores the color correction table CT. In this case, the color correction unit 120 corresponds to the calibration means described in the claims.

  According to the third embodiment, the same effects (1) to (3) as in the first embodiment can be obtained in the calibration system 3 including the printer 4, the scanner 5, and the host computer 6.

  Although the first to third embodiments have been described above, the following modifications may be made.

  (Modification 1) In the second embodiment, the determination in step S220 is performed using the average value of the target pixel and the adjacent pixel. The determination in step S220 may be performed using the value. Even in this case, the image region MG corresponding to the position detection mark M can be detected with higher accuracy, and calibration can be performed appropriately.

  (Modification 2) In the third embodiment, the read image data read by the scanner 5 is output to the printer 4 via the host computer 6, but the printer 4 and the scanner 5 directly As a connected system configuration, the scanner 5 may output read image data to the printer 4.

  (Modification 3) In the above-described embodiment, the case where four-cycle electrophotographic printing is performed has been described as an example. However, a configuration in which electrophotographic image formation is performed by a tandem method may be used. Alternatively, the printing may be performed using various printing methods such as an inkjet method and a thermal transfer method.

1 is a diagram illustrating a configuration of a copying apparatus according to a first embodiment. The figure which showed the structure of the printing engine. The figure which showed the function structure of the controller. The figure which showed an example of the patch sheet. The figure which showed an example of the color conversion table. The flowchart which showed the procedure of the process at the time of calibration execution. Explanatory drawing for demonstrating image formation of a position detection mark. The flowchart which showed the procedure of the position detection mark search process. Explanatory drawing for demonstrating a position detection mark search process. Explanatory drawing for demonstrating a 2nd Example. The figure which showed the structure of the calibration system which concerns on a 3rd Example.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Copy apparatus as image forming apparatus, 2 ... Host computer, 3 ... Calibration system, 4 ... Printer as image forming apparatus, 5 ... Scanner as reading means, 7 ... Printer driver, 8 ... Scanner driver, 10 ... Print engine as image forming means, 20... Scanner unit as reading means, 21... Document table, 22... Carriage, 23... Scanning mechanism, 30 ... operation panel, 40. RAM, 44 ... EEPROM as storage unit, 45 ... Communication I / F, 46 ... Engine I / F, 47 ... Scanner I / F, 48 ... Panel I / F, 100 ... Drawing processing unit, 110 ... Conversion unit Color conversion unit 120... Color correction unit as a calibration unit 130 130 Halftone processing unit 140 140 Color correction table as a calibration unit Generation unit, PD ... patch image data, TD ... target data, TT ... color conversion table, TTa ... color conversion table for normal printing as a second correspondence table, TTb ... calibration as a first correspondence table Color conversion table, CT ... color correction table, PG ... patch image as calibration image, P ... patch, M ... position detection mark as position detection image.

Claims (7)

An image forming apparatus that forms an image using a black color material, a cyan color material, a magenta color material, and a yellow color material,
A calibration image based on a predetermined gradation value, and a position detection image arranged at a predetermined position with respect to the calibration image are formed using the color material, and the calibration image and the position detection image are formed. Image forming means for recording the image on a print medium;
Calibration means for calibrating density output characteristics of the image forming means based on the density difference between the density of the calibration image recorded on the print medium and the density determined in advance for the calibration image;
The image forming unit forms the position detection image having a substantially maximum density for each of the plurality of color materials, and superimposes and records the position detection images of the respective colors on the print medium. . The image forming apparatus according to claim 1.
The calibration image including a plurality of patches each having a predetermined color and gradation value, and the position detection image having a combination of a color and a gradation value different from the plurality of patches, red, green, A storage unit for storing calibration image data expressed by blue gradation values;
The gradation value of the calibration image data is associated with the gradation values of cyan, magenta, yellow and black, and the gradation value of the position detection image is substantially the maximum for each color of cyan, magenta, yellow and black. A first correspondence table associated with gradation values;
Conversion means for performing conversion processing on the calibration image data according to the first correspondence table;
The image forming apparatus forms the calibration image and the position detection image based on the calibration image data after the conversion process. The image forming apparatus according to claim 2.
The first correspondence table is:
In the calibration image data, the position detection image is represented by a predetermined gradation value excluding the gradation value having the same value for each color of red, green, and blue,
The first correspondence table associates a gradation value having the same value for each color of red, green, and blue with a gradation value corresponding to a single black color material, and represents a predetermined gradation value representing the position detection image. The image forming apparatus is characterized in that a cyan color material, a magenta color material, and a yellow color material are associated with gradation values representing composite black. The image forming apparatus according to claim 2 or 3,
A second correspondence table that associates the gradation values of red, green, and blue with the gradation values of cyan, magenta, yellow, and black; and
The image forming apparatus, wherein the conversion unit performs a conversion process according to the second correspondence table during normal printing, and performs a conversion process according to the first correspondence table during calibration. The image forming apparatus according to any one of claims 1 to 4,
The image forming apparatus includes a reading unit that generates read image data by reading the calibration image and the position detection image recorded on the print medium,
The read image data is bitmap data having gradation values for each of a large number of pixels arranged in a matrix,
The calibrating unit sequentially pays attention to the pixels of the read image data, and the gradation corresponding to the position detection image based on the gradation value of the attention pixel of interest and the gradation value of the pixel adjacent to the attention pixel. The read image to be acquired as a density of a calibration image from a relative position with respect to the searched position detection image by searching an image region corresponding to the position detection image by determining whether or not the value is a value. An image forming apparatus, wherein the density of a calibration image recorded on the printing medium is acquired by detecting a position of data. The image forming apparatus according to claim 5.
The calibration means determines whether or not the gradation value of the pixel of interest and the average value of the gradation values of pixels adjacent to the pixel of interest are gradation values corresponding to the position detection image. An image forming apparatus searching for an image area corresponding to the position detection image. A calibration method for an image forming apparatus that forms an image using a black color material, a cyan color material, a magenta color material, and a yellow color material,
A calibration image based on a predetermined gradation value, and a position detection image arranged at a predetermined position with respect to the calibration image are formed using the color material, and the calibration image and the position detection image are formed. Forming an image on a print medium; and
A calibration step for calibrating density output characteristics of image formation based on a density difference between a density of the calibration image recorded on the print medium and a density predetermined for the calibration image,
In the image forming step, the position detection image having a substantially maximum density is formed for each of the plurality of color materials, and the position detection images of the respective colors are overlapped and recorded on the print medium. Calibration method.

Images