JP6260200B2 - Image forming apparatus and image shift correction method - Google Patents

Description

  The present invention relates to an image forming apparatus that corrects a skew generated in an image to be formed when a raster image is formed by a scanning method, and an image shift correction method that corrects an image shift that is secondarily generated by the skew correction.

2. Description of the Related Art Conventionally, in an image forming apparatus such as a printer that records an image on a recording medium using image data processed from an original image, a method of forming a raster image by a scanning method has been widely adopted.
The scanning method is based on a technique of forming a raster image by writing pixels arranged in the main scanning direction on the recording medium with writing image data while moving the recording medium in the sub-scanning direction. The main scanning direction and the sub scanning direction are orthogonal to each other. Further, the recording medium includes a recording sheet (also simply referred to as “sheet”) and an image carrier (for example, a post-recording member) interposed before recording on the sheet.

  As an image forming apparatus using the above method, for example, lighting of a light source such as a laser is controlled by writing image data, and a light beam to be emitted is reflected by a rotating polygon mirror, and then to a rotating photosensitive drum as a recording medium. An apparatus for forming an image by projecting and scanning and exposing the surface of a photoreceptor is known. Here, main scanning occurs due to the rotation of the polygon mirror, and sub scanning occurs due to the rotation of the photosensitive drum. The image formed on the photosensitive drum is transferred to a sheet as a final recording medium.

In such a light beam scanning type image forming apparatus, there are variations in characteristics of optical elements such as lenses used for light beam scanning and mirrors of rotary polygon mirrors, and assembly accuracy of the optical elements in the apparatus. Depending on such component characteristics and assembly accuracy, the scanning line of the light beam may be tilted or bent with respect to the feeding direction (sub-scanning direction) of the photoreceptor surface.
As a result of the inclination and bending of the scanning light beam, when an image is formed with a plurality of colors, a shift occurs between the colors, which is perceived as a color shift. In particular, in a method in which individual photosensitive members are subjected to scanning exposure for each color component color, misalignment between colors is likely to occur, and there is a high need for means for eliminating or suppressing the misregistration.
In the following description, the above-described inclination or bending of the scanning line or a positional deviation or distortion appearing in an image formed thereby is referred to as “skew”.

Patent Document 1 can be cited as a conventionally proposed technique for correcting a skew generated in scanning exposure.
The skew correction described in Patent Document 1 is performed by a method of correcting image data used for laser lighting (light emission) control for writing an image with a light beam on a main scanning line. Specifically, the image data to be corrected is divided into a plurality of areas in the main scanning direction, and the image data is shifted in the sub-scanning direction in the direction opposite to the direction in which the skew is generated in each divided area. By turning on the light source with the image data subjected to such processing, no skew or bending occurs on the paper, and the skew is corrected.

Japanese Patent Application Laid-Open No. H10-228561 proposes a method for dealing with factors that cause image quality deterioration such as a step (edge) newly generated in an image as a result of skew correction by the above method. The level difference described above is caused by the image shift when the skew correction is performed on the image data whose value is reduced using a dither matrix having a small period. In this case, image quality deterioration such as generation of streak noise at the position where the image is shifted, or change in color tone in the region before and after the image is shifted may occur in the mixed color image.
In Patent Document 1, in order to reduce image quality degradation such as streak noise that occurs when there is a step of one pixel, the detection of the generated step and the density of the pixel column on the main scanning line of the detected step portion. A correction method is employed.

However, the conventional technique shown in Patent Document 1 that corrects a step as an image shift that is secondarily generated in an image that has been skew-corrected by shifting an image is a technique that deals with correction of a step of one pixel. Lacks adaptability to other steps. This is because the amount of shift required for skew correction differs depending on the type of dither matrix used for reducing the value. That is, depending on the type of dither matrix, a shift of a plurality of pixels is necessary for skew correction, and a step of a plurality of pixels is generated.
Therefore, the conventional technology cannot correct the step according to the situation where adaptation to a step other than one pixel is required.

Note that there is an invention of a prior application (Patent Document 2) of the present applicant (hereinafter referred to as “prior art”) regarding skew correction in response to a situation where a shift of a plurality of pixels other than one pixel is required.
This prior art shows that information for specifying the type of the dither matrix is acquired, and a method of changing the shift amount according to the type of the dither matrix specified by the acquired information is shown.
According to the above technique employed in the prior art, even if image data is shifted in units of one pixel, a shift in units of a plurality of pixels that suppresses abnormal images with respect to a dither matrix that cannot suppress abnormal images such as streaks. Shift image data by amount. That is, an appropriate skew correction is performed by determining a shift amount adapted to the type of dither matrix.

  An object of the present invention is to perform main scanning on an original image that is a correction target when correcting a skew that may occur in an image when a raster image is formed by a scanning method by image shift in the sub-scanning direction in units of the number of pixels. The image shift that is secondarily caused by skew correction between the image divided range divided in the direction and the image divided range adjacent to the image divided range is not only one pixel but also a predetermined plurality of pixels. It is to be able to correct.

In the present invention, when pixels arranged in the main scanning direction based on the original image are written on a recording medium by a scanning method to form a raster image, the original image is corrected in the main scanning direction in order to correct a skew generated in the raster image. An image forming apparatus for generating image data for writing by shifting an image of an image division range to be corrected by one or more predetermined pixel units in the sub-scanning direction for each image division range divided into Image deviation detecting means for detecting an image deviation in units of a predetermined number of pixels, which is generated by correcting skew between the images in the image division range adjacent to and in contact with the image division range to be corrected, from the image data for correction The image shift correction unit corrects the image shift with respect to the image data for writing of the image in the image division range to be corrected on the condition that the image shift is detected by the image shift detection unit. Possess an image deviation correcting means for performing over data processing, the image shift detection means, the pixel pattern represented by a pixel group in a predetermined area straddling the boundary of the image division areas in contact with each other adjacent the, one connection A means for determining the presence or absence of the step according to whether or not a pixel pattern condition corresponding to a step of a predetermined number of pixels in the image is met, and outputting the determined presence or absence of the step as an image shift detection result, The detecting means sets one of a plurality of types of steps having different numbers of pixels as the predetermined number of pixels, and supports each step so that the pass / fail for various steps can be determined by changing the number of pixels to be set. When one of the steps that can be changed is set, the predetermined pixel pattern conditions are switched according to the set number of steps. An image forming apparatus to make use Te.

  According to the present invention, an image shift secondary caused by the skew correction can be corrected not only for a single pixel but also for a predetermined plurality of pixels. Therefore, depending on the processing conditions of the writing image data, the skew correction may be performed. Even when an image shift of a plurality of pixels is required, deterioration in image quality in the formed image can be suppressed, and deterioration in image quality of an image formed due to a difference in processing conditions of image data for writing does not occur. High image quality can be maintained.

1 is a diagram illustrating a configuration of a printer engine according to an image forming apparatus of an embodiment. FIG. 2 is a diagram for explaining a configuration of an exposure unit having a scanning exposure apparatus as a main part viewed from the direction A in FIG. 1 and an image writing operation in the exposure unit. It is a figure explaining the correction | amendment principle of a skew. It is a figure which shows the structure of the image data processing system of a writing control part (FIG. 2) with the flow of data. It is a figure explaining the level | step difference which arises when image data is shifted per 2 pixels in skew correction. It is a figure which shows the structure of the image data processing system of a skew correction process part (FIG. 4) with the flow of data. It is a figure which shows the structure of the image data processing system of a level | step difference correction | amendment part (FIG. 6) with the flow of data. It is a figure explaining the pixel matrix of the data processing object in a level | step difference correction | amendment part (FIG. 7). It is a figure explaining the determination conditions of the level | step difference which generate | occur | produces when image data is shifted per pixel in skew correction. It is a figure explaining the determination conditions of the level | step difference which generate | occur | produces when image data is shifted per 2 pixels in skew correction. It is a figure explaining the case where the level | step difference (lower right) of 2 pixel units which generate | occur | produces in skew correction | amendment is correct | amended on the right side of a shift position. It is a figure explaining the case where the level | step difference (upward to the right) 2 units which generate | occur | produces in skew correction | amendment is correct | amended on the right side of a shift position. It is a figure explaining the case where the level | step difference (lower right) which arises in skew correction | amendment is correct | amended on the left side of a shift position. It is a figure explaining the case where the level | step difference (upward to the right) of a 2 pixel unit which generate | occur | produces in skew correction | amendment is correct | amended on the left side of a shift position. It is a figure explaining the case where the level | step difference of 2 pixel units which generate | occur | produces in skew correction | amendment is correct | amended on both sides of a shift position. It is a figure explaining an example in the case of making the line which obtains a reference value into one line below the attention line, when correcting the level difference of 2 pixels which arises in skew correction. It is a figure explaining the case where the line which obtains a reference value is made 2 lines below an attention line, when correcting the level difference of 2 pixels which arises in skew correction. It is a figure which shows the flow of a skew correction process.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
The image forming apparatus of the present embodiment is an example of an apparatus that forms an image by a scanning method using electrophotographic technology. In this image forming apparatus, lighting of the laser light source is controlled by writing image data, a light beam emitted from the laser light source is incident on the scanning optical system, and is projected as a scanning beam for exposing the photosensitive member as a recording medium. Thus, an image is formed on the photosensitive surface. Here, the pixel array is written by scanning the light beam in the main scanning direction, and a two-dimensional scanning method in which a photosensitive member as a recording medium is sent in the sub-scanning direction (perpendicular to the main scanning direction). A raster image is formed.

This image forming apparatus has a function of correcting skew that may occur when a raster image is formed on a recording medium by the above scanning method. In the skew correction, the image data for writing in the image division range to be corrected is shifted pixel by pixel in the sub-scanning direction so as to cancel the influence of the skew generated in the image in the image division range obtained by dividing the original image in the main scanning direction. It depends on the method.
Further, according to the skew correction that shifts the image in units of pixels, an image shift may occur as a result of the skew correction between the images in the image division range adjacent to and in contact with the image division range that is the correction target. This image shift is caused by shifting the images in the image division range in the sub-scanning direction at the time of skew correction, and a continuous image in the original image is a boundary where the image division ranges of the image subjected to skew correction are in contact with each other (“image data This occurs in the form of a step in the “shift position”. Here, a step as an image shift caused by skew correction is a problem to be solved.

The level difference as the image shift becomes a cause of deterioration of image quality that is secondarily generated in the formed image whose image quality is improved by performing the skew correction. Therefore, the image quality can be further improved by performing correction for suppressing or mitigating the influence of the step.
As described in the above [Background Art], there is a conventional technique (see Patent Document 1) for correcting image shifts that are secondarily generated in an image subjected to skew correction. However, this conventional technique is a solution for correcting the image shift of one pixel, and lacks adaptability to image shifts other than one pixel.
In view of this, the present image forming apparatus is provided with a correction function that takes into account adaptation to image shifts in units of a predetermined number of pixels of single and plural pixels.

The image forming apparatus according to the present embodiment will be described by taking an image forming apparatus such as a printer, a copier, or a multifunction peripheral (MFP) that forms an image by a scanning method using electrophotographic technology. Further, a method other than the electrophotographic technique can be similarly implemented as long as it is an image forming apparatus that forms a raster image by writing pixels by a scanning method.
A recording medium on which pixels are written by a scanning method is not limited to a mode in which an image formed on a photoconductor as a first recording medium is transferred to a sheet as a second recording medium, as in electrophotography. Instead, the image may be directly recorded on the paper by the scanning method.

The image forming apparatus of this embodiment will be described with reference to the illustrated configuration example.
FIG. 1 is a diagram illustrating a configuration of a printer engine according to the image forming apparatus 100 of the present embodiment. The printer engine shown in FIG. 1 employs an image forming method that requires correction of image misalignment that occurs secondarily in the image subjected to the skew correction described above. Is.
In FIG. 1, an image forming apparatus 100 is an engine of a tandem type color printer based on electrophotography, and includes an exposure unit, a color image forming unit 112, a transfer unit 122, and the like, which are main parts of a scanning exposure apparatus 102.
The color image forming unit 112 includes image forming process units (image forming units) 104, 106, 108, and 110 for respective colors of magenta (M), cyan (C), yellow (Y), and black (K).
The transfer unit 122 includes an endless intermediate transfer belt 114 as a component.
Each image forming process unit 104, 106, 108, 110 of the color image forming unit 112 includes a photosensitive drum 104a, 106a, 108a, 110a, respectively. Around each of these photosensitive drums, chargers 104b, 106b, 108b, 110b, developing devices 104c, 106c, 108c, 110c, primary transfer rollers 104d, 106d, 108d, 110d, and the like are arranged.

  The scanning exposure apparatus 102 includes a light source and a scanning optical system. A laser diode (LD) 206 (see FIG. 2), which is a semiconductor laser element, is used as a light source, and a light (laser) beam emitted from the LD 206 is passed through a scanning optical system. The scanning optical system includes a polygon mirror (rotating polygon mirror) 102c, which is a deflector, an fθ lens 102b, a reflection mirror 102a, and the like. The laser beam emitted from the light source is deflected by the polygon mirror 102c and incident on the fθ lens 102b, thereby scanning the photosensitive surface of each photosensitive drum at a substantially constant speed in the main scanning direction.

In this embodiment, the number of laser beams corresponding to the colors M, C, Y, and K is generated, and after passing through the fθ lens 102b, reflected by the reflecting mirror 102a. Therefore, four LDs 206 are provided, the polygon mirror 102c is provided in two stages coaxially, and the fθ lens 102b is provided in two stages on both sides of the polygon mirror 102c.
Each laser beam is shaped through the WTL lens 102d, then deflected again by the plurality of reflecting mirrors 102e, and emitted as four laser beams L used for exposure. Each laser beam L follows the surface to be scanned (hereinafter also simply referred to as “surface”) of the photoconductive drums 104a, 106a, 108a, 110a of the image forming process units 104, 106, 108, 110 along the respective axial directions. Irradiation while scanning in the main scanning direction.
Since irradiation of the laser beam L onto the surface of each photoconductive drum 104a, 106a, 108a, 110a is performed using a plurality of optical elements as described above, the timing is synchronized in the main scanning direction and the sub-scanning direction. Is done.
The “main scanning direction” is defined as the scanning direction of the laser beam, and the “sub-scanning direction” is orthogonal to the main scanning direction. In this image forming apparatus 100, the photosensitive drums 104a, 106a, 108a, and 110a rotate. Direction, that is, the moving direction of the drum photosensitive surface.

Each of the photosensitive drums 104a, 106a, 108a, and 110a includes a photoconductive layer including at least a charge generation layer and a charge transport layer on a conductive drum such as aluminum. The photoconductive layers of the photosensitive drums 104a, 106a, 108a, and 110a are charged with surface charges applied by the chargers 104b, 106b, 108b, and 110b configured by corotrons, scorotrons, charging rollers, or the like. .
In FIG. 1, the surface of the charged photoconductive layer of each of the photosensitive drums 104a, 106a, 108a, 110a that rotates counterclockwise is emitted from the laser beam L emitted from the LD whose lighting is controlled by the image data for writing from the scanning exposure apparatus 102. By doing so, scanning exposure is performed. By image writing with the laser beam L, a two-dimensional electrostatic latent image is formed on the surface of the photosensitive drum.
In this embodiment, formation of the electrostatic latent image and a toner image to be described later is started in the order of K, Y, C, and M. The order of forming the electrostatic latent image of each color and a toner image to be described later is as follows. This is not a limitation. Therefore, the arrangement order of the image forming process units 104, 106, 108, 110 for each color is not limited to the order shown.

The electrostatic latent images formed on the surfaces of the photosensitive drums 104a, 106a, 108a, and 110a are respectively developed by the developing units 104c, 106c, 108c, and 110c, and are K, Y, C, and M as developers of the respective colors. The toner is developed with each toner, and a toner image of each color is formed.
Each developing device 104c, 106c, 108c, 110c includes a developing sleeve, a developer supply roller, a regulating blade, and the like.
The developed toner images of the respective colors are sequentially superimposed and transferred in the order of K, Y, C, and M on the intermediate transfer belt 114 moving in the arrow B direction in the primary transfer portion. In the primary transfer portion, the photosensitive drums 104a, 106a, 108a, and 110a face the primary transfer rollers 104d, 106d, 108d, and 110d, respectively, with the intermediate transfer belt 114 interposed therebetween. A transfer bias voltage is applied to the primary transfer rollers 104d, 106d, 108d, and 110d.

The intermediate transfer belt 114 is stretched around the conveyance rollers 114a, 114b, and 114c, and one of the intermediate transfer belts 114 is rotated in the direction indicated by the arrow B (hereinafter referred to as “rotation”) by the conveyance rollers 114a or 114c, which are driving rollers. The intermediate transfer belt 114 carries a full-color toner image based on K, Y, C, and M toner images that are sequentially transferred on the surface of the intermediate transfer belt 114, and conveys the toner image to the secondary transfer unit.
The secondary transfer unit includes a secondary transfer belt 118 that is rotated in the direction indicated by arrow C by the transport rollers 118a and 118b. The conveyance roller 114b of the intermediate transfer belt 114 also functions as a secondary transfer counter roller.
A sheet-like recording medium (paper) 124 such as transfer paper or a plastic sheet is supplied to the secondary transfer unit from a recording medium storage unit 128 such as a paper feed cassette by a conveyance roller 126. Then, a secondary transfer bias is applied to the conveying roller 114 b that also functions as a secondary transfer counter roller, and the full-color toner image carried on the intermediate transfer belt 114 is held by suction on the secondary transfer belt 118. Transfer to the recording medium 124.

The recording medium 124 onto which the full-color toner image has been transferred is conveyed to the fixing device 120 by the rotation of the secondary transfer belt 118 in the direction of arrow C.
The fixing device 120 fixes the toner image on the recording medium 124 by sandwiching the recording medium 124 at the nip portion between the fixing roller 121 and the pressure roller 123 and conveying the recording medium 124 with the transferred toner image while heating and pressing. The recording medium 124 that has passed through the fixing device 120 is discharged to the outside of the image forming apparatus 100 as a printed matter 132.
After the toner image is transferred to the recording medium, the transfer residual toner is removed from the intermediate transfer belt 114 by a cleaning unit 116 including a cleaning blade to prepare for the next image forming process.

FIG. 2 is a diagram for explaining the configuration of the exposure unit having the scanning exposure apparatus 102 as a main part viewed from the direction A in FIG. 1 and the image writing operation in the exposure unit.
In FIG. 2, the exposure unit includes a scanning exposure apparatus 102 and an LD lighting control unit 20.
The scanning exposure apparatus 102 includes a scanning optical system including an LD 206 as a light source, a polygon mirror 102c, an fθ lens 102b, and the like as constituent elements.
In the scanning exposure apparatus 102 shown in FIG. 2, the laser beam from the LD 206 is deflected and scanned by the polygon mirror 102c, and the surface of the photosensitive drum 104a is exposed while being scanned by the laser beam L that has passed through the fθ lens 102b.
The polygon mirror 102c is rotationally driven at a peripheral speed of several thousand to several tens of thousands rpm by a polygon motor (not shown).

In order to detect the laser beam L deflected and scanned by the polygon mirror 102c at a predetermined position before the image writing start position on the main scanning line, an optical sensor (PD, synchronization detection sensor) such as a reflection mirror 208 and a photodiode. 210. Note that the above-mentioned predetermined position is outside the image area, and is actually a position where the photosensitive drum 104a as a recording medium is not projected. An image writing start position in each main scanning direction is controlled by a synchronization detection signal from the synchronization detection sensor 210.
In the scanning exposure apparatus 102, four LDs 206 are provided for each color, and as shown in FIG. 1, the polygon mirror 102c is provided in two upper and lower stages (the stage is shared by laser beams for two colors), Four laser beams L can be deflected and scanned. Four fθ lenses 102b and four reflection mirrors 102a are also provided. The light source may be provided with a surface emitting laser (VCSEL) instead of the LD 206.
As shown in FIG. 1, the photosensitive drum 104a is also provided with the photosensitive drums 104a, 106a, 108a, 110a for the image forming process sections 104, 106, 108, 110 for the respective colors.

In addition, the image forming apparatus 100 is a printer engine that responds to various print requests using image data input through different input means such as a scanner or a network I / F (interface), such as an MFP having a composite function. To form an image. The controller 11 shown in FIG. 2 has a function as a main controller that collectively performs image forming operations in response to various printing requests as described above and controls the entire apparatus (see the controller 11 in FIG. 4).
The controller 11 puts the write control unit 12 under control and causes the write control unit 12 to perform write control in response to various print requests.
The writing control unit 12 generates writing image data for controlling the lighting of the LD 206 based on the image data received together with the print command from the controller 11 and outputs the writing image data to the LD lighting control unit 20. The image data for writing for controlling the lighting of the LD 206 may be a small value or a multi-value. In the case of binary, it is possible to adopt a method of generating pixels by ON / OFF control of lighting, and in the case of multiple values, a method of generating pixels by light amount control.

  By the way, a control system comprising the LD lighting control unit 20 and the writing control unit 12 for controlling the lighting of the LD 206, and the controller 11 as the higher level control unit can be configured by a computer. That is, the computer includes a CPU (Central Processing Unit) for executing instructions of the software program, ROM (Read Only Memory), RAM (Random Access Memory), and NVRAM (Non Volatile RAM) storage means as hardware. As an element of The ROM is a memory that stores programs, data, and the like used by the CPU to realize the operation and processing of the control system. The RAM is a memory used as a memory for temporarily storing data generated by the program or a work memory for storing data necessary for the operation of the software program. The NVRAM is a non-volatile memory that stores management information for managing the control system.

  When the control system is configured by a computer, a program and control data for executing an LD control operation including a lighting control capable of correcting a step caused by skew correction, which will be described later, are transmitted via various recording media. Install on your computer. The CPU can execute an intended control operation by driving the installed program and using the installed control data.

Here, skew correction performed in the writing control will be described.
FIG. 3 is a diagram for explaining a skew correction principle.
In FIG. 3, A, B, C, and D respectively represent image data used for printing or writing or an image printed or written on a recording medium as pixel rows arranged on the horizontal axis in the main scanning direction. The vertical axis in the figure is the sub-scanning direction. Regions 1 to 4 indicate the division ranges of the image divided in the main scanning direction, and are regions to be corrected.
Hereinafter, a method for correcting the bending and inclination of the print image by shifting the image data in the sub-scanning direction will be described with reference to FIG.

When the original image is printed using writing image data of pixel rows arranged in a straight line in the main scanning direction as shown in FIG. 3A, on the image surface of the photosensitive member or on the finally output paper, As shown in FIG. 3B, the pixel is displaced in the sub-scanning direction, and the toner image is bent or inclined. This is because the position of the image is shifted due to the characteristics of the optical system such as a lens or mirror that transmits and reflects the light beam emitted from the light source and the skew generated by the tilt of the optical element generated in the assembly process. .
In an apparatus that forms an image by superimposing a plurality of color toner images, the bend and inclination differ depending on the system of each color, so that color misregistration occurs between colors and image quality is impaired.

Therefore, as shown in FIG. 3C, the writing image data of the original image is shifted in the sub-scanning direction in accordance with the amount of misalignment due to skew, and printing is performed using the shifted writing image data. Correct the tilt. In this case, the direction in which the writing image data of the original image is shifted is opposite to the direction in which the inclination or the like is generated. In the example of FIG. 3, the image of the image plane that is not subjected to shift correction (FIG. 3B) has an upward slope from region 1 to region 3 and a downward slope from region 3 to region 4. Therefore, the image data for writing after the shift is shifted downward from the region 1 toward the region 3, the region 3 is not shifted, and the region 4 is located above the region 3 as shown in FIG. 3C. The shift is performed for each area.
The amount to be shifted at this time is the smallest unit of the image data in the sub-scanning direction, that is, every line unit that is the smallest unit capable of exposing the beam, that is, every pixel unit formed in the sub-scanning direction.

By performing the shift in units of lines (pixels) in this way, as shown in FIG. 3D, even if the optical system or the like is bent or tilted, it is bent or tilted in the sub-scanning direction on the image surface of the photoreceptor or on the paper. And a toner image subjected to skew correction can be formed.
Normally, reading of image data from a memory storing image data used for writing is performed for each line to be written. However, when performing image data shift for this skew correction, as described above, sub-scanning is performed by changing the line of the image read from the memory for each of the regions 1, 2, 3, and 4 divided in the main scanning direction. Output data as if the image was shifted in the direction. Since the image data for writing is processed according to such a procedure, the image data to be shifted is the data handled by using the half-valued half-tone image rather than the multi-value half-tone image. Since the size of the memory becomes smaller, the size of the memory can be reduced.

The image forming apparatus includes an image data processing system that performs processing in accordance with the above-described skew correction principle. This image data processing system is configured as a processing system of the writing control unit (FIG. 2).
FIG. 4 is a diagram showing the configuration of the image data processing system of the writing control unit 12 (FIG. 2) together with the data flow. FIG. 4 shows a configuration of a form applied to a copying machine or a multi-function machine equipped with a printer engine (FIGS. 1 and 2) for forming a color image.
In FIG. 4, a controller 11 functions as a main controller that controls the entire image forming apparatus, and includes a printer controller 21, a scanner controller 22, and an image processing unit 23 as components.

The printer controller 21 is for receiving image data transmitted from an external device such as a personal computer (hereinafter referred to as “PC”) via a network. The printer controller 21 transfers the received image data to the image processing unit 23.
The scanner controller 22 is for acquiring image data by reading a document with a scanner (not shown). The scanner controller 22 transfers the acquired image data to the image processing unit 23.
The image processing unit 23 generates various image data for generating image data that can be used for writing by the printer engine in accordance with the image data received by the printer controller 21 or the image data acquired by the scanner controller 22. Apply processing.

The writing control unit 12 receives the image data transferred from the image processing unit 23 of the controller 11, performs various processes for generating the writing image data on the received image data, and then performs this image processing. An image signal used for writing is generated from the data and transmitted to the LD lighting control unit 20.
The LD lighting control unit 20 is provided in the exposure unit, and controls the lighting of the LD 206 that emits a laser beam that irradiates the surface of the photosensitive drum 104a and writes an image.

The writing control unit 12 includes an input image control unit 31 and a line memory 32 at an input stage that receives image data transferred from the image processing unit 23.
The writing image processing unit 34 performs various image processing for generating image data used for lighting control based on the image data stored in the line memory 32.
The LD data output unit 35 is provided in an output stage that outputs an image signal used for writing to the LD lighting control unit 20 and can set an output frequency very finely, for example, a voltage controlled oscillator (VCO: Voltage Controlled Oscillator). A device such as a clock generator is used.
The input image control unit 31 receives the image data transferred from the image processing unit 23, stores the received image data in the line memory 32, and transfers the stored image data to the skew correction processing unit 33. The line memory 32 sequentially stores the image data transferred from the image processing unit 23.

The skew correction processing unit 33 performs skew correction on the image data used for writing in accordance with the principle described above with reference to FIG.
In the present embodiment, a raster image is formed by pixel rows sequentially written in a line in the main scanning direction at regular intervals in the sub-scanning direction, and image data used for this image writing is generated. Therefore, in order to cope with the present skew correction, in the present embodiment, the image data for forming the raster image is stored in the line memory 32 for each line in units of one line in the main scanning direction.

In addition, the skew correction processing unit 33 uses the image data for each line in the main scanning direction stored in the line memory 32 as an image range for skew correction. And a correction process is performed as necessary for each image division range.
This skew correction processing corrects the skew generated when the toner image is formed by shifting the original image data in the sub-scanning direction in the opposite direction to the skew that may occur in the image to be formed for each image division range. .
Since FIG. 4 shows a data processing system common to each color, the system is actually configured for each of the four colors. At this time, it is necessary to shift the skew correction in accordance with the same reference between the systems of the respective colors in order to prevent the positional deviation between the colors.
In addition, as a method of generating image data or an image signal used for lighting control of the LD 206 as image data for writing at the time of performing skew correction, an existing technique can be adopted, and thus description thereof is omitted here.

Next, correction of image misalignment that occurs secondarily in an image subjected to skew correction will be described.
This image shift that occurs secondarily due to the skew correction is dealt with by a correction function that considers adaptation to image shift in units of a predetermined number of pixels in the image forming apparatus.
Here, this image shift that is secondarily caused by skew correction will be described by taking as an example a step generated in a pixel unit in the formed image.

FIG. 5 is a diagram for explaining a step generated when image data is shifted in units of two pixels in skew correction. The image in the figure shows an example of fine lines formed in the main scanning direction.
FIG. 5A shows original image data before performing shift correction. A pixel row of one line in the main scanning direction has a configuration in which high-density pixels are continuously arranged on the line. An image having a configuration in which four lines are continuously arranged in the scanning direction is shown. Since the image data in the figure is before the shift correction, a continuous image, that is, a thin line formed in the main scanning direction, is displayed at the “image data shift position” shown in the middle of the pixel row in the main scanning direction. This is an image to be constructed, and no step is generated.
FIG. 5B shows image data after the shift correction is performed, and is an example in which the image data shown in FIG. 5A is shifted by two lines (two pixels) in the sub-scanning direction. Since the image data in the figure is after the shift correction, the “image data shift position” shown in the middle of the pixel row in the main scanning direction is originally connected to form a thin line formed in the main scanning direction. A step as an image shift of two pixels is generated in an image that should be.

As a cause of image quality deterioration such as streak and color unevenness caused by skew, there are image processing conditions of image data for writing, for example, conditions such as a dither matrix used for a value reduction process.
Focusing on the dither matrix, a skew correction method for preventing deterioration of image quality such as streak and color unevenness caused by the skew has already been proposed (see Patent Document 2, hereinafter referred to as “prior art”).
In this prior art, when the image data is shifted to correct the skew, the amount by which the image data is shifted is changed according to the shape of the dither matrix employed. In other words, depending on the shape of the dither matrix adopted, even if this is not possible with one pixel, the condition for eliminating streaks and color unevenness caused by skew is satisfied by shifting two pixels, and the image quality can be improved. It is a countermeasure. In this coping method, the amount of skew correction shift is determined by the image data reduction processing conditions such as the type of dither matrix.

The example of FIG. 5 shows a case where it is selected to shift the image data in units of two pixels in order to correct the skew. In this case, by performing skew correction, correction that suppresses the negative influence due to skew, such as a color shift that is originally intended, is made when viewed as a whole image. However, paying attention to the image data shift position, when the original image is a fine line or an image edge, a step corresponding to the shift amount at the shift position, that is, a step corresponding to two pixels in the example of FIG. 5 occurs. Become.
When a dither pattern having a shift amount of 2 pixels for which skew correction functions effectively is selected, a new element that degrades image quality is generated due to a step as an image shift that is secondarily generated by skew correction. The level difference generated in the image becomes larger as the number of pixels representing the level difference becomes larger, and image quality deterioration becomes worse.

In view of this, the present image forming apparatus is provided with a step correction function that adapts to a step of each predetermined number of pixels and suppresses deterioration in image quality.
This step correction is performed as part of the skew correction process. First, a step of detecting a step from the corrected image data is performed, and then a step of correcting the step detected in the previous step is performed. Basic.
In the step of detecting the step, a predetermined amount of step corresponding to the shift amount in the skew correction is detected from the image data subjected to the skew correction. Further, in the step of correcting the step, the image data of the step portion is corrected and the image data used for writing the image is output so as to suppress the deterioration of the image due to the step for the step detected in the previous step.

Here, a data processing system that performs processing in accordance with the above-described basics of the level difference correction processing will be described with reference to the accompanying drawings.
FIG. 6 is a diagram showing the configuration of the image data processing system of the skew correction processing unit 33 (FIG. 4) together with the data flow.
In FIG. 6, the skew correction processing unit 33 (see FIG. 4) includes a data selector 331, a skew output control unit 332, and a step correction unit 333. Since the level difference correction is a process for the image data on which the skew correction has been performed, the level difference correction unit 333 is provided in the subsequent stage in the skew correction processing unit 33 as shown in the figure.

The data selector 331 selects the image data transferred from the input image control unit 31 (FIG. 4) and transfers it to the step correction unit 333.
As described above, the image forming apparatus 100 sequentially writes pixels in the main scanning direction and performs an operation of forming an image as a pixel row arranged on the line at regular intervals in the sub-scanning direction. To form a raster image. In addition, skew correction and correction of a step generated secondarily by the skew correction are performed on the image data in which the raster image formed by this scanning method is written.
Therefore, the data selector 331 in this embodiment holds a total of five lines, that is, a designated line in each line of the pixel column formed at regular intervals in the sub-scanning direction and the two lines above and below the specified line. It is possible to output to the correction unit 333.
Which line is selected and output from a plurality of lines (5 lines in the present embodiment) by the data selector 331 is determined by information from the skew output control unit 332.

The skew output control unit 332 outputs a line select signal to the data selector 331. This line select signal includes information on a shift position and a shift direction of skew correction obtained by an image misalignment detection unit (not shown) due to skew and a shift amount corresponding to a dither matrix in order to perform skew correction processing. And generated.
Each information of the shift position according to the skew correction shift position, shift direction, and dither matrix necessary for generating the line select signal is acquired for each image division range (see regions 1 to 4 in FIG. 3).
In the present embodiment, the shift amount corresponding to the dither matrix is information managed by the controller 11 as the upper control unit, and the skew output control unit 332 receives this information from the controller 11. Further, the skew output control unit 332 acquires the skew correction shift position and shift direction from the image data received through the input image control unit 31.
Note that the skew correction processing itself in the present embodiment can be performed by using an existing technique.

In addition, the skew output control unit 332 sends a step correction area width signal to the step correction unit 333 and a shift direction and shift information that is a source of the line select signal output to the data selector 331 along with this signal. Outputs the shift amount.
The step correction unit 333 determines a correction range from the input step correction area width signal, performs step correction according to the shift amount on the image data input through the data selector 331, and writes the image processing unit 34. (See FIG. 4). In order to perform the level difference correction according to the shift amount, shift information in the skew correction input from the skew output control unit 332 is used.
Here, the step correction area width signal is a signal indicating the step correction area width which is the number of pixels as a unit of the step correction (density correction) (see the embodiment in FIG. 11).
Note that the number of lines output from the data selector 331 in accordance with the processing in the step correction unit 333 is 5 lines in the present embodiment as described above, but is not limited to this, and as an arbitrary value of 2 lines or more Can be configured.

Here, the step correction will be described in more detail.
The correction processing of the step portion generated in the image after the skew correction is configured by a step detection process, a correction range determination process, and a density correction process for the image data after the skew correction process.
FIG. 7 is a diagram showing the configuration of the image data processing system of the step correction unit 333 (FIG. 6) together with the data flow.
The step correction unit 333 of this embodiment performs a correction process of the step portion using the image data for five lines input from the data selector 331.
As shown in FIG. 7, the step correction unit 333 includes a correction density calculation unit 333c, a step detection unit 333d, a correction range determination unit 333a, and a step correction output unit 333s.

Image data subjected to skew correction output from the data selector 331 (see FIG. 6) is input to the correction density calculator 333c and the step detector 333d.
The level difference detection unit 333d corresponds to the image shift detection unit of the present invention, and the pixel at the shift position (the right pixel of the “image data shift position” in FIG. 5B) in the input image data after skew correction is sequentially set as the target pixel. Based on the result of determining whether or not the pixel of interest is a stepped pixel constituting the stepped portion, that is, the presence or absence of the stepped portion, processing for detecting the stepped portion is performed. The step detection process will be described in detail later with reference to examples.
The correction density calculation unit 333c performs density calculation to calculate a density value (referred to as a “correction density value”) to be changed by the target pixel using a pixel group including a target pixel detected as a step and pixels above and below the target pixel. Processing for outputting the density calculation result to the step correction output unit 333 s is performed. The correction density calculation process will be described in detail later with reference to examples.

When the step detection unit 333d determines that the target pixel is a step pixel, the correction range determination unit 333a performs processing for correcting a step, that is, a range of pixels to be subjected to density value change processing (“ A process of determining a “step correction range” or simply “correction range” is performed.
In the present embodiment, the step correction range is a fixed range in the pixel row aligned in the main scanning direction with respect to the step pixels. A step correction flag indicating the step correction range determined by the correction range determination unit 333a is output as a correction range signal to the correction density calculation unit 333c and the step correction output unit 333s.
The level difference correction output unit 333s performs density correction in the level difference correction range determined by the correction range determination unit 333a using the correction density value output from the correction density calculation unit 333c, and writes the density-corrected image as a writing image. The data is output to the processing unit 34.
By the processing of the step correction unit 333, only the pixels of the step portion that needs to be corrected can be converted into image data that forms an image in which the density value smoothly changes and output.

Hereinafter, the process of each part of the level | step difference correction | amendment part 333 is demonstrated more concretely.
Image data for five lines after skew correction output from the data selector 331 is input to the step detection unit 333d.
The step detection unit 333d sequentially determines, for each pixel of interest, whether or not the pixel of interest is a step pixel with respect to the input image data for five lines, by sequentially using the pixels in the image shift portion by skew correction as the pixel of interest. The determination is made based on the density value of the image data (pixel group). The image shift portion by skew correction is an image portion of the image division range shifted for skew correction in the image division range (see regions 1 to 4 in FIG. 3).

Here, the step detection unit 333d determines whether or not the pixel is a step pixel as follows. In the following, an example will be described in which discrimination is performed with reference to an area of 5 lines in the sub-scanning direction and 8 pixels in the main scanning direction.
FIG. 8 is a diagram for explaining a pixel matrix that is a data processing target in the level difference correction unit 333 (FIG. 7).
FIG. 8A shows a raster image in which a pixel row is written by a scanning method. The rectangular frame indicated by a solid line in FIG. 8A is a pixel area of 5 lines × 8 pixels referred to when determining a stepped pixel. is there.
Accordingly, the 5-line image input to the level difference detection unit 333d is accumulated in a 5-line × 8-pixel register.
As shown in FIG. 8B, the level difference detection unit 333d first divides each line into 10 areas AJ by dividing each line into 2 in the main scanning direction, as shown in FIG. 8B. The area is the unit of processing. Each area of AJ includes 4 pixels, and numbers 0, 1, 2, and 3 are attached to each pixel in order of the main scanning direction.

The level difference detection unit 333d calculates the density of each area, that is, the density of a pixel group composed of a plurality of pixels in the area, and is approximately at the center of the area of 5 lines × 8 pixels from the density value of each area obtained. Then, it is determined whether or not the pixel F0 in the area F is a stepped pixel.
Here, in the level difference detection unit 333d of the present embodiment, the above calculation of the density values of the areas A to J is performed by adding the density values of the four pixels in each area. It is assumed that the density value of each pixel takes 0-15 values as 4 bits. Moreover, although the case where the density values of four pixels in one area are added is illustrated, the number of pixels to which the density is added in the area is not limited to this number.
In addition, the process of adding the density values of the four pixels is illustrated as a simple addition, but is not limited thereto, and a method of adding the weight values to the density values of the respective pixels may be used.

Next, the level difference detection unit 333d determines whether each area belongs to a high density region or a low density region from the density value of each area calculated based on the density value of the pixel group in the area. .
The density determination of this area is based on a method in which the calculated density value of each area is compared with a preset threshold value, and when it is equal to or higher than the threshold value, it is determined as low density when it is smaller than the threshold value. Can be adopted. In this determination method, the threshold value may be one value, but may be a method using two threshold values provided with a first threshold value ThH on the high concentration side and a second threshold value ThL on the low concentration side. In the method using two threshold values, the level difference detection unit 333d determines that the density value of each area is equal to or higher than the first threshold value ThH as high density, and the case where the density value is equal to or lower than the second threshold value ThL as low density. As a result, a three-stage determination is performed. In the embodiment described later, a case where two threshold values are used is illustrated.

Next, the step detection unit 333d determines whether or not the pixel of interest F0 is a step pixel from the density values and shift directions of the eight areas.
Here, the shift direction is determined by information on which line in the line memory 32 each of the previous pixel of the target pixel F0 and the target pixel F0 outputs. When shifted, the shift direction is determined from the information of the output line because the lines are output from different lines of the line memory 32 before and after the shift position. In the present embodiment, the step detection unit 333d determines the direction by the skew output control unit 332, and the step detection unit 333d receives the result.

  The step detection unit 333d determines that the target pixel F0 is a step pixel based on the following conditions. In other words, depending on the combination of two step-up states, right-up and down-right, that appear in the pixels in the 5 line × 8 pixel region determined by the shift direction, and the density of the areas A to J, the high density to the low density and the low density to the high density. The pixel pattern of the step in any state that changes to density is also represented. Therefore, it can be determined whether or not the target pixel F0 is a stepped pixel depending on whether or not the pixel pattern of the pixel group in the 5 lines × 8 pixel region matches any pixel pattern condition of this combination.

FIG. 9 is a diagram for explaining a determination condition for a step generated when image data is shifted in units of one pixel in skew correction.
FIG. 10 is a diagram for explaining a determination condition for a level difference that occurs when image data is shifted in units of two pixels in skew correction.
The “image image” in the bottom column of FIG. 9 and FIG. 10 is an image image representing a pixel pattern of a step by a pixel group in a 5 line × 8 pixel region (see FIG. 8B).
Since FIG. 9 is an image image representing a step of one pixel, two patterns are provided for each of the lower right side and the upper right side, resulting in a total of four patterns.
The combinations of the densities of the areas AJ of the four patterns are shown as conditions 1 to 4 in the vertical column in FIG. The conditions 1 to 4 are composed of a high density and a low density where the area A-J has no intermediate density due to two thresholds provided with a first threshold ThH on the high density side and a second threshold ThL on the low density side. This is a condition indicating a pixel pattern of one pixel step by pixel group.
If the density of the area A-J satisfies any one of the conditions 1 to 4, it is determined that the target pixel F0 is a step pixel of one pixel step shown in the corresponding “image image”.

Since FIG. 10 is an image image representing a level difference of 2 pixels, there are 4 patterns for each of right-down and right-up, and there are a total of 8 patterns.
The combinations of the densities of the areas A-J in each of the eight patterns are shown as conditions 1 to 8 in the vertical column in FIG. Conditions 1 to 8 are composed of a high density and a low density where the area A-J has no intermediate density due to two thresholds provided with a first threshold ThH on the high density side and a second threshold ThL on the low density side. This is a condition indicating that the pixel pattern is a two-pixel step by pixel group.
If the density of the area A-J satisfies any one of the conditions 1 to 8, it is determined that the pixel of interest F0 is a step pixel of a two-pixel step indicated by the corresponding “image image”.
In the two-pixel step, the patterns used for detection are eight patterns that are twice the one-pixel step (four patterns). This is because each of the two upper and lower (sub-scanning direction) pixels forming the step becomes a step pixel to be detected. In the step correction according to the present embodiment, the step correction (details will be described later) corresponding to each detected step pixel. ).

Note that FIG. 9 showing the determination condition corresponding to the one pixel step and FIG. 10 showing the determination condition corresponding to the two pixel step use the common first threshold ThH and second threshold ThL. Different thresholds may be used. For example, since a two-pixel step is more visible than a one-pixel step, adopting a threshold that makes it easier to determine that the step is a step can make the step less noticeable. To change.
Further, when the target pixel F0 does not satisfy the determination conditions shown in FIG. 9 and FIG. 10, even the shift position is not detected, and the step detection unit 333d needs to correct the target pixel F0 for the step correction. It is determined that there is no non-step pixel.

By the way, in the image forming apparatus 100, when processing image data used for image formation based on image data input with a print request, processing under different conditions may be required for each input. It is necessary to stand on the assumption that.
That is, for the image data input by the image forming apparatus 100, a value reduction process such as a dithering process performed at the time of data output can be performed only by means predetermined by the device (image forming apparatus) (for example, one piece of image data). Dither matrix) is not used. Usually, when using corresponding processing conditions prepared on the device side according to the conditions instructed in the print request, or processing conditions adapted to the type of image obtained by analyzing the input image on the device side, A method of selecting and using from among prepared types is adopted.

As described above, when the image data is processed under processing conditions such that the dither matrix used for the value reduction process is different for each input, the skew correction shift amount determined by the type of the dither matrix also changes. As a result, the image data after the skew correction process changes.
Under such a process, even when the step is corrected for the image data after skew correction, the corresponding correction process needs to be changed depending on the input image data to be corrected.
One of the conditions that requires the correction process to be changed is the step detection process condition. As described above, the change is required according to the number of pixels of the step.
For this reason, the step detection unit 333d acquires information on the number of pixels of a step that can be generated as a result of skew correction performed on the input image data to be processed in the process of detecting the step, and sets the obtained number of pixels of the step. The corresponding detection process is performed.

As described above, the step detection unit 333d can acquire the shift amount received from the skew output control unit 332 as the pixel number information of the step, so that the step detection process is performed according to the condition selected by the setting of the obtained pixel number. Do.
That is, the step detection unit 333d corresponds to the number of pixels acquired from the processing conditions (see FIGS. 9 and 10) corresponding to the difference in the number of step pixels prepared for the step detection process. A condition used for determining whether or not the pixel level difference is acceptable is selected, and the process is switched to the selected condition. The above-described conditions used for determination apply the pixel pattern condition of FIG. 9 for one pixel step and the pixel pattern condition of FIG. 10 for two pixel step.
As described above, the process of determining whether or not the target pixel F0 is the stepped pixel of this number of pixels is performed according to the condition selected based on the number of stepped pixels, and the detection result of the step is obtained.

Since the detected level difference causes deterioration in image quality, level difference correction is performed by changing the density value of a predetermined pixel in the level difference part in order to suppress the influence of the level difference.
Predetermined pixels to be processed for step correction are pixels arranged on a main scanning line (hereinafter referred to as “target line”) including the target pixel F0 when the target pixel F0 is determined as a step pixel by the step detection unit 333d. In the column, the pixels are in a predetermined range as a correction range.
The level difference correction is intended to smooth the level difference and suppress deterioration of image quality by changing the density value of the pixels included in the correction range.

FIG. 11 is a diagram for explaining a case where a level difference (downward to the right) in units of two pixels that occurs in skew correction is corrected on the right side of the shift position.
FIG. 11 is an image showing a step generated when image data in which six lines of high-density pixel rows arranged in the main scanning direction are consecutive in the sub-scanning direction is shifted in units of two pixels in skew correction. This figure is similar to FIG. 5B in that it represents a two-pixel step occurring in a thin line image.
FIG. 11A is an image before level difference correction. In the same figure, the correction indicated by the “target line” including the target pixel F0 (see FIG. 8) subject to step determination, the “reference line” and “areas 0 to 4” referred to in correction density calculation (described later) The range and the step correction area width of each “area” are shown.
FIG. 11B shows an image after the step correction. The difference between FIG. 11B and FIG. 11A is that the result of changing the density value of the pixel in the correction range of the target line is added.

Here, an outline of the step correction processing performed by the step correction unit 333 by determining the correction range will be described with reference to FIG. 11 showing an embodiment of images before and after the step correction.
The correction range determination unit 333a receives a step correction area indicating a step correction area width (see FIG. 11A) that determines the number of pixels that is a processing unit of the step correction (density correction) from the skew output control unit 332 (see FIG. 6). Receives a width signal.
When the target pixel F0 is determined to be a step pixel by the step detection process by the step detection unit 333d, the correction range determination unit 333a determines the number n of areas corresponding to the step correction area width, thereby correcting the correction range. Is decided. That is, when the number of areas n is determined, the correction range is determined by “step difference correction area width × area number n” (in the example of FIG. 11A, the number of areas n = 5 and the step correction area width is 2 pixels. Pixel is the correction range).

The correction range determination unit 333a turns on the step correction flag when a step is detected, and clears a counter that counts the step correction range. The step correction flag is a flag indicating whether or not a fixed number of pixels from the step pixel in the main scanning direction is the correction range, and the storage location of the flag is secured in a storage medium such as a memory.
When the step correction flag is ON, the correction range is indicated. When the step correction flag is OFF, the correction range is not indicated.
The correction range determination unit 333a outputs the step correction flag to the correction density calculation unit 333c and the step correction output unit 333s as a correction range signal.

In order to explain the outline of the step correction processing performed by the step correction unit 333, reference is made to FIG.
Next, the level difference correction by changing the pixel density will be described in more detail based on a specific example.
The embodiment of the level difference correction shown in FIG. 11 shows a correction method in the case where the level difference in units of two pixels is generated in a downward-sloping manner. When the step occurs at the lower right, the step at the image data shift position is detected when the target pixel F0 satisfies the condition 1 and the condition 6 (see FIG. 10).
In addition, even a level difference in units of two pixels may rise to the right instead of the lower right in FIG.

FIG. 12 is a diagram for explaining a case where a step (upper right) in units of two pixels generated in skew correction is corrected on the right side of the shift position.
Note that FIG. 12 is the same as FIG. 11 except that the level difference in FIG.
In the correction method in the case where the level difference in units of two pixels in FIG. 12 is generated at the upper right, the level difference at the image data shift position is detected when the target pixel F0 is in condition 4 and condition 7 (see FIG. 10). ) Is satisfied.

Thus, there are two lines of interest with the pixel of interest F0, as shown in FIGS. The level difference is corrected by changing the density of the pixel on the target line.
The corrected density calculation unit 333c multiplies the density values of the pixel of the target line and the pixel of the reference line by a weighting coefficient and adds them to calculate the density value of the target line pixel after density correction.
The reference line is a line that is referred to when calculating the density value to be changed of the pixel of the target line that is the target of the step correction. Which line above and below the target line is used as a reference line depends on the shift direction of the skew correction. When calculating the correction density value on the right side of the image data shift position, as shown in FIG. 11, if the step is lowered to the right (the right side is shifted downward), the lower line is used as the reference line, as shown in FIG. If the step is raised to the right (the right side is shifted upward), the upper line is set as the reference line.

A supplementary description will be given of a method for calculating the density value of the target line pixel.
The pixel whose density is changed is in the correction range on the line of interest, and is the pixel on the right side of the image data shift position in the examples of FIGS.
Since the correction range is areas 0 to 4 and the step correction area width is 2 pixels, the pixel density is changed for 10 pixels on the right side from the image data shift position.
In the density calculation, the density value of the pixel of the target line and the pixel of the reference line are respectively multiplied by a weighting coefficient, and the value obtained from the pixel of each line is added to obtain a calculated density value.

The weighting coefficient assigned to each pixel of the target line and each pixel of the reference line is shown in the form of a table associated with areas 0 to 4 in the center of each of FIGS. 11 and 12. As described in this weighting coefficient value table, the value is gradually increased with increasing distance from the image data shift position with respect to the pixel of the target line, and gradually with increasing distance from the image data shift position with respect to the pixel of the reference line. The value is made smaller. Note that the same weighting coefficient table is used in FIGS. 11 and 12.
The density value of each pixel calculated by the above calculation method for each area of the correction range using this weighting coefficient value is the value of the correction target pixel of the target line, and the level difference that was a two-pixel level before correction is smoothed. Change the density value to FIG. 11B and FIG. 12B showing the image after correction represent a state in which the level difference is smoothed by gradually changing the pixel density of the pixel rows arranged in the target line in the main scanning direction.

The example in FIGS. 11 and 12 is a method of performing correction on the pixel on the right side of the image data shift position, but the same correction can be performed on the pixel on the left side of the image data shift position. . This embodiment will be described next.
FIG. 13 is a diagram for explaining a case where a two-pixel unit step (downward right) that occurs in skew correction is corrected on the left side of the shift position.
The embodiment of the step correction shown in FIG. 13 shows a correction method in the case where the step in units of two pixels is generated with a downward slope. When the level difference occurs at the lower right, the level difference is detected at the image data shift position in the example of FIG. 11 in the above-described embodiment, that is, the pixel of interest F0 is in condition 1 and condition 6 (see FIG. 10). May be satisfied. However, unlike the previous embodiment (FIG. 11), the example of FIG. 13 is a case where the pixel of interest F0 satisfies the conditions 2 and 5 (see FIG. 10). This is because, in a two-pixel step, there are two step pixels in the sub-scanning direction, so even if the step occurs with the same downward slope, step detection is performed when each pixel becomes the target pixel F0. Note that this occurs at a two-pixel step regardless of the shift direction.

In addition, even a level difference in units of two pixels may rise to the right instead of the lower right in FIG.
FIG. 14 is a diagram for explaining a case where a step (upward to the right) in units of two pixels generated in skew correction is corrected on the left side of the shift position.
Note that FIG. 14 is the same as FIG. 13 except that the level difference in FIG.
In the correction method in the case where the level difference in units of two pixels in FIG. 14 is generated at the upper right, the level difference at the image data shift position is detected when the target pixel F0 is condition 3 and condition 8 (see FIG. 10). ) Is satisfied.

In this way, there are two lines of interest with the pixel of interest F0, as shown in FIGS. The level difference is corrected by changing the density of the pixel on the target line.
The corrected density calculation unit 333c multiplies the density values of the pixel of the target line and the pixel of the reference line by a weighting coefficient and adds them to calculate the density value of the target line pixel after density correction.
The reference line is a line that is referred to when calculating the density value to be changed of the pixel of the target line that is the target of the step correction. Which line above and below the target line is used as a reference line depends on the shift direction of the skew correction. When calculating the correction density value on the left side of the image data shift position, as shown in FIG. 13, if the step is lowered to the right (the right side is shifted downward), the upper line is used as a reference line, as shown in FIG. If the step is raised to the right (the right side is shifted upward), the lower line is set as the reference line.

A supplementary description will be given of a method for calculating the density value of the target line pixel.
The pixel whose density is changed is in the correction range on the line of interest, and is the pixel on the left side of the image data shift position in the examples of FIGS. 13 and 14.
Since the correction range is areas 0 to 4 and the step correction area width is 2 pixels, the pixel density is changed for 10 pixels on the left side from the image data shift position.
In the density calculation, the density value of the pixel of the target line and the pixel of the reference line are respectively multiplied by a weighting coefficient, and the value obtained from the pixel of each line is added to obtain a calculated density value.

The weighting coefficient assigned to each pixel of the target line and each pixel of the reference line is shown in the form of a table associated with areas 0 to 4 in the center of each of FIGS. 13 and 14. As described in this weighting coefficient value table, the value is gradually increased with increasing distance from the image data shift position with respect to the pixel of the target line, and gradually with increasing distance from the image data shift position with respect to the pixel of the reference line. The value is made smaller. The same weighting coefficient value table is used in FIGS.
The density value of each pixel calculated by the above calculation method for each area of the correction range using this weighting coefficient value is the value of the correction target pixel of the target line, and the level difference that was a two-pixel level before correction is smoothed. Change the density value to FIG. 13B and FIG. 14B showing the images after correction represent a state in which the level difference is smoothed by gradually changing the pixel density of the pixel rows arranged in the target line in the main scanning direction.

As shown in the step detection condition in FIG. 10, one two-pixel step generated at the shift position as a result of skew correction is detected as a step pixel when two specific pixels become the target pixel, A step correction corresponding to the step detection is performed.
The step correction corresponding to each step detection follows the above-described method (see FIGS. 11 to 14). If one is on the left side of the image data shift position, the other is on the right side. The level difference is corrected by changing the pixel density.
As an embodiment, it may be possible to simplify the processing by correcting one side of the image data shift position, but it is desirable to improve the image quality on both sides.

FIG. 15 is a diagram for explaining a case where a level difference of 2 pixels generated in skew correction is corrected on both sides of the shift position.
FIG. 15 shows an embodiment in which the embodiment shown in FIGS. 11 and 12 that are corrected on the right side of the image data shift position and the embodiment shown in FIGS. 13 and 14 that are corrected on the left side of the image data shift position are combined. FIG. 15A is an image before the step correction is performed, and FIG. 15B is an image after the step correction is performed.
FIG. 15B reflects the density value change result for the pixels in the correction range set in the target line including the stepped pixels detected under condition 8 in FIG.

Two step pixels are determined by the step detection unit 333d with respect to one two-pixel step, and step correction is performed according to each detected step pixel. In the step correction, the step is corrected on either the left or right side of the image data shift position corresponding to each step pixel.
The correction density calculation unit 333c corrects the correction density value for performing the above-described step correction on the right side and the left side of the image data shift position described above with reference to FIGS. 11, 12, 13, and 14. Calculation is performed according to the density value calculation method.
The correction density values calculated on the left and right sides of the image data shift position are finally combined by the step correction output unit 333s.
By doing in this way, as shown in FIG. 15B, the left and right step correction of the image data shift position can be performed. As shown in the figure, in the image after the level difference correction, the level difference generated in the thin line becomes smooth throughout and the level difference becomes difficult to visually recognize, so that the image quality improvement effect is enhanced.

Further, depending on the input image to be processed, there may be a difference in the number of pixels, such as a one-pixel difference or a two-pixel difference, so it is desirable to adopt a configuration corresponding to such a case.
As a configuration corresponding to the steps with different numbers of pixels, first, it is necessary to prepare respective detection conditions (see FIGS. 9 and 10) in order to detect the steps with the number of pixels in the step detection unit 333d. . Since the skew correction shift amount or the like applied to the input image data to be processed can be received from the skew output control unit 332, if the detection condition is switched according to the shift amount, the step detection unit 333d has a step having a different number of pixels. Can be detected.

Further, the step correction needs to correspond to steps having different numbers of pixels. For the correction of the two-pixel step, the method described above with reference to FIGS. 11 to 14 is employed.
When one pixel step is detected by preparing to detect one pixel step, a means for correcting the one pixel step is prepared, and correction corresponding to this step is performed. Note that a method for correcting one pixel step already exists as described in Patent Document 1 above, and can be dealt with by adopting this method.
However, in the present embodiment described above, the density of the one pixel step is corrected by the same process as the step correction on both the left and right sides of the image data shift position with respect to the two pixel step described above with reference to FIGS. It can be corrected. That is, there is an advantage that the density correction processing means for two pixel steps can be shared with one pixel step.

Next, another embodiment in the process of changing the density value of the pixel of the target line in order to correct the step will be described.
In the corrected density calculation unit 333c of the image forming apparatus 100, when calculating the pixel density to be changed, the density value obtained by referring to the density value of the pixels on the reference line is used to calculate the density value to be changed.
In each of the above embodiments, the reference line is one line above the target line (see FIGS. 12 and 13) or one line below (see FIGS. 11 and 14).
However, if the reference line is determined to be one line above or below the target line, depending on the type of dither matrix used to reduce the value of the input image data to be processed, a secondary abnormality may occur due to the step correction. Since the image is generated, the image may be deteriorated.

FIG. 16 is a diagram illustrating an example of a case where a line for obtaining a reference value is set one line below a target line when correcting a level difference in units of two pixels that occurs in skew correction.
FIG. 16 shows an example in which an abnormal image is generated secondarily with the step correction.
FIG. 16A is an image before the step correction is performed. In the figure, the input original image data is subjected to halftone (decrease) processing by a dither matrix with a period of 2 for the pixel line of the horizontal line (main scanning line), and this image data is processed in units of 2 pixels. It is the image which performed the skew correction by shifting. Therefore, a two-pixel step is generated by the skew correction.

FIG. 16B shows an image after the step correction is performed on the image data of FIG. 16A. The step correction performed at this time is performed under the same conditions as in the embodiment described above with reference to FIG. That is, the pixel density value to be changed is calculated using the same weighting factor for each area in the same correction range, with one line below the target line as a reference line.
In FIG. 16B showing the result of the step correction under such conditions, the arrangement of the pixels in the correction range in which the density value has been changed is disturbed. That is, if the pixel density correction is performed by calculating the same pixel density value as in the embodiment described with reference to FIG. 11, the periodicity of the pixels is lost. For this reason, the stepped portion appears to be thick due to the influence of the pixel added by changing the density value, and even if the step can be corrected, an abnormal image is secondarily generated.

Therefore, in order to suppress an abnormal image that is secondarily generated by using a dither matrix with a period 2 pixel column in the main scanning line, the following method is adopted in the present embodiment.
The main point of this method is that when the skew correction is performed by performing the shift by the number of pixels corresponding to the image processed by the dither matrix having a period of a plurality of pixels, the density value of the pixel that is the target of the step correction is calculated. The purpose is to select a pixel column suitable for the shift amount as a pixel column to be referred to for calculation.
The adaptation to the shift amount in the example of FIG. 16 is performed by correcting the pixel level difference caused by the shift amount of 2 pixels according to the dither matrix of period 2, so refer to the line of the pixel column separated by 2 pixels that adapts to the shift amount. Determine as a line.

FIG. 17 is a diagram for explaining a case where a line for obtaining a reference value is two lines below the target line when correcting a level difference in units of two pixels that occurs in skew correction.
FIG. 17A is an image before the step correction is performed. This figure is an image obtained by processing the pixel row of the main scanning line with the dither matrix having a period of 2 for the input original image data, and performing skew correction by shifting the image data by 2 pixels. is there.
Therefore, a two-pixel step is generated by the skew correction. Note that the image data itself shown in FIG. 17A is the same as that shown in FIG. 16A.

  FIG. 17B is an image after the step correction is applied to the image data of FIG. 17A. The step correction performed at this time is processed using a dither matrix with a period of 2 for the pixel column of the main scanning line, so that the reference line is two lines below the target line. Moreover, the weighting coefficient used for each area of a correction range and a correction range is the same conditions as the said embodiment, ie, embodiment described with reference to FIG. The method for calculating the density value of the pixels in the correction range is also the same as in the above embodiment.

In the present embodiment, since the reference line is changed according to the shift amount input to the step correction unit 333, it is possible to change the reference line according to the cycle of the dither matrix to be used. Therefore, the pixels in the correction range in which the density value is changed by the density correction can be matched with the cycle of the dither matrix.
By performing density correction in accordance with such a dither matrix cycle, the pixel cycle of the correction range after the step correction is such that the pixel cycle is the dither matrix cycle so that it can be seen in the pixel array of the target line in FIG. 17B. It will be in the state which kept. For this reason, the stepped portion does not appear to be thick due to the influence of the pixel added by changing the density value (see FIG. 16B), and the occurrence of an abnormal image can be suppressed.

Next, a procedure of skew correction processing performed by the skew correction processing unit 33 in the writing control unit 12 of the present embodiment will be described.
FIG. 18 is a flowchart illustrating the skew correction process.
When the controller 11 as the main control unit receives a print request via the internal printer controller 21 or the like, it issues a print instruction to the write control unit 12 in response to the request. The write control unit 12 receives the original image data to be printed (print output) together with the print instruction, and uses the image data (image data for writing) used by the LD lighting control unit 20 for image writing according to the content of the instruction. ).
In the present embodiment, the skew correction processing is performed by the skew correction processing unit 33 as part of the image data processing of the writing control unit 12 in this embodiment.

Hereinafter, the procedure of the skew correction processing performed by the skew correction processing unit 33 will be described with reference to FIG.
After confirming that the image data of the number of lines to be corrected is input, the skew correction process is started. First, information on the shift amount determined by the dither matrix or the like used for the value reduction process is acquired ( Step S101). In the present embodiment, the controller 11 that is the upper control unit manages information on the shift amount corresponding to the dither matrix, and therefore acquires this information from the controller 11.
Next, the image data to be processed is shifted in the sub-scanning direction for each image division range (unit area for skew correction) divided in the main scanning direction according to the shift amount acquired in step S101 (step S102). This skew correction processing is described in the operation description of the skew output control unit 332 described above, and more detailed description is given in Patent Document 2, so please refer to it.

Next, a process for detecting a level difference caused by the image shift is performed on the image data on which the skew correction process has been performed.
In the step detection, the detection condition varies depending on the shift amount when the skew correction is performed (see FIGS. 9 and 10). Therefore, whether the condition is selected according to the shift amount is determined, and the detection result is obtained.
Therefore, as a processing procedure, first, a step determination condition that is a step detection condition corresponding to the shift amount is selected (step S103). Since the determination condition for detecting the step is variable depending on the shift amount, the condition is selected according to the shift amount. The pixel pattern condition of FIG. 9 is used for one pixel step of a shift amount of one pixel (line), and the pixel pattern determination condition of FIG. 10 is used for two pixel steps of a shift amount of two pixels (line). Apply.

Next, according to the selected determination condition, a step determination is performed by determining whether or not the pixel pattern of the predetermined region of the target pixel specified in the pixel row of the processing target image sequentially and the pixel pattern representing the step as the determination condition are acceptable. A result is obtained (step S104).
In the above description of the present embodiment, the step detection is performed on the image after the image is shifted by the skew correction process. However, the present invention is not limited to this, and the shift amount is acquired in advance. If so, a processing method for detecting a step directly from the original image data may be adopted.

Since the process branches depending on the result of the step detection, that is, whether or not there is a step, whether or not there is a step is confirmed from the result of the step detection (step S105).
If there is no step in step S105 (step S105-NO), no processing is required, so the processing of this flow is terminated and the image data is output to the next processing as it is.

On the other hand, if there is a step in step S105 (step S105-YES), the processing is performed according to the step correction processing procedure.
At the beginning of the step correction process, a correction range is determined according to the shift amount (step S106).
The correction range is different in the range required in the correction process according to the difference in level determined by the shift amount, that is, whether the level difference is one pixel level or two pixel level. Therefore, it is necessary to determine the correction range. In other words, adapting to either one pixel step or two pixel step, and in the two pixel step, there are two target pixels for which a step detection has been made for one step, and the correction is set within the target line Naturally, the range also changes, so the range to be adapted to each is decided. The details of the process for determining the correction range are described in the description of the operation of the correction range determination unit 333a described above.

Next, as a level difference correction process, a corrected density value is calculated to change the density value to smooth the level difference for the pixels within the correction range determined in step S106, and the calculated density value is applied to the corresponding pixel. (Step S107). At this time, the image data after the correction processing of the correction range including the pixel array of the correction density value is combined with the original image data, that is, the image data after skew correction, and is output to the processing unit at the next stage.
Note that details of the processing for calculating the correction density value are described in the description of the operation of the correction range determination unit 333a described above, so refer to it.
The level difference correction processing for the image data in step S107 is completed, and the processing of this flow ends.

  11 .. Controller, 12 .. Write control unit, 20 .. LD lighting control unit, 23 .. Image processing unit, 31 .. Input image control unit, 32 .. Line memory, 33 .. Skew correction processing unit, 34 ··· Write image processing unit, 331 ··· Data selector, 332 ··· Skew output control unit, 333 ·· Step correction unit, 333c · · Correction density calculation unit, 333d · · Step detection unit, 333a ··· Correction range Determining unit, 333 s, step difference output unit, 35, LD data output unit, 100, image forming apparatus, 104a, 106a, 108a, 110a, photosensitive drum, 102, scanning exposure apparatus, 206, LD .

JP 2010-246098 A Japanese Patent Application No. 2013-17470

Claims (8)

An image obtained by dividing the original image in the main scanning direction in order to correct the skew generated in the raster image when writing the pixels aligned in the main scanning direction on the recording medium based on the original image by the scanning method to form the raster image. An image forming apparatus that generates image data for writing by shifting an image in an image division range to be corrected by a predetermined number of pixels in the sub-scanning direction for each division range,
An image shift for detecting an image shift in units of a predetermined number of pixels, which is caused by correcting a skew between both images in the image split range adjacent to and in contact with the image split range to be corrected, from the image data for writing Detection means;
An image on which data processing for correcting the image shift is performed on the image data for writing of the image in the image division range to be corrected on condition that the image shift is detected by the image shift detection unit. It possesses a deviation correction processing means,
The image shift detection means is a pixel pattern condition in which a pixel pattern represented by a pixel group in a predetermined region straddling a boundary between adjacent image division ranges adjacent to each other corresponds to a step of a predetermined number of pixels generated in a continuous image Is a means for determining the presence or absence of the step according to whether or not it fits, and outputting the presence or absence of the determined step as a detection result of image shift,
The image shift detection means sets one of a plurality of types of steps having different numbers of pixels as the predetermined number of pixels, and determines whether each step is acceptable by changing the number of pixels to be set. An image forming apparatus that prepares a pattern condition corresponding to a step and switches and uses a predetermined pixel pattern condition according to the set number of steps when one of the changeable steps is set . The image forming apparatus according to claim 1,
The image shift detection means, changeable and the setting of the step, to that image forming apparatus as defined in accordance with the image processing conditions of the write image data shift amount of the image is determined in the correction of the skew . The image forming apparatus according to claim 2,
The image forming apparatus the image processing conditions of the image data for the write is KIND der dithering process. The image forming apparatus according to any one of claims 1 to 3,
The image misalignment correction processing means includes a processing range determining means for determining a range in which data processing for correcting image misalignment is performed, and a range determined by the processing range determining means so as to suppress deterioration in image quality due to the image misalignment. an image forming apparatus Ru provided with a pixel density changing means for changing the density of the pixel. The image forming apparatus according to claim 4.
The processing range determination means fits a range on which data processing is performed within the image division range determined in the skew correction in a pixel row arranged in a main scanning direction including a portion of a stepped portion caused by the image shift. Range and
The pixel density changing unit is changed based on the density of the pixel column in the range determined by the processing range determining unit and the density obtained by referring to the pixel column arranged in the main scanning direction in the vicinity of the pixel column. an image forming apparatus Ru comprising means for calculating the concentration. The image forming apparatus according to claim 5 .
Said processing range determination unit, the range subjected to data processing, adjacently image forming apparatus as stipulated on both sides of the range of the boundary of the image division areas in contact with each other. The image forming apparatus according to claim 5 or 6,
When calculating the density to be changed by the pixel density changing unit , the pixel row to be referred to is selected by selecting a pixel row that is adapted to the shift amount of the image in the image division range to be corrected in the skew correction. the image forming apparatus you way. An image obtained by dividing the original image in the main scanning direction in order to correct the skew generated in the raster image when writing the pixels aligned in the main scanning direction on the recording medium based on the original image by the scanning method to form the raster image. An image shift correction method in an image forming apparatus that generates image data for writing by shifting an image of an image division range to be corrected by a predetermined number of pixels in the sub-scanning direction for each division range,
An image shift for detecting an image shift in units of a predetermined number of pixels, which is caused by correcting a skew between both images in the image split range adjacent to and in contact with the image split range to be corrected, from the image data for writing A detection process;
An image on which data processing for correcting the image shift is performed on the image data for writing of the image in the image division range to be corrected on the condition that the image shift is detected in the image shift detection step. possess a deviation correction process,
The image shift detection step is a pixel pattern condition in which a pixel pattern represented by a pixel group in a predetermined region straddling the boundary of adjacent image division ranges that are adjacent to each other corresponds to a step in a predetermined number of pixels generated in a continuous image The presence or absence of the step is determined by whether or not it fits, and the presence or absence of the determined step is output as an image misalignment detection result,
In the image shift detection step, one of a plurality of types of steps having different numbers of pixels is set as the predetermined number of pixels, and each pass / fail can be determined by changing the number of pixels to be set. Image misalignment correction that prepares pattern conditions corresponding to steps and switches and uses predetermined pixel pattern conditions according to the set number of steps when one of the steps that can be changed is set Way .

Images