JP2015198364A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device in which respective gradation correction conditions for plural halftone processing are created with higher precision.SOLUTION: A density sensor 117 measures the density of a pattern formed as a toner image by an image forming engine 101. A table creator 307 creates a gradation correction condition for halftone processing executed on a pattern image on the basis of a measurement result of the halftone processing executed on the pattern image. Particularly, the table creator 307 feeds back the measurement result of the halftone processing executed on the pattern image to the gradation correction condition for the halftone processing excluding the halftone processing executed on the pattern image on the basis of the dot precision degree of the halftone processing executed on the pattern image and the dot precision degree of the halftone processing excluding the halftone processing executed on the pattern image.

Description

  The present invention relates to an image forming apparatus.

  In an image forming apparatus, it is known that the density of an image varies from a desired density based on various factors. In addition, an image forming apparatus including a plurality of halftone processes (dither processes) needs to periodically create appropriate gradation correction conditions for each halftone process to bring the image density close to the desired density.

  In Patent Documents 1 and 2, a toner pattern is formed using one halftone process, the density of the pattern is detected, density fluctuations of other halftone processes are predicted, and a gradation is determined for each of a plurality of halftone processes. It describes that a tone correction condition is created. In these inventions, the pattern may be formed by one halftone process among a plurality of halftone processes, which is advantageous in that the toner consumption can be reduced.

JP 2013-013045 A JP 2013-031162 A

  By the way, there are various types of halftone processing such as those with coarse dot definition and those with fine dot definition. It is difficult to predict the density of a pattern subjected to halftone processing with fine dot definition from the density of a pattern subjected to halftone processing with coarse dot definition. For example, it is difficult to predict the density of a pattern that has been subjected to halftone processing with a high number of screen lines from the density of a pattern that has undergone halftone processing with a low number of screen lines. This is because the density of a pattern subjected to halftone processing with a low number of lines is insensitive to density fluctuation, but the density of a pattern subjected to halftone processing with a high number of lines is sensitive to density fluctuation. As described above, it is impossible to predict the density of the pattern subjected to the high tone number halftone process from the density of the pattern subjected to the low tone number halftone process.

  Accordingly, an object of the present invention is to create the gradation correction conditions for each of a plurality of halftone processes with higher accuracy.

The present invention is, for example,
Correction means for correcting image data using gradation correction conditions;
Processing means for performing halftone processing on the corrected image data;
Forming means for forming an image based on the image data subjected to the halftone processing;
Measuring means for measuring a pattern image formed by the forming means;
Creating means for creating gradation correction conditions for halftone processing applied to the pattern image based on the measurement result of the pattern image;
The creating means applies the pattern image to the pattern image based on the dot definition of the halftone process applied to the pattern image and the dot definition of the halftone process other than the halftone process applied to the pattern image. An image forming apparatus is provided, wherein the measurement result of the halftone processing is fed back to a gradation correction condition for halftone processing applied to the pattern image.

  According to the present invention, the gradation correction conditions for each of a plurality of halftone processes can be created more accurately.

Schematic diagram of image forming apparatus Diagram showing the configuration of the print system The figure which shows the preparation method of the gradation correction table Diagram showing density characteristics for each number of lines Diagram showing the concept of latent image Diagram showing the difference in dot size depending on the number of lines The figure which shows an example of the pattern formed between paper The figure which shows an example of a feedback rate table Flow chart showing dot definition determination processing The figure which shows an example of the characteristic of several halftone processing Flowchart showing tone correction table creation processing The figure which shows an example of the characteristic of several halftone processing The figure which shows an example of a feedback rate table Flow chart showing feedback rate determination process Diagram showing an example of feedback rate

(Image forming device)
In FIG. 1, the image forming apparatus 100 includes four image forming stations 120, 121, 122, and 123 that constitute an image forming engine. The mechanical configurations of the image forming stations 120, 121, 122, and 123 are common except for the color of the toner. The primary charger 111 uniformly charges the surface of the photosensitive drum 105. The laser scanner unit 107 irradiates the surface of the photosensitive drum 105 with laser light from the laser 108 to form an electrostatic latent image corresponding to the image data. The developing device 112 develops the electrostatic latent image using toner to form a toner image. The toner image is primarily transferred from the photosensitive drum 105 to the intermediate transfer belt 106 and further transferred to the transfer material 110 by the transfer roller 114. The transfer material 110 may be called a recording material, a recording medium, a paper, a sheet, a transfer paper, or the like. The fixing devices 150 and 160 fix the unfixed toner image to the transfer material 110. The operation unit 180 includes a display device that displays information and an input device that inputs information. Switching of halftone processing is instructed through the operation unit 180. The density sensor 117 has a light emitting part that emits light and a light receiving part that receives reflected light from the toner image, and outputs a signal (measurement data) corresponding to the density.

(Image processing unit)
In FIG. 2, the image forming apparatus 100 includes a printer controller 300 and an image forming engine 101. The host computer 301 is connected to the bus 319 through the host I / F unit 302. The operation unit 180 is connected to the bus 319 via the panel I / F unit 311. The external memory 181 is connected to the bus 319 through the memory I / F unit 312. The control code received by the host I / F unit 302 is temporarily held in the input / output buffer 303. The printer controller CPU 313 controls each unit connected via the bus 319. The ROM 304 stores a control program executed by the printer controller CPU 313 and a program containing control data. The ROM 304 is a nonvolatile memory and may be a rewritable memory. The RAM 309 is used as a work memory. Typical functions realized by the printer controller CPU 313 executing a program include an image information generation unit 305, a condition determination unit 306, and a table creation unit 307. The image information generation unit 305 generates various image objects based on data received from the host computer 301. A RIP (Raster Image Processor) unit 314 develops an image object into a bitmap image. The color processing unit 315 performs color conversion of the RGB bitmap image to generate a YMCK bitmap image. A gradation correction unit 316 performs gradation correction of each image of YMCK. The halftone processing unit 317 applies pre-designated pseudo halftone processing such as a dither matrix method and an error diffusion method to the tone-corrected image data. The image data output from the halftone processing unit 317 is transferred to the image forming engine 101 through the engine I / F unit 318. The engine control CPU 102 controls the laser scanner unit 107 and the like based on the image data to form an image.

  The condition determination unit 306 determines a maximum density (Vcont: development contrast) based on measurement data of a pattern image (hereinafter simply referred to as a pattern). More specifically, the condition determining unit 306 uses the measurement data obtained by the density sensor 117 to obtain an image density characteristic with respect to the developing bias, and calculates a developing bias and an exposure amount that can obtain the maximum density from the characteristic. To the engine control CPU 102. Thereby, the maximum density is kept constant. The table creation unit 307 performs gradation correction based on the result of the maximum density adjustment. The table storage unit 310 stores the gradation correction conditions created by the table creation unit 307. The gradation correction condition is held as, for example, a gradation correction table (such as a gamma lookup table (γLUT)). A gradation correction unit 316 executes gradation correction of image data using a gradation correction table.

(Method for creating a gradation correction table using a pattern)
The table creation unit 307 forms a pattern subjected to the halftone process and measures the density in accordance with the halftone process identification signal notified from the printer controller CPU 313. Then, the table creation unit 307 creates (updates or corrects) a gradation correction table corresponding to the halftone process based on the measurement data (measurement result). Further, the table creation unit 307 creates each gradation correction table corresponding to another halftone process that has not been applied to the pattern. At this time, the measurement data obtained for the halftone process applied to the pattern is fed back to the gradation correction table for other halftone processes. In particular, the present embodiment is characterized in that the feedback rate is determined in consideration of the relationship between the dot definition of halftone processing applied to the pattern and the dot definition of halftone processing not applied to the pattern image. is there.

  Here, it is assumed that the gradation correction table is composed of a first γLUT and a second γLUT. The first γLUT is a table for correcting comparatively long-term density fluctuations, and is created or updated immediately after the image forming apparatus 100 is turned on or when environmental conditions change greatly. The second γLUT is a table for correcting relatively short-term density fluctuations, and is created every time one image is formed. The first γLUT is formed using, for example, 10 patterns formed and the measurement results. On the other hand, the second γLUT is created by measuring the density of, for example, four patterns formed between sheets. The sheet interval is between the preceding toner image and the succeeding toner image on the intermediate transfer belt 106. That is, a pattern is formed using a section that is not transferred to the transfer material 110. Note that in order to improve the number of images (throughput) formed per unit time, it is necessary to narrow the gap between sheets, so the number of patterns that can be formed between sheets is also reduced. However, since the measurement data can be obtained with high frequency by forming a pattern between the sheets, the density fluctuation in a short period can be corrected even with a small number of patterns.

  As shown in FIG. 3A, the first γLUT1 is a table for correcting the input / output relationship so that the output result for the input signal IN matches the target density characteristic TGT. Immediately after the first γLUT1 is created, the second γLUT2 is reset to a table such as a unit matrix that does not correct the input / output relationship.

  FIG. 3B shows the measured densities of the four patterns formed between the sheets in order to create the second γLUT. As can be seen from FIG. 3B, the measured densities of the four patterns are deviated from the target density characteristic TGT, and thus must be corrected by the second γLUT2. As shown in FIG. 3C, the table creation unit 307 obtains the difference ΔD between the measurement data and the target density characteristic TGT, and creates the second γLUT2 so as to correct the difference ΔD. The table creation unit 307 combines the first γLUT and the second γLUT, stores the combined γLUT in the gradation correction unit 316, and prepares for the next image formation.

  In this way, a composite γLUT for a certain halftone process is created, but the composite γLUT for the remaining halftone process may be updated without using a pattern. For example, when the table creation unit 307 forms a pattern subjected to halftone processing with fine dot definition, the measurement result is obtained in the composite γLUT of halftone processing with a coarser dot definition than that of the halftone processing. Reflect. On the other hand, when the table creation unit 307 forms a pattern subjected to halftone processing with coarse dot definition, the composite γLUT for halftone processing with a finer dot definition than the dot definition of the halftone processing is used. Do not reflect measurement results. In other words, the composite γLUT for halftone processing with finer dot definition is not updated and is not involved in the pattern. As a result, a composite γLUT for each halftone process can be created with high accuracy. As a result, the reproducibility of the gradation in the image subjected to each halftone process will be good.

(Details of issues and features of this embodiment)
FIG. 4 shows a VD characteristic which is a relationship between the development contrast potential (difference between development bias and exposure potential: hereinafter referred to as Vcont) and density. Here, a comparison is made between a halftone process with a high line number (242 lpi) with fine dot definition and a halftone process with a low line number (106 lpi) with a coarse dot definition. The VD characteristic for 106 lpi is a clean downward convex curve. On the other hand, the V-D characteristic for 242 lpi is a so-called S-shaped curve characteristic in which the density is difficult to appear in the highlight region, the density changes abruptly in the middle tone, and the density does not change in the shadow portion. When deterioration of the photosensitive drum 105 or fluctuation of the light source of the laser scanner unit 107 occurs, density fluctuation is likely to occur. This is because the output density is not linearly related to the development contrast potential Vcont determined using a solid image having a uniform density, and an S-shaped curve is often drawn. Furthermore, this phenomenon depends on whether the dot contrast in the latent image is sufficiently secured. However, in the following, in order to refer to the density variation due to the fineness of the dots, the latent image when the dots are formed will be described.

  FIG. 5A shows a latent image potential in a highlight area to which halftone processing is applied. FIG. 5B shows the latent image potential of the halftone portion to which the low tone number halftone processing is applied. FIG. 5C shows the latent image potential of the halftone portion to which the high tone number halftone processing is applied. As shown in FIG. 5A, it is difficult to increase the latent image potential in the highlight area. This is because the number of pixels per dot decreases as the number of halftone lines (dot interval) increases. On the other hand, dot contrast can be increased in halftone processing with a low number of lines. This is because the number of pixels per dot is large.

  FIG. 6 shows an example of an image to which halftone processing with a high number of lines is applied and an image to which halftone processing with a low number of lines is applied. The number of dots (area) for 2400 dpi 106 lpi is 50 pixels. The number of dot pixels for 2400 dpi 242 lpi is 13 pixels. Thus, since the 106 lpi halftone processing dot is larger than the 242 lpi halftone processing dot, the latent image potential is stable, and even if the characteristics of the image forming engine 101 slightly change, the influence is not easily affected.

  As shown in FIGS. 5B and 5C, in the halftone, the dark potential Vd (referred to as the inter-dot dark potential) that exists between adjacent dots decreases. However, the bright potential Vl is maintained. As a result, the dot contrast of the high tone number halftone process is lower than the dot contrast of the low tone number halftone process. The gradation of density should be expressed with an area gradation (dot area) that is faithful to the image data. However, since the inter-dot dark potential is lowered, a sharp dot latent image is not formed. Since the density starts to increase even between the dots when the latent image potential exceeds a certain threshold value (development bias potential), the density gradient becomes steep. Due to such an event, the characteristic for the high line number is an S-shaped curve, and the characteristic for the low line number is a clean downward convex curve.

  As described above, even if the characteristics of the image forming engine 101 are the same, the density characteristics differ depending on the definition of the dots. When the charging potential (dark potential Vd) of the photosensitive drum fluctuates, it is possible to detect the change in density at a high number of lines, but it is not possible to detect the change in density at a low number of lines. . Therefore, it cannot be predicted from the measurement data of the pattern formed using the halftone processing with the low number of lines that the measurement data of the pattern formed using the halftone processing with the high number of lines is fluctuating. That is, based on the measurement data of the pattern formed by using the halftone process with the low number of lines, it is not possible to accurately generate the gradation correction table for the halftone process with the high number of lines.

  Therefore, in this embodiment, when the measurement data of a pattern to which halftone processing with coarse dot definition is applied is acquired, the gradation correction table for halftone processing with relatively fine dot definition is not updated. . On the other hand, when measurement data of a pattern to which halftone processing with fine dot definition is applied is acquired, the gradation correction table for halftone processing with relatively coarse dot definition is updated using the measurement data. As a result, each gradation correction table for a plurality of halftone processes can be created more accurately.

(Pattern formation sequence)
The pattern formation between the sheets will be described with reference to FIG. For simplicity of explanation, a black pattern will be described, but in actuality, patterns of other colors of CMY are also arranged in the main scanning direction. In this case, the density sensor 117 includes four sets of light emitting units and light receiving units so that four patterns can be detected. Alternatively, a set of light emitting units and light receiving units may be provided, and patterns of other colors may be formed from the 17th page onward. The former is superior from the viewpoint of the update frequency of the gradation correction table, but the latter is superior from the viewpoint of reducing the number of parts.

  As shown in FIG. 7, A4 size images from the first sheet to the 16th sheet are formed, and patterns A1 to D4 are formed between the sheets. The patterns A1 to A4, B1 to B4, C1 to C4, and D1 to D4 are periodically and repeatedly formed. That is, the pattern A1 is formed again after the pattern D4. Of the codes assigned to identify the pattern, A through D indicate the type of halftone processing (identification information), and 1 through 4 indicate the density level.

  FIG. 8A shows the relationship between halftone processing applied to form a pattern and halftone processing reflecting the measurement result. Halftone processing A is error diffusion, halftone processing B is low-line number dither processing, halftone processing C is dither processing set for copy processing, and halftone processing D is high-line number dither processing. Dither processing. The halftone processes B and D are used in a print job instructed from the host computer 301, for example. Error diffusion is used, for example, in facsimile reception processing. Here, it is assumed that the dot definition is high in the order of A, B, C, and D. As is clear from FIG. 8A, when the measurement data of the pattern to which halftone processing with coarse dot definition is applied is acquired, the gradation correction table for halftone processing with relatively fine dot definition is updated. Not. When the measurement data of a pattern to which halftone processing with fine dot definition is applied is acquired, the gradation correction table for halftone processing with relatively coarse dot definition is updated using the measurement data.

  As shown in FIG. 7, in the section described as the first set, a pattern is formed by the halftone processing unit 317 applying error diffusion. The density sensor 117 measures the density of each pattern and outputs measurement data to the table creation unit 307. The table creation unit 307 reflects the measurement data only in the error diffusion gradation correction table. This is because the dot definition of error diffusion is lower than the dot definition of other halftone processes. In the second set section, the halftone processing unit 317 applies a low line number dither to form a pattern. The measurement data output from the density sensor 117 is reflected in the tone correction table for low line number dither and the error correction tone correction table whose dot definition is coarser than the dot definition of low line number dither. . In the third set section, the halftone processing unit 317 applies a dither for copy processing, thereby forming a pattern. The measurement data is reflected in the tone correction table for copy processing, the error correction tone correction table for dot definition coarser than the copy processing dither dot definition, and the tone correction table for low-line number dither. Is done. In the fourth set section, the halftone processing unit 317 applies a high line number dither to form a pattern. Since the dot definition of the high line number dither is the finest, the measurement data is reflected in the gradation correction tables for all halftone processing.

(Dot definition determination method)
Here, the definition of dot definition will be described. The flow shown in FIG. 9 is a conceptual flow and is not executed in the image forming engine 101. However, this flow can be programmed and incorporated.

  The number of lines can be grasped by performing one-dimensional processing after performing 2D-FFT (two-dimensional fast Fourier transform) and further performing peak detection. The periodicity can also be grasped from the result of peak detection. Whether the growth method is a line or a dot can be grasped by analyzing a 2D-FFT processed image. Therefore, the dot definition may be calculated according to the flow shown in FIG. In this embodiment, it is assumed that the dot definition is analyzed in advance before creating the gradation correction table, and the analysis result is held in the ROM 304 or the like as a table as shown in FIG.

In S001, the dither (halftone processing) used for normal image formation is grasped. Normal image formation refers to forming an image to be transferred to the transfer material 110. In the present embodiment, four types of halftone processing are illustrated, but the number of halftone processing may be five or more, or may be three or less. The halftone processes A to D of the present embodiment are assumed to have characteristics as shown in FIG. In S002, the halftone process is classified into a periodic pattern and an aperiodic pattern. In the example shown in FIG. 10, the non-periodic pattern is only halftone processing A, and the periodic pattern is halftone processing B to D. S003 to S006 are periodicity pattern determination flows. In S003, the number of lines of halftone processing is grasped, and in S004, the order of roughness of each halftone process in the periodic pattern is determined based on the number of lines. At this point, the following order is coarse: Low line number (halftone process B) <Copy (halftone process C) <High line number (halftone process D): There are two halftone processes with the same number of fine lines If so, in step S005, the dot definition is determined based on the growth method. That is, it is determined that the line growth type dot definition is coarser than the dot concentration type dot definition. In S006, the dot size of 5% density is grasped for the halftone processing of each periodic pattern type. In step S009, the dot definition is compared between the periodic pattern type halftone process and the non-periodic pattern type halftone process, and the final order is determined.

  For a non-periodic pattern type dither, in step S007, the dot size having a density of 5% is grasped for the halftone processing of each non-periodic pattern type. If there are a plurality of non-periodic pattern type halftone processes, the order of dot definition among the plurality of non-periodic pattern type halftone processes is determined according to the dot size in S008. It is determined that the dot definition is coarser in descending order of dot size. In step S009, the dot definition is compared between the periodic pattern type halftone process and the non-periodic pattern type halftone process, and the final order is determined. In particular, the overall ranking is determined based on a dot size of 5% density.

As shown in FIG. 10, image data to which halftone processing A (error diffusion) is applied is formed with a resolution of 600 dpi. However, since the resolution of the image forming engine 101 is 2400 dpi, the image processor performs resolution conversion from 600 dpi to 2400 dpi using the nearest neighbor method. For halftone processing A to D, comparison is executed with a dot size aligned in units of 2400 dpi. As a result, since the error size is the largest dot size, the dot definition is determined as follows.
Coarse: Halftone process A (error diffusion) <Halftone process B (low line number) <Halftone process C (copy) <Halftone process D (high line number): Fine, that is, halftone process A (error diffusion) The dot definition ranks 4th. Halftone processing B (low number of lines) dot definition ranks third. In the halftone process C (copy), the dot definition ranks second. In the halftone process D (high line number), the dot definition ranks first.

  As described above, the dot definition of each halftone process is determined in advance, and a table as shown in FIG. 8A is created. The dot definition indicates a relative order of a plurality of halftone processes that are held, and is not an absolute index.

(Calculation flow)
A process of updating the gradation correction table while a plurality of images are continuously formed will be described with reference to FIG. It is assumed that the first γLUT has already been created and the second γLUT is created here. The method for creating the first γLUT is the same as the method for creating the second γLUT. As described above, when the second γLUT is updated, a composite γLUT is created by combining with the first γLUT.

  In step S010, the printer controller CPU 313 determines whether an image formation instruction has been input. For example, if a copy instruction is input from the operation unit 180, a print instruction is received from the host computer 301, or a facsimile incoming signal is received, the printer controller CPU 313 determines that an image formation instruction has been issued. , Go to S011. The printer controller CPU 313 activates the table creation unit 307 to execute the following processing.

  In step S011, the table creation unit 307 grasps halftone processing used in the halftone processing unit 317 for forming a pattern. As described with reference to FIG. 7, the application order of the plurality of halftone processes included in the halftone processing unit 317 is determined in advance. Further, each time a pattern is formed, the table creation unit 307 executes a count from A1 to D4 and grasps which gradation level pattern is formed by which halftone process. Therefore, the table creation unit 307 can determine which halftone process is applied by referring to the count value.

  In step S012, the table creation unit 307 determines whether the halftone process is error diffusion. If the halftone processing is error diffusion, the process proceeds to S013. In step S013, the table creation unit 307 specifies error diffusion to the halftone processing unit 317 and supplies image data for forming a pattern to the halftone processing unit 317. When the pattern A1 is formed, image data of gradation level 1 is supplied to the halftone processing unit 317. The halftone processing unit 317 applies error diffusion to the image data. The engine control CPU 102 of the image forming engine 101 controls the image forming engine 101 according to the image data output from the halftone processing unit 317 to form a pattern on the intermediate transfer belt 106. Thereafter, the process proceeds to S019.

  If it is determined in S012 that the halftone process is not error diffusion, the process proceeds to S014. In step S014, the table creation unit 307 determines whether the halftone process is a low line number type halftone process. If the halftone process is a low line number type halftone process, the process proceeds to S015. In step S <b> 015, the table creation unit 307 designates the low tone number type halftone processing in the halftone processing unit 317 and supplies image data for forming a pattern to the halftone processing unit 317. The halftone processing unit 317 applies low line number type halftone processing to the image data. The engine control CPU 102 of the image forming engine 101 controls the image forming engine 101 according to the image data output from the halftone processing unit 317 to form a pattern on the intermediate transfer belt 106. Thereafter, the process proceeds to S019.

  If it is determined in S014 that the halftone process is not a low line number type halftone process, the process proceeds to S016. In step S016, the table creation unit 307 determines whether the halftone process is a halftone process for copying. If the halftone process is a halftone process for copying, the process proceeds to S017. In step S <b> 017, the table creation unit 307 specifies halftone processing for copying in the halftone processing unit 317 and supplies image data for forming a pattern to the halftone processing unit 317. A halftone processing unit 317 applies a halftone process for copying to image data. The engine control CPU 102 of the image forming engine 101 controls the image forming engine 101 according to the image data output from the halftone processing unit 317 to form a pattern on the intermediate transfer belt 106. Thereafter, the process proceeds to S019.

  If it is determined in S016 that the halftone process is not a halftone process for copying, the process proceeds to S018. In step S <b> 018, the table creation unit 307 designates the high tone number type halftone processing to the halftone processing unit 317 and supplies image data for forming a pattern to the halftone processing unit 317. A halftone processing unit 317 applies high line number type halftone processing to image data. The engine control CPU 102 of the image forming engine 101 controls the image forming engine 101 according to the image data output from the halftone processing unit 317 to form a pattern on the intermediate transfer belt 106. Thereafter, the process proceeds to S019.

  In step S <b> 019, the table creation unit 307 measures the pattern mobility using the density sensor 117. In S020, the table creation unit 307 measures the difference ΔD between the target density characteristic TGT and each measurement data. As described with reference to FIG. 3C, here, the difference ΔD is obtained for each of the four measurement data.

  In step S021, the table creation unit 307 refers to the feedback rate table stored in the ROM 304 or the like, acquires a feedback rate for each halftone process corresponding to the halftone process used to form the pattern, and calculates the difference ΔD. 'Determine. By the way, if the difference ΔD is used for calculation as it is, an excessive correction table may be created. This is because the pattern measurement data may include various error factors (dirt of the intermediate transfer belt 106, background unevenness, etc.) that may occur during pattern detection. That is, if measurement data acquired at a high frequency as in this embodiment is reflected in the correction table with 100%, a continuous output density step (hereinafter referred to as a control step) may occur. Therefore, the maximum value of the feedback rate is set to 50%. The maximum value can be changed as appropriate.

  FIG. 8B shows an example of the feedback rate table. As shown in FIG. 8B, the feedback rate is calculated from the relationship between the dot definition of the halftone process used when forming the pattern and the dot definition of the halftone process that was not used when forming the pattern. Has been determined. Since the maximum value of the feedback rate is set to 50%, the idea of suppressing the control step by gradually applying correction is incorporated.

The table creation unit 307 obtains the difference ΔD ′ using the following formula, for example.
ΔD ′ = feedback rate × ΔD
For example, when error diffusion is applied to form a pattern, 50% of ΔD is reflected in the error diffusion correction table. Also, ΔD for the pattern to which error diffusion is applied is not reflected in the remaining halftone processing correction tables. This is because the feedback rate is set to 0%. When a pattern is formed by applying a high line number type halftone process, 50% of ΔD is reflected in its own correction table. Further, 10% of ΔD for the pattern to which the high line number type halftone processing is applied is reflected in the remaining halftone processing correction table. This is because the feedback rate is set to 10%.

  In S022, the table creation unit 307 creates a second γLUT using the difference ΔD ′ for each halftone process. In step S023, the second γLUT is set in the gradation correction unit 316. Thus, each gradation correction table of the plurality of halftone processes is based on the relationship between the dot definition of the halftone process applied to the pattern image and the dot definition of the halftone process not applied to the pattern image. Created. Therefore, it is possible to create a gradation correction table for each of a plurality of halftone processes with higher accuracy.

  In the above description, the dot definition is determined based on the number of lines and the dot size at the 5% area ratio. However, this determination method is only an example, and the resolution at the time of halftone processing may be included as another index. For example, for dot dispersion dither and error diffusion, the minimum dot size is determined by the resolution. Therefore, the dot definition may be determined by comparing the resolutions between a plurality of halftone processes.

A determination method that places emphasis on a highlight area where density fluctuation is conspicuous may be employed.
<1>: Dot size of 5% density part (excluding line screen)
<2>: Number of lines <3>: Growth method Coarse: Line screen type <Dot concentration type <Dot dispersion type>: Fine Here, the priority in the determination method is <1>, <2>, <3>. As the line screen, there are a growth method in which the highlight region is changed to dot growth and then a line is formed in the halftone region, and a method of growing in a line from the beginning. Therefore, it is difficult to define the dot size because the dots also overlap on the line. Further, since the dots form overlapping lines, the latent image is stabilized from a certain density. Therefore, if the number of lines is the same, it is determined that the line growth is rougher than the dot growth.

<Example 2>
In the second embodiment, a response when the halftone processing is switched by the user will be described. Printers for POD (print on demand) often switch halftone processing based on the printing application. Also, there are about 5 to 10 halftone processes held by one printer. These halftone processes can be arbitrarily changed by the user through the operation unit 180.

  All of the four halftone processes shown in FIG. 8B may not be selectable by the user, but may be selectable only for the low line number type and high line number type for printing. In a copy job (copying process), moire tends to occur on a document. For this reason, moire may be conspicuous if the user selects an arbitrary halftone process just because the user wants to change the texture. Therefore, in the copying process, the halftone process set for the copying process often cannot be changed to another type of halftone process.

  Moreover, since error diffusion has little merit for the user, a change to error diffusion may not be permitted. This is because as the resolution is increased, the dot size in the highlight area becomes smaller, the stability is deteriorated, and the roughness is conspicuous. In addition, if the resolution is rough, the texture becomes uncomfortable and the user becomes conscious of roughness. Therefore, switching to error diffusion is not allowed for the user.

  As shown in FIG. 12, for the halftone processing used for the print job, the user can select either the low line number type or the high line number type. In FIG. 12, each column indicates the type of halftone processing. The seven halftone processes are arranged in the order of coarse dot definition. 106 lpiDot has a dot definition of 7, which is the coarsest. 242 lpiDot has the finest dot definition of 1, and is the finest. When the user arbitrarily switches the number of lines, the table creation unit 307 refers to the table shown in FIG. 12 and compares the dot definition of each halftone process. The table creation unit 307 modifies the feedback rate table shown in FIG. 8B according to the comparison result. For example, assume that the low line number type halftone processing is switched to 106 lpiDot and the high line number type halftone processing is switched to 170 lpiDot. An example of the feedback rate table obtained in this case is shown in FIG. The feedback rate table shown in FIG. 8C is sorted according to dot definition. Therefore, halftone processing A is 106 lpiDot, halftone processing B is error diffusion, halftone processing C is 170 lpiDot, and halftone processing D is for copy processing. When the halftone processing A is 106 lpiDot, the dot definition is the coarsest (large), so the pattern measurement data is reflected only in the own tone correction table. On the other hand, since the halftone processing for copy processing, which is the halftone processing D, has the finest (smaller) dot definition, the measurement data is reflected in each gradation correction table based on each feedback rate.

  As described above, when the type of halftone processing is changed by the user, the printer controller CPU can correct the feedback rate table based on the dot definition. Therefore, even if the type of halftone processing is changed, the gradation correction table corresponding to each halftone processing can be appropriately corrected.

<Example 3>
In the third embodiment, a process when the halftone process for printing is switched over a plurality of times in a short time will be described. As described in the second embodiment, the halftone processing for printing can be changed by the user. For this reason, when the halftone processing for printing is switched in a short time, the idea of gradually correcting as described in the first embodiment does not hold. In addition, how much ΔD is generated for the switched halftone processing depends on the dot definition. In the first and second embodiments, the feedback rate of halftone processing that is not used for pattern printing is uniformly set to 10%. Therefore, when the halftone processing for printing is frequently switched, ΔD changes each time, and a density step at the time of switching may occur.

  Therefore, in the third embodiment, it is assumed that the user frequently changes the halftone processing for printing, and measurement data on the low line number type and the high line number type that can be selected as the halftone processing for printing by the user are obtained. No feedback is given to other halftone processes. This can be realized by storing a feedback rate table as shown in FIG. For example, the measurement data of the high line number type halftone process D and the measurement data of the low line number type halftone process B are not reflected in the gradation correction tables of other halftone processes. By doing so, even if the user frequently switches halftone processing, a density step is less likely to occur.

<Example 4>
In the fourth embodiment, a configuration in which a feedback rate to halftone processing that is not applied to a pattern is changed based on the relationship of dot definition will be described. The halftone processing for printing can be changed by the user. The dot definition is defined for a plurality of halftone processes held by the halftone processing unit 317, the relative magnitude relation of the dot definition is grasped, and the feedback rate as shown in FIG. 13A is determined. be able to. As described in the third embodiment, the degree of influence on the variation in characteristics of the image forming engine 101 is different based on the dot definition. In other words, the gradation correction table can be created with higher accuracy by adding the relationship between the dot definition of the halftone process as the feedback source and the dot definition of the halftone process as the feedback destination to the feedback rate. Become. In the fourth embodiment, the dot rate is added to each of the seven types of halftone processing described in the second embodiment, and the feedback rate to the other halftone processing table is determined.

  In S030 shown in FIG. 14, the table creation unit 307 determines whether or not the halftone processing has been switched by the user. If the halftone process has been switched, the process proceeds to S031. In S031, the table creation unit 307 grasps which halftone process is changed to which halftone process. Since the user instructs which halftone process to change to which halftone process through the operation unit 180, the table creation unit 307 performs grasping based on this instruction. In S032, the table creation unit 307 creates a provisional feedback table according to the dot definition order. FIG. 13B shows an example of a provisional feedback table. In S033, the table creation unit 307 determines undecided feedback rates FB1 to FB6 in the provisional feedback table. The feedback rates FB1 to FB6 can be determined from the following equation, for example.

FBi = FBb × 1 / (F1-F2)
Here, FBi is feedback rates FB1 to FB6. FBb is a basic feedback rate, for example, 30%. F1 is the dot definition of halftone processing involved in pattern formation. F2 is the dot definition of halftone processing that is not involved in pattern formation. According to this equation, if the difference between F1 and F2 is small, it means that the dot reproducibility of halftone processing involved in pattern formation is close to the dot reproducibility of halftone processing not involved in pattern formation. That is, the difference Δ for the halftone process related to the pattern formation is close to the difference Δ for the halftone process not related to the pattern formation. On the contrary, if the difference between F1 and F2 is large, it means that the dots of halftone processing that are not involved in pattern formation are relatively rough. That is, ΔD of halftone processing that was not involved in pattern formation is often smaller than ΔD of halftone processing that is involved in pattern formation. Therefore, a formula for calculating the feedback rate was constructed so that ΔD was smaller than ΔD obtained from the pattern.

  In step S <b> 034, the table creation unit 307 generates a feedback rate table to which the calculated feedback rates FB <b> 1 to FB <b> 6 are applied, and stores them in the ROM 304. FIG. 15 shows an example of the calculation result of the feedback rate. FIG. 13C shows an example of a final feedback rate table.

<Summary>
As described above, the gradation correction unit 316 applies gradation correction to the image data. A halftone processing unit 317 applies a halftone process designated among a plurality of halftone processes to the image data subjected to the gradation correction by the gradation correction unit 316. The image forming engine 101 forms a toner image corresponding to the image data output from the halftone processing unit 317. The density sensor 117 measures the density of the pattern formed as a toner image by the image forming engine 101. The table creation unit 307 creates a gradation correction table for halftone processing applied to the pattern image based on the measurement result of the halftone processing applied to the pattern image. As described with reference to the first to fourth embodiments, the table creation unit 307 includes the dot definition of the halftone process applied to the pattern image and the dots of the halftone process other than the halftone process applied to the pattern image. Based on the definition, the measurement result of the halftone process applied to the pattern image is fed back to the gradation correction table for the halftone process other than the halftone process applied to the pattern image. Thereby, each gradation correction table of the plurality of halftone processes is created with high accuracy.

  As described in the first embodiment, the table creation unit 307 determines that the dot definition of the halftone process applied to the pattern image is the dot definition of the halftone process other than the halftone process applied to the pattern image. If it is finer, the measurement result of the halftone processing applied to the pattern image is fed back to the gradation correction table for halftone processing other than the halftone processing applied to the pattern image. Further, the table creation unit 307 applies a pattern image to the pattern image if the dot definition of the halftone process applied to the pattern image is coarser than the dot definition of the halftone process other than the halftone process applied to the pattern image. The measurement result of the applied halftone processing is fed back to the gradation correction table for halftone processing other than the halftone processing applied to the pattern image. There is no feedback from halftone processing with coarse dot definition to halftone processing with fine dot definition, but there is feedback from halftone processing with fine dot definition to halftone processing with coarse dot definition. Thereby, the gradation correction table is created with high accuracy.

  The ROM 304 and the RAM 309 may function as a storage unit that stores a feedback rate corresponding to a combination of halftone processing performed on the pattern image and halftone processing other than halftone processing performed on the pattern image. The table creation unit 307 may feed back the measurement result to the gradation correction table based on this feedback rate. By determining the feedback rate in advance, the time and energy for calculating the feedback rate can be reduced.

  As described with reference to the fourth embodiment, the table creation unit 307 includes the dot definition of the halftone process applied to the pattern image and the dot definition of the halftone process other than the halftone process applied to the pattern image. It may function as a determination unit that determines the feedback rate based on the difference between the two. In particular, the table creation unit 307 may increase the feedback rate as the difference is smaller. This is because if the difference is small, it means that the dot reproducibility of halftone processing involved in pattern formation is close to the dot reproducibility of halftone processing not involved in pattern formation.

  As described with reference to the third embodiment, the feedback rate from the halftone process that can be switched by the user to another halftone process may be zero. In other words, the halftone process applied to pattern formation may be a halftone process that is not allowed to be switched to another halftone process by the user (cannot be switched). As a result, the control step can be reduced.

  As described with reference to FIG. 9, the dot definition of the halftone process of the periodic pattern type may be determined in advance based on the dot interval or the number of lines. The dot definition of the halftone processing of the periodic pattern type may be determined in advance based on the dot size at a predetermined density. The dot definition of the non-periodic pattern type halftone processing may be determined in advance based on the dot size at a predetermined density. The dot definition of the halftone process may be determined in advance based on a dot growth method. This is because these parameters all contribute to dot definition.

  As described in the second embodiment, the table creation unit 307 reconstructs the feedback rate table by recalculating the feedback rate when the type of halftone processing that can be selected in the halftone processing unit is changed. May be. Therefore, even if the type of halftone processing is changed, the gradation correction table corresponding to each halftone processing can be appropriately corrected.

  117... Density sensor, 101... Image forming engine, 307.

Claims (12)

Correction means for correcting image data using gradation correction conditions;
Processing means for performing halftone processing on the corrected image data;
Forming means for forming an image based on the image data subjected to the halftone processing;
Measuring means for measuring a pattern image formed by the forming means;
Creating means for creating gradation correction conditions for halftone processing applied to the pattern image based on the measurement result of the pattern image;
The creating means applies the pattern image to the pattern image based on the dot definition of the halftone process applied to the pattern image and the dot definition of the halftone process other than the halftone process applied to the pattern image. An image forming apparatus, wherein the measurement result of the halftone process is fed back to a gradation correction condition for halftone processing that has not been applied to the pattern image. The creating means includes
If the dot definition of halftone processing applied to the pattern image is finer than the dot definition of halftone processing other than halftone processing applied to the pattern image, the halftone processing applied to the pattern image Feeding back the measurement results of the processing to gradation correction conditions for halftone processing other than halftone processing applied to the pattern image;
If the dot definition of the halftone process applied to the pattern image is coarser than the dot definition of the halftone process other than the halftone process applied to the pattern image, the halftone process applied to the pattern image The image forming apparatus according to claim 1, wherein the measurement result of the processing is fed back to a gradation correction condition for halftone processing other than the halftone processing applied to the pattern image. Storage means for storing a feedback rate corresponding to a combination of halftone processing applied to the pattern image and halftone processing other than halftone processing applied to the pattern image;
The image forming apparatus according to claim 1, wherein the creating unit feeds back the measurement result to a gradation correction condition based on the feedback rate.   Determining means for determining the feedback rate based on a difference between dot definition of halftone processing applied to the pattern image and dot definition of halftone processing other than halftone processing applied to the pattern image; The image forming apparatus according to claim 3, further comprising:   The image forming apparatus according to claim 4, wherein the determination unit increases the feedback rate as the difference is smaller.   The image forming apparatus according to claim 5, wherein the determination unit recalculates the feedback rate when the type of halftone processing that can be selected by the processing unit is changed. The processing means is configured to perform halftone processing selected from a plurality of halftone processes on the image data, and halftone processing that can be switched by a user as the halftone processing performed on the image by the processing means And halftone processing that cannot be switched by the user,
The image forming apparatus according to claim 3, wherein a feedback rate from halftone processing that can be switched by the user to another halftone processing is set to zero.   The image forming apparatus according to claim 7, wherein the halftone process performed on the pattern image is a halftone process that cannot be switched by the user.   9. The image forming apparatus according to claim 1, wherein the dot definition of the halftone processing of the periodic pattern type is determined in advance based on the dot interval or the number of lines.   The image forming apparatus according to claim 9, wherein the dot definition of the halftone process of the periodic pattern type is further determined in advance based on a dot size at a predetermined density.   11. The image forming apparatus according to claim 1, wherein the dot definition of the non-periodic pattern type halftone processing is determined in advance based on a dot size at a predetermined density.   12. The image forming apparatus according to claim 1, wherein the dot definition of the halftone processing is further determined in advance based on a dot growth method.

Images