JP6587217B2 - Image forming apparatus, image forming method, and image forming program - Google Patents

Description

  The present invention relates to an image forming apparatus, an image forming method, and an image forming program, and more particularly to a technique for improving reproducibility of a smooth gradation change.

  A typical image forming apparatus can form an image on a print medium using CMYK toner according to RGB input image data. In the image forming process, gamma correction is performed in order to cancel the non-linear characteristics of the image forming process and realize a linear image reproduction. However, the non-linear characteristics of image forming processing sometimes cause an excessively large output gradation difference (also called gradation skip) with respect to the minimum input gradation difference, resulting in streaks or pseudo contours in the image. Appears and may degrade image quality. For example, Japanese Patent Application Laid-Open No. H10-228667 adds a suitable noise data to the output image data and outputs the gradation skip, which is a problem when outputting a gradation image. It proposes a technology to effectively suppress it.

JP 2000-134475 A

  However, the addition of noise data is a method of blurring streaks and pseudo contours that occur due to gradation skipping with noise and making it inconspicuous, so there is a possibility that the reproducibility of smooth gradation changes such as gradation images may be deteriorated was there.

  The present invention has been made in view of such a situation, and provides a technique for improving the reproducibility of a smooth gradation change such as a gradation image while suppressing image quality deterioration due to gradation skip. Objective.

  An image forming apparatus according to the present invention includes a gamma correction unit that performs gamma correction on input image data, and an image on a print medium with a density gradation value corresponding to predetermined image forming characteristics based on the gamma-corrected image data. An image forming unit to be formed and the input image data are analyzed, and a gradation skip in which an adjacent gradation value difference that is a difference between the adjacent gradation gradation values exceeds a preset threshold value is detected. A gradation skip determining unit that determines whether or not the gradation skip occurs in the image, and the gamma correction unit reduces the adjacent gradation value difference when it is determined that the gradation skip occurs. The density gradation value is adjusted.

  An image forming method of the present invention includes a gamma correction step for gamma correcting input image data, and an image on a print medium with a density gradation value corresponding to a predetermined image forming characteristic based on the gamma corrected image data. An image forming step to be formed and the input image data are analyzed, and a gradation skip in which an adjacent gradation value difference that is a difference between the adjacent gradation gradation values exceeds a preset threshold value is detected. A gradation skip determination step for determining whether or not the image occurs in the image, and the gamma correction step is configured such that the adjacent gradation value difference is reduced when it is determined that the gradation skip occurs. The step of adjusting the density gradation value.

  The image forming program of the present invention controls an image forming apparatus having an image forming unit that forms an image on a printing medium with a density gradation value corresponding to a predetermined image forming characteristic, based on gamma-corrected image data. . The image forming program analyzes the input image data and a gamma correction unit that performs gamma correction on the input image data, and sets an adjacent gradation value difference that is a difference between the density gradation values adjacent to the gradation. The image forming apparatus is caused to function as a gradation skip determining unit that determines whether or not a gradation skip exceeding a threshold value is generated in the image, and the gamma correction unit causes the gradation skip to occur. If it is determined, the density gradation value is adjusted so that the adjacent gradation value difference is reduced.

  According to the present invention, it is possible to improve the reproducibility of a smooth gradation change such as a gradation image while suppressing image quality deterioration caused by gradation skip.

2 is a block diagram illustrating a functional configuration of the image forming apparatus 1 according to an embodiment of the present disclosure. 1 is a cross-sectional view illustrating an overall configuration of an image forming apparatus 1 according to an embodiment. It is a flowchart which shows the content of the gamma table calibration process procedure which concerns on one Embodiment. 3 is a graph showing a gamma curve of the image forming apparatus 1 according to an embodiment in comparison with a typical gamma curve. 4 is a graph showing a mechanism of gradation jump occurrence in the image forming apparatus 1 according to an embodiment. 6 is a graph showing a gradation skip suppression gamma curve and a gradation skip suppression mechanism according to an embodiment. It is a flowchart which shows the content of the gamma correction process procedure which concerns on one Embodiment.

  Hereinafter, modes for carrying out the present invention (hereinafter referred to as “embodiments”) will be described with reference to the drawings.

  FIG. 1 is a block diagram showing a functional configuration of an image forming apparatus 1 according to an embodiment of the present invention. The image forming apparatus 1 includes a control unit 10, an image forming unit 20, a storage unit 40, and an image reading unit 50. The image reading unit 50 reads an image from a document and generates an image data ID that is digital data. The image data ID may be given from a personal computer (not shown) outside the image forming apparatus 1 or the like. The image data ID is also called input image data.

  The image forming unit 20 includes a color conversion processing unit 21 having a gamma correction unit 211, a halftone processing unit 22, a calibration density sensor 28, an exposure unit 29, and a photosensitive drum (image carrier) that is an amorphous silicon photosensitive member. Body) 30c-30k, developing parts 100c-100k, and charging parts 25c-25k. The color conversion processing unit 21 performs gamma correction by the gamma correction unit 211 after color-converting the image data ID, which is RGB data, into CMYK data. The halftone processing unit 22 performs halftone processing on the CMYK data to generate CMYK halftone data.

  The control unit 10 includes a gradation skip determination unit 11 and a calibration processing unit 12. The functions of the gradation skip determination unit 11 and the calibration processing unit 12 will be described later. The control unit 10 includes main storage means such as RAM and ROM, and control means such as MPU (Micro Processing Unit) and CPU (Central Processing Unit). The control unit 10 also has controller functions related to various I / O, USB (Universal Serial Bus), bus, and other hardware interfaces, and controls the entire image forming apparatus 1.

  The storage unit 40 is a storage device including a hard disk drive or a flash memory that is a non-temporary recording medium, and stores a control program and data for processing executed by the control unit 10 and the color conversion processing unit 21. In the present embodiment, the storage unit 40 further stores calibration image data CD for forming a CMYK gradation calibration adjustment patch and two gamma tables 41a and 41b.

  Gamma correction is performed using either one of the two gamma tables 41a and 41b. The gamma table 41a is a gamma table giving priority to gradation reproducibility (also called a gradation priority table). The gamma table 41b is a gamma table (also referred to as a gradation difference priority table) that prioritizes the reproducibility of gradation differences. Details of the two gamma tables 41a and 41b will be described later.

  FIG. 2 is a cross-sectional view illustrating the overall configuration of the image forming apparatus 1 according to the embodiment. The image forming apparatus 1 of the present embodiment is a tandem type color printer. In the image forming apparatus 1, photosensitive drums (image bearing members) 30m, 30c, 30y, and 30k are arranged in a row in the casing 70 so as to correspond to the respective colors of magenta, cyan, yellow, and black. Developing units 100m, 100c, 100y, and 100k are disposed adjacent to the photosensitive drums 30m, 30c, 30y, and 30k, respectively.

  The photosensitive drums 30m, 30c, 30y, and 30k are irradiated with laser beams Lm, Lc, Ly, and Lk for the respective colors from the exposure unit 29. By this irradiation, electrostatic latent images are formed on the photosensitive drums 30m, 30c, 30y, and 30k. The developing units 100m, 100c, 100y, and 100k attach the toner to the electrostatic latent images formed on the surfaces of the photosensitive drums 30m, 30c, 30y, and 30k while stirring the toner. Thereby, the developing process is completed, and toner images of the respective colors are formed on the surfaces of the photosensitive drums 30c to 30k.

  The image forming apparatus 1 has an endless intermediate transfer belt 27. The intermediate transfer belt 27 is stretched around the tension roller 24, the driving roller 26a, and the driven roller 26b. The intermediate transfer belt 27 is driven to circulate by the rotation of the driving roller 26a.

  For example, a black toner image on the photosensitive drum 30k is primarily transferred to the intermediate transfer belt 27 by sandwiching the intermediate transfer belt 27 between the photosensitive drum 30k and the primary transfer roller 23k, and the intermediate transfer belt 27 being driven to circulate. The The same applies to the three colors cyan, yellow, and magenta. The calibration density sensor 28 is disposed at a position where the density of each color image primarily transferred to the intermediate transfer belt 27 can be measured.

  A full color toner image is formed on the surface of the intermediate transfer belt 27 by performing primary transfer so as to be superimposed on each other at a predetermined timing. Thereafter, the full-color toner image is secondarily transferred to the printing paper P supplied from the paper feed cassette 60 and then fixed to the printing paper P.

  FIG. 3 is a flowchart showing the contents of a gamma table calibration processing procedure according to an embodiment. The gamma table calibration processing procedure calibrates the gamma table 41a that prioritizes accurate tone reproducibility, and generates a gamma table 41b that prioritizes smooth tone change reproducibility as needed. It is a procedure to do.

  In step S110, the calibration processing unit 12 of the control unit 10 causes the image forming unit 20 to form an adjustment patch for CMYK gradation calibration. The CMYK gradation calibration adjustment patch is formed on the intermediate transfer belt 27 based on the calibration image data read from the storage unit 40. The CMYK gradation calibration adjustment patches are a plurality of patches having different densities in CMYK colors in stages.

  In step S120, the calibration processing unit 12 measures the density of the CMYK gradation calibration adjustment patch using the calibration density sensor 28, and outputs a density gradation value. In step S130, the calibration processing unit 12 calibrates the gamma table 41a.

  FIG. 4 is a graph showing the gamma curve G1 of the image forming apparatus 1 according to the embodiment in comparison with a typical gamma curve G0. FIG. 4A shows a target value T, a typical image forming characteristic E0, and a typical gamma curve G0. The target value T indicates the target linear input / output characteristic. The horizontal axis indicates any input gradation value of CMYK. The vertical axis indicates any output gradation value (density gradation value) of CMYK.

  A typical gamma curve G0 is configured as a look-up table (LUT), for example, and is calibrated to function as follows. A typical gamma curve G0 outputs an intermediate gradation value 64 (A3 point) at a point (A2 point) corresponding to the input gradation value 128 (A1 point). A typical image forming characteristic E0 (a model of a typical image forming apparatus) is an image having a density corresponding to the density gradation value of the output gradation value 128 (A5 point) according to the intermediate gradation value 64 (A4 point). (A6 point). This density gradation value matches the target value T for the input gradation value 128 (point A1) of the gamma curve G0.

  As a result, the gamma curve G0 can achieve a linear input / output characteristic as a whole by canceling the non-linear characteristic of the typical image forming characteristic E0. In other words, the gamma table calibration processing procedure is a procedure for calibrating the gamma curve G0 so that the typical image forming characteristic E0 and the gamma curve G0 as a whole realize linear input / output characteristics.

  On the other hand, FIG. 4B shows the target value T and the image forming characteristic E1 of the image forming apparatus 1 according to the embodiment. A calibrated gamma curve G1 of the image forming apparatus 1 is shown. The gamma curve G1 is a calibration value for the gamma table 41a.

  In step S140, the gradation skip determination unit 11 of the control unit 10 analyzes the calibration value of the gamma table 41a and determines whether there is a possibility of gradation skip. In this example, the gradation skip determination unit 11 determines that there is a possibility of gradation skip in the gradation region Z.

  FIG. 5 is a graph illustrating a mechanism of occurrence of gradation skip in the image forming apparatus 1 according to the embodiment. 5 expands the gradation region Z, which is a region having a high gradation value in FIG. 4B, in accordance with the environmental variation in addition to the image formation characteristic E1 of the image forming apparatus 1 according to the embodiment. An image forming characteristic E2 having slightly changed characteristics is shown. However, in this example, fluctuations are emphasized for easy understanding.

  The gamma curve G1 is calibrated assuming the image forming characteristic E1. In the image forming characteristic E1, a density gradation value 254 (point B2) is detected with respect to an input gradation value (intermediate gradation value) 228 (point B1), and an input gradation value (intermediate gradation value) 255 is detected. Thus, the density gradation value 255 is detected. Therefore, the gamma curve G1 is calibrated so as to return the intermediate gradation value 228 for the input gradation value 254 and to return the intermediate gradation value 255 for the input gradation value 255.

  As a result, the gamma curve G1 can realize linear input / output characteristics integrally with the image forming characteristics E1. However, the gamma curve G1 has an intermediate gradation value according to an increase of one gradation from 254 to 255 (adjacent gradation value) of the input gradation value according to the saturation of the gradation value in the image forming characteristic E1. From 228 to 255, the intermediate gradation value changes abruptly by 27 gradations.

  However, for example, a state is assumed in which the characteristic changes from the image forming characteristic E1 to the image forming characteristic E2 in accordance with an environmental change or a change with time. The image forming characteristic E2 outputs a density gradation value 243 (B3 point) with respect to an input gradation value (intermediate gradation value) 228 (B1 point), and with respect to an input gradation value (intermediate gradation value) 255. The density gradation value 255 is output.

  As described above, the gamma curve G1 has the density gradation value 243 according to the increase of one gradation from 254 to 255 (adjacent gradation value) of the input gradation value according to the characteristic variation in the image forming characteristic E2. To 255, there is a rapid change by 12 gradations (adjacent gradation value difference). This sudden density change becomes a factor of image quality deterioration as a streak resulting from gradation skip. Such a difference between density gradation values adjacent to each other is called an adjacent gradation value difference.

  FIG. 6 is a graph showing a gradation skip suppression gamma curve G2 and a gradation skip suppression mechanism according to an embodiment. FIG. 6A shows two image forming characteristics E1 and E2 and a tone skip suppression gamma curve G2 according to an embodiment. FIG. 6B shows a mechanism of gradation skip suppression by the gradation skip suppression gamma curve G2.

  In this example, the gamma curve G2 is configured by suppressing the inclination (adjacent gradation value difference) so that the maximum increase is about 5 gradations according to the increase of 1 gradation for easy understanding. Yes. Therefore, as shown in FIG. 6B, the gamma curve G2 returns the intermediate gradation value 205 for the input gradation value 254 (point C1), and the intermediate gradation value for the input gradation value 255. 208 (C2 point) is returned.

  The image forming characteristic E1 is an image having a density such that an adjacent tone value difference of 0.5 occurs with respect to an input tone value (intermediate tone value) 205 and an input tone value (intermediate tone value) 208. Form (C3 point, C4 point). On the other hand, the image forming characteristic E2 has such a density that an adjacent tone value difference 1.2 occurs between the input tone value (intermediate tone value) 205 and the input tone value (intermediate tone value) 208. An image is formed (C5 point, C6 point). As described above, the gamma curve G2 can realize gamma correction with high robustness that does not cause the gradation difference to increase suddenly even if the image forming characteristic E1 of the image forming apparatus 1 changes.

  As described above, the gamma curve G2 has robustness with respect to the characteristic change of the image forming characteristics of the image forming apparatus 1, and has the property of suppressing the gradation skip by giving priority to the reproducibility of the smooth gradation change. is doing. However, the gamma curve G2 is a cause of image formation at a density lower than the target gradation value having linearity with respect to the input gradation value.

  Therefore, the gamma curve G2 has a property as a gradation difference priority table that is a table that prioritizes gradation differences over gradation values, and is called a gradation difference priority table. On the other hand, the gamma curve G1 has a property as a gradation priority table that prioritizes gradation values over gradation differences, that is, a table that prioritizes accurate gradation reproducibility, This is called a gradation priority table. Note that the gamma curve G1 is referred to as a gradation priority table for convenience even when there is no possibility of gradation skipping and a gradation difference priority table is not generated.

  In step S140, the gradation skip determination unit 11 analyzes the calibration value of the gamma table 41a and determines that there is a possibility of gradation skip. The process proceeds to step S150, and the possibility of gradation skip is determined. If it is determined that there is no, the process proceeds to step S170. The gradation skip determination unit 11 performs the determination using at least one image formation characteristic model (in this example, the image formation characteristic E2) assuming a characteristic variation set in advance as described above.

  In step S150, the calibration processing unit 12 executes a gradation skip suppression table generation process. In the gradation skip suppression table generation process, the calibration processing unit 12 adjusts the calibration value of the gamma table 41a so that the slope of the curve does not become excessively large, and generates the gamma table 41b. The gamma table 41b is generated, for example, by adjusting the gamma table 41a so that an increase of 5 gradations has a maximum increase rate (tilt) with respect to an increase of 1 gradation of input gradation values. As described above, the gamma table 41b is configured such that the difference between adjacent gradation values (in this example, 5 gradations, but 10 gradations) may be equal to or less than the threshold Th (in this example, 10 gradations). ing.

  In step S160, the calibration processing unit 12 executes a gradation skip flag setting process. The gradation skip flag setting process is a process of setting a flag indicating that the gamma table 41b has been generated. The gradation skip flag is used in gamma correction processing (described later).

  In step S170, the calibration processing unit 12 executes a gamma table update process. In the gamma table update process, the calibration processing unit 12 rewrites the gamma table 41a stored in the storage unit 40 with the data after calibration, and newly stores the gamma table 41b in the storage unit 40, or uses the data after calibration. rewrite. As a result, the color conversion processing unit 21 can use the gradation priority table and the gradation difference priority table.

  Note that the gradation skip determining unit 11 determines that the image forming characteristic specified in the gamma table calibration processing procedure is different from the range of the image forming characteristic model that assumes a preset characteristic variation. The formation characteristic model may be modified to expand the range of characteristic fluctuation.

  FIG. 7 is a flowchart showing a gamma correction processing procedure according to an embodiment. In step S210, the color conversion processing unit 21 performs color conversion processing. In the color conversion process, the color conversion processing unit 21 converts the image data ID, which is RGB data, into CMYK data as described above. The CMYK data is a target density gradation value of each CMYK toner.

  In step S220, the gradation skip determination unit 11 checks whether or not a gradation skip flag is set. If the gradation skip flag is set, the process proceeds to step S230. If the gradation skip flag is not set, the process proceeds to step S250. Note that the setting of the gradation skip flag is not essential, and the gradation skip determination unit 11 may calibrate so as to determine whether or not there is a possibility of gradation skip (step S140).

  In step S250, the gradation skip determination unit 11 performs a gradation priority table selection process. This is because the gradation priority table (gamma table 41a) has no possibility of gradation skip and the gradation skip suppression table (gamma table 41b) has not been generated.

  In step S230, the color conversion processing unit 21 executes an adjacent tone difference calculation process. The adjacent gradation difference is an output gradation value that increases as one gradation of the input gradation value increases. In other words, the adjacent gradation difference is a difference between output gradation values corresponding to input gradation values adjacent to the gradation.

  In step S240, the gradation skip determining unit 11 determines whether or not there is a possibility of gradation skip. Specifically, the gradation skip determination unit 11 determines whether there is an adjacent gradation difference that exceeds a preset threshold Th (for example, 10 gradations).

  The color conversion processing unit 21 analyzes the CMYK data and determines whether or not gradation skip occurs due to the use of the gamma table 41a. Specifically, for example, when the maximum input gradation value of CMYK data is 224 or less, the gradation region Z (see FIG. 4B) that may cause gradation skipping is not used. The gradation skip determination unit 11 determines that there is no possibility of gradation skip. On the other hand, when a gradation region Z that has a possibility of gradation skip is used, the gradation skip determination unit 11 determines that there is a possibility of gradation skip.

  If the gradation skip determination unit 11 determines that there is no possibility of gradation skip, the process proceeds to step S250. If it is determined that there is a possibility of gradation skip, the process proceeds to step S260. Proceed. In step S260, the gradation skip determination unit 11 executes a gradation difference priority table selection process. In the gradation difference priority table selection process, the gradation skip determination unit 11 selects a gradation difference priority table (gamma table 41b).

  In step S270, the gamma correction unit 211 performs gamma correction processing. In the gamma correction processing, the color conversion processing unit 21 performs gamma correction using the gradation difference priority table for an input image that may have gradation skipping, and there is no possibility of gradation skipping. Gamma correction can be performed on the input image using the gradation priority table.

  As described above, the image forming apparatus 1 according to the embodiment includes a tone priority table that prioritizes accurate tone reproducibility and tone difference priority that prioritizes smooth tone change reproducibility. Gamma correction is executed by properly using the table. As a result, the image forming apparatus 1 is capable of smooth gradation change such as a gradation image while suppressing image quality deterioration due to gradation skipping with high robustness even if the image formation characteristic varies according to environmental variation or the like. Reproducibility can be improved.

  The present invention can be implemented not only in the above embodiment but also in the following modifications.

  Modification 1: In the above embodiment, gamma correction is performed after the color conversion process, but it is not always necessary to perform gamma correction after the color conversion process. For example, the image forming apparatus may be configured to simultaneously perform color conversion and gamma correction using a color conversion table configured so that gamma correction can be performed together with color conversion processing.

  The color conversion table includes a first color conversion table including a gradation priority table and a second color conversion table including a gradation difference priority table. The color conversion processing unit 21 appropriately selects and uses the first color conversion table and the second color conversion table.

  Modification 2: In the above embodiment, the gradation priority table and the gradation difference priority table are properly used to improve the reproducibility of a smooth gradation change such as a gradation image while suppressing deterioration in image quality due to gradation skipping. I am letting. However, the present invention is not limited to such a method. For example, for an input gradation value in which occurrence of gradation skip is predicted, a correction table is prepared in advance so that the difference between adjacent gradation values is reduced. The image forming apparatus may be configured to suppress adjacent tone value differences. As described above, the gamma correction unit may be configured to suppress the gradation skip by adjusting the density gradation value so that the adjacent gradation value difference becomes small.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 10 Control part 11 Gradation skip judging part 12 Calibration processing part 20 Image forming part 21 Color conversion processing part 211 Gamma correction part 28 Calibration density sensor 29 Exposure part 40 Storage part 50 Image reading part 60 Paper feed cassette 70 Enclosure

Claims (8)

An image forming apparatus,
A gamma correction unit for gamma correction of input image data;
An image forming unit that forms an image on a print medium with a density gradation value corresponding to a predetermined image forming characteristic based on the gamma-corrected image data;
Analyzing the input image data, whether or not a gradation skip occurs in the image in which an adjacent gradation value difference, which is a difference between the density gradation values adjacent to each other in gradation, exceeds a preset threshold value A gradation skip determining unit for determining whether or not
With
When it is determined that the gradation skip occurs, the gamma correction unit adjusts the density gradation value so that the adjacent gradation value difference becomes small. The image forming apparatus according to claim 1,
The gradation skip determination unit is an image forming apparatus that performs the determination on the assumption of a variation in the image forming characteristics. The image forming apparatus according to claim 1, wherein
The gamma correction unit can use a tone priority table and a tone difference priority table, and when the tone skip determination unit determines that a tone skip occurs when the tone priority table is used, , To suppress the gradation skip using the gradation difference priority table,
The gradation forming priority table is an image forming apparatus configured such that the adjacent gradation value difference is equal to or less than the threshold value. The image forming apparatus according to claim 3, wherein
The gradation priority table is included in the first color conversion table,
The gradation difference priority table is included in the second color conversion table,
The gamma correction unit has a color conversion table including the first color conversion table and the second color conversion table. When the first color conversion table is used, the gradation skip occurs when a gradation skip occurs. An image forming apparatus that, when determined by the determination unit, uses the second color conversion table to color-convert RGB data representing RGB gradation values to generate CMY data representing CMY gradation values. . The image forming apparatus according to claim 3, further comprising:
A plurality of patches having different densities in stages, and a calibration processing unit that calibrates the gradation priority table based on the densities of the formed patches;
The gradation skip determination unit analyzes the gradation priority table and generates the gradation difference priority table in the calibration processing unit when the adjacent gradation value difference that exceeds the threshold exists. An image forming apparatus. The image forming apparatus according to claim 5, further comprising:
The image forming apparatus, wherein the gamma correction unit selects the gradation difference priority table in response to detection of a gradation skip flag indicating that the gradation difference priority table is available. An image forming method comprising:
A gamma correction process for gamma correcting the input image data;
An image forming step of forming an image on a print medium with a density gradation value corresponding to a predetermined image forming characteristic based on the gamma-corrected image data;
Analyzing the input image data, whether or not a gradation skip occurs in the image in which an adjacent gradation value difference, which is a difference between the density gradation values adjacent to each other in gradation, exceeds a preset threshold value A gradation skip determination step for determining whether or not
With
The image forming method, wherein the gamma correction step includes a step of adjusting the density gradation value so that the adjacent gradation value difference is reduced when it is determined that the gradation skip occurs. An image forming program for controlling an image forming apparatus having an image forming unit that forms an image on a print medium with a density gradation value corresponding to a predetermined image forming characteristic based on gamma-corrected image data. ,
A gamma correction unit that performs gamma correction on the input image data; and the input image data is analyzed, and an adjacent gradation value difference that is a difference between the adjacent gradation gradation values exceeds a preset threshold value. Causing the image forming apparatus to function as a gradation skip determination unit that determines whether or not a gradation skip occurs in the image;
The gamma correction unit is an image forming program for adjusting the density gradation value so that the adjacent gradation value difference is reduced when it is determined that the gradation skip occurs.