JP2015192412A - Image forming apparatus and control method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform image formation and output with an appropriate density.SOLUTION: An electrophotographic image forming apparatus includes: color conversion means which applies first color conversion processing to a pixel having a first image attribute and applies second color conversion processing to a pixel having a second image attribute; and total toner quantity control means for generating an image signal configured so as not to exceed a predetermined toner loading quantity in the case of image formation on the basis of pixel values converted by the color conversion means. The first color conversion processing includes density adjustment processing for adjusting an image density corresponding to at least a pixel value of a maximum density so as to form an image region including the pixel value of the maximum density as a solid image in the image formation. The second color conversion processing is color conversion processing considering a density change caused by the density adjustment processing in such a manner that the total toner quantity control means can generate an image signal through signal processing common for both the pixel value converted by the first color conversion processing and the pixel value converted by the second color conversion processing.

Description

  The present invention relates to an image forming technique for controlling image density in an output image.

  An electrophotographic system is known as an image recording system used in an image forming apparatus such as a printer or a copying machine. In the electrophotographic system, a latent image is formed on a photosensitive drum using a laser beam and developed with a charged color material (hereinafter referred to as toner). The image is recorded by transferring and fixing the developed toner image on paper. By the way, in the electrophotographic system, processes such as laser exposure, latent image formation on a photoreceptor, toner development, toner transfer to a paper medium, and fixing by heat in an electrophotographic process are the temperature, humidity, and components around the apparatus. Susceptible to changes over time. Therefore, the amount of toner finally fixed on the paper changes. Such instability is also known to occur in other systems such as an ink jet recording system and a thermal transfer system.

  Therefore, there is a method of correcting the characteristics of the image forming unit by outputting a test pattern image and measuring the density of the output pattern. As a non-linear characteristic correction method here, a lookup table (LUT) is generally used. When the correction is performed, in order to satisfy the gradation characteristic aimed by the apparatus, the correction is performed aiming at producing a smooth gradation characteristic to the maximum density targeted by the apparatus. For this reason, the image signal before conversion by the LUT is a signal having characteristics output with specific gradation characteristics (target gradation). However, depending on the state of the apparatus, a concentration exceeding the maximum concentration may be output, and the excess may be suppressed by the LUT.

  However, when the upper limit value is suppressed, an image in which jaggy or fine lines are interrupted may be output when a maximum density text or line is output. Therefore, Patent Document 1 discloses a method for allowing the user to select an LUT for forcibly correcting the upper limit value to the maximum value so as to ensure that the maximum signal input value is output at the maximum density.

  On the other hand, if the amount of toner per unit area exceeds a certain amount, toner fixing failure and scattering occur, and not only image quality is impaired but also there is a concern about damage to the apparatus body. For this reason, control is normally performed such that the total amount of toner per unit area is always less than a certain fixed amount. For example, in the case of a color printer, in order to suppress the excessive reduction, there is a method of reducing the total amount per unit area by changing the combination of toners of four colors of cyan, magenta, yellow, and black. Specifically, a method is known in which cyan, magenta, and yellow are reduced and black is increased to reduce the total amount of toner while suppressing color change and gradation deterioration.

JP 2001-94784 A

  As described above, under conditions where the upper limit of the toner amount per unit area is determined, if the output exceeds the maximum density targeted by the device in order to reduce jaggies, an image such as poor fixing or scattering Defects can occur. However, in general, the jaggy and discontinuity reduction processing is executed for text attribute pixels and graphic attribute pixels, and is not executed for image attribute pixels. Therefore, in order to control the maximum density in order to prevent the occurrence of image defects, it is necessary to switch processing for each pixel attribute, and as a result, switching of necessary hardware resources and software control increases, resulting in high cost. .

  The present invention has been made in view of the above-described problems, and an object thereof is to provide a technique that enables more suitable image formation output.

  In order to solve the above-described problems, an image forming apparatus according to the present invention has the following configuration. That is, in an electrophotographic image forming apparatus that forms an image on a recording medium based on an input image, a first color conversion process is performed on a pixel value of a pixel having a first image attribute for the input image. And applying a second color conversion process to a pixel value of a pixel having a second image attribute different from the first image attribute, and converted by the color conversion means A total toner amount control unit that generates an image signal based on a pixel value so as not to exceed a predetermined toner amount during the image formation, and the first color conversion process Includes a density adjustment process for adjusting an image density corresponding to at least the pixel value of the maximum density so that an image region having the pixel value of the maximum density is formed as a solid image in the image formation. 2 color conversion That is, the total toner amount control means performs the image processing by common signal processing for both the pixel value converted by the first color conversion processing and the pixel value converted by the second color conversion processing. This is a color conversion process in consideration of a change in image density due to the density adjustment process so that a signal can be generated.

  According to the present invention, it is possible to provide a technique that enables more suitable image formation output.

1 is a block diagram illustrating a schematic configuration of an image forming apparatus according to a first embodiment. 1 is a cross-sectional view of an image forming apparatus according to a first embodiment. It is a figure which shows the mode of the binarization using the dither method exemplarily. It is a figure which shows the flow of the various processes performed by the image process part. 7 is a flowchart of a toner total amount control main process. It is a flowchart of correction | amendment LUT production | generation. It is a figure which shows the test pattern image utilized in correction | amendment LUT production | generation. It is a figure which shows an example of the result of the density | concentration measurement of a test pattern image. It is a figure which shows an example of the characteristic of correction | amendment LUT. It is a figure which shows the example of a correction | amendment of the coefficient in a color conversion process (1st Embodiment). 6 is a flowchart for explaining correction coefficient setting (updating) processing (first embodiment); It is a figure which shows the example of correction | amendment of the coefficient in a color conversion process (2nd Embodiment). It is a flowchart explaining the setting (update) process of the coefficient of correction | amendment (2nd Embodiment). It is a figure which shows an example of correction | amendment LUT which does not require terminal point correction | amendment. It is a figure which shows an example of the setting of a mixing ratio.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. The following embodiments are merely examples, and are not intended to limit the scope of the present invention.

(First embodiment)
As a first embodiment of an image forming apparatus according to the present invention, an electrophotographic color image forming apparatus will be described below as an example.

<Device configuration>
FIG. 1 is a block diagram illustrating a schematic configuration of the image forming apparatus according to the first embodiment. The image forming apparatus (printer) includes an image reading unit 101, an image processing unit 102, a storage unit 103, a CPU 104, and an image output unit 105. Note that the image forming apparatus is configured to be connectable via a network or the like to a server that manages image data, a personal computer (PC) that instructs execution of printing, or the like.

  The image reading unit 101 reads an image of a document and outputs image data. The image processing unit 102 converts print information including image data input from the image reading unit 101 or the outside into intermediate information (hereinafter referred to as “object”), and stores it in an object buffer of the storage unit 103. Here, each object has any image attribute of text, graphic, or image. Further, the image processing unit 102 generates bitmap data based on the buffered object and stores it in the buffer of the storage unit 103. At that time, pseudo halftone processing such as color conversion processing, density adjustment processing, toner total amount control processing, video count processing, printer gamma correction processing, and dithering is performed. Details of each process will be described later.

  The storage unit 103 includes a ROM, a RAM, a hard disk (HD), and the like. The ROM stores various control programs and image processing programs executed by the CPU 104. The RAM is used as a reference area or work area in which the CPU 104 stores data and various types of information. The RAM and HD are used for the above-described object buffer and the like. For example, the image forming apparatus accumulates image data of a document over a plurality of sorted pages, and prints out a plurality of copies. The image output unit 105 forms and outputs a color image on a recording medium such as recording paper.

  FIG. 2 is a cross-sectional view of the image forming apparatus according to the first embodiment. The image reading unit 101 is a functional unit that optically reads the document 204 and generates image data. The image reading unit 101 irradiates the original 204 placed between the original table glass 203 and the original pressure plate 202 with the light of the lamp 205. Reflected light from the original 204 is guided to mirrors 206 and 207, and an image is formed on the three-line sensor 210 by the lens 208. Here, the lens 208 is provided with an infrared cut filter 231. Then, a mirror unit including the mirror 206 and the lamp 205 is moved at a speed V and a mirror unit including the mirror 207 is moved in the arrow direction (right direction in FIG. 2) at a speed V / 2 by a motor (not shown). That is, the mirror unit is moved in a direction (sub-scanning direction) perpendicular to the scanning direction (main scanning direction) of the three-line sensor 210 to scan the entire surface of the document 204.

  A three-line sensor 210 composed of a three-line CCD color-separates input light information, reads each color component of red (R), green (G), and blue (B), and performs image processing on the color component signal. Send to part 102. Each of the CCDs constituting the three-line sensor 210 has, for example, a light-receiving element for 5000 pixels, and a short side direction (297 mm) of an A3 size document that is the maximum size of a document that can be placed on the document table glass 203. ) At a resolution of 600 dpi. The standard white plate 211 is for correcting data read by the CCDs 210-1 to 210-3 constituting the three-line sensor 210. The standard white plate 211 is a plate that exhibits substantially uniform reflection characteristics within the visible light range.

  The image processing unit 102 electrically processes an image signal (RGB color space signal) input from the three-line sensor 210 to obtain cyan (C), magenta (M), yellow (Y), and black (K). Each color component signal is generated, and the generated CMYK color component signals are sent to the image output unit 105. The image processing unit 102 also performs the same processing on image data input from the outside, and sends the generated CMYK color component signals to the image output unit 105. At this time, the image output to the image output unit 105 is a CMYK image that has been subjected to pseudo intermediate processing such as dithering.

  The C, M, Y, or K image signal sent from the image reading unit 101 is sent to the laser driver 212 of the image output unit 105. The laser driver 212 modulates and drives the semiconductor laser element 213 according to the input image signal. The laser beam output from the semiconductor laser element 213 scans the photosensitive drum 217 via the polygon mirror 214, the f-θ lens 215, and the mirror 216, and forms an electrostatic latent image on the photosensitive drum 217.

  The developing unit includes a magenta developing unit 219, a cyan developing unit 220, a yellow developing unit 221 and a black developing unit 222. The four developing devices sequentially come into contact with the photosensitive drum 217, whereby the electrostatic latent image formed on the photosensitive drum 217 is developed with the corresponding color toner to form a toner image. The recording paper supplied from the recording paper cassette 225 is wound around the transfer drum 223, and the toner image on the photosensitive drum 217 is transferred to the recording paper. The recording paper onto which the four color toner images of C, M, Y, and K are sequentially transferred in this way passes through the fixing unit 226, and is discharged outside the apparatus after the toner image is fixed. .

<Prerequisite Technology for Image Forming Apparatus>
-Toner reduction When the maximum density of the target of the device is assumed to be "1.5", the toner amount becomes excessive when the density of "1.6" comes out of the device in a certain environment. It will be. In particular, the technique described in Patent Document 1 may reduce jaggy and the like, but may exceed the upper limit of the amount of recording material (toner or the like) per unit area. Therefore, a toner reduction process that limits the total amount of toner per unit area may be performed.

  However, if the total amount of toner per unit area is determined on the assumption that the toner density has shifted to the maximum density, the toner will be excessively reduced when the toner density shifts to a lower density. Usually, the greater the amount of toner used, the darker the image density. As a result, the color becomes deeper and more preferable color reproduction is possible. Therefore, it is said that the image quality is better as the toner reduction amount is kept to a minimum.

  For example, assuming that the maximum value of each color assumed by the apparatus is 100%, that is, the maximum is 400% for four colors, but the limit value of the total amount per unit area is limited to 250%. If it is assumed that the density fluctuation fluctuates in a direction that is 20% darker than the target, 120% of the toner amount assumed by the worst apparatus per color is output. Assuming that, it is conceivable to limit the limit value to about 250 × 0.8 = 200% in advance, but in this case, the color and gradation are deteriorated.

Video Count Processing In the electrophotographic system using a two-component developer that is widely used, the amount of toner replenishment is important in order to suppress the change in the amount of toner fixed on paper. The two-component developer is a developer mainly composed of toner (non-magnetic) and carrier (magnetic). In development using a two-component developer, the toner charge amount is adjusted by adjusting the proportion of toner in the developer so that an electrostatic image formed under a predetermined charge / exposure condition is developed with a predetermined amount of applied toner. Is kept constant. For example, when the toner charge amount is decreased, the amount of applied toner is increased and the image density is increased even in the same electrostatic image. Conversely, when the toner charge amount is increased, the image density is decreased even in the same electrostatic image.

  Therefore, in order to keep the ratio of the toner in the developer constant, it is necessary to replenish the toner as much as consumed. However, since the amount of toner actually consumed cannot be directly measured, there are various known consumption amount estimation methods. One of the important methods is a method called video count. The video count counts the predicted toner consumption of the entire image by converting the image signal having the characteristic output at the target gradation before the conversion processing using the above-described LUT into the predicted toner consumption. It is.

・ Problem Since the conversion to the predicted toner consumption in the video count is based on the premise that the image is output at the assumed target gradation, when the toner is reduced, the accurate count value is not obtained. As a result, since the actual gradation characteristics have increased, the consumed toner amount becomes larger than the consumed amount predicted from the video count value, and as a result, the replenishment amount becomes smaller than the ideal value. Then, the ratio of the toner density in the developer is changed, and a phenomenon in which the output density is continuously reduced occurs, so that stable gradation characteristics cannot be obtained.

  From the above, it is desirable to reduce jaggies and breaks while limiting the amount of toner per unit area. Further, it is desirable that the video count can be normally performed. However, reduction of jaggies and interruptions is necessary for pixels of text attributes and graphic attributes, but not for pixels of image attributes. However, when processing is switched for each pixel attribute, as a result, the required hardware resources and software control are increased, resulting in higher costs. For this reason, a method that requires less switching of necessary hardware resources and software control is desired.

<Image processing unit>
FIG. 4 is a diagram illustrating the flow of various processes executed by the image processing unit. Specifically, color conversion processing, density adjustment processing, toner total amount control processing, video count processing, gamma correction processing, and pseudo halftone processing are shown. The image processing unit performs processing based on the image itself and attribute information (text, image, graphic) for each pixel attached to the image.

  In the color conversion process 401, gradation image data (input image) expressed in RGB input from the network or the image reading unit 101 is converted into a CMYK color space depending on the image output unit 105 such as a toner color material. This process is performed using a three-dimensional lookup table (LUT). The three-dimensional LUT does not hold the relationship between all RGB and output CMYK values, but holds the relationship between specific RGB values called grid points and output CMYK values. For example, when RGB is input with an 8-bit value (0 to 255), it is set as a grid point position obtained by dividing each component direction of RGB into 15 sections. In that case, 4096 (= 16 × 16 × 16) lattice points and the corresponding CMYK values are the coefficients of the three-dimensional LUT. CMYK values for RGB values other than the grid points are obtained by a known interpolation calculation.

  Here, the color conversion processing includes black single color guarantee color conversion processing 403 for converting black so that it is expressed by K toner single color, and normal color conversion processing for expressing black by mixing CMY toners other than K toner. There are two types of color conversion processing 402. For example, the black single color guarantee color conversion processing 403 is applied to pixel values of text attributes and graphic attributes that are likely to have noticeable scattering and color misalignment around text and lines. On the other hand, the normal color conversion process 402 is applied to the pixel value of the image attribute pixel to express deep black.

  In the three-dimensional LUT, the converted CMYK values are set to have a specific target relationship (target gradation) with respect to the toner density on each paper surface. The target gradation differs depending on the design concept of each device, and the relationship between the CMYK value and the toner density may be linear or non-linear. If the value of CMYK is multi-valued data (for example, 8-bit value), if the value is “0”, the density is “0”, if the value is “255”, the maximum density targeted by the printer, and other values, A density corresponding to the target gradation is output. Hereinafter, this CMYK signal value is referred to as a “target gradation CMYK signal value”. When correcting a one-dimensional LUT of gamma correction processing described later in order to avoid jaggies and broken images, the output density indicated by the target gradation CMYK signal value changes, and it is necessary to deal with it. Become.

  The density adjustment process 404 adjusts the density of each CMYK toner. For example, the density is adjusted using a known method such as linear conversion processing or one-dimensional LUT. When the density adjustment is performed, the density changes, so that the total toner amount and the toner consumption amount change compared to immediately after color processing.

  The total toner amount control process 405 includes a toner density conversion process 406, a total toner amount control main process 407, and a toner density reverse conversion process 408. In the toner density conversion processing 406, the target gradation CMYK signal value is converted into a toner amount. In the total toner amount control main processing 407, total amount restriction processing is performed by performing replacement processing of CMY toner with K toner. Further, in the following description, the toner amount is expressed by setting the target maximum density output to “100%”. In the toner density reverse conversion process 408, the CMYK toner amount is returned to the target gradation CMYK signal value. Details will be described later. The output signal of the total toner amount control process 405 is an input for video count processing and gamma correction processing.

  The video count process 409 includes a toner consumption conversion process 410 and a count process 411. The toner consumption amount conversion process 410 converts the target gradation CMYK signal value into a toner consumption amount. The count process 411 counts the toner consumption for each color (toner consumption calculation means). The counted toner consumption information is sent from the image processing unit 102 to the image output unit 105 and used for toner supply processing in the image output unit 105.

  The gamma correction process 412 performs a gamma correction process on the output signal of the toner total amount control process 405 for each color. By this gamma correction processing, the non-linearity of the gradation of each color depending on the image output unit 105 is absorbed, and correction is performed so that the relationship between the target gradation CMYK signal value and the target gradation is maintained. This process is performed by a one-dimensional LUT. A method for creating this one-dimensional LUT will be described later.

  In pseudo halftone processing 413, halftone processing (pseudo halftone processing) is performed on the CMYK image after gamma correction. This process is performed by a known dither process or an error diffusion process. This process binarizes the input multivalued image data. Thereafter, the image that has been subjected to the pseudo-halftone process is transferred from the image processing unit 102 to the image output unit 105, whereby the image can be output.

  FIG. 3 is a diagram exemplarily showing the state of binarization by halftone processing using the dither method. FIG. 3B shows an example in which halftoning is performed on the multi-valued image in FIG. 3A and binarized. Similarly, the results of processing using FIGS. 3C, 3E, and 3G as inputs are as shown in FIG. 3D, f, and h, respectively.

  As described above, in the halftone process, the pseudo gradation is reproduced by forming an image of a predetermined dot pattern. Therefore, as long as the maximum value is not input, some missing dot occurs, and depending on the input image, the image area is output as a jaggy or a broken line. Hereinafter, the image having the maximum value is referred to as “solid”. In other words, this solid output is the highest density output without jaggies or interruptions.

<Operation of the device>
The printer state changes due to the influence of the installation environment and durability. Therefore, in the above-described processing, it is necessary to update setting values for the color conversion processing 401, the total toner amount control processing 405, the video count processing 409, and the gamma correction processing 412 according to the printer state. Therefore, in the following, update of setting values used in each process will be described. In addition, details of each process and a specific setting value calculation method will be described in detail.

  FIG. 11 is a flowchart for explaining correction coefficient setting (updating) processing. All of the setting processing is realized by the CPU 104 executing a predetermined control program.

  In step S <b> 1101, the CPU 104 generates a one-dimensional LUT coefficient used in the gamma correction process 412. As described above, the toner density (toner amount) on the recording paper changes due to the influence of the temperature and humidity around the apparatus and the aging of the components. In order to compensate for the instability of the gradation caused by this change, a gamma correction process is used. In other words, in the gamma correction process, it is necessary to update (calibrate) the coefficient so as to follow the change in the toner density caused by the environmental change, instead of continuing to operate the LUT coefficient with the initial value.

  FIG. 6 is a flowchart of the correction LUT generation. FIG. 7 is a diagram showing a test pattern image used in generating a correction LUT.

  In step S601, the printer forms and outputs a test pattern image on recording paper. The test pattern image includes, for example, a total of 32 (= 8 × 4) rectangular areas corresponding to eight levels of toner adhesion rate for each color of CMYK. In the following, the rectangular areas of the 8-level toner adhesion rate will be referred to as 0th to 7th patches, respectively. At this time, the input values (output values of the gamma correction processing 412) in the pseudo halftone processing 413 for the 0th to 7th patches are as follows. That is, the 0th patch is designated with a signal value “0”, the 1st patch is designated with a signal value “36”, the 2nd patch is designated with a signal value “73”, and the 7th patch is designated with a signal value “255” at equal intervals. This is arranged for each color of CMYK, only the pseudo halftone process 413 is applied, and the test pattern image output from the printer and formed on the recording paper shows the original gradation of the printer in the current environment. It becomes.

  In step S602, the image reading unit 101 reads a test pattern image formed on a recording sheet and acquires it as RGB images. At this time, using the R signal for the C toner patch, the G signal for the M toner and K toner patches, and the B signal for the Y toner patches makes it possible to obtain gradation with high accuracy. Become.

  In step S603, the image processing unit 102 averages the read signal value corresponding to each patch by the patch area, and calculates the representative read signal value of each patch. In step S604, the image processing unit 102 converts the obtained patch read signal value into a density. When the read signal is read as a linear signal with respect to the reflectance, the conversion to the density is logarithmic conversion. For example, when the input luminance is represented by an 8-bit value, the following equation can be used.

D = −255 × log 10 (S / 255) /1.6
This is a conversion formula in which the luminance signal S is normalized so that D = 255 when the document density is “1.6”. When D exceeds “255”, it is necessary to limit the value to “255” or less. Of course, instead of calculating using the above formula, conversion may be performed using a table in which luminance is input and density is output.

  FIG. 8 is a diagram illustrating an example of the result of density measurement of a test pattern image. In FIG. 8, the measured density for each patch in the test pattern is indicated by ◯. Here, the result of plotting the density of the 0th patch in the order of D0 and the density of the 1st patch in the order of D1 is shown. From the obtained result, the density when this printer outputs a solid can be specified by D7. Since the information of the original tone characteristics of the printer including the solid output density (hereinafter referred to as DMax) (D0 to D7 density information) is used in the subsequent processing, the CPU 104 holds the information in the storage unit 103. Keep it.

When the prime gradation characteristic of the printer is S (x), the inverse transformation is S_inv (y). When the target gradation characteristic expressed by the curve 82 is F (x) and the correction function is M (x), the characteristic of F (x) is given by M (x) = S_inv (F (x)). Can output an image. (S (M (x) = F (x) is obtained if the corrected signal is output with the original characteristic.)
In step S605, the CPU 104 generates a correction LUT equivalent to the correction function M (x). Actually, with respect to the prime gradation characteristics, values of points other than 8 are linearly interpolated or multi-dimensionally interpolated to obtain S [x] as the prime gradation characteristics LUT, and from there, the inverse transformation LUT What is necessary is just to obtain | require S_inv [y]. As for the target gradation characteristics, F [x] indicating the target gradation characteristics in relation to the LUT is calculated, and M [x] = S_inv [F [x]] is obtained as the correction LUT. The restrictions for obtaining the inverse conversion LUT will be described when describing the coefficient correction method of the normal color conversion process 402 described later. By using the correction LUT generated in this way, the target gradation can be obtained by correcting and outputting the CMYK value. This target is a fixed target that is assumed in advance by the printer. By correcting for this target, the same gradation state is guaranteed regardless of the color and gradation characteristics of the printer. . Since the corrected LUT coefficient is used in subsequent processing, the CPU 104 holds it in the storage unit 103.

  FIG. 9 is a diagram illustrating an example of the characteristics of the correction LUT. Referring to the solid curve 91 indicating the relationship between the input signal (before correction) and the output signal (after correction), the output value for the input value “255” is not “255”. This indicates that, when outputting a solid image, the maximum density targeted by the printer is exceeded. In addition, when gamma correction is performed using a solid as an input, it means that the dots are partially missing (that is, not solid) by dither processing.

  In step S606, the CPU 104 further corrects the output value at the end point of the correction LUT (near the maximum input value). That is, this further correction reflects a correction amount for ensuring that the input of “255” is solid. Hereinafter, this processing is referred to as end point correction, and an LUT obtained by the end point correction is referred to as an end point correction LUT. When the output value at the end point of the correction LUT is “255” from the beginning, the correction LUT and the end point correction LUT are the same, and the correction LUT is used as the end point correction LUT as it is.

  FIG. 14 is a diagram illustrating an example of a correction LUT that does not require terminal point correction. The correction LUT 1401 has an output value “255” with respect to the input value “255”. That is, since the input value “255” is output as a solid, there is no need to correct the end point. In addition, when the end point is corrected only for black, the correction LUT is used as the end point correction LUT as it is for the color for which the end point correction is not performed.

  As a specific end point correction method, a line (dashed line 92 in FIG. 9) that linearly connects the input maximum value 255 to the output maximum value 255 from the output Wo corresponding to the input value Wi is a new correction table. And This correction example is only an example, and the output signal may be corrected so that the density increases linearly from the Wo toward the end point instead of the output signal. By performing the end point correction processing in this way, it is possible to correct the end point while maintaining the continuity of the gradation although the gradation is partially deviated from the original target. The generated end point correction LUT is a coefficient used in the gamma correction processing 412. Further, the CPU 104 holds the data in the storage unit 103 for use in subsequent processing.

  In this way, by creating and holding the end point correction LUT used in the gamma correction process 412, it is possible to satisfactorily reproduce an image to be output as a solid image such as a line or text. In this example, the image reading unit 101 of the apparatus is used as the density calculating unit. However, the density value may be acquired by other means such as a densitometer.

  In step S1102, the CPU 104 determines whether or not the end point correction has been performed. For example, the CPU 104 refers to the correction LUT and the end point correction LUT held in the storage unit 103, and determines that the end point correction is performed when one of the CMYK colors is not the same. The process proceeds to step S1103. On the other hand, if the correction LUT and the end point correction LUT match for all colors, it is determined that the end point correction has not been performed, and the process advances to step S1108.

  In step S1103, the CPU 104 generates a color conversion processing coefficient considering the end point correction. The end point correction process is a process for preventing dots from being lost in a pixel region whose input is “255” by outputting a solid image as described above. Therefore, it is desirable that the end point correction LUT is applied to a pixel having a text attribute or a graphic attribute that the text or line is desired to avoid being interrupted. On the other hand, when the end point correction LUT is applied to the image attribute region, the tone characteristics change from the original target tone in the high density portion, and the amount of consumed toner increases. Therefore, it is not desirable to apply the end point correction to the pixel having the image attribute. However, when the presence / absence of the end point correction process is switched according to the attribute of the pixel, the output gradation characteristics assumed by the CMYK signal after the color conversion process differ according to the attribute of the pixel. Therefore, not only the gamma correction processing but also the total toner amount control processing 405 and the video count processing 409 need to be switched. Therefore, additional hardware resources or software control becomes complicated.

  Therefore, in the first embodiment, the color conversion process is devised to prevent the increase in hardware resources and the complexity of software control, while the end points of the image attribute pixels and other attribute pixels are set. Switch the effect of correction. Specifically, the coefficient of the normal color conversion process 402 applied to the pixel having the image attribute is corrected so as to remove the influence of the end point correction in the subsequent stage. Then, the end point correction LUT is applied by the gamma correction processing 412 and output. With such a configuration, it is not necessary to switch the applied coefficient between the image attribute and other attributes in the gamma correction process 412. Further, the CMYK signal after the color conversion process 401 is a signal having gradation characteristics on the premise that the end point correction LUT is applied to all attributes. Therefore, it is not necessary to switch the processing according to the attribute of the pixel in the toner total amount control processing 405 and the video count processing 409 as well.

  Hereinafter, a specific coefficient correction method of the normal color conversion process 402 will be described. If the coefficient of the normal color conversion process 402 is corrected according to the following formula, the coefficient is determined in consideration of the end point correction.

ODM ′ [r, g, b] = E_inv [M [ODM [r, g, b]]]
here,
M [x]: Output of specific color of correction LUT before end point correction at input x E [x]: Output of specific color of end point correction LUT at input x E_inv [y]: y = Inverse LUT that outputs the value of x at E [x] (however, if there are a plurality of x, the center value is obtained, and if there is no corresponding x, it is obtained by interpolation from x having a value of near y) )
ODM [r, g, b]: Output of a specific color at a grid point (r, g, b) of a three-dimensional LUT for normal color conversion ODM ′ [r, g, b]: For normal color conversion after correction This is an output of a specific color at a lattice point (r, g, b) of the three-dimensional LUT.

If the coefficients of the normal color conversion process 402 after correction are input to the end point correction LUT,
E [ODM ′ [r, g, b]] = E [E_inv [M [ODM [r, g, b]]]] ≈M [ODM [r, g, b]]
It becomes. Therefore, it can be seen that the coefficient is a desired correction. Note that the last equality is set to “≈” because it is a problem of bit accuracy and does not always return to the original value by the inverse transformation LUT. Since the above calculation is correction of the output value of a specific color, actually, it is necessary to perform the same calculation for all colors (CMYK) of all grid points of the three-dimensional LUT.

  FIG. 10 is a diagram illustrating an example of coefficient correction in the color conversion process. Here, one grid point of a three-dimensional LUT in which 16 grid points are arranged on each of the RGB axes and a total of 4096 (= 16 × 16 × 16) grid points exist is shown as an example. . Here, the lattice points correspond to R = 17, G = 34, and B = 0, and the coordinates of the lattice points are (r, g, b) = (1, 2, 0). First, for cyan, ODM [1,2,0] = 160. Then, M [160] = 148. If E [150] = 148, then E_inv [148] = 150. Therefore, ODM ′ [r, g, b] = E_inv [M [ODM [1, 2, 0]]] = 150. Magenta and yellow are not corrected because the end point is not corrected. For black, ODM [1,2,0] = 219. Then, M [219] = 197. If E [201] = 197, then E_inv [197] = 201. Therefore, ODM ′ [r, g, b] = E_inv [M [ODM [1, 2, 0]]] = 201.

  In step S <b> 1104, the CPU 104 generates a coefficient for the total toner amount control process 405 considering the end point correction. First, how to obtain the coefficient of the toner density conversion process 406 will be described. If the terminal point correction is not performed, the CMYK signal value output at the target gradation is obtained. Therefore, if the target gradation is converted to the target gradation with the maximum density of 100%, the toner amount and Become. For this reason, the target gradation LUT for conversion to the target gradation with the maximum density of 100% is the default coefficient. When the end point correction is performed, the CMYK signal value is not output in the original target gradation, and it is necessary to consider it. That is, U [S [E [x]]] is a coefficient of the toner density conversion process 406 in consideration of the end point correction.

  Here, the end point correction LUT for the input x is E [x], the LUT created by interpolating the original gradation characteristics of the printer held in the storage unit 103 is S [x], and the unit system conversion LUT is U Let [x]. The unit system conversion LUT is an LUT for converting into a unit system in which the maximum density of the target gradation is 100%. Therefore, the maximum output of U [S [E [x]]] becomes a value exceeding 100% due to the influence of the end point correction. This coefficient is a CMYK value on the premise that the end point correction LUT is applied in the gamma correction processing 412 even for the pixel of the image attribute, and can be used in common with the pixel of the text attribute and the graphic attribute. It is. Further, since the toner density conversion process 406 is common for the image signals generated based on the pixels of the respective attributes, all the pixels are also shared in the subsequent toner total amount control main process 407 and the toner density reverse conversion process 408. Can be processed. That is, it is possible to save hardware resources and simplify software control.

  When priority is given to the simplicity of calculation, the target gradation LUT may be multiplied by a gain and used as a coefficient for the toner density conversion processing 406. The gain at that time may be “Dmax / the maximum density of the target gradation” or “the maximum value of the end point correction LUT / the maximum value of the correction LUT”. In the above description, one toner color has been described, but the above processing is performed for all CMYK colors. In addition, the relationship between the toner density and the toner amount may be different for each toner color. In this case, the coefficient (LUT) of the toner density conversion process 406 for each color may be multiplied. This gain needs to be set for each model according to the toner to be used.

  FIG. 5 is a flowchart of a main toner amount control main process according to the first embodiment. This processing is performed with reference to all CMYK colors in units of pixels. In step S501, the image processing unit 102 inputs CMYK (C1, M1, Y1, K1) as an input to the total toner amount control main process 407, and in S502, calculates the total value SUM1. Here, CMYK (C1, M1, Y1, K1) is data of one pixel unit of the CMYK image after the toner density conversion process performed in the toner density conversion process 406, and is a CMYK toner amount.

  In step S503, the image processing unit 102 reads the LIMIT (limit value) 550 and compares it with SUM1. Here, LIMIT (limit value) 550 is a limit value of the toner amount that can be fixed, and is defined by a numerical value such as “250%”, for example. If the toner amount exceeding the LIMIT (limit value) 550 is to be fixed, there is a concern that the image quality of the output image is deteriorated or the image output unit 105 is damaged. Therefore, the final total toner amount needs to be LIMIT (limit value) 550 or less.

  Therefore, when SUM1 is LIMIT (limit value) 550 or less, CMYK (C1, M1, Y1, K1) is determined as CMYK (C3, M3, Y3, K3) in step S513. Here, CMYK (C3, M3, Y3, K3) is data of one pixel unit of the output CMYK image of the toner total amount control main processing 407. On the other hand, when SUM1 is larger than LIMIT (limit value) 550, the UCR value is calculated by the equation shown in step S505. Here, the UCR value affects the CMY toner reduction value and the K increase value. In the total toner amount control 304, in order to minimize the reduction amount of the toner amount, the half of the amount exceeding the limit value or the smallest value among C1, M1, and Y1 is set as the UCR value.

  In step S506, the image processing unit 102 calculates K2 among C2, M2, Y2, and K2, which are values after the first total toner amount restriction. As K2, a value obtained by adding a UCR value to K1 is basically used. However, a value exceeding the maximum value (K_MAX%) of the K conversion LUT in the toner density conversion process 406 cannot be set. This is because K_MAX corresponds to the toner density when the maximum value is output in the gamma correction processing 412. Therefore, if K_MAX% is exceeded, the value of K_MAX% is set to K2. This K_MAX is a coefficient in the total toner amount control main process generated in step S1104.

  In step S507, the image processing unit 102 reduces the values of C1, M1, and Y1, and calculates the values of C2, M2, and Y2. Here, the difference between the value K2 calculated in step S506 and the value K1 is set as a reduction value. In step S508, CMYK (C2, M2, Y2, K2) in which the total amount of toner calculated by the above processing is reduced is derived. In step S509, the image processing unit 102 calculates SUM2 by summing C2, M2, Y2, and K2.

  In step S510, the image processing unit 102 reads the LIMIT (limit value) 550 and compares it with SUM2. If SUM2 is LIMIT (limit value) 550 or less, the image processing unit 114 determines CMYK (C2, M2, Y2, K2) as CMYK (C3, M3, Y3, K3) in step S512. On the other hand, if SUM2 is larger than LIMIT (limit value) 550, in step S511, the image processing unit 102 sets the value of K2 as it is as K3, and subtracts K2 from LIMIT (limit value) 550 and C2. A coefficient is calculated from the total value of M2 and Y2. By multiplying C2, M2, and Y2 by the calculated coefficient, C3, M3, and Y3 in which the toner amount is reduced are calculated, and CMYK (C3, M3, Y3, and K3) is determined. In step S514, CMYK (C2, M2, Y2, K2) in which the total amount of toner determined by the above processing is reduced is output.

  Finally, how to obtain the coefficient of the toner density reverse conversion process 408 will be described. In the toner density reverse conversion process 408, the toner amount may be returned to the CMYK value on the assumption that the gamma correction process 412 is applied. That is, the inverse conversion LUT of the conversion LUT of the toner density conversion process 406 becomes the coefficient of the toner density reverse conversion process 408. The inverse transformation LUT may be obtained in the same manner as the inverse transformation LUT of the end point correction LUT described in S1103. When there is no end point correction, the coefficient of the toner density conversion process 406 is a fixed value, so the coefficient of the toner density reverse conversion process 408 is also a fixed value.

  By using the generated coefficient in the total toner amount control processing 405, the total value of the CMYK toner amount considering the change in the output gradation characteristic due to the end point correction is set to a value not exceeding the value set as LIMIT. It becomes possible to update. As a result, it is possible to guarantee a toner amount equal to or less than the total toner amount assumed for the apparatus in units of pixels. As a result, it is possible to prevent image defects such as fixing failure and scattering.

  In the above description, a mechanism has been described in which each color can be processed as it is when the amount of toner exceeds 100%. When the amount of toner exceeding 100% cannot be handled due to hardware restrictions or the like, normalization may be performed so that the maximum amount of toner (CMYK_MAX) in all colors becomes 100%. Specifically, the gain of 100 / CMYK_MAX may be applied to the conversion LUT of the toner density conversion process 406 and the toner density reverse conversion process 408, and the LIMIT (limit value) 550 and K_MAX in the toner total amount control main process 407. For example, when the maximum toner amount of CMYK is 100%, 110%, 100%, and 120%, CMYK_MAX is 120%. When the LIMIT (limit value) 550 is 240%, the LIMIT (limit value) 550 after normalization may be set by multiplying 100/120 and 200%. Similarly, other coefficients may be normalized by multiplying by 100/120.

  In step S <b> 1105, the CPU 104 generates a coefficient for the toner consumption amount conversion process 410 in consideration of the end point correction. Due to factors such as transfer efficiency, the output density and the toner consumption amount do not always coincide completely. Therefore, the conversion LUT from the toner density to the toner consumption is C [x]. When the end point correction LUT for the input x is E [x] and the LUT created by interpolating the original gradation characteristics of the printer held in the storage unit 103 is S [x], C [S [E [ x]]] is the conversion LUT of the toner consumption conversion process 410 in consideration of the end point correction. C [x] is obtained in advance by actual measurement for each model. If there is a relationship between the toner density and the toner consumption only up to the target gradation, extrapolation is performed to derive the toner consumption of the portion where the density is higher than the target gradation for end point correction. . If the calculation is simplified, S [M [x]] may be used as the toner consumption as it is. The coefficient obtained here is a CMYK value on the premise that the end point correction LUT is applied in the gamma correction processing 412 even for the pixel having the image attribute. Therefore, both the image attribute pixel and the text attribute or graphic attribute pixel can be used in common. That is, it is possible to save hardware resources and simplify software control.

  In step S1106, the CPU 104 stores the coefficients created in steps S1103 to S1105 in the storage unit 103. That is, it is set as a coefficient used in image processing after the next job. Then, the flow ends.

  In step S <b> 1108, the CPU 104 stores a default coefficient prepared in advance in the storage unit 103. That is, it is set as a coefficient used in image processing after the next job. Note that the default coefficient is a coefficient that can be determined in advance on the assumption that the toner density characteristic comes out in accordance with the target gradation. For example, the default coefficient is stored in the storage unit 103 in advance at the time of manufacture. is there. Then, the flow ends.

  By using the coefficient set (updated) by the above processing, the signal space after color conversion is handled as a signal having a common target gradation characteristic even when the end point correction is performed. Is possible. That is, the toner total amount control process and the video count process can be handled in a common signal processing / signal space.

  As described above, according to the first embodiment, an image signal having common target gradation characteristics can be generated in the signal space after color conversion. That is, the toner total amount control process and the video count process can be performed in a common signal space. With this configuration, even if the image to be processed contains attributes that you want to output with end point correction (for example, text attributes or graphic attributes) and attributes that you want to output without correction (image attributes), complicated switching processing is required. Is no longer needed. Therefore, it is possible to perform the end point correction, the total toner amount control process, and the video count process together while suppressing an increase in necessary hardware resources and software control load.

  In the above description, the description has been made focusing on the density of the end point (near the maximum input value). However, the density of the specific color may be increased overall. In this case, the LUT that is darkened as a whole may be read as the overall density correction LUT, and the end point correction LUT may be read as the overall density correction LUT.

  In the above description, as an update method of the one-dimensional LUT for gamma correction, an example of outputting to paper and performing calibration using a reader or a densitometer has been described. However, only a specific density range is obtained using a sensor in the main body. Even simple calibration such as correction can be realized.

  Furthermore, in the above description, an example in which the same processing is performed for all CMYK colors has been described, but it is also possible to apply only to a specific color. By limiting to a specific color, the influence of the end point correction becomes small, and it becomes possible to give priority to gradation and color. For example, a specific color is designated as K, which has little influence on the color balance and has a large influence of jaggies and interruptions.

(Second Embodiment)
In the second embodiment, an example in which the coefficient of the black single color guarantee color conversion process 403 is corrected together with the correction of the coefficient of the normal color conversion process 402 will be described. Note that the configuration of the image forming apparatus and the appearance of the apparatus are the same as those in the first embodiment, and a description thereof will be omitted. In the following, a coefficient correction method of the black single color guaranteeing color conversion processing 403, which is different from the first embodiment, will be mainly described.

  FIG. 13 is a flowchart for explaining correction coefficient setting (updating) processing (second embodiment). Since only step S1303 is different from the first embodiment (FIG. 11), only step S1303 will be described.

  In step S1303, the CPU 104 generates a color conversion processing coefficient considering the end point correction. The most important purpose of the end point correction processing is to avoid jaggies and breaks in fine lines for text and lines drawn in a single color. For this reason, it is not highly necessary to correct the end point of a portion that is mixed with other toner even if the pixel has a text attribute or a graphic attribute. On the other hand, end point correction can avoid jaggies and breaks in fine lines, but it also has the adverse effect of increasing the toner consumption and changing the color of the mixed color portion. Therefore, in the black single color guarantee color conversion processing 403, the coefficient is corrected so that the strength of the end point correction is increased for a portion close to a single color and the strength of the end point correction is decreased for a mixed color portion.

  A specific coefficient correction method of the black single color guarantee color conversion processing 403 will be described. If the coefficient of the black single color guarantee color conversion process 403 is corrected according to the following formula, the coefficient takes into consideration the color mixture condition and the end point correction.

B_ODM ′ [r, g, b] = E_inv [M [B_ODM [r, g, b]]] × W [sum] + B_ODM [r, g, b] × (1−W [sum])
here,
M [x]: Output of specific color of correction LUT before end point correction at input x E [x]: Output of specific color of end point correction LUT at input x E_inv [y]: y = Inverse transformation LUT that outputs the value of x at E [x] (however, if there are a plurality of x values, the center value is obtained, and if there is no corresponding x value, it is obtained by interpolation from x having a near y value)
B_ODM [r, g, b]: Output of specific color of grid point (r, g, b) in 3D LUT of black color guarantee color conversion B_ODM ′ [r, g, b]: Black monochrome guarantee after correction Output of specific color of grid point (r, g, b) in three-dimensional LUT for color conversion sum: Summation of grid point (r, g, b) other than specific color in three-dimensional LUT of color conversion for black single color guarantee Value (for example, when K is a specific color, the values of C, M, and Y are added together)
W [sum]: LUT that returns a ratio to be output without end point correction according to the value of the color mixture sum
It is.

  For the portion of E_inv [M [B_ODM [r, g, b]]] in the above formula, the first embodiment is the same except that the three-dimensional LUT is a three-dimensional LUT for black single color guarantee color conversion. This is the same as the coefficient correction method of the normal color conversion process 402 in FIG. Therefore, E_inv [M [B_ODM [r, g, b]]] is a correction value for output without termination correction when the termination point correction LUT is used in the gamma correction processing 412. This correction value and the output value of the default B_ODM [r, g, b] that will be output with termination correction correction are mixed and output at a ratio W [sum].

  The ratio W [sum] is an LUT in which the proportion of correction for output without termination correction is increased in proportion to the total value of colors other than the specific color. That is, when the CMYK output is 8 bits, this LUT returns “0” when the input is “0”, and returns “1” when the input is “765 (= 255 × 3)” which is the maximum value. LUT. For intermediate input, considering the toner consumption, a value that makes the output “1” as soon as possible may be set for each model so as not to make a step or gradation jump.

  FIG. 15 is a diagram illustrating an example of setting the mixing ratio W [sum]. In FIG. 15, since the mixing ratio is set to “1” at an early stage, a large area is output without the end point correction, and the color does not change in the area and the toner consumption amount is large. Will not Increase. Note that the calculation based on the above formula is correction of the output value of a specific color, and in fact, it is necessary to perform the same calculation for all colors (CMYK) of all grid points of the three-dimensional LUT.

  FIG. 12 is a diagram illustrating an example of coefficient correction in the color conversion process (second embodiment). 12 (a) and 12 (b) show, as an example, one grid point of a three-dimensional LUT having 16 grid points on the RGB axes and a total of 16 × 16 × 16 = 4096 grid points.

FIG. 12A shows lattice points corresponding to R = 17, G = 0, and B = 0, and the coordinates of the lattice points are (r, g, b) = (1, 0, 0). . First, cyan, magenta, and yellow are not corrected because end point correction is not performed. For black, B_ODM [1, 0, 0] = 240. Then, M [240] = 223. If E [230] = 223, then E_inv [223] = 230. As a result, the correction value at the time of output corresponding to no end point correction is E_inv [M [B_ODM [r, g, b]]] = 230. The total value of CMY is 18, and sum = 18. W [18] is 0.1. Therefore,
B_ODM ′ [r, g, b] = E_inv [M [B_ODM [r, g, b]]] × W [sum] + B_ODM [r, g, b] × (1−W [sum])
= 230 × 0.1 + 240 × 0.9 = 239
It becomes. In the example of FIG. 12A, since the degree of single color is strong, the correction value is strongly influenced by the end point correction.

FIG. 12B shows lattice points corresponding to R = 51, G = 17, and B = 34, and the coordinates of the lattice points are (r, g, b) = (3, 1, 2). . First, cyan, magenta, and yellow are not corrected because end point correction is not performed. For black, B_ODM [3,1,2] = 206. Then, M [206] = 181. When E [196] = 181, E_inv [181] = 196. As a result, the correction value at the time of output corresponding to no end point correction is E_inv [M [B_ODM [r, g, b]]] = 196. The total value of CMY is 337, and sum = 337. And W [337] is 0.8. Therefore,
B_ODM ′ [r, g, b] = E_inv [M [B_ODM [r, g, b]]] × W [sum] + B_ODM [r, g, b] × (1−W [sum])
= 196 × 0.8 + 206 × 0.2 = 198
It becomes. In the example of FIG. 12B, since the degree of color mixture is strong, the correction value enables output that suppresses the influence of end point correction. Here, an example in which the end point correction is performed only for the K toner has been described, but the same method may be applied to the case where the C, M, and Y toners are targets. The correction of the coefficients in the normal color conversion process 402 is the same as that in the first embodiment described above, and a description thereof will be omitted.

  As described above, according to the second embodiment, in addition to the effects of the first embodiment described above, the influence of the end point correction is minimized, and jaggies and discontinuities in text and graphics monochrome portions are suppressed. Is possible. In addition, since the influence of the end point correction is suppressed for the mixed color portion, an increase in toner consumption and a change in color are suppressed.

  In the above description, the description has been made focusing on the density of the end point (near the maximum input value). However, the density of the specific color may be increased overall. In this case, the LUT that is darkened as a whole may be read as the overall density correction LUT, and the end point correction LUT may be read as the overall density correction LUT.

(Other examples)
The present invention can also be realized by executing the following processing. That is, software (program) that realizes the functions of the above-described embodiments is supplied to a system or apparatus via a network or various storage media, and a computer (or CPU, MPU, or the like) of the system or apparatus reads the program. It is a process to be executed.

DESCRIPTION OF SYMBOLS 101 Image reading part; 102 Image processing part; 103 Storage part; 105 Image output part

Claims (10)

An electrophotographic image forming apparatus that forms an image on a recording medium based on an input image,
A pixel value of a pixel having a second image attribute different from the first image attribute by applying a first color conversion process to a pixel value of a pixel having the first image attribute for the input image Color conversion means for applying the second color conversion processing to
A total toner amount control means for generating an image signal configured so as not to exceed a predetermined applied toner amount during the image formation based on the pixel value converted by the color conversion means;
Have
In the first color conversion process, at least a density for adjusting an image density corresponding to the pixel value of the maximum density so that an image area having the pixel value of the maximum density is formed as a solid image in the image formation. Including the adjustment process,
The second color conversion process is common to both the pixel value converted by the first color conversion process and the pixel value converted by the second color conversion process by the toner total amount control means. An image forming apparatus, wherein color conversion processing is performed in consideration of a change in image density by the density adjustment processing so that the image signal can be generated by the signal processing. Toner consumption calculating means for calculating toner consumption based on the image signal generated by the toner total amount control means;
Determining means for determining the amount of toner to be replenished based on the toner consumption calculated by the toner consumption calculating means;
The image forming apparatus according to claim 1, further comprising: Gamma correction means for performing gamma correction on the image signal generated by the total toner amount control means;
Halftone processing means for applying halftone processing based on the image signal gamma corrected by the gamma correction means;
The image forming apparatus according to claim 1, further comprising: The first image attributes are a text attribute and a graphic attribute;
The image forming apparatus according to claim 1, wherein the second image attribute is an image attribute. A first determining means for determining a first look-up table (LUT) used for the first color conversion processing;
The first determination unit reflects the correction amount in the density adjustment processing on the LUT determined based on the gradation characteristics in the image formation and the target target gradation characteristics, thereby allowing the first determination section to reflect the correction amount in the density adjustment processing. The image forming apparatus according to claim 1, wherein one LUT is determined. The color conversion unit uses the LUT determined by the first determination unit for the color component on which the density adjustment process is performed in the first color conversion process, and uses the first color. The image forming apparatus according to claim 5, wherein an LUT specified in advance is used for a color component that has not been subjected to the density adjustment process in the conversion process. The density adjustment process is configured to reduce the intensity of adjustment when a pixel to be processed is a mixed color as compared with a case where the pixel is a single color. Image forming apparatus. The color conversion means converts the pixel value of the input image expressed in the RGB color space into a pixel value expressed in the CMYK color space,
8. The toner total amount control unit generates an image signal configured so as not to exceed a predetermined amount of applied toner during the image formation for each color of CMYK. The image forming apparatus according to claim 1. A control method of an electrophotographic image forming apparatus that forms an image on a recording medium based on an input image,
A pixel value of a pixel having a second image attribute different from the first image attribute by applying a first color conversion process to a pixel value of a pixel having the first image attribute for the input image A color conversion step of applying the second color conversion process to
A total toner amount control step for generating an image signal configured so as not to exceed a predetermined amount of applied toner during the image formation based on the pixel value converted by the color conversion step;
Including
In the first color conversion process, at least a density for adjusting an image density corresponding to the pixel value of the maximum density so that an image area having the pixel value of the maximum density is formed as a solid image in the image formation. Including the adjustment process,
The second color conversion process is common to both the pixel value converted by the first color conversion process and the pixel value converted by the second color conversion process in the toner total amount control step. A method for controlling an image forming apparatus, comprising: color conversion processing in consideration of a change in image density due to the density adjustment processing so that the image signal can be generated by the signal processing.   A program for causing a computer to function as each unit of the image forming apparatus according to any one of claims 1 to 8.