JP2009017411A - Image forming apparatus, and image processing apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To correct image formation conditions for suppressing color change using image data represented by a colorimetric system different from a colorimetric system by which a read is made by an image read means, even when the image data is input. <P>SOLUTION: Gray scale values of original image data representing an original image and gray scale values of inspection image data representing an output image to be formed on a form based upon the original image data are read out to generate a cumulative histogram C1 using the gray scale values of the original image data and a cumulative histogram C2 using the gray scale values of the inspection image data. Based upon the generated cumulative histograms C1 and C2, a correction gray scale characteristic T2 representing the correspondence between input gray scale values as gray scale values of the histogram C2 for respective cumulative frequencies obtained by dividing cumulative frequencies of 0 to 100% by 256 and output gray scale values as gray scale values of the histogram C1 for the cumulative frequencies is found to generate and update a gray scale correction table. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus and an image processing apparatus.

When printing a plurality of copies of the same document using an electrophotographic image forming apparatus or printing a document of a plurality of pages, there is a problem that color variations occur and the hue of each section or each page varies. As main causes of this problem, it is conceivable that the charge amount control of the toner is unstable, and that the photodischarge curve (PIDC) changes due to light fatigue of the photoreceptor due to continuous printing. Therefore, as a countermeasure against this problem, the electrophotographic image forming apparatus includes a calibration unit.
Conventional calibration means forms a color patch with an arbitrary halftone dot area ratio with unfixed toner on a photosensitive member or intermediate transfer member, and performs calibration by measuring with a density sensor or printing a test chart. The image forming means is calibrated from the output and measurement. In this calibration, various image forming conditions (such as toner charge amount, photoreceptor surface potential, exposure unit light amount, gradation correction lookup table, etc.) are corrected. However, in the above-described measurement by the density sensor, the calibration accuracy is poor because the density fluctuations of the unfixed toner and the fixed toner are different, and the number of colors and the halftone dot area ratio that can be produced by the color patch are limited. In addition, in the method of forming a color patch of unfixed toner in the original image writing area of the photoconductor or intermediate transfer member, or in the method of outputting a test chart for measuring the density of the fixed toner, the user temporarily outputs a printout. Since it was necessary to stop, productivity was reduced.
In order to solve these problems, for example, Patent Document 1 updates a gradation correction table for correcting color fluctuations using gradation values extracted from image data generated by reading a predetermined test image. Techniques to do this are disclosed.
JP 2001-045295 A

By the way, in the technique described in Patent Document 1 described above, gradation values of RGB colors extracted from R (red), G (green), and B (blue) image data read by image reading means such as a scanner device. The tone correction table is updated based on the above. Therefore, when image data of a color system other than the RGB color system, for example, YMCK color system (K is black) is input from an external device, the tone correction table is updated using the image data. There was a problem that could not.
The present invention has been made in view of such a background, and even when image data expressed in a color system different from the color system read by the image reading unit is input, the image data Is used to correct image forming conditions for suppressing color fluctuations.

  In order to solve the above problems, the present invention provides a first acquisition unit that acquires image data representing an original image and the image data acquired by the first acquisition unit in a first color system. A first conversion unit that converts the tone value into image data, and a tone value of each color of the first color system from the image data obtained by the conversion of the first conversion unit for each pixel; The output image is recorded using a plurality of color materials corresponding to the first color system in accordance with an image forming condition including a first extracting unit that extracts the image data and image data acquired by the first acquiring unit. Image forming means formed on a medium, second acquisition means for acquiring image data representing an output image formed by the image forming means as inspection image data, and the inspection acquired by the second acquisition means The image data is stored in the first A second conversion means for converting the image data to which gradation values are expressed in a color system, and the inspection image data obtained by the conversion of the second conversion means, from each color scale of the first color system. A second extraction unit that extracts a tone value for each pixel, and a first value that represents a cumulative number of pixels in each gradation value using the gradation value of each pixel extracted by the first extraction unit. Generation means for generating a cumulative frequency distribution and generating a second cumulative frequency distribution representing the cumulative number of pixels at each gradation value using the gradation value of each pixel extracted by the second extraction means. And calculating means for calculating a correspondence relationship between the gradation value in the first cumulative frequency distribution and the gradation value in the second cumulative frequency distribution for each cumulative frequency representing the cumulative number of pixels. Based on the correspondence calculated by the calculation means, the image formation To provide an image forming apparatus characterized by comprising a correction means for correcting the image forming conditions to stage follow.

  In the present invention, the calculation means normalizes the maximum cumulative frequency in the first cumulative frequency distribution and the second cumulative frequency distribution to be the gray level of the output image, and normalizes the cumulative For each frequency, a correspondence relationship between a gradation value or approximate gradation value in the first cumulative frequency distribution and a gradation value or approximate gradation value in the second cumulative frequency distribution may be calculated.

  In the present invention, the calculating means includes the gradation value in one of the first cumulative frequency distribution or the second cumulative frequency distribution and the other cumulative frequency having an equal value or an approximate value to the cumulative frequency representing the gradation value. A correspondence relationship with the gradation value in the distribution may be calculated.

  In the present invention, there is provided ground color specifying means for specifying a gradation value representing the color of the recording medium on which no output image is formed by the image forming means, and the generating means is extracted by the second extracting means. The second cumulative frequency distribution may be generated using a gradation value obtained by removing a gradation value representing the color of the recording medium specified by the ground color specifying means from among the determined gradation values.

  In the present invention, the color reading unit reads the color of the recording medium before the output image is formed by the image forming unit or the color of the storage medium in the area where the output image is not formed by the image forming unit. The ground color specifying means specifies a gradation value representing a color read by the color reading means as a gradation value representing a color of the recording medium on which an output image is not formed by the image forming means. May be.

  In the present invention, the ground color specifying means obtains a gradation value that maximizes a correlation coefficient between the first cumulative frequency distribution and the second cumulative frequency distribution, and calculates the gradation value of the recording medium. You may specify as a gradation value showing a color.

  In the present invention, the generation means further generates a first frequency distribution representing the number of pixels in each gradation value using the gradation value of each pixel extracted by the first extraction means, Using the gradation value of each pixel extracted by the second extraction unit, a second frequency distribution representing the number of pixels in each gradation value is generated, and the ground color specifying unit is configured to A gradation value that maximizes the correlation coefficient between the frequency distribution and the second frequency distribution may be obtained, and the gradation value may be specified as a gradation value representing the color of the recording medium.

  In the present invention, reflected light when the original image or the output image is irradiated with light, the reflected image including visible light and invisible light is received to read the image, and the second color specification An image reading unit that generates image data in which gradation values are expressed by a system, and the first acquisition unit or the second acquisition unit may acquire the image data generated by the image reading unit. Good.

  In the present invention, using a plurality of color materials corresponding to the first color system, a gradation pattern forming means for forming a gradation pattern having a predetermined halftone dot area ratio on the surface of the photoreceptor or the intermediate transfer belt, Measuring means for measuring the gradation value of the gradation pattern formed by the gradation pattern forming means in the first color system, and the calculating means calculates the gradation value measured by the measuring means. In consideration of the above, the correspondence relationship may be calculated.

  In the present invention, the first acquisition unit determines whether or not attribute information representing a predetermined dot pattern is added to the acquired image data, and the generation unit includes the acquired image. When it is determined by the first acquisition unit that attribute information representing a predetermined dot pattern is added to the data, the predetermined dot pattern is added to the image data acquired by the first acquisition unit. The dot pattern is arranged using the gradation value extracted by the first extraction unit from the image data to which the image data to be represented is added, or the gradation value of the pixel representing the predetermined dot pattern. The first cumulative frequency distribution may be generated using a gradation value added to the gradation value extracted by the first extraction means from a position.

  In the present invention, the calculating unit includes a determining unit that determines whether the number of gradations of the gradation value included in the image data acquired by the first acquiring unit is greater than or equal to a predetermined threshold. If the determination unit determines that the number of gradations is smaller than the predetermined threshold, the correspondence relationship may not be calculated.

  In the present invention, the generation means may calculate the first cumulative frequency distribution based on the gradation values respectively extracted by the first extraction means from the plurality of image data acquired by the first acquisition means. And generating the second cumulative frequency distribution based on the gradation values respectively extracted by the second extraction means from each of the inspection image data corresponding to the plurality of image data. Good.

  In the present invention, there is provided switching means for switching the operation mode of the own apparatus to the first operation mode or the second operation mode, and the first conversion means is configured such that the operation mode of the own apparatus is the first operation mode. In some cases, the image data acquired by the first acquisition means is converted into image data in which gradation values are expressed in the first color system, and the operation mode of the device is the second operation. When the mode is selected, the image data acquired by the first acquisition means is output without being converted into image data in which gradation values are expressed in the first color system, and the calculation means When the operation mode of the device itself is the second operation mode, the correspondence relationship may not be calculated.

  Further, the present invention provides a first acquisition unit that acquires image data representing an original image, and the image data acquired by the first acquisition unit is expressed by a gradation value in a first color system. A first conversion means for converting into the image data, and a first gradation value for each pixel of the first color system from the image data obtained by the conversion of the first conversion means. And an output image formed on a recording medium using a plurality of color materials corresponding to the first color system in accordance with image forming conditions including image data acquired by the extracting means and the first acquiring means. A second acquisition unit that acquires image data to be expressed as inspection image data, and an image in which gradation values are expressed in the first color system of the inspection image data acquired by the second acquisition unit Second conversion means for converting to data; and the second conversion means. Second extraction means for extracting the gradation value of each color of the first color system for each pixel from the inspection image data obtained by the conversion of the means, and each pixel extracted by the first extraction means Using the first cumulative frequency distribution representing the cumulative number of pixels in each gradation value and the gradation value of each pixel extracted by the second extraction means. Generating means for generating a second cumulative frequency distribution representing the cumulative number of pixels in the gradation value; for each cumulative frequency representing the cumulative number of pixels, the gradation value in the first cumulative frequency distribution; A calculating unit that calculates a correspondence relationship with the gradation value in the second cumulative frequency distribution, and a correction unit that corrects the image forming condition based on the correspondence relationship calculated by the calculating unit. An image processing apparatus is provided.

  According to the present invention, even when image data expressed in a color system different from the color system read by the image reading unit is input, the image data is used to suppress color variation. The image forming conditions can be corrected.

[Embodiment]
FIG. 1 is a diagram illustrating a configuration of an image forming apparatus 1 according to the present embodiment. As illustrated in FIG. 1, the image forming apparatus 1 includes a control unit 10, a display operation unit 20, a communication unit 30, an image processing unit 40, an image forming unit 50, and an image reading unit 60. Yes. The control unit 10 includes a CPU (Central Processing Unit) and a memory, and the CPU controls each unit of the image forming apparatus 1 by executing various programs stored in the memory. The display operation unit 20 includes, for example, a touch panel and operation buttons. The display operation unit 20 displays an image corresponding to the image signal supplied from the control unit 10, receives an operation by the user, and controls a signal corresponding to the operation. Supplied to the unit 10.

  The communication unit 30 is an interface for transmitting / receiving data to / from an external device connected via a network. For example, PDL data described in a page description language (PDL) is transmitted from the external device. The PDL data transmitted from the external device is supplied to the control unit 10 and the image processing unit 40 by the communication unit 30. The image processing unit 40 includes a calculation unit such as a CPU and an application specific integrated circuit (ASIC) and a memory, and performs various types of image processing. In the image processing unit 40, gradation correction processing for correcting the gradation of an image is performed using, for example, a gradation correction table stored in a memory.

  The image forming unit 50 is supplied using toners of Y (yellow), M (magenta), C (cyan), and K (black) colors corresponding to the YMCK color system (first color system). An output image corresponding to the image data to be recorded is formed on a sheet as a recording medium. More specifically, the image forming unit 50 includes a paper supply unit 51, a paper transport unit 52, image forming units 53Y, 53M, 53C, and 53K corresponding to each color of YMCK, an intermediate transfer belt 54, and a secondary transfer unit. 55 and a fixing device 56. The paper supply unit 51 stores paper of a predetermined size. The paper transport unit 52 transports the paper stored in the paper supply unit 51 to the secondary transfer unit 55, and the paper output from the secondary transfer unit 55 passes through the fixing device 56 and the image reading unit 60. Transport to the paper exit.

  Each of the image forming units 53Y, 53M, 53C, and 53K includes a photosensitive drum 531, a charging unit 532, an exposure unit 533, a developing unit 534, and a primary transfer unit 535. The photosensitive drum 531 is an image holding body that holds a toner image, and is uniformly charged to a charging potential determined by the charging unit 532. The exposure unit 533 forms an electrostatic latent image by irradiating the photosensitive drum 531 with light corresponding to the image data of each color of YMCK. The developing unit 534 performs development by supplying toner of each color of YMCK to the electrostatic latent image to form a toner image on the surface of the photosensitive drum 531. The primary transfer unit 535 primarily transfers the toner image on the surface of the photosensitive drum 531 to the intermediate transfer belt 54 by a potential difference with the photosensitive drum 531. In this way, the toner image primarily transferred to the intermediate transfer belt 54 is moved in accordance with the circumferential movement of the intermediate transfer belt 54, and is secondarily transferred to the sheet conveyed from the sheet supply unit 51 by the secondary transfer unit 55. The The fixing device 56 applies heat and pressure to the paper conveyed from the secondary transfer unit 55 to fix the toner image on the paper. Then, the sheet that has undergone such a fixing process is conveyed to the image reading unit 60 through a sheet conveying path.

  The image reading unit 60 irradiates light on a document set by a user or a sheet conveyed through a fixing process, and receives reflected light, thereby outputting an output image formed on the document or the sheet. Read and generate image data in which gradation is expressed in the RGB color system (second color system). More specifically, the image reading unit 60 includes a light source 61, a reflection mirror 62, an imaging lens 63, and an image sensor 64. The light source 61 irradiates light on a document set by a user or on a sheet conveyed through a fixing process. The reflection mirror 62 guides the reflected light reflected by the paper to the imaging lens 63 when light is emitted from the light source 61. The imaging lens 63 focuses the reflected light on the image sensor 64. The reflected light is color-separated into R, G, B and invisible light by an R (red), G (green), and B (blue) filter and a visible light cut filter provided in front of the image sensor 64. Each image is formed on the corresponding image sensor 64. The image sensor 64 is a four-color image sensor provided for each of RGB and invisible light, receives reflected light imaged by the imaging lens 63, and R, G, B and invisible light according to the received light amount. Image data is generated.

  As described above, in the image forming apparatus 1, the image processing unit 40 performs gradation correction processing according to the gradation correction table. Here, the correction table update processing in the image processing unit 40 will be described in detail. FIG. 2 is a diagram illustrating functions provided in the image processing unit 40. As shown in the figure, the image processing unit 40 includes a document image data acquisition unit 41, a first conversion unit 42, a first extraction unit 43, an image formation condition correction unit 44, and an inspection image data acquisition unit 45. A second conversion unit 46, a second extraction unit 47, and a gradation correction table generation unit 48. The gradation correction table update process includes a gradation value of document image data representing a document image provided by a user, and a gradation value of inspection image data representing an output image formed on a sheet based on the document image data. And based on. Next, the tone correction table update process will be described in detail with reference to FIG.

First, a process for extracting gradation values from document image data representing a document image will be described.
For example, when PDL data representing a document image is transmitted from an external device, the PDL data is supplied to the image processing unit 40 by the communication unit 30. The document image data acquisition unit 41 of the image processing unit 40 interprets the PDL data and acquires document image data in which gradation values are expressed in YMCK (step S100).

  The first conversion unit 42 converts the YMCK document image data acquired by the document image data acquisition unit 41 into YMCK (first color system) document image data in the toner color space used in the image forming unit 50. Conversion is performed (step S110). This is a process considering the case where the color space of the document image data is different from the color space of the toner used in the image forming apparatus 1. For example, when the color represented by the document image data is expressed in the color space of ink for offset printing, the color space of the document image data matches the color space of the toner used in the image forming unit 50. do not do. Therefore, it is necessary to convert the YMCK image data in the color space of the document image data into YMCK document image data in the toner color space used in the image forming unit 50. Such conversion processing by the first conversion unit 42 is performed based on a color conversion matrix, an LUT (look-up table), or the like stored in advance.

  The first extraction unit 43 extracts the gradation value of each color of YMCK for each pixel from the YMCK (first color system) document image data obtained by the conversion of the first conversion unit 42 (step S120). In this example, gradation values of each color of YMCK representing 256 gradations from 0 to 255 are extracted. In the following description, it is assumed that the smaller the gradation value, the lower the lightness and the darker the color, and the higher the gradation value, the higher the lightness and the lighter the color.

  The image forming condition correction unit 44 performs gradation correction on the document image data from which the gradation value is extracted by the first extraction unit 43, using the gradation correction table stored in the memory (step S130). That is, the image forming condition corrected by the image forming condition correcting unit 44 is the content of the gradation correction table for correcting the gradation of the document image data. The document image data that has been subjected to gradation correction by the image forming condition correction unit 44 is supplied to the image forming unit 50. The image forming unit 50 uses the YMCK toner to form an output image corresponding to the supplied document image data on the sheet (step S140). In this example, an output image having a screen line number of 300 lpi (lines per inch) is formed on a sheet. Then, the paper on which the output image is formed by the image forming unit 50 is conveyed to the image reading unit 60 through the paper conveyance path. The image reading unit 60 reads the output image of the conveyed paper and generates image data expressed in RGB (second color system) and invisible light gradation (step S150). In this example, image data with a resolution of 200 dpi is generated. The resolution of the image data generated by the image reading unit 60 is to increase the processing speed and to generate a highly accurate gradation correction table when performing a gradation correction table generation process described later. In order not to read the halftone dot structure, the number of screen lines of the output image formed by the image forming unit 50 is set to be lower. In this manner, the image data generated by the image reading unit 60 is supplied to the image processing unit 40 as inspection image data.

Next, processing for extracting gradation values from inspection image data will be described.
The inspection image data acquisition unit 45 of the image processing unit 40 acquires RGB (second color system) and invisible inspection image data generated by the image reading unit 60 (step S160). The second conversion unit 46 converts the RGB and invisible inspection image data acquired by the inspection image data acquisition unit 45 into YMCK inspection image data (step S170). In this conversion process, the K inspection image data is converted based on the invisible light inspection image data, so that high-accuracy YMCK inspection image data can be obtained even if there is inking. The second extraction unit 47 extracts the gradation value of each color of YMCK for each pixel from the YMCK (first color system) image data obtained by the conversion of the second conversion unit 46 (step S180). In this example, in the same manner as described above, gradation values of each color of YMCK representing 256 gradations from 0 to 255 are extracted.

Next, a process for generating a gradation correction table based on the gradation value extracted by the first extraction unit 43 and the gradation value extracted by the second extraction unit 47 will be described.
First, the gradation correction table generation unit 48 uses the gradation values extracted by the first extraction unit 43 to generate a histogram H1 that represents the number of pixels in each gradation value. Further, the gradation correction table generation unit 48 generates a histogram H2 representing the number of pixels in each gradation value using the gradation values extracted by the second extraction unit 47 (step S190). FIG. 4 is a diagram showing a histogram H1 generated based on the gradation value extracted by the first extraction unit 43 and a histogram H2 generated based on the gradation value extracted by the second extraction unit 47. It is. The horizontal axis in the figure represents the gradation value, and the vertical axis in the figure represents the number of pixels in relative frequency (%).

  Subsequently, the gradation correction table generation unit 48, based on the generated histograms H1 and H2, a cumulative histogram C1 (first cumulative frequency distribution) representing the cumulative number of pixels in each gradation value, and a cumulative histogram C2. (Second cumulative frequency distribution) is generated (step S200). FIG. 5 shows the cumulative histograms C1 and C2. The horizontal axis in the figure represents the gradation value, and the vertical axis in the figure represents the cumulative number of pixels in cumulative frequency (%). Here, the cumulative histogram will be described. For example, in the histogram H2 described above, the number of pixels with a gradation value of “0” is 0.2% of the total number of pixels, and the number of pixels with a gradation value of “1” is the entire number. It is assumed that the number of pixels is 3.2 (%) and the number of pixels having a gradation value of “2” is 0.8 (%) of the total number of pixels. In this case, the generated cumulative histogram C2 has a cumulative frequency of the gradation value “0” of 0.2 (%) (a frequency indicating the number of pixels having the gradation value “0”), and the gradation value “0”. The cumulative frequency of 1 ”is 0.2 (%) (the cumulative frequency of the gradation value“ 0 ”) and 3.2 (%) (the frequency indicating the number of pixels whose gradation value is“ 1 ”). 3.4 (%), the cumulative frequency of the gradation value “2” is 3.4 (%) (the cumulative frequency of the gradation value “1”) and 0.8 (%) (the gradation value is “2”). (Frequency representing the number of pixels) is 4.2 (%). In this way, cumulative histograms C1 and C2 representing the cumulative frequency at each gradation value from 0 to 255 are generated.

  As shown in the figure, the cumulative histogram C2 is deviated from the cumulative histogram C1. For example, the gradation value having a cumulative frequency of 40 (%) is 60 in the cumulative histogram C2, but 115 in the cumulative histogram C1. This indicates that color variation occurs in the image forming unit 50 and the image forming unit 50 has gradation characteristics that cannot be corrected by the current gradation correction table stored in the memory. Subsequently, the gradation correction table generation unit 48 normalizes the cumulative histogram C2 and the cumulative frequency in the cumulative histogram C1 so that the maximum value of the cumulative frequency becomes 256, which is the gradation number of the output image formed by the image forming unit 50, For each normalized cumulative frequency, a corrected gradation characteristic T2 representing the correspondence between the gradation value in the cumulative histogram C2 and the gradation value in the cumulative histogram C1 is calculated. That is, the cumulative frequency from 0 to 100 (%) is divided into 256, and for each of the divided cumulative frequencies, the input gradation value that is the gradation value in the cumulative histogram C2 and the gradation value in the cumulative histogram C1 are used. A corrected gradation characteristic T2 representing a correspondence relationship with a certain output gradation value is calculated. FIG. 6 is a diagram showing the corrected gradation characteristic T2 calculated in this way. In this figure, the gradation characteristic of the image forming unit 50 in which the color variation has occurred is represented by gradation characteristic T1, and the target gradation characteristic is represented by reference gradation characteristic T0.

  Then, the gradation correction table generation unit 48 generates a gradation correction table that represents the correspondence between each input gradation value and its output gradation value in a table format based on the calculated corrected gradation characteristic T2. (Step S210). This gradation correction table is stored in the memory of the image processing unit 40 as a new correction table used for gradation correction by the image forming condition correction unit 44. Such a correction table update process is performed every time the image forming process is performed in the image forming apparatus 1.

  Accordingly, even when image data expressed in the YMCK color system different from the RGB color system read by the image reading unit 60 is input, gradation correction is performed using the image data. The tone correction table can be updated.

[Modification]
The above is the description of the embodiment, but the contents of this embodiment can be modified as follows. Moreover, you may combine the following each aspect suitably.

(1) In the above-described embodiment, in order to generate a more accurate gradation correction table, the gradation value representing the ground color of the paper on which the output image is not formed is specified by the image forming unit 50, and the second A cumulative histogram may be generated using tone values excluding the tone values representing the ground color of the specified paper from the tone values extracted by the extraction unit 47. Such a process of removing the ground color of the paper includes the gradation value of the ground color of the paper on which the output image corresponding to the image data is formed, and the gradation value of the background portion where the image is not formed in the document image. If the two do not match, a significant effect is exhibited. FIG. 7 is a diagram showing a histogram representing the gradation values of each ground color for papers A, B, and C having different ground colors. This histogram indicates that there is a gradation value representing the background color of the paper in the vicinity of the gradation value having a peak frequency level. For example, the histogram of the paper A in the figure has a peak frequency near the gradation value 250. Therefore, it can be seen that the gradation value representing the background color of the paper A is around 250. Similarly, the histogram of the paper B in the figure has a frequency peak around the gradation value 253, so the gradation value representing the ground color of the paper B is around 253, and the histogram of the paper C is Since the frequency is peaked around the gradation value 240, it can be seen that the gradation value representing the ground color of the paper C is around 240.

  For example, an output image similar to that in the above-described embodiment is formed on recycled paper having a gradation value representing the background color of 200, and the gradation value of the inspection image data representing the output image is set in the same manner as described above. Assume that a cumulative histogram is generated using In this case, the generated cumulative histogram should originally have the characteristics shown in C2 of FIG. 5 described above, but as shown in C2 ′ of FIG. %) Is a cumulative histogram. This is because the gradation value representing the ground color of the recycled paper on which the output image is formed is 200. Further, the corrected gradation characteristic obtained from this cumulative histogram is supposed to be the characteristic indicated by T2 shown in FIG. 6, but becomes the characteristic indicated by T2 'in FIG. For this reason, the accuracy of the gradation correction table generated based on this corrected gradation characteristic is deteriorated.

Next, processing for removing the ground color of the paper will be described in detail.
For example, the above-described image processing unit 40 is provided with a ground color specifying unit that specifies a gradation value representing the ground color of the paper. The ground color specifying unit uses the image reading unit 60 and the color of the paper before the output image is formed by the image forming unit 50 or the region where the output image is not formed by the image forming unit 50 (margin). By reading the color of the paper, a gradation value representing the ground color of the paper stored in the paper supply unit 51 is acquired. For example, “average value of gradation values representing the ground color of the paper” − “paper The gradation value representing the ground color of the paper is specified using the mathematical formula “standard deviation of the gradation value representing the ground color” × 2. For example, if the paper stored in the paper supply unit 51 is the paper C shown in FIG. 7, the average value is 240 and the standard deviation is 1.2, so 240−1.2 × 2 = 237.6. A gradation value of 237 obtained by rounding down the decimal point of the calculation result is specified as a gradation value representing the ground color of the paper. The gradation value representing the background color of the paper may be a gradation value designated by the user, or may be set in advance when only a predetermined paper is used.
The gradation correction table generation unit 48 removes the gradation value representing the ground color of the paper specified by the above-described ground color specifying unit from the cumulative histogram using the gradation value extracted by the second extraction unit 47. . For example, when the gradation value representing the background color of the paper specified by the background color specifying unit is 200, the gradation of 200 to 255 is obtained from the cumulative histogram using the gradation value extracted by the second extraction unit 47. Remove the value. Then, the gradation value of 0 to 199 is extended to 0 to 255, and the linear interpolation is performed between them, and the gradation value is normalized so that the cumulative frequency becomes 100. For the gradation value of the document image data representing the document image, the gradation value of the background portion in the document image is set to 255 in advance. As a result, a cumulative histogram from which the gradation value representing the background color of the paper is removed is calculated, and a highly accurate gradation correction table is generated.

  Further, the ground color specifying unit described above is a level that represents the ground color of the paper based on a correlation coefficient between a cumulative histogram generated based on the original image data and a cumulative histogram generated based on the inspection image data. A key value may be specified. More specifically, the ground color specifying unit first includes a cumulative histogram of a predetermined number of gradations in the document image data and a cumulative histogram of the predetermined number of gradations in the inspection image data. A correlation with a cumulative histogram of a predetermined number of gradations up to the set provisional gradation value is calculated. For example, assuming that the predetermined number of gradations is 96 gradations and the provisional gradation value set as the background color of the paper is 245, the accumulated histogram shown in FIG. A correlation coefficient between the cumulative histogram C1 for 96 gradations and the cumulative histogram C2 ′ for 96 gradations with gradation values from 150 to 245 is calculated. Similarly, assuming that the temporary gradation value representing the background color of the paper is 239, the cumulative histogram C1 for 96 gradations with gradation values of 160 to 255 and the 96th floor with gradation values of 144 to 239. A correlation coefficient with the cumulative histogram C2 ′ of the distribution is calculated. FIG. 10 is a diagram showing the correlation coefficient calculated in this way. The vertical axis of the figure represents the correlation coefficient, and the horizontal axis of the figure represents the provisional gradation value set as the background color of the paper. The ground color specifying unit specifies the provisional gradation value that maximizes the correlation coefficient as a gradation value that represents the ground color of the paper. In this figure, since the temporary gradation value is 205 and the correlation coefficient is the maximum, this 205 is specified as the gradation value representing the ground color of the paper. Similarly, the ground color specifying unit is configured to express the ground color of the paper based on the correlation coefficient between the histogram generated based on the document image data and the histogram generated based on the inspection image data. A key value may be specified.

  Further, the ground color specifying unit described above calculates a ratio of pixels having a gradation value of 255 representing a background portion in the document image among pixels constituting the document image data, and an inspection image corresponding to the document image data. In the cumulative histogram based on the data, the gradation value of the cumulative frequency of the calculated ratio may be specified as the gradation value representing the background color of the paper. For example, when the ratio of the pixels representing the background portion in the document image among the pixels constituting the document image data is 20%, the cumulative frequency in the cumulative histogram based on the inspection image data corresponding to the document image data is A gradation value of 80% is specified as a gradation value representing the background color of the paper.

  In the above-described method, the gradation value representing the ground color of the paper specified by the ground color specifying unit is removed from the cumulative histogram using the gradation value extracted by the second extracting unit 47. The gradation value representing the ground color of the paper specified by the ground color specifying unit may be removed from the inspection image data. More specifically, the gradation correction table generation unit 48 sets the gradation value included in each pixel extracted by the second extraction unit 47 for each color representing the ground color of the paper identified by the ground color identification unit. Whether the gradation value is exceeded is checked. If the gradation value is exceeded, the gradation value is removed from the inspection image data. For example, when the gradation values representing the background color of the paper specified by the background color specifying unit are R = 240, G = 240, and B = 240, the gradation values included in a certain pixel are R = 250, G = Assuming 250 and B = 250, the gradation value included in the pixel is removed from the inspection image data as a gradation value representing the ground color of the paper. Note that the gradation value of the background portion in the document image is removed in advance for the gradation value of the document image data representing the document image. Even when this method is employed, a highly accurate gradation correction table can be generated.

(2) Forgery prevention tracking dot images may be added to the document image for the purpose of preventing forgery. This anti-counterfeit tracking dot image is a plurality of dot images dispersed and arranged in the original image. For example, the date and time of image formation and the identification information of the image forming apparatus 1 are expressed by the positional relationship of each dot. ing. In the above-described embodiment, when the attribute information indicating the forgery prevention tracking dot is added to the document image data acquired by the document image data acquisition unit 41, the attribute information indicating the forgery prevention tracking dot is added. When an output image is formed on a sheet based on the original image data, forgery prevention tracking dots that did not exist in the original original image data appear in the output image. For this reason, an error corresponding to forgery prevention tracking dots is generated between the cumulative histogram based on the inspection image data representing the output image and the cumulative histogram based on the document image data, and the level generated based on the cumulative histogram is generated. The accuracy of the tone correction table will deteriorate. Therefore, in order to generate a highly accurate gradation correction table, the first extraction unit 43 extracts the gradation value of each pixel from the state in which image data representing the forgery prevention tracking dot image is added to the document image data. A cumulative histogram is generated using the extracted gradation values. Then, the gradation correction table generation unit 48 generates a gradation based on the cumulative histogram based on the original image data to which image data representing anti-counterfeit tracking dots is added and the cumulative histogram based on the inspection image data representing the output image. A tone correction table is generated.

Next, a process for adding image data representing the anti-counterfeit tracking dot will be described in detail. For example, the document image data acquisition unit 41 determines whether or not a document image is formed by adding attribute information representing anti-counterfeit tracking dots to the acquired document image data. When a document image is formed by adding attribute information representing anti-counterfeit tracking dots, the document image data acquisition unit 41 means the image formation date and time, the identification information of the image forming apparatus 1 and the like for the acquired document image data. Add anti-counterfeit tracking dot image data. Then, the first extraction unit 43 extracts gradation values from the original image data to which image data of anti-counterfeit tracking dots is added, and the gradation correction table generation unit 48 adds image data representing anti-counterfeit tracking dots. A cumulative histogram is generated using the gradation values extracted by the first extraction unit 43 from the original document image data.
If it is determined that attribute information representing anti-counterfeit tracking dots is included, the gradation correction table generating unit 48 does not add image data representing anti-counterfeit tracking dots to the original image as described above. Adds the gradation value representing each dot image of the anti-counterfeit tracking dot to the gradation value extracted by the first extraction unit 43 from the document image data at the position where each dot image is arranged, and the addition A cumulative histogram may be generated using a later gradation value. Thereby, a highly accurate gradation correction table is generated.

(3) In the above-described embodiment, the image formation condition correction unit 44 updates the gradation correction table based on the correction gradation characteristic T2, but the image formation condition to be corrected is the gradation. It is not limited to the correction table. For example, the image forming condition correction unit 44 determines the amount of light emitted by the exposure unit 533 or the potential difference between the photosensitive drum 531 and the primary transfer unit 535 or the secondary transfer unit 55 based on the corrected gradation characteristic T2. Alternatively, the toner charge amount, the charging potential of the photosensitive drum 531, or the fixing temperature of the fixing device 56 may be changed. In other words, the image forming condition correction unit 44 only needs to change the image forming condition based on the corrected gradation characteristic T2. As the image forming condition, the above-described gradation correction table or the exposure unit 533 emits light. Such as the amount of light, the potential difference between the photosensitive drum 531 and the primary transfer portion 535 or the secondary transfer portion 55, the toner charge amount, the charging potential of the photosensitive drum 531 or the fixing temperature of the fixing device 56. Various conditions that affect formation can be applied.

(4) In the embodiment described above, the image forming condition correction unit 44 calculates the corrected gradation characteristic T2 based on the correspondence between the gradation value in the cumulative histogram C2 and the gradation value in the cumulative histogram C1. It was. On the other hand, using a plurality of toners corresponding to YMCK (first color system), a gradation pattern having a predetermined dot area ratio is formed on the surface of the photosensitive drum 531 or the intermediate transfer belt 54, and The gradation value of the formed gradation pattern is measured by the density sensor, and the image forming condition correction unit 44 calculates the gradation correction specification T2 in consideration of the gradation value of the gradation pattern measured by the density sensor. May be. Thereby, the accuracy of the gradation correction table can be further improved.

(5) In the embodiment described above, one cumulative histogram C1 generated from one image data acquired by the document image data acquisition unit 41 and one cumulative image generated from inspection image data corresponding to the image data. A gradation correction table has been generated based on the histogram C2. In contrast, for example, an average value of cumulative histograms generated from a plurality of image data acquired by the document image data acquisition unit 41 and an average of cumulative histograms generated from inspection image data corresponding to the image data. A gradation correction table may be generated based on the value. Thereby, the accuracy of the gradation correction table can be further improved.

(6) In the above-described embodiment, the image reading unit 60 generates R, G, B, and invisible light image data. In the second conversion unit, the K inspection image data is an invisible light inspection image. It was converted based on the data. On the other hand, the image reading unit 60 may generate only RGB image data, and the K inspection image data may be calculated from the YMC image data obtained by converting the RGB image data using a predetermined formula. Accordingly, even when the image reading unit 60 included in the image forming apparatus 1 is configured not to generate invisible light image data, the same effects as those of the above-described embodiment can be obtained.

(7) In the embodiment described above, the correction table update process is performed every time the image forming process is performed in the image forming apparatus 1. In contrast, it is determined whether or not the number of gradation values extracted from the image data acquired by the document image data acquisition unit 41 is equal to or greater than a predetermined threshold value. The tone correction table update process may not be performed. For example, when the predetermined threshold value is 50, when document image data having a gradation value range of 100 to 130 or document image data having a gradation value range of 180 to 200 is acquired, the number of gradations Is below the threshold value of 50, the correction table update process is not performed. As a result, the gradation correction table is not updated when document image data that is not suitable for the correction table update processing is acquired, so that the gradation correction table can be held with high accuracy.

(8) In the above-described embodiment, the correction table update process is performed every time the image forming process is performed in the image forming apparatus 1. On the other hand, when the image forming process is performed in an operation mode not suitable for the correction table update process, the correction table update process need not be performed. For example, the image forming apparatus 1 is provided with an operation mode for performing image formation as it is without converting the acquired document image data into YMCK (first color system) by the first conversion unit 42. When the image reading unit 60 of the forming apparatus 1 does not generate invisible light image data but generates only R, G, B image data, when an operation in the operation mode described above is designated by the user. The correction table update process may not be performed. This example corresponds to a form in which the YMCK signal specified by the user is output without conversion. In this case, even for pixels having the same YMCK value, the inking rate specified by the user may not be unified in the original image data, and the inking rate is determined from the inspection image data read by the image reading unit 60. It may not be determined accurately. As a result, the reliability of the K signal decreases. As a result, the gradation correction table is not updated in an operation mode that is not suitable for the correction table updating process, so that the gradation correction table can be held with high accuracy.

(9) In the embodiment described above, the first conversion unit 42 uses the YMCK tone value in the color space of the document image data acquired by the document image data acquisition unit 41 as the color of the toner used in the image forming unit 50. It has been converted to YMCK (first color system) gradation value in space. On the other hand, the first conversion unit 42 converts the RGB document image data generated by reading the document image set by the user by the image reading unit 60 into YMCK (first color system) document image data. May be converted to The first conversion unit 42 converts the original image data of the L * a * b * color system obtained by converting the original image data of RGB generated by reading the original image by the image reading unit 60 into the YMCK (first image data). 1 color system) may be converted into original image data.

  In the above-described embodiment, the inspection image data acquisition unit acquires the generated RGB inspection image data by reading the output image formed on the sheet by the image reading unit 60. On the other hand, the inspection image data acquisition unit may acquire inspection image data representing an output image formed by the image forming apparatus 1 from the communication unit 30. For example, the user causes a sheet on which an output image is formed by the image forming apparatus 1 to be read by an image reading apparatus different from the image forming apparatus 1 and generates inspection image data representing the output image. Then, the user transmits the inspection image data from the image reading apparatus to the communication unit 30 of the image forming apparatus 1. In this case, if the inspection image data transmitted to the communication unit 30 is YMCK inspection image data, the second conversion unit 46 converts the inspection image data into the YMCK (first image data) corresponding to the color space of the image forming apparatus 1. 1 inspection color data). If the transmitted inspection image data is RGB inspection image data, and the inspection image data is converted into L * a * b * colorimetric inspection image data, the L * a * b * table is displayed. Color system inspection image data is converted into YMCK (first color system) inspection image data. Even with this configuration, the same effect as the above-described embodiment can be obtained.

(10) In the embodiment described above, the gradation correction table generation unit 48 sets the maximum value of the cumulative frequency in the cumulative histogram C2 and the cumulative histogram C1 to 256, which is the number of gradations of the output image formed by the image forming unit 50. For each normalized cumulative frequency, a corrected gradation characteristic T2 representing the correspondence between the gradation value in the cumulative histogram C2 and the gradation value in the cumulative histogram C1 is calculated. On the other hand, the gradation correction table generation unit 48 determines the gradation value in one of the cumulative histogram C2 or the cumulative histogram C1, and the other cumulative frequency distribution having an equal value or an approximate value to the cumulative frequency representing the gradation value. A corrected gradation characteristic T2 representing a correspondence relationship with the gradation value at may be calculated. Further, the gradation correction table generation unit 48 does not perform the above-described normalization, and for each cumulative frequency in the cumulative histogram C2 and the cumulative histogram C1, the gradation value in the cumulative histogram C2 and the gradation value in the cumulative histogram C1 The corrected gradation characteristic T2 representing the correspondence relationship between the two may be calculated. The gradation correction table generation unit 48 may calculate the corrected gradation characteristic T2 using the approximate gradation value in the cumulative histogram C2 and the approximate gradation value in the cumulative histogram C1.

(11) In the above-described embodiment, the image processing unit 40 is built in the image forming apparatus 1, but the image processing unit 40 may operate alone as an image processing apparatus.

1 is a diagram illustrating a configuration of an image forming apparatus 1. FIG. It is a figure which shows the function with which the image process part 40 is provided. It is a flowchart which shows the update process of a gradation correction table. It is a figure which shows the histograms H1 and H2. It is a figure which shows this cumulative histogram C1 and C2. It is a figure which shows the correction | amendment gradation characteristic T2. 6 is a cumulative histogram showing gradation values representing the ground colors of sheets A, B, and C. It is a figure which shows the accumulation histogram C2 'concerning a modification. It is a figure which shows the correction | amendment gradation characteristic T2 'concerning a modification. It is a figure which shows the correlation coefficient of histogram C1 and C2 '.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 10 ... Control part, 20 ... Display operation part, 30 ... Communication part, 40 ... Image processing part, 41 ... Original image data acquisition part, 42 ... 1st conversion part, 43 ... 1st extraction part, 44 ... Image formation condition correction unit, 45 ... Inspection image data acquisition unit, 46 ... Second conversion unit, 47 ... Second extraction unit, 48 ... Gradation correction table generation unit, 50 ... Image formation unit, 51 ... Paper supply unit , 52 ... Paper transport unit, 53 ... Image forming unit, 531 ... Photosensitive drum, 532 ... Charging unit, 533 ... Exposure unit, 534 ... Development unit, 535 ... Primary transfer unit, 54 ... Intermediate transfer belt, 55 ... Secondary Transfer unit 56... Fixing device 60. Image reading unit 61. Light source 62 62 Reflecting mirror 63 Image forming lens 64 Image sensor

Claims (14)

First acquisition means for acquiring image data representing an original image;
First conversion means for converting the image data acquired by the first acquisition means into image data in which gradation values are expressed in a first color system;
First extraction means for extracting, for each pixel, a gradation value of each color of the first color system from image data obtained by the conversion of the first conversion means;
Image forming means for forming an output image on a recording medium using a plurality of color materials corresponding to the first color system, in accordance with image forming conditions including image data acquired by the first acquiring means;
Second acquisition means for acquiring image data representing an output image formed by the image forming means as inspection image data;
Second conversion means for converting the inspection image data acquired by the second acquisition means into image data in which gradation values are expressed in the first color system;
Second extraction means for extracting, for each pixel, a gradation value of each color of the first color system from inspection image data obtained by the conversion of the second conversion means;
Using the gradation value of each pixel extracted by the first extraction means, a first cumulative frequency distribution representing the cumulative number of pixels in each gradation value is generated, and extracted by the second extraction means Generating means for generating a second cumulative frequency distribution that represents the cumulative number of pixels in each gradation value using the gradation value of each pixel that has been generated;
Calculating means for calculating a correspondence between a gradation value in the first cumulative frequency distribution and a gradation value in the second cumulative frequency distribution for each cumulative frequency representing the cumulative number of the pixels;
An image forming apparatus comprising: a correcting unit that corrects the image forming condition that the image forming unit should follow based on the correspondence calculated by the calculating unit. The calculation means normalizes the maximum value of the cumulative frequency in the first cumulative frequency distribution and the second cumulative frequency distribution to be the gradation number of the output image, and for each normalized cumulative frequency, The correspondence relationship between the gradation value or approximate gradation value in the first cumulative frequency distribution and the gradation value or approximate gradation value in the second cumulative frequency distribution is calculated. Image forming apparatus. The calculation means includes a gradation value in one of the first cumulative frequency distribution or the second cumulative frequency distribution and a gradation in the other cumulative frequency distribution having an equal value or an approximate value to a cumulative frequency representing the gradation value. The image forming apparatus according to claim 1, wherein a correspondence relationship with the value is calculated. A ground color specifying means for specifying a gradation value representing a color of the recording medium on which an output image is not formed by the image forming means;
The generating means uses the gradation value obtained by excluding the gradation value representing the color of the recording medium specified by the ground color specifying means from the gradation values extracted by the second extracting means. The image forming apparatus according to claim 1, wherein the second cumulative frequency distribution is generated. A color reading unit that reads the color of the recording medium before the output image is formed by the image forming unit or the color of the storage medium in the area where the output image is not formed by the image forming unit;
The ground color specifying means specifies the gradation value representing the color read by the color reading means as the gradation value representing the color of the recording medium on which no output image is formed by the image forming means. The image forming apparatus according to claim 4. The ground color specifying means obtains a gradation value that maximizes a correlation coefficient between the first cumulative frequency distribution and the second cumulative frequency distribution, and uses the gradation value to represent a color of the recording medium. The image forming apparatus according to claim 4, wherein the image forming apparatus is specified as a tone value. The generation means further generates a first frequency distribution representing the number of pixels in each gradation value by using the gradation value of each pixel extracted by the first extraction means, and the second frequency distribution Using the gradation value of each pixel extracted by the extracting means, a second frequency distribution representing the number of pixels in each gradation value is generated,
The ground color specifying means obtains a gradation value that maximizes a correlation coefficient between the first frequency distribution and the second frequency distribution, and uses the gradation value as a gradation value representing the color of the recording medium. The image forming apparatus according to claim 4, wherein the image forming apparatus is specified as follows. The reflected light when the original image or the output image is irradiated with light, and the reflected image including visible light and invisible light is received to read the image, and the gradation value is determined in the second color system. Comprising image reading means for generating image data in which
The image forming apparatus according to claim 1, wherein the first acquisition unit or the second acquisition unit acquires image data generated by the image reading unit. A plurality of color materials corresponding to the first color system, a gradation pattern forming means for forming a gradation pattern having a predetermined halftone dot area ratio on the surface of the photoreceptor or the intermediate transfer belt;
Measuring means for measuring the gradation value of the gradation pattern formed by the gradation pattern forming means in the first color system;
The image forming apparatus according to claim 1, wherein the calculation unit calculates the correspondence relationship in consideration of a gradation value measured by the measurement unit. The first acquisition unit determines whether or not attribute information representing a predetermined dot pattern is added to the acquired image data.
The generating means includes
When it is determined by the first acquisition unit that attribute information representing a predetermined dot pattern is added to the acquired image data, the image data acquired by the first acquisition unit is added to the image data acquired by the first acquisition unit. Using the gradation value extracted by the first extraction means from the image data to which the image data representing the predetermined dot pattern is added, or the gradation value of the pixel representing the predetermined dot pattern, The first cumulative frequency distribution is generated using a gradation value added to a gradation value extracted by the first extraction unit from a position where a dot pattern is arranged. Image forming apparatus. Determination means for determining whether or not the number of gradation values included in the image data acquired by the first acquisition means is greater than or equal to a predetermined threshold;
The image forming apparatus according to claim 1, wherein the calculation unit does not calculate the correspondence when the determination unit determines that the number of gradations is smaller than the predetermined threshold. . The generating means includes
Generating the first cumulative frequency distribution based on the gradation values respectively extracted by the first extraction unit from the plurality of image data acquired by the first acquisition unit;
Further, the second cumulative frequency distribution is generated based on the gradation values respectively extracted by the second extraction unit from each of the inspection image data corresponding to the plurality of image data. The image forming apparatus according to claim 1. Switching means for switching the operation mode of the device to the first operation mode or the second operation mode;
The first conversion means includes
When the operation mode of the apparatus is the first operation mode, the image data acquired by the first acquisition unit is converted into image data in which gradation values are expressed in the first color system. And
When the operation mode of the device is the second operation mode, the image data acquired by the first acquisition unit is converted into image data in which gradation values are expressed in the first color system. Output without conversion,
The image forming apparatus according to claim 1, wherein the calculation unit does not calculate the correspondence when the operation mode of the apparatus is the second operation mode. First acquisition means for acquiring image data representing an original image;
First conversion means for converting the image data acquired by the first acquisition means into image data in which gradation values are expressed in a first color system;
First extraction means for extracting, for each pixel, a gradation value of each color of the first color system from image data obtained by the conversion of the first conversion means;
In accordance with image forming conditions including image data acquired by the first acquisition means, image data representing an output image formed on a recording medium using a plurality of color materials corresponding to the first color system, Second acquisition means for acquiring as inspection image data;
Second conversion means for converting the inspection image data acquired by the second acquisition means into image data in which gradation values are expressed in the first color system;
Second extraction means for extracting, for each pixel, a gradation value of each color of the first color system from inspection image data obtained by the conversion of the second conversion means;
Using the gradation value of each pixel extracted by the first extraction means, a first cumulative frequency distribution representing the cumulative number of pixels in each gradation value and the second extraction means Generating means for generating a second cumulative frequency distribution representing the cumulative number of pixels in each gradation value using the gradation value of each pixel;
Calculating means for calculating a correspondence relationship between a gradation value in the first cumulative frequency distribution and a gradation value in the second cumulative frequency distribution for each cumulative frequency representing the cumulative number of pixels;
An image processing apparatus comprising: a correcting unit that corrects the image forming condition based on the correspondence calculated by the calculating unit.

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