JP5225934B2 - Image forming apparatus and program - Google Patents

Description

  The present invention relates to an image forming apparatus and the like.

  In an electrophotographic image forming apparatus, in order to form a toner image, a one-component developing system using one-component toner, and a two-component developing apparatus using a two-component developer including a non-magnetic toner and a magnetic carrier It is divided roughly into.

  The one-component development method is suitable for downsizing, but is not suitable for high-speed development. Therefore, in a high-speed and long-life image forming apparatus, a two-component development device is almost adopted. In this type of developing apparatus using a two-component developer, the carrier itself in the two-component developer is not consumed and remains in the apparatus, but does not decrease. However, the toner is consumed and decreased. Therefore, in order to stabilize the image quality, the toner is appropriately replenished so that the toner concentration of the two-component developer is kept constant.

  In addition, the image forming apparatus changes the print image quality under the influence of deterioration of the photoconductor and developer and changes in environmental conditions. Therefore, various image quality adjustments (image density adjustment, process, etc.) are required to prevent this phenomenon. Recently, there is a technique in which those adjustment modes are implemented even when the image forming apparatus is not used for a long time.

  In recent years, in an image forming apparatus, there is an increasing need for printing with high image quality in gray scale or color. For this reason, in recent image forming apparatuses, halftone density correction is performed as a technique of image quality adjustment in addition to high density correction determined by binary values. Here, a method using a dither pattern is known as one of methods for correcting halftone density. In addition, not only the central dither pattern (main dither) but also a correction method using a plurality of dither patterns is known.

  For example, although it is set to 600 dpi in normal use, if it is possible to switch between 800 dpi and 1200 dpi as a way to obtain a high-definition image, the dither pattern for 600 dpi is used as the main dither, and 800 dpi is used. The dither pattern is A dither, the dither pattern for 1200 dpi is B dither, and the dither pattern is selected according to the output dpi.

  In Patent Document 1, multi-level dither processing is performed on an input image signal to convert it into a signal of a predetermined level number, and a signal of a predetermined level number output by the multi-level dither unit. An image processing apparatus having level conversion means for converting to a corresponding predetermined density level and image forming means for forming an image based on a predetermined density level signal output by the level conversion means, A correction image forming means for forming a corrected image by the forming means; a density detecting means for detecting the density of the formed corrected image; and the level conversion based on the density of the corrected image detected by the density detecting means. Control means for setting an intermediate density level among the predetermined density levels in the means, and when the image is formed by the multi-value dither method, The image processing apparatus is disclosed that such concentration levels maintained.

JP-A-9-326926

  Here, when halftone density correction is performed using a dither pattern, it is ideal that the output density can be faithfully reproduced with respect to the input density, that is, has a linear relationship. For example, as shown in FIG. 11, when the tone curve of the input density and the output density is shown, it is ideal that it is a straight line (ideal line). However, the actual relationship between the input density and the output density (that is, the γ characteristic) is as shown in FIG. 12A, and the density adjustment is performed so that this becomes a linear relationship.

  However, for example, when comparing dither A generated at 800 dpi (FIG. 12B) and dither B generated at 1200 dpi (FIG. 12C) with the main dither generated at 600 dpi. Therefore, different γ characteristics are obtained. As can be seen from these, the γ characteristic is slightly different depending on each dither.

  Japanese Patent Application Laid-Open No. 2004-228561 discloses that when the power is turned on, it is converted into a predetermined gradation so as to set an optimum halftone level under the environment, and is modulated and output. However, it is necessary to perform it every time the power is turned on. .

  In addition, since the dither pattern used in the halftone density adjustment in the conventional image forming apparatus performs the halftone density adjustment for only one main dither, the resolution for reading the document is different, and other dither patterns are used. However, it could not be said that the density adjustment was appropriately performed.

  In view of the above-described problems, an object of the present invention is to perform image quality adjustment more efficiently by performing halftone density adjustment of each dither pattern at an appropriate timing in an image forming apparatus having a plurality of dither patterns. An image forming apparatus that can be executed is provided.

In view of the above-described problems, the image forming apparatus of the present invention includes a dither pattern storage unit that stores a plurality of dither patterns according to an input image, and performs dither processing using the dither pattern on the input image. subjected, in an image forming apparatus for forming a pseudo-halftone image, the dither pattern storing means, as the dither pattern, the main dither pattern, stores and one or more sub-dither pattern, the pseudo intermediate Based on the detection result of the density detection means for detecting the density of the toned image, and the detection result of the density detection means, halftone density adjustment is executed only for the main dither pattern at a predetermined timing, and from the predetermined timing. with a small timing, the actual the halftone density adjustment to dither patterns stored in the dither pattern storage means A halftone density adjusting means for, characterized in that it comprises a.

In the image forming apparatus of the present invention, the halftone density adjusting means, the halftone density adjustment, and executes after a high density adjustment.

  The image forming apparatus of the present invention further includes a dither pattern correction coefficient storage unit that stores a correction coefficient for the main dither pattern for the sub dither pattern, and the sub dither pattern is the main dither pattern. It is calculated based on the dither pattern and the correction coefficient.

In the image forming apparatus of the present invention, the halftone density adjusting means, when executing the half-tone density correction for pairs to the all of the dither pattern, and said main dither pattern and the sub-dither pattern When the correction coefficient is updated based on the comparison result and the halftone density adjustment is executed only for the main dither pattern, the sub dither pattern is changed based on the main dither pattern and the correction coefficient. It is characterized by updating .

  In the image forming apparatus of the present invention, the dither pattern is stored according to the resolution of the input image.

  In the image forming apparatus of the present invention, the predetermined timing is when an environmental condition of the image forming apparatus is greatly changed.

The program of the present invention has a dither pattern storage unit that stores a plurality of dither patterns in accordance with an input image, performs dither processing on the input image using the dither pattern, and outputs a pseudo halftone image In the computer, the dither pattern storage means stores a main dither pattern and one or a plurality of sub dither patterns as the dither pattern, and a density detection function for detecting the density of the pseudo halftone image And halftone density adjustment only for the main dither pattern at a predetermined timing based on the detection result of the density detection function , and the dither pattern storage means at a timing less than the predetermined timing. halftone density adjusting function to perform the halftone density adjustment for all the dither patterns stored in Program for, to realize.

  According to the present invention, since halftone density correction is periodically performed on all dither patterns and updated to an optimum value for each dither pattern, it is possible to maintain clean image quality.

  According to the present invention, since the halftone density adjustment is performed after the high density adjustment, the halftone density correction is performed after securing the solid density, so that the image quality is further stabilized.

  In addition, according to the present invention, by obtaining the relational expression for the main dither pattern, halftone correction is performed using the updated relational expression at normal times, so that the adjustment time can be shortened. .

  In addition, according to the present invention, the predetermined timing is implemented when the environmental condition changes greatly. Since environmental conditions have a large effect on halftone density, it is possible to maintain a beautiful image quality by detecting the change.

  According to the invention, the regular time interval is reduced according to the degree of deterioration of the developer. Since the degree of change of the developer with time increases, it is possible to maintain a good image quality by performing halftone density correction in accordance with the change.

  Further, according to the present invention, since the degree of change differs for each color of the developer, an image forming apparatus that maintains color image quality can be obtained by performing halftone density correction for each color.

FIG. 2 is a cross-sectional view for explaining the configuration of the image forming apparatus according to the present embodiment. 1 is a functional configuration diagram for explaining an image forming apparatus according to an embodiment. It is a figure for demonstrating the structure of the dither pattern in this embodiment. It is a figure for demonstrating the structure of the correction coefficient in this embodiment. It is a figure for demonstrating the density adjustment implementation interval in this embodiment. It is a figure for demonstrating the flow of the process in this embodiment. It is a gamma characteristic figure in each dither. It is a figure for demonstrating correction | amendment of main dither. It is a figure for demonstrating the difference of the gamma characteristic of a main dither and another dither. It is a figure for demonstrating the relationship regarding the dither A and the dither B. FIG. It is a figure for demonstrating the conventional gamma characteristic. It is a figure for demonstrating the conventional gamma characteristic.

  Hereinafter, an image forming apparatus including a developing device according to the best embodiment of the present invention will be described in detail with reference to the drawings.

[Device configuration]
FIG. 1 is an explanatory diagram showing a simple configuration of a color image forming apparatus employing the image forming method of the present invention. The image forming apparatus 100, which is a main body apparatus, forms multi-color and single-color images on a sheet based on read image data of a document or image data transmitted via a network or the like. Therefore, the image forming apparatus 100 includes an exposure unit E, a photosensitive drum 101 (101a to 101d), a developing device 102 (102a to 102d), a charging roller 103 (103a to 103d), a cleaning unit 104 (104a to 104d), Intermediate transfer belt 11, primary transfer roller 13 (13a to 13d), secondary transfer roller 14, fixing device 15, paper transport paths P1, P2, P3, paper feed cassette 16, manual paper feed tray 17, paper discharge tray 18, etc. It has.

  The image forming apparatus 100 corresponds to the hues of four colors of cyan (C), magenta (M), and yellow (Y), which are three subtractive primary colors obtained by color separation of black (K) and a color image. Image formation is performed in the image forming units Pa to Pd using the image data. The image forming units Pa to Pd have the same configuration. For example, the black image forming unit Pa includes a photosensitive drum 101a, a developing device 102a, a charging roller 103a, a transfer roller 13a, a cleaning unit 104a, and the like. The image forming portions Pa to Pd are arranged in a line in the moving direction (sub-scanning direction) of the intermediate transfer belt 11.

  The charging roller 103 is a contact-type charger that uniformly charges the surface of the photosensitive drum 101 to a predetermined potential. Instead of the charging roller 103, a contact type charger using a charging brush or a non-contact type charger using a charging wire may be used. An exposure unit E that is an exposure apparatus of the present embodiment includes a semiconductor laser, a polygon mirror 4, a first reflection mirror 7, a second reflection mirror 8, and the like (not shown), and each of hues of black, cyan, magenta, and yellow. Each of the photosensitive drums 101a to 101d is irradiated with a light beam such as a laser beam modulated by the image data. On each of the photosensitive drums 101a to 101d, an electrostatic latent image is formed based on image data of each hue of black, cyan, magenta, and yellow.

  The developing device 102 supplies toner to the surface of the photosensitive drum 101 on which the electrostatic latent image is formed, and develops the electrostatic latent image into a toner image. Each of the developing devices 102a to 102d contains toner of each hue of black, cyan, magenta, and yellow, and the electrostatic latent image of each hue formed on each of the photosensitive drums 101a to 101d is black, cyan. Then, the toner images are visualized as magenta and yellow toner images. The cleaning unit 104 removes and collects toner remaining on the surface of the photosensitive drum 101 after development and image transfer.

  The intermediate transfer belt 11 disposed above the photosensitive drum 101 is stretched between the driving roller 11a and the driven roller 11b to form a loop-shaped movement path. The outer peripheral surface of the intermediate transfer belt 11 faces the photosensitive drum 101d, the photosensitive drum 101c, the photosensitive drum 101b, and the photosensitive drum 101a in this order. Primary transfer rollers 13a to 13d are arranged at positions facing the respective photosensitive drums 101a to 101d with the intermediate transfer belt 11 interposed therebetween. Each of the positions where the intermediate transfer belt 11 faces the photosensitive drums 101a to 101d is a primary transfer position. The intermediate transfer belt 11 is formed of a film having a thickness of about 100 to 150 μm.

  The primary transfer rollers 13a to 13d have constant voltage control of a primary transfer bias opposite to the charging polarity of the toner in order to transfer the toner images carried on the surfaces of the photosensitive drums 101a to 101d onto the intermediate transfer belt 11. Applied. As a result, the toner images of each hue formed on the photosensitive drum 101 (101a to 101d) are sequentially transferred to the outer peripheral surface of the intermediate transfer belt 11, and a full color toner image is formed on the outer peripheral surface of the intermediate transfer belt 11. Is done.

  However, when image data of only a part of the hues of yellow, magenta, cyan, and black is input, some of the four photosensitive drums 101a to 101d corresponding to the hue of the input image data. Only the photosensitive drum 101 forms an electrostatic latent image and a toner image. For example, when forming a monochrome image, an electrostatic latent image and a toner image are formed only on the photosensitive drum 101 a corresponding to the black hue, and only the black toner image is transferred to the outer peripheral surface of the intermediate transfer belt 11. Is done.

  As an example, each of the primary transfer rollers 13a to 13d is configured such that a shaft surface made of a metal (for example, stainless steel) having a diameter of 8 to 10 mm is covered with a conductive elastic material (for example, EPDM, urethane foam, or the like). In addition, a high voltage is uniformly applied to the intermediate transfer belt 11 by a conductive elastic material.

  The toner image transferred to the outer peripheral surface of the intermediate transfer belt 11 at each primary transfer position is conveyed to a secondary transfer position that is a position facing the secondary transfer roller 14 by the rotation of the intermediate transfer belt 11. The secondary transfer roller 14 is pressed against the outer peripheral surface of the intermediate transfer belt 11 whose inner peripheral surface is in contact with the peripheral surface of the driving roller 11a at a predetermined nip pressure during image formation. When the paper fed from the paper feed cassette 16 or the manual paper feed tray 17 passes between the secondary transfer roller 14 and the intermediate transfer belt 11, the polarity of the toner charged on the secondary transfer roller 14 is opposite to that of the toner. The high voltage is applied. As a result, the toner image is transferred from the outer peripheral surface of the intermediate transfer belt 11 to the surface of the sheet.

  Of the toner adhering to the intermediate transfer belt 11 from the photosensitive drum 101, the toner remaining on the intermediate transfer belt 11 without being transferred onto the paper is removed by the cleaning unit 12 in order to prevent color mixing in the next process. Collected.

  The sheet on which the toner image has been transferred is guided to the fixing device 15, and passes between the heating roller 15a and the pressure roller 15b to be heated and pressurized. As a result, the toner image is firmly fixed on the surface of the paper. The paper on which the toner image is fixed is discharged onto the paper discharge tray 18 by the paper discharge roller 18a.

  In the image forming apparatus 100, a substantially vertical direction for feeding the paper stored in the paper cassette 16 to the paper discharge tray 18 between the secondary transfer roller 14 and the intermediate transfer belt 11 and via the fixing device 15. A paper transport path P1 is provided. In the paper transport path P1, a pickup roller 16a that feeds the paper in the paper cassette 16 one by one into the paper transport path P1, a transport roller r that transports the fed paper upward, and the transported paper is predetermined. A registration roller 19 that guides between the secondary transfer roller 14 and the intermediate transfer belt 11 at the timing and a paper discharge roller 18 a that discharges the paper to the paper discharge tray 18 are disposed.

  Further, inside the image forming apparatus 100, a paper conveyance path P2 in which a pickup roller 17a and a conveyance roller r are arranged is formed between the manual paper feed tray 17 and the registration roller 19. Further, a paper transport path P3 is formed between the paper discharge roller 18a and the upstream side of the registration roller 19 in the paper transport path P1.

  The paper discharge roller 18a is rotatable in both forward and reverse directions. When the second side image is formed during single-sided image formation for forming an image on one side of the paper and double-sided image formation for forming an image on both sides of the paper, the paper discharge roller 18a is rotated forward. Driven in the direction, the paper is discharged to the paper discharge tray 18. On the other hand, when the first side image is formed in the double-sided image formation, the discharge roller 18a is driven in the normal rotation direction until the rear end of the paper passes through the fixing device 15, and then reversely rotated with the rear end of the paper sandwiched. Driven in the direction, the sheet is guided into the sheet conveyance path P3. As a result, the paper on which the image is formed on only one side when the double-sided image is formed is guided to the paper transport path P1 with the front and back surfaces and the front and rear ends reversed.

  The registration roller 19 feeds a sheet fed from the sheet cassette 16 or the manual feed tray 17 or conveyed via the sheet conveying path P 3 at a timing synchronized with the rotation of the intermediate transfer belt 11. 14 and the intermediate transfer belt 11. For this reason, the registration roller 19 stops rotating when the operation of the photosensitive drum 101 and the intermediate transfer belt 11 is started, and the sheet fed or conveyed prior to the rotation of the intermediate transfer belt 11 has the front end at the registration roller 19. The movement in the sheet conveyance path P1 is stopped in a state where the sheet 19 is in contact with the sheet 19. Thereafter, the registration roller 19 is a position where the secondary transfer roller 14 and the intermediate transfer belt 11 are in pressure contact with each other, at a timing when the front end portion of the sheet and the front end portion of the toner image formed on the intermediate transfer belt 11 face each other. Start spinning.

  Note that, during full color image formation in which image formation is performed in all of the image forming portions Pa to Pd, the primary transfer rollers 13a to 13d press the intermediate transfer belt 11 against all of the photosensitive drums 101a to 101d. On the other hand, at the time of monochrome image formation in which image formation is performed only in the image forming portion Pa, only the primary transfer roller 13a is brought into pressure contact with the photosensitive drum 101a.

[Function configuration]
Next, the functional configuration of the image forming apparatus 100 will be described with reference to FIG. As shown in FIG. 2, the image forming apparatus 100 includes a control unit 110, an image quality adjustment unit 120, a storage unit 130, an image reading unit 140, an image forming unit 145, a number counter unit 150, and a number setting unit. 155, an environmental condition detection unit 160, and a patch density measurement unit 170 are connected via a bus.

  The control unit 110 is a functional unit that performs various operations and controls of the image forming apparatus 100. Each process is realized by reading and executing various programs stored in the image forming apparatus 100. Here, the control part 110 is comprised by CPU (Central Processing Unit) which performs a calculation, for example.

  The image quality adjustment unit (density adjustment unit) 120 is a functional unit for executing various processes (for example, process control) for adjusting the image quality when an image is formed. At a predetermined timing, the image quality adjustment unit 120 executes image quality adjustment processing.

  Here, the image quality adjustment unit 120 in the present embodiment includes a calculation unit 122 and a dither comparison unit 124. The calculation unit 122 is a functional unit that executes various calculation processes performed during the image quality adjustment process, and generates, for example, each dither pattern. The dither comparison unit 124 is a functional unit that performs a comparison process of a main dither and a dither pattern (sub dither) as a sub corresponding to each resolution.

  In the present embodiment, description will be made assuming that dither A is used as a main dither generated at 600 dpi, dither A is used as a sub dither generated at 800 dpi, and dither B is used as a sub dither generated at 1200 dpi.

  The storage unit 130 is a functional unit for storing the setting state of the image forming apparatus 100 and storing image data. Here, the storage unit 130 is configured by any storage device such as a semiconductor memory, a hard disk drive, and an optical disk drive, for example. Further, various data and programs for operating the image forming apparatus 100 are also stored. The control unit 110 executes a control process by reading and executing the control program stored in the storage unit 130.

  The storage unit 130 stores a dither pattern 132 and a dither pattern correction coefficient 134. The dither pattern 132 stores each dither pattern (main dither and sub dither). For example, as shown in FIG. 3, the output value of the main dither and each sub dither is stored as a dither pattern corresponding to the input value.

  The dither pattern correction coefficient 134 stores the relationship between the main dither and each sub dither. For example, as shown in FIG. 4, the difference from the main dither is stored according to the input value. For example, when the input value is “2”, the dither A is output as “+2” from the output value of the main dither, and the dither B is output as “−1” from the output value of the main dither.

  In the present embodiment, the conversion between the output of the main dither and the output of the other sub dither is described as being executed using a table, but may be calculated using a conversion formula.

For example, assuming that the output value of the main dither is X, the output value of the dither A is Y _A , and the output value of the dither B is Y _B , the dither A has an input value range of a to b,
Y _A = S ₁ × X ² + T ₁ × X + U
The dither B is expressed as follows when the range of input values is c to d:
Y _B = S ₂ × X ³ + T ₂ × X ² + U ₂ × X + V ₂
Each dither may be generated by substituting a value into the above equation. Further, S ₁ , T ₁ , U ₁ , S ₂ , T ₂ , U ₂ , and V ₂ are constants, and each dither can be adjusted by correcting the constants.

  The image reading unit 140 is a functional unit that reads an image recorded on a recording sheet and generates image data. The image reading unit 140 is composed of, for example, a scanner. The image data generated by the image reading unit 140 is sent to the storage unit 130 and stored therein.

  The image forming unit 145 is a functional unit that forms an image based on the image data stored in the storage unit 130 and records (prints) the image on a recording sheet.

  The number counter unit 150 is a functional unit for counting the number of images formed in the image forming apparatus 100. In the present embodiment, the image quality adjustment process is performed by the image quality adjustment unit 120 by being read by the environmental condition detection unit 160 and referring to the counter. Further, the number setting unit 155 can set the number of images to be formed.

  The environmental condition detection unit 160 is a functional unit for executing image adjustment processing (process control / procedure control) according to the apparatus environment in the image forming apparatus 100. In the present embodiment, as an example, the number of images formed by the image forming apparatus 100 is used as an environmental condition.

  Specifically, as shown in FIG. 5, for example, the density adjustment is performed every 2000 sheets until the total number of printed sheets is 20,000 (A4), but every 1600 sheets between 2 and 40,000 sheets. Adjust the density. When the number is 60,000 or more, the density is adjusted every 1000 sheets. This is because the development characteristics such as chargeability tend to decrease due to the use of the developer, so that the output image density changes. In view of this, it is possible to form an image with more stable image quality by shortening the density adjustment interval that is periodically performed as the use of the developer progresses compared to the initial period.

  In addition to the timing described above, the density adjustment timing is a process that is appropriately executed, such as “when the environment changes”, “when the power is turned on”, “when the color balance is adjusted”, and “when the consumables are replaced”.

  The patch density measurement unit 170 is a functional unit for measuring the patch density used when the calculation unit 122 performs calculations in the image quality adjustment process. Depending on the patch density output by the patch density measuring unit 170, image quality adjustment processing is appropriately performed.

[Process flow]
Next, a basic processing method of the image forming apparatus 100 in the present embodiment will be described based on the flowchart of FIG.

  First, it is determined whether the image quality density adjustment should be performed by determining whether the power is turned on, when the number of print jobs has passed, or when the environmental change has changed significantly (step S10).

  If it is determined that it is time to adjust the density (step S10; Yes), high density adjustment is performed (step S12). Here, the high density adjustment is a conventionally used process, and a detailed description thereof is omitted. For example, toner patches are created on the photosensitive member or on the transfer bell with several development potential conditions. The toner adhesion amount is detected by a density sensor. A development potential condition is determined so that a predetermined toner adhesion amount is obtained based on the detection result. In the same control, not only the development potential but also conditions such as the photosensitive member charging potential, exposure laser power, and transfer potential may be determined.

  This high density adjustment is carried out in order to always obtain an optimum density with respect to a change in image density caused by a change in the state of the photosensitive member, developer, or the like.

  Next, it is determined whether or not it is time to execute halftone density adjustment for all dither patterns (step S14). Specifically, various timings are conceivable, such as when normal density adjustment (process control) has passed several times, when the apparatus has been used for a predetermined time or more, and when consumables have been replaced. Here, it is assumed that halftone density adjustment of all dithers is executed every time density adjustment is executed three times. That is, in the density adjustment, the process moves to step S22 twice and to step S16 once.

  When halftone density adjustment is performed for all dither patterns (step S14; Yes), halftone density adjustment is performed for all sub dithers including the main dither (step S16). As a result, the dither pattern 132 is updated.

  Thereafter, each updated dither is compared by the dither comparator 124 to see the correlation with the main dither (step S18), and the dither pattern correction coefficient 134 is updated based on the comparison result (step S20).

  If it is determined in step S14 that all halftone density adjustments are not to be executed (step S14; No), only the main dither density adjustment is performed (step S22). Thereby, only the main dither of the dither pattern 132 is updated first.

Subsequently, each sub dither is updated based on the updated main dither (step S24). Based on the output value of the main dither updated in step S22 and the correction coefficient stored in the dither pattern correction coefficient 134, the output value of each sub dither is calculated, and the dither pattern 132 is updated.
Therefore, it is possible to perform appropriate density adjustment for halftone density adjustment in each sub dither, usually only by adjusting the main dither.

[Example]
An image forming method for expressing halftone density using a plurality of dither patterns in the case where the above-described processing or the like is used will be described.

  First, as shown in FIG. 12, the γ characteristic for each dither is obtained at the initial stage of the image forming apparatus 100.

  More specifically, FIG. 7 is a graph showing the correlation between the main dither and each sub dither. As shown in FIG. 7A, the relationship between the dither A and the main dither is not linear (coefficient 1) but is slightly different, and the relationship is A1. In the case of a linear relationship, all correction coefficients are “0”. Further, as shown in FIG. 7B, the relationship between the dither B and the main dither is not linear, and the relationship is obtained as B1.

  After that, when it is time to adjust halftone density, the γ characteristic is newly renewed for only the main dither. For example, as shown in FIG. 8, the newly obtained γ characteristic is a solid line with respect to the previously obtained γ characteristic (dashed line).

  However, for the sub-dither (dither A and B in the present embodiment), the γ characteristic is not newly re-established, but is initially obtained and a new main dither and numerical value are obtained using the relationship (A1, B1). The halftone density correction was performed by subdithering.

  For this reason, the other sub-dithers deviated from the optimum γ characteristics with the passage of time, and accurate halftone density adjustment was not performed. That is, as shown in FIG. 9, as the output density with respect to the input density, a nearly perfect linear characteristic is secured for the main dither, but the other dither has been deviated.

  Therefore, in the image forming apparatus 100 of this embodiment, the dither pattern is read again and the dither pattern is updated by performing halftone density adjustment on other sub dithers periodically. That is, as with the main dither, the dither pattern is updated by taking the γ characteristic again in each sub dither, and the relationship (correction coefficient) between the dither pattern γ characteristic of the sub dither obtained again and the γ characteristic of the main dither is calculated. Execute the process.

  Accordingly, for dither A, for example, the relationship updated from A1 to A2 in FIG. 10A is changed, and for dither B, the relationship updated from B1 to B2 in FIG. 10B is changed. . That is, since the relationship between the main dither and each sub dither is also updated periodically, an appropriate sub dither can always be obtained based on the main dither, and a clean halftone density can be maintained.

  If density adjustment is always performed for all dither patterns, perfect halftone density adjustment can be obtained, but conversely, execution time is required and toner consumption is high because toner patches are created. The problem will happen. Therefore, in the present embodiment, the density adjustment of all dither patterns is executed periodically (for example, according to the environment detected by the environmental condition detection unit 160), and usually only for one main dither pattern. The density is adjusted.

[Modification]
The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and the design and the like within the scope of the present invention are also within the scope of the claims. include.

  In the above-described embodiment, the dither pattern is switched according to the resolution. However, the dither pattern may be changed according to the characteristics of the document to be read. For example, a normal “character / photo mode” in which characters and photos are mixed as the main dither, a “character mode” in which the characters are centered as the dither A, and a “photo mode” in which the photos are the center as the dither B, respectively. It is also possible to correspond to the original.

  In the above-described embodiment, the environmental condition detection unit 160 uses the number of printed sheets as an environmental condition. However, the environmental condition detection unit 160 may execute it using other conditions (for example, the internal temperature of the image forming apparatus 100). Of course.

  In the above-described embodiment, the relationship between the main dither and each sub dither has been described as storing a conversion table and generating each sub dither based on the conversion table. However, the conversion formula with the main dither may be used. Of course it is good.

In this case, when the relationship between the main dither and each sub dither is updated in step S20 of FIG. 6, the parameter of the conversion formula is changed. By changing the parameters, it is possible to appropriately generate each dither.

  In addition to realizing the functions of the above-described image forming apparatus and other functions in hardware, it can be realized by loading a computer program having the above-described functions into the memory of the computer. This computer program is stored in a magnetic disk, semiconductor memory or other recording medium provided in the image forming apparatus. Each function described above is realized by being loaded from the recording medium into a memory of a computer provided in the image forming apparatus and executed.

DESCRIPTION OF SYMBOLS 11 Intermediate transfer belt 15 Fixing apparatus 100 Image forming apparatus 101 Photosensitive drum 102 Developing apparatus 103 Charging roller 104 Cleaning unit 110 Control part 120 Image quality adjustment part (density adjustment part)
122 arithmetic unit 124 dither comparison unit 130 storage unit 132 dither pattern 140 image reading unit 145 image forming unit 150 sheet number counter unit 155 sheet number setting unit 160 environmental condition detection unit 170 patch density measurement unit

Claims (7)

In an image forming apparatus that includes a dither pattern storage unit that stores a plurality of dither patterns according to an input image, performs dither processing on the input image using the dither pattern, and forms a pseudo halftone image.
The dither pattern storing means, as the dither pattern, the main dither pattern, stores and one or more sub-dither pattern,
Density detecting means for detecting the density of the pseudo halftone image;
Based on the detection result of the density detection means, halftone density adjustment is executed only for the main dither pattern at a predetermined timing, and stored in the dither pattern storage means at a timing less than the predetermined timing. a halftone density adjusting means for executing the halftone density adjustment for all the dither patterns being,
An image forming apparatus comprising: The image forming apparatus according to claim 1, wherein the halftone density adjustment unit performs the halftone density adjustment after the high density adjustment. A dither pattern correction coefficient storage means for storing a correction coefficient for the main dither pattern for the sub dither pattern;
The image forming apparatus according to claim 1, wherein the sub dither pattern is calculated based on the main dither pattern and the correction coefficient. The halftone density adjusting means, when executing the half-tone density correction for pairs to the all of the dither patterns, updating the correction coefficient based on a comparison result between said main dither pattern and the sub-dither pattern and, when executing the half-tone density adjustment only to the main dither pattern, claim 3, characterized in that updating the sub-dither pattern based on said main dither pattern and the correction factor The image forming apparatus described in 1.   The image forming apparatus according to claim 1, wherein the dither pattern is stored according to a resolution of an input image.   The image forming apparatus according to claim 1, wherein the predetermined timing is a time when an environmental condition of the image forming apparatus is greatly changed. A computer that has a dither pattern storage unit that stores a plurality of dither patterns according to an input image, performs dither processing on the input image using the dither pattern, and outputs a pseudo halftone image.
The dither pattern storing means, as the dither pattern, the main dither pattern, stores and one or more sub-dither pattern,
A density detection function for detecting the density of the pseudo halftone image;
Based on the detection result of the density detection function, halftone density adjustment is executed only for the main dither pattern at a predetermined timing, and stored in the dither pattern storage means at a timing less than the predetermined timing. a halftone density adjusting function to perform the halftone density adjustment for all the dither patterns being,
A program to realize

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