JP2017208811A - Image processing apparatus, image forming apparatus, image processing method, and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the level difference in gradation occurring in correcting vertical streaks.SOLUTION: An image processing apparatus of the present invention comprises a storage part and a correction part. The storage part includes a plurality of halftone cells indicating a set of pixels in which gradation is expressed with two or more values and stores, for each of a plurality of positions in a main scanning direction of image data in which the gradation of the halftone cells is expressed by pulse-surface-area modulation, correction information associated with a correction value having a larger number of gradations than that of the halftone cells and indicating a value for acquiring a target output. The correction part corrects the image data on the basis of the correction information.SELECTED DRAWING: Figure 5

Description

  The present invention relates to an image processing apparatus, an image forming apparatus, an image processing method, and a program.

  2. Description of the Related Art Conventionally, an input image is used to eliminate image defects such as streaks that occur in the same direction as the feeding direction of a recording medium (for example, paper) in an image forming apparatus (hereinafter sometimes referred to as “vertical streaks”). A technique for correcting the density value of data is known.

  For example, Patent Document 1 discloses a plurality of basic configurations in addition to the gradation characteristics (relationship between input gradation values and output gradation values) of each primary color when an image of primary colors (basic composition colors) is printed. Disclosed is a technique for determining the gradation characteristics of each primary color when printing a multi-order color image in which colors are superimposed, creating a correction table based on these, and correcting the density value of input image data. ing.

  Here, depending on the number of gradations of the image data, the gradations may not be expressed smoothly. For example, it is difficult to connect all the gradations smoothly with an 8-bit gradation number. In such a case, if the gradation of the 8-bit image data is changed by the correction using the correction table, a gradation step is increased due to the change, and the vertical stripes are conspicuous. .

  In order to solve the above-described problem and achieve the object, the present invention includes a plurality of halftone cells indicating a set of pixels in which gradation is expressed by two or more values, The number of gradations in the main scanning direction of the image data in which the gradation of the halftone cell is expressed is larger than the number of halftone cells, and a correction value indicating the value for obtaining the target output is supported. The image processing apparatus includes a storage unit that stores the added correction information, and a correction unit that corrects the image data based on the correction information.

  According to the present invention, it is possible to suppress gradation steps that occur when correcting vertical stripes.

FIG. 1 is a diagram for explaining tone jump. FIG. 2 is a diagram for explaining a conventional problem. FIG. 3 is a diagram illustrating an example of the hardware configuration of the MFP. FIG. 4 is a diagram for explaining the input image data. FIG. 5 is a diagram illustrating an example of functions of the MFP. FIG. 6 is a diagram illustrating an example of the correction information. FIG. 7 is a flowchart illustrating an operation example of the MFP. FIG. 8 is a diagram illustrating a unit of a composite halftone cell that performs area gradation processing in input image data. FIG. 9 is a flowchart illustrating an example of processing for determining whether or not each halftone cell included in one composite halftone cell is a correction target. FIG. 10 is a diagram illustrating an example of the correction flag information. FIG. 11 is a diagram illustrating an example of correction flag information. FIG. 12 is a diagram illustrating an example of the correction information. FIG. 13 is a diagram illustrating a result of multiplying the correction flag and the correction value for each halftone cell. FIG. 14 is a diagram illustrating an example of correspondence information. FIG. 15 is a diagram showing final correction data. FIG. 16 is a diagram illustrating images of image data before and after correction. FIG. 17 is a flowchart illustrating an example of processing when correction is performed by obtaining final correction data. FIG. 18 is a diagram illustrating an example of the correction information.

  Hereinafter, embodiments of an image processing apparatus, an image forming apparatus, an image processing method, and a program according to the present invention will be described in detail with reference to the accompanying drawings. Hereinafter, as an example of an image forming apparatus to which the present invention is applied, a multifunction peripheral (MFP) will be described as an example. However, the present invention is not limited to this. Note that a multifunction peripheral is a device having a plurality of different functions such as a copy function, a scanner function, a print function, and a fax function.

  Before describing the specific contents of the embodiment, an outline of the present invention will be described. In order to create an 8-bit gradation (which can be expressed in 256 ways) in the image forming apparatus, image processing is performed so as to obtain a smooth gradation by performing screening. As shown in FIG. In the logarithm, it may occur that not all gradations are smoothly connected. In the present specification, the portion that cannot be smoothly connected is referred to as “tone jump” (see FIG. 1).

  There are few image forming apparatuses that can express gradation by the shading of one pixel itself. An example is a sublimation printer. Normally, as represented by offset printing, it is common to have only 1-bit information (binary information) indicating whether or not ink is attached to one pixel. In the laser exposure type electrophotographic method, there is a method in which the exposure time of one pixel is time-divided to express a gradation of about 16 values (4 bits), but an 8-bit gradation is expressed by one pixel. I can't. In view of this, in an image forming apparatus such as offset printing or electrophotographic method, an area gradation method is adopted in which the gradation of an image is represented by a set area of pixels such as halftone cells.

  In general, an image input to the image forming apparatus is represented by 8-bit gradation using area gradation, and an output image is generally represented by 8-bit gradation using area gradation. . However, in an image forming apparatus that corrects image defects (density unevenness and streaks) by converting input image data, if there is a tone jump in a gradation image that is not corrected for image defects (see FIG. 1, all gradation images are If the image cannot be connected smoothly, the correction may cause many fine streaks due to the tone jump, and the streaks may stand out from the uncorrected image.

  An image in which streaks are more conspicuous than an uncorrected image is a uniform image having a halftone near the gradation where a tone jump occurs. In the uncorrected halftone uniform image, the tone of the original image does not change, so the tone jump is not visible, but when performing the above correction, the tone jump is performed to change the tone of the original image. On the contrary, it may stand out as a fine streak (see FIG. 2).

  The present invention increases the number of gradations of correction values for obtaining a target output and smoothly connects gradations at correction positions for the purpose of eliminating tone jumps that occur when correcting vertical stripes. Thereby, the occurrence of tone jump can be suppressed. Hereinafter, specific contents of the embodiments will be described in detail with reference to the accompanying drawings.

(First embodiment)
FIG. 3 is a diagram illustrating an example of a hardware configuration of the MFP 1 according to the present embodiment. As illustrated in FIG. 3, the MFP 1 includes a CPU 10, an image input unit 20, a reading unit 30, a calibration memory 40, a storage unit 50, and an image output unit 60.

  The CPU 10 controls the overall operation of the MFP 1.

  The image input unit 20 receives image data from the host device. In the following description, image data received from the host device may be referred to as “input image data”. In this example, the input image data is image data expressed with 8-bit gradation. More specifically, as shown in FIG. 4, the input image data represents a set of pixels (in the example of FIG. 4, a set of 16 × 16 = 256 pixels) in which gradation is expressed with two or more values. This is image data that includes a plurality of halftone cells (halftone dots) and represents the gradation of each halftone cell (in this example, 8-bit gradation) by the area gradation method. In FIG. 4, the gradation of each pixel is exemplified as a case where the gradation is expressed in binary (represented by 1 bit). However, the present invention is not limited to this. For example, the gradation is expressed in multiple values (represented by multiple bits). It may be a form.

  The description of FIG. 3 is continued. The reading unit 30 is a device for optically reading an image formed on a recording medium (for example, paper) by the image output unit 60 under the control of the CPU 10, and includes, for example, a line sensor. As the configuration of the reading unit 30, various known configurations of scanner devices can be used.

  The calibration memory 40 is a device that stores in advance a test image used for calibration processing for generating correction information described later.

  The storage unit 50 stores correction information obtained by the calibration process. More specifically, the storage unit 50 associates a correction value for obtaining a target output with a larger number of gradations than the halftone cell for each of a plurality of positions in the main scanning direction of the input image data. The correction information is stored. Here, the main scanning direction refers to a direction orthogonal to the feeding direction (sub-scanning direction) of the recording medium (paper in this example).

  The image output unit 60 is an apparatus for forming image data such as input image data (corrected input image data described later) and a test image stored in the calibration memory 40 on a recording medium under the control of the CPU 10. It is.

  More specifically, the image output unit 60 is a device for fixing a toner image corresponding to image data such as input image data and a test image stored in the calibration memory 40 on a recording medium. The CMYK toners are mounted on the image output unit 60, and an image forming unit including a photoreceptor, a charger, a developing unit, and a photoreceptor cleaner, an exposure unit, and a fixing unit are mounted on each toner. Yes. The image output unit 60 irradiates a light beam from the exposure device according to the image data to form a toner image corresponding to each toner on the photosensitive member, and transfers the toner image formed on the photosensitive member to the intermediate transfer belt. After (primary transfer), the toner image transferred onto the intermediate transfer belt is transferred to the recording medium (secondary transfer), and the toner image transferred onto the recording medium is transferred at a temperature within a predetermined range by a fixing device. Fix by heating and pressing. As a result, an image is formed on the recording medium.

  Since the configuration of the image output unit 60 as described above is well known, a detailed description thereof is omitted here. As the configuration of the image output unit 60, various known configurations of printer engines can be used. Note that the recording medium is not limited to paper.

  FIG. 5 is a diagram illustrating an example of functions that the MFP 1 of the present embodiment has. As illustrated in FIG. 5, the MFP 1 includes a creation unit 101, an input image data reception unit 102, a correction unit 103, and an output control unit 104. For convenience of explanation, FIG. 5 mainly illustrates functions related to the present embodiment, but the functions of the MFP 1 are not limited to these.

  The creation unit 101 creates the correction information described above. In the embodiment, the creation unit 101 performs a calibration process so that the number of gradations is larger than that of a halftone cell and a target output is obtained for each of a plurality of positions in the main scanning direction of image data. Correction information in which values are associated is created. For example, the creation unit 101 can create correction information based on the read luminance of a test pattern obtained by forming a test image prepared in advance on a recording medium. The acquisition of the correction value will be described in more detail. The correction value acquisition need not be performed during printing, and is performed offline. The image output unit 60 of the MFP 1 (digital printing machine) outputs a chart having a uniform density in a direction perpendicular to the paper feed direction. The chart incorporates a plurality of process colors and gradations of each color, and outputs them under conditions without correction. At this time, in the output chart, density unevenness generated from the MFP 1 is output on the chart. The output chart is read by a scanner, and unevenness and streaks in the paper feed direction of the MFP 1 are read for each color and each gradation. The correction value can be calculated so as to cancel the read unevenness and streak data.

  Here, the number of gradations of the correction value is larger than the number of gradations of the image data (8 bits), and in this example is 9 bits. That is, the correction value can represent 512 gradations. However, the present invention is not limited to this. Here, the 9-bit correction value corresponding to the 8-bit correction value “1” (indicating that the gradation value of the image data is increased by “1”) is also “1”, and the 9-bit correction value Is assumed to be settable to 512 values from -127 to 128 in 0.5 increments.

  For example, as shown in FIG. 6A, the correction information is in a lookup table (LUT) format in which a 9-bit correction value is associated with each halftone cell (halftone dot) that divides input image data into a plurality of pieces. It may be information. In this example, the density (resolution) of pixels included in the halftone cell is 300 dpi, but is not limited thereto. The correction information created by the creation unit 101 is stored in the storage unit 50, for example.

  The input image data receiving unit 102 receives input image data from the host device. The correction unit 103 corrects the image data based on the above correction information. In this example, the correcting unit 103 corrects the input image data received by the input image data receiving unit 102 based on the correction information generated by the generating unit 101.

  When the correction value is associated with any position in the main scanning direction in the correction information described above, the correction unit 103 is arranged in the sub-scanning direction orthogonal to the main scanning direction at the position. A value of a pixel included in a set of halftone cells corresponding to the number of gradations of the value is corrected according to the correction value. For example, in FIG. 6A, a correction value of −0.5 is associated with each halftone cell (halftone dot) located in the third column from the left. In this example, since the area of one halftone cell is an area that can express an 8-bit gradation, an area equivalent to two halftone cells is required to express a 9-bit gradation. That is, in this example, the number of halftones corresponding to the number of gradations of the correction value is “2”, and one half is formed by two halftone cells adjacent in the sub-scanning direction (in the following, , This set of halftone cells may be referred to as a composite halftone cell).

  Then, the correcting unit 103 varies the 8-bit correction value for each of the plurality of halftone cells constituting the set so that a 9-bit correction value is obtained for one set (composite halftone cell) as a whole. As described above, when the 9-bit correction value is −0.5 value, two halftone cells (two halftone cells adjacent in the sub-scanning direction) constituting one set (composite halftone cell) The correction value of -0.5 value is implement | achieved in the one whole group by setting any one correction value to 0 value and making the other correction value -1 value. In this way, as shown in FIG. 6B, the correction values of the correction target halftone cells adjacent to each other in the sub-scanning direction are made different from each other, thereby preventing the same correction portion from continuing in the vertical direction. This can suppress the occurrence of tone jump.

  The output control unit 104 illustrated in FIG. 5 performs control to output input image data that has been corrected by the correction unit 103. More specifically, the output control unit 104 controls the image output unit 60 so that a toner image corresponding to the corrected input image data is formed on the recording medium.

  The functions of the MFP 1 described above (creating unit 101, input image data receiving unit 102, correction unit 103, output control unit 104, etc.) are realized by the CPU 10 executing a program stored in the storage unit 50 or the like. However, it is not limited to this. For example, at least a part of the functions of the MFP 1 (creating unit 101, input image data receiving unit 102, correction unit 103, output control unit 104, etc.) is realized by a dedicated hardware circuit (semiconductor integrated circuit or the like). It may be.

  FIG. 7 is a flowchart illustrating an operation example of the MFP 1 of the present embodiment. As shown in FIG. 7, first, the input image data receiving unit 102 receives input image data from the host device (step S1). Next, the correction | amendment part 103 correct | amends the input image data received by step S1 based on the above-mentioned correction information (step S2). Next, the output control unit 104 performs control to output the input image data corrected in step S2 (step S3).

  As described above, in this embodiment, the gradation of the correction position can be smoothly connected by increasing the number of gradations of the correction value for obtaining the target output. Therefore, according to the present embodiment, it is possible to suppress a gradation step (tone jump) that occurs when correcting vertical stripes.

  For example, when the number of vertical stripes when the 8-bit input image data is an A3 size full-scale halftone image is compared between the conventional example and this embodiment, the number of vertical stripes in the conventional example is 12. On the other hand, the number of vertical stripes in this embodiment is zero.

(Second Embodiment)
In the second embodiment, an example of how to specify a correction target and how to calculate a correction value will be described in more detail. Description of parts common to the above-described first embodiment will be omitted as appropriate.

  In the input image data shown in FIG. 8, a frame surrounded by a thick line represents a unit of a composite halftone cell for area gradation processing (in this example, one composite halftone cell is composed of four halftone cells). ) Here, for each composite halftone cell, it is determined whether or not each halftone cell constituting the composite halftone cell is a correction target, and the density of adjacent columns and rows for each halftone cell is determined. If equal, it is determined to be a correction target.

  FIG. 9 is a flowchart illustrating an example of processing for determining whether or not each halftone cell included in the composite halftone cell is a correction target. As shown in FIG. 9, the correction unit 103 includes four halftones in which the density value of the determination target halftone cell constitutes one composite halftone cell (a composite halftone cell including the determination target halftone cell). It is determined whether or not the density value of the halftone cell adjacent to the determination target halftone cell in the column direction coincides with that of the cell (step S11). When the result of step S11 is negative (step S11: No), the correction unit 103 determines that the determination target halftone cell is not the correction target, and sets the correction flag to “0” (step S12).

  When the result of step S11 is affirmative (step S11: Yes), the correction unit 103 has the density value of the determination target halftone cell adjacent to the determination target halftone cell among the four halftone cells in the row direction. It is determined whether or not it matches the density value of the halftone cell (step S13). If the result of step S13 is negative (step S13: No), the process proceeds to step S12 described above. On the other hand, when the result of step S13 is affirmative (step S13: Yes), the correction unit 103 determines that the halftone cell to be determined is a correction target, and sets the correction flag to “1” (step S14). .

  A determination example for the composite halftone cell 200 in the upper right of FIG. 8 is shown. Of the four halftone cells constituting the composite halftone cell 200, the halftone cell in the first row and the first column is referred to as "No. 1 halftone cell", and the halftone cell in the first row and the second column is designated as “No. 2 halftone cell”, the second row and first column halftone cell is called “No. 3 halftone cell”, and the second row and second column halftone cell is “No. 4”. "Halftone cell". In this example, the density value of “No. 3 halftone cell” indicates “gray”, and the density values of the other halftone cells indicate “white”.

  The density value of “No. 1 halftone cell” is the same as the density value of “No. 2 halftone cell” adjacent in the row direction (density value indicating white), but adjacent to the column direction “ The density value of “No. 3 halftone cell” indicates gray and the density value is different. Therefore, “No. 1 halftone cell” is not subject to correction. The density value of “No. 2 halftone cell” is the same as the density value of “No. 1 halftone cell” adjacent in the row direction (density value indicating white), and “No. The density value of “.4 halftone cell” is the same. Therefore, “No. 2 halftone cell” is a correction target. The density value of “No. 3 halftone cell” is different from the density value of “No. 4 half tone cell” adjacent in the row direction, and the density value of “No. 1 half tone cell” adjacent in the column direction is different. Also different from the value. Therefore, “No. 3 halftone cell” is not subject to correction. The density value of “No. 4 halftone cell” is the same as the density value of “No. 2 halftone cell” adjacent in the column direction, but “No. 3 halftone cell adjacent in the row direction”. The density value of “” indicates gray and the density value is different. Therefore, “No. 4 halftone cell” is not subject to correction. The above determination is performed for all halftone cells, and the correction target is represented by “1” (the correction flag is set to “1”) and the non-correction target is represented by “0” (the correction flag is set to “0”). FIG. Information in which a correction flag is set for each halftone cell as shown in FIG. 10 may be referred to as “correction flag information”.

  Next, a method for determining a final correction value from the correction flag information and the correction information will be described. FIG. 11 is a diagram showing an example of correction flag information. In this example, the correction flag information is information indicating a correction flag for each halftone cell. FIG. 12 is a diagram showing an example of the correction information. The correction information in this example is information indicating a correction value (in this example, the remainder after 8-bit correction) for each halftone cell. In this example, the correction unit 103 obtains the product of the correction flag and the correction value indicated by the correction information for each halftone cell. FIG. 13 is a diagram showing a result of multiplying the correction flag and the correction value for each halftone cell by using the correction flag information shown in FIG. 11 and the correction information shown in FIG.

  In this example, for each composite halftone cell, the composite halftone cell is configured using the values of the odd-numbered row and the odd-numbered column (the first row and the first column of the four halftone cells configuring the composite halftone cell). The four correction values corresponding to the four halftone cells to be one-to-one are determined. Here, as shown in FIG. 14, four halftone cells constituting a composite halftone cell including the halftone cell for each candidate value indicating a correction value candidate that can be taken by the odd-numbered and odd-numbered halftone cells. For each composite halftone cell, with reference to the correspondence information that associates the final correction values of each, the correction value of the first row and first column of the composite halftone cell (multiplication of the correction flag and the correction value) A final correction value (final correction value of each of the four halftone cells constituting the composite halftone cell) associated with the result is obtained. In this way, the final correction value for each halftone cell can be determined. In this embodiment, the odd-numbered and odd-numbered halftone cell candidate values are used to determine the final correction value. However, the present invention is not limited to this. For example, the halftone cell candidate values at other positions are used. As a key, it is also possible to determine the final correction value of each of the four halftone cells constituting the composite halftone cell including the halftone cell. In short, the correspondence information includes the halftone cell candidate value corresponding to any position of the composite halftone cell and each of a plurality (four in this example) of halftone cells constituting the composite halftone cell. Any information that associates the final correction value may be used. FIG. 15 is a diagram showing final correction data indicating the final correction value of each halftone cell in the above example. This final correction data is added to the original image (input image data). FIG. 16A shows an image of input image data before correction, and FIG. 16B shows an image after correction.

  FIG. 17 is a flowchart illustrating an example of processing when correction is performed by obtaining final correction data. As shown in FIG. 17, the correction unit 103 obtains the product of the correction value and the correction flag for each halftone cell (step S20). Next, the correction | amendment part 103 calculates | requires final correction data using the above-mentioned correspondence information (step S21). The specific contents are as described above. Next, the correction unit 103 adds final correction data to the input image data (step S22).

(Third embodiment)
Next, a third embodiment will be described. In the present embodiment, when the correction value is associated with any position in the main scanning direction, the correction unit 103 increases the resolution in the sub-scanning direction at that position. In addition, the correction unit 103 is included in a set of halftone cells of a number corresponding to the number of gradations of the correction value arranged in the sub-scanning direction of the position, as in the first embodiment. The pixel value is corrected according to the correction value.

  In the first embodiment, the number of area gradations is increased using two halftone cells (halftone dots) adjacent to each other in the sub-scanning direction in order to create a 9-bit correction value. As in the embodiment, for example, in order to create a 10-bit correction value, it is necessary to use four halftone cells. When the pixel resolution is 300 dpi, the size of the four halftone cells corresponds to 0.34 mm.

  For example, in the case of high-resolution image data such as 1200 dpi, there is no particular problem even when four halftone cells are replaced with one composite halftone cell. However, for example, in the case of 300 dpi image data, when an observer observes an image from a position 300 mm away, the sensitivity to the density fluctuation in the period near 1 mm / cycle is high, and a period of about 0.3 mm / cycle is easily perceived. It becomes a cycle. For this reason, if the resolution of the image is 300 dpi and the number of gradations is increased using four halftone cells, the period may be easily perceived.

  Therefore, in this embodiment, the resolution in the sub-scanning direction is doubled to 600 dpi, the size of the four halftone cells is reduced to 0.17 mm, and an area gradation is created with a period of 0.17 mm / cycle. As a result, it is possible to suppress gradation steps that occur when correcting vertical stripes while reducing sensitivity to density fluctuations.

  FIG. 18A is a diagram illustrating an example of correction information before increasing the resolution. FIG. 18B is a diagram illustrating an example of correction information after increasing the resolution. The correction value “0.25” in the sixth column from the left in FIG. 18A corresponds to the sixth unit column from the left in FIG. 18B. Here, (upper left, upper right, lower left, lower right) ) In the order of (1, 0, 0, 0).

  Similarly for other correction values, the correction value “−0.75” before increasing the resolution is (−1, −1, −1, 0) using four pixels constituting one unit. Represent. The correction value “−0.5” before increasing the resolution is expressed as (−1, −1, 0, 0) using four pixels. The correction value “−0.25” before increasing the resolution is expressed as (−1, 0, 0, 0) using four pixels. The correction value “0.5” before increasing the resolution is expressed as (1, 1, 0, 0) using four pixels. The correction value “0.75” before increasing the resolution is expressed as (1, 1, 1, 0) using four pixels. As described above, a gradation corresponding to 10 bits can be corrected with an 8-bit gradation by combining pixel areas.

  For example, when the number of vertical stripes when the 8-bit input image data is an A3 size full-scale halftone image is compared between the conventional example and this embodiment, the number of vertical stripes in the conventional example is 12. On the other hand, the number of vertical stripes in this embodiment is zero.

  Although the embodiments according to the present invention have been described above, the present invention is not limited to the above-described embodiments as they are, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. Various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the above-described embodiments. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

  The program executed by the MFP 1 (CPU 10) of each of the above-described embodiments is a file in an installable format or an executable format, and is a CD-ROM, flexible disk (FD), CD-R, DVD (Digital Versatile Disk). ), A computer-readable recording medium such as a USB (Universal Serial Bus), or the like, or may be provided or distributed via a network such as the Internet. Various programs may be provided by being incorporated in advance in a ROM or the like.

1 MFP
10 CPU
20 Image input unit 30 Reading unit 40 Calibration memory 50 Storage unit 60 Image output unit 101 Creation unit 102 Input image data reception unit 103 Correction unit 104 Output control unit

Japanese Patent No. 4661376

Claims (6)

A plurality of halftone cells representing a set of pixels in which gradation is expressed by two or more values, and a plurality of image data in which the gradation of each halftone cell is expressed by the area gradation method in the main scanning direction A storage unit that stores correction information in which a correction value indicating a value for obtaining a target output is associated with more gradations than the halftone cell for each position;
A correction unit that corrects the image data based on the correction information.
Image processing device. When the correction value is associated with any position in the main scanning direction, the correction unit includes the correction value arranged in the sub-scanning direction orthogonal to the main scanning direction at the position. Correcting the values of the pixels included in the set of halftone cells corresponding to the number of gradations according to the correction value;
The image processing apparatus according to claim 1. When the correction value is associated with any position in the main scanning direction, the correction unit increases the resolution in the sub-scanning direction at the position.
The image processing apparatus according to claim 2. A plurality of halftone cells representing a set of pixels in which gradation is expressed by two or more values, and a plurality of image data in which the gradation of each halftone cell is expressed by the area gradation method in the main scanning direction A storage unit that stores correction information in which a correction value indicating a value for obtaining a target output is associated with more gradations than the halftone cell for each position;
A correction unit that corrects the image data based on the correction information.
Image forming apparatus. A plurality of halftone cells representing a set of pixels in which gradation is expressed by two or more values, and a plurality of image data in which the gradation of each halftone cell is expressed by the area gradation method in the main scanning direction A correction step of correcting the image data on the basis of correction information in which the number of gradations is larger than that of the halftone cell and a correction value indicating a value for obtaining a target output is associated with each position. ,
Image processing method. On the computer,
A plurality of halftone cells representing a set of pixels in which gradation is expressed by two or more values, and a plurality of image data in which the gradation of each halftone cell is expressed by the area gradation method in the main scanning direction A correction step for correcting the image data is performed on the basis of correction information in which the number of gradations is larger than that of the halftone cell and a correction value indicating a value for obtaining a target output is associated with each position Program to let you.