JP2010217795A - Image forming apparatus, image forming method and program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To fully reduce a stringy noise and obtain a high-quality image while preventing color shift due to skew between each colors by skew correction. <P>SOLUTION: An image forming apparatus includes: a pattern matching section 211 in which densities of a pixel of interest and pixels around the pixel of interest are calculated when the pixel of interest of correction target is at a division position in the main scanning direction based on pixels of an image data which underwent the skew correction, and generation of density deviation is detected based on the calculated densities, a sift direction due to the skew correction, and a predetermined pattern; and a dither correction output section 221 in which density correction of the image data is performed based on the detected density deviation when the generation of density deviation is detected. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

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

  Conventionally, the demand for speeding up of color copiers has increased, so that the electrostatic latent image forming portion including the photoconductor is classified into 4 (cyan (C), magenta (M), yellow (Y), black (K)). Tandem color copiers arranged in parallel are the mainstream. In this tandem system, the alignment technique between the colors is an important issue because of its configuration.

  For this reason, in a tandem type color copying machine, a predetermined toner pattern is formed on the transfer belt with toner of each color, and this toner pattern is detected using an optical sensor, so that the color between each color can be detected. Some have a function to reduce color misregistration by calculating the amount of misregistration for each factor such as registration misalignment, magnification misalignment, skew misalignment in the main scanning direction and sub-scanning direction, and performing feedback correction to match each other. Many.

  Further, this correction process is performed when the power is turned on, when the environment such as temperature changes, or when a predetermined number of sheets have been printed, so that the color misregistration amount is always controlled to be within a predetermined range. Among the color misregistration amounts, the main and sub registration misregistration can be corrected by adjusting the timing of laser light writing on the photoconductor, and the main scanning magnification can be corrected electrically by adjusting the pixel clock. .

  Regarding the skew of the laser beam for performing scanning exposure, there are a method of correcting mechanically and a method of correcting an output image by image processing. In the mechanical correction method, correction is realized by having an adjustment mechanism that displaces the mirror inside the laser beam writing unit. However, an actuator such as a mirror displacement motor is required for automatic execution, and this causes an increase in cost. There is a problem that the laser beam writing unit cannot be made small.

  On the other hand, the method of correcting by image processing is to read the image from the line memory while switching the reading position so that a part of the image is accumulated in the line memory and the output image is shifted in the direction opposite to the skew and output. By doing so, the skew between the colors is corrected. In this case, since it is only necessary to add a line memory to the image processing unit in accordance with the correction range, there is an advantage that it can be realized at a relatively low cost compared to mechanical correction. In addition, the method of correcting by image processing is effective as a method of reducing not only skew but also bending caused by the characteristics of the lens inside the laser beam writing unit.

  Patent Document 1 discloses an image recording apparatus that reduces skew by such image processing. In the image recording apparatus according to Patent Document 1, it is detected whether or not a target pixel is a shift position, and a change point of an adjacent pixel in the main scanning direction and a pixel pattern around the target pixel at the shift position are predetermined. When the pattern matches, the density correction corresponding to the set of the matched pixel pattern and the change position of the adjacent pixel in the main scanning direction is performed on the target pixel, so that streak-like noise can be reduced.

  However, the skew between the colors can be reduced. However, when the adjacent relationship between the pixels changes at the image shift position to cause a density shift, and a density shift is significant, a streak-like noise image is generated. There is a problem that this noise image becomes particularly prominent with respect to an image expressed by pseudo gradation processing such as dithering.

  Further, the technique of Patent Document 1 can prevent the generation of streak noise images to some extent, but in some cases, traces of density correction may be noticeable. For example, in the case of a low density halftone image expressed by various dither matrices or an image expressed by a distributed or line-type dither matrix, there is a problem that the image quality is significantly deteriorated.

  The present invention has been made in view of the above, and is capable of obtaining a high-quality image by sufficiently reducing streak-like noise while preventing color shift due to skew between colors by skew correction. An object is to provide a forming apparatus, an image forming method, and a program.

  In order to solve the above-described problem and achieve the object, the present invention is an image forming apparatus that divides image data of one line in the main scanning direction and performs skew correction of the image data, and the skew-corrected image Based on the data pixels, when the target pixel to be corrected is at a division position in the main scanning direction, the density of the target pixel and pixels around the target pixel are calculated, and the calculated density and shift by skew correction are calculated. Detection means for detecting the occurrence of density deviation based on the direction and a predetermined pattern, and a plurality of pixels in the vicinity of the division position based on the detected density deviation when the density deviation is detected. Correction means for performing density correction to compensate for the difference in output area.

  The present invention also provides an image forming method executed by the image forming apparatus and a program for causing a computer to execute the image forming method.

  According to the present invention, when the target pixel to be corrected is at a division position in the main scanning direction, the density of the target pixel and the surrounding pixels of the target pixel are calculated based on the skew-corrected image data pixel. If the occurrence of density deviation is detected based on the calculated density, the shift direction by skew correction, and a predetermined pattern, and if the occurrence of density deviation is detected, based on the detected density deviation When performing skew correction by image processing by performing density correction that compensates for differences in the output areas of a plurality of pixels in the vicinity of the division position, only a location where a density deviation is detected, i.e., where a streak-like noise is conspicuous. And the density value of the pixel near the division position is corrected. Therefore, according to the present invention, it is possible to obtain a high-quality image by sufficiently reducing streak-like noise while preventing color shift due to skew between colors by skew correction. That is, according to the present invention, it is possible to prevent the generation of a streak-like noise image due to image shift and to achieve high-quality image correction in which traces due to density correction are not noticeable.

FIG. 1 is a front view of an image processing unit, an exposure unit, and a transfer belt for explaining the image forming principle of a color copying machine. FIG. 2 is a perspective view of the transfer belt showing a state in which a correction pattern is formed. FIG. 3 is a block diagram showing an example of the structure of a mechanism for performing write control and misregistration correction of a color copying machine. FIG. 4 is a block diagram illustrating an example of the configuration of the write control unit. FIG. 5 is a flowchart showing a processing procedure for positional deviation correction. FIG. 6 is a flowchart illustrating an example of a printing processing procedure. FIG. 7 is a timing chart showing an example of writing timing correction in the sub-scanning direction by the writing control unit. FIG. 8A is a diagram illustrating an example of a misregistration correction pattern formed on the transfer belt. FIG. 8B is a diagram illustrating an example of a skew amount calculation method. FIG. 9A is a diagram illustrating an example of the skew amount of each color based on the K color when the resolution in the sub-scanning direction is 600 dpi. FIG. 9B is a diagram illustrating an example of the skew correction amount in the case of FIG. FIG. 10A is a diagram illustrating an example of a skew correction method (a skew correction amount calculation method). FIG. 10B is a diagram illustrating an example of a skew correction method (a skew correction amount calculation method). FIG. 10C is a diagram illustrating an example of a skew correction method (a skew correction amount calculation method). FIG. 10-4 is a diagram illustrating an example of the skew correction method (skew correction amount calculation method). FIG. 10-5 is a diagram illustrating an example of the skew correction method (skew correction amount calculation method). FIG. 10-6 is a diagram illustrating an example of the skew correction method (skew correction amount calculation method). FIG. 11A is a diagram illustrating another example of the skew correction method (skew correction amount calculation method). FIG. 11B is a diagram illustrating another example of the skew correction method (skew correction amount calculation method). FIG. 11C is a diagram illustrating another example of the skew correction method (skew correction amount calculation method). FIG. 11D is a diagram illustrating another example of the skew correction method (skew correction amount calculation method). FIG. 11-5 is a diagram illustrating another example of the skew correction method (skew correction amount calculation method). FIG. 11-6 is a diagram illustrating another example of the skew correction method (skew correction amount calculation method). FIG. 12A is a timing chart at the time of skew correction of the line memory. FIG. 12B is a timing chart at the time of skew correction of the line memory. FIG. 13 is a schematic diagram showing storage areas of the line memories 128K, 128M, 128Y, and 128C. FIG. 14 is a block diagram showing an example of the configuration of the write control unit when only one line memory is provided. FIG. 15 is a schematic diagram illustrating an example of an image subjected to dither processing. FIG. 16 is a schematic diagram illustrating a state in which the pixel adjacency changes at the image shift position. FIG. 17 is a schematic diagram for explaining a change in the toner adhesion area depending on the image shift position. FIG. 18A is an explanatory diagram of an image image having a density subjected to dither processing and an image image when shifted downward. FIG. 18B is an explanatory diagram of another image image with a dithered density and an image image when shifted downward. FIG. 19 is an explanatory diagram showing a white streak-like noise image. FIG. 20 is an explanatory diagram showing another noise image having a white stripe shape. FIG. 21 is a block diagram illustrating a functional configuration of the skew correction processing unit. FIG. 22 is a block diagram illustrating a functional configuration of the dither correction unit 181. FIG. 23 is a schematic diagram for explaining the contents of the dither correction processing. FIG. 24 is a flowchart showing the procedure of the dither correction process. FIG. 25 is a flowchart illustrating the procedure of density deviation detection processing. FIG. 26 is a diagram illustrating an example of a matching pattern used in the pattern matching process and dither correction. FIG. 27 is an explanatory diagram showing an example of a dither matrix. FIG. 28 is an explanatory diagram showing another example of the dither matrix. FIG. 29 is an explanatory diagram showing an example of a line-type dither matrix. FIG. 30 is an explanatory diagram showing an example of a distributed dither matrix. FIG. 31 is an explanatory diagram showing a state in which the density values of pixels near the target pixel are input to the input pixel pattern. FIG. 32 is an explanatory diagram showing a state where the input pixel pattern and the pattern D match. FIG. 33 is an explanatory diagram illustrating an example in which density correction is performed on an image using density values. FIG. 34 is an explanatory diagram illustrating another example in which density correction is performed on an image using density values. FIG. 35 is a schematic diagram illustrating an output image example of image data on which dither correction has been performed. FIG. 36 is a schematic diagram illustrating another output image example of image data on which dither correction has been performed. FIG. 37 is a schematic diagram illustrating another output image example of image data on which dither correction has been performed. FIG. 38 is a schematic diagram illustrating an example of a state when the image data illustrated in FIG. 35 is viewed. FIG. 39 is a schematic diagram illustrating an example of a state when the image data illustrated in FIG. 36 is viewed. FIG. 40 is a schematic diagram illustrating an example of a state when the image data illustrated in FIG. 37 is viewed. FIG. 41 is a schematic diagram illustrating an example of an input pixel pattern. FIG. 42 is an explanatory diagram showing a multi-line matrix for each different screen.

  Exemplary embodiments of an image forming apparatus, an image forming method, and a program according to the present invention are explained in detail below with reference to the accompanying drawings. In the following embodiment, an example in which the image forming apparatus of the present invention is applied to a color copying machine will be described. However, the present invention is not limited to a color copying machine, and may be applied to any apparatus that performs image processing such as a multifunction machine in which a plurality of functions such as a facsimile, a scanner function, a copy function, a fax machine, and a printer are housed in one housing. be able to.

  First, the image forming principle of the color copying machine of this embodiment will be described with reference to FIG. FIG. 1 is a front view of an image processing unit, an exposure unit, and a transfer belt for explaining the image forming principle of a color copying machine. A color copying machine is an apparatus that forms an image on transfer paper by electrophotographic image formation.

  In this color copying machine, four image forming units 1Y, 1M, 1C, and 1K in an image processing unit 1 that form images of different colors (Y, M, C, and K) are transferred as transfer media. It is a tandem type arranged in a line along the transfer belt 3 that conveys the paper 2. The transfer belt 3 is installed between a driving roller 4 that rotates and a driven roller 5 that rotates and is driven to rotate in the direction of the arrow in the drawing by the rotation of the driving roller 4. Below the transfer belt 3, a paper feed tray 6 in which the transfer paper 2 is stored is provided. The transfer sheet 2 at the uppermost position among the transfer sheets 2 stored in the sheet feed tray 6 is fed toward the transfer belt 3 during image formation, and is attracted onto the transfer belt 3 by electrostatic adsorption. The sucked transfer paper 2 is conveyed to the image forming unit 1Y, where Y-color image formation is first performed.

  The image forming units 1Y, 1M, 1C, and 1K include photosensitive drums 7Y, 7M, 7C, and 7K, and chargers 8Y, 8M, 8C, and 8K arranged around the photosensitive drums 7Y, 7M, 7C, and 7K, respectively. And developing devices 10Y, 10M, 10C, and 10K, photoconductor cleaners 11Y, 11M, 11C, and 11K, and transfer devices 12Y, 12M, 12C, and 12K.

  The surface of the photosensitive drum 7Y of the image forming unit 1Y is uniformly charged by the charger 8Y, and then exposed by the exposure unit 9 with the laser beam LY corresponding to the Y color image, thereby forming an electrostatic latent image. The The formed electrostatic latent image is developed by the developing device 10Y, and a toner image is formed on the photosensitive drum 7Y. This toner image is transferred to the transfer paper 2 by the transfer device 12Y at a position (transfer position) where the photosensitive drum 7Y and the transfer paper 2 on the transfer belt 3 are in contact with each other. Color) image is formed. In the photoreceptor drum 7Y after the transfer, unnecessary toner remaining on the drum surface is cleaned by the photoreceptor cleaner 11Y to prepare for the next image formation.

  In this way, the transfer paper 2 onto which the single color (Y color) has been transferred by the image forming unit 1Y is conveyed to the image forming unit 1M by the transfer belt 3. Similarly, the M color toner image formed on the photosensitive drum 7M is transferred onto the transfer paper 2 in a similar manner. Thereafter, the transfer paper 2 is further conveyed sequentially to the image forming unit 1C and the image forming unit 1K, and similarly, the formed C and K color toner images are transferred to the transfer paper 2, whereby the transfer paper 2 is transferred onto the transfer paper 2. A color image is formed.

  Then, the transfer paper 2 on which the color image is formed through the image forming unit 1K is peeled off from the transfer belt 3, fixed by the fixing device 13, and then discharged.

  In a tandem type color copying machine, alignment between colors is important because of its configuration. Color misregistration between colors includes registration misregistration in the main scanning direction (direction parallel to the rotation axis of the photosensitive drums 7K, 7M, 7C, and 7Y), and sub-scanning direction (photosensitive drums 7K, 7M, 7C, and 7Y). Registration deviation in the direction perpendicular to the rotation axis, main scanning magnification deviation, skew deviation, and the like. Therefore, in this color copying machine, before the actual color image forming operation is performed on the transfer paper 2, the misregistration correction between the colors using the correction pattern is performed.

  FIG. 2 is a perspective view of the transfer belt showing a state in which a correction pattern is formed. In this color copying machine, a correction pattern 14 for correcting color misregistration of each color is formed on the transfer belt 3 by the image forming units 1Y, 1M, 1C, and 1K in order to correct misregistration. Detection is performed by detection sensors 15 and 16 for detection. In the example of FIG. 2, a plurality of detection sensors 15 and 16 are arranged at both ends of the transfer belt 3 in the main scanning direction, and the transfer belt 3 has correction patterns corresponding to the arrangement positions of the detection sensors 15 and 16. 14 is formed. Such a correction pattern 14 is detected when the transfer belt 3 rolls in the conveying direction shown in the figure and sequentially passes through the detection sensors 15 and 16. When this correction pattern 14 is detected, various deviation amounts (main scanning magnification deviation amount, main scanning registration deviation amount, sub-scanning registration deviation amount, skew deviation amount, distortion amount) are calculated from the detection result. An arithmetic process is performed, and the correction amount of each shift component is calculated from the color shift amount.

  Next, a block related to the control operation of the color copying machine and its operation will be described. FIG. 3 is a block diagram showing an example of the structure of a mechanism for performing write control and misregistration correction of a color copying machine. Processing units that perform misregistration correction processing in this color copying machine include detection sensors 15 and 16, a printer controller 111, a scanner controller 112, an engine control unit 113, and LD (Laser Diode) control for each color of K, M, C, and Y. 114K, 114M, 114C, 114Y.

  The detection sensors 15 and 16 are for detecting the correction pattern 14 transferred to the transfer belt 3 in order to calculate the positional deviation of each color image. The detection sensors 15 and 16 detect the position of the correction pattern 14 and output an analog detection signal to the engine control unit 113.

  The printer controller 111 is for receiving image data transmitted from an external device (for example, a personal computer (hereinafter referred to as PC)) via a network. The printer controller 111 transfers the received image data to the image processing unit 124.

  The scanner controller 112 is for acquiring a document image read by a scan (not shown). The scanner controller 112 transfers the acquired image data to the image processing unit 124.

  The engine control unit 113 is roughly divided into a pattern detection unit 121, a CPU (Central Processing Unit) 122, a RAM (Random Access Memory) 123, an image processing unit 124, and a writing control unit 125.

  The pattern detection unit 121 amplifies the detection signals output from the detection sensors 15 and 16, converts the amplified analog detection signal into digital data, and stores the converted digital data in the RAM 123.

  The CPU 122 calculates a color misregistration amount from digital data that is a detection signal of the position of the correction pattern 14 stored in the RAM 123, and calculates a color misregistration correction amount for correcting the color misregistration from the calculated color misregistration amount. Here, the amount of color misregistration includes the amount of distortion of each color, the amount of magnification error in the main scanning direction, the amount of misregistration in the main scanning direction, and the amount of misregistration in the sub scanning direction (hereinafter referred to as main / sub resist misregistration amount), skew. There is a deviation amount. Further, as the color misregistration correction amount, from these various misregistration amounts, the distortion correction amount of each color, the main scanning magnification correction amount, the main scanning direction registration correction amount, and the sub scanning direction registration correction amount (hereinafter referred to as main / sub registration amounts). Correction amount) and skew correction amount.

  Further, the CPU 122 performs distortion of the Y, M, and C colors when the K color is used as a reference color based on the resolution of the image data and the calculated distortion amount of each color (Y, M, C, K). The line amount is calculated, and the number of lines in the line memory is determined based on the distortion line amount of each color with respect to these reference colors. The reference color refers to a color that serves as a reference position when calculating the distortion amount of each color. In this example, the K color is used as the reference color.

  The RAM 123 is for temporarily storing digital data of the correction pattern 14 acquired from the pattern detection unit 121 via the CPU 122. The RAM 123 may be replaced with a nonvolatile memory, and the digital data of the correction pattern 14 may be stored in the nonvolatile memory.

  The image processing unit 124 performs various image processing according to each image data received by the printer controller 111 or each image data acquired from the scanner controller 112. The image processing unit 124 receives the sub-scan timing signal (K, M, C, Y) _FSYNC_N for each color transmitted from the writing control unit 125, and receives the main scanning gate signal (K, M, C, Y) for each color. Y) _IPLGATE_N, the sub-scan gate signal (K, M, C, Y) _IPFGATE_N, and the image signal (K, M, C, Y) _IPDATA_N associated with these synchronization signals are transmitted to the write control unit 125.

  The writing control unit 125 receives the image data transferred from the image processing unit 124, performs various writing processes on the received image data to generate an image signal (K, M, C, Y) _LDDATA, and each LD control unit 114K, 114M, 114C, 114Y.

  The LD control units 114K, 114M, 114C, and 114Y are provided in the exposure unit 9, and control the irradiation of the laser beams LY, LM, LC, and LK to the photosensitive drums 7Y, 7M, 7C, and 7K by the exposure unit 9. Is for. By irradiating the laser beams LY, LM, LC, and LK, toner images are formed on the photosensitive drums 7Y, 7M, 7C, and 7K. The formed toner image is transferred to the transfer paper 2 and output.

  An outline of color image forming processing in such a color copying machine will be described. The printer image from the PC is processed by the printer controller 111, and the copy image is processed by the scanner controller 112, and transferred to the image processing unit 124 of the engine control unit 113. The image processing unit 124 performs various types of image processing according to each image data, converts the image data into color image data, and transfers the image data to the writing control unit 125. The writing control unit 125 generates a printing timing for each color, receives image data in accordance with the sub-scanning timing, performs various writing image processing, converts it to LD light emission data, and converts each color LD control unit 114K, 114M, 114C. , 114Y emits LD and forms an image on the photosensitive drum.

  Here, the write control unit 125 in the engine control unit 113 will be further described with reference to FIG. FIG. 4 is a block diagram illustrating an example of the configuration of the write control unit. The write control unit 125 is roughly divided into K, M, C, and Y color write control units 126K, 126M, 126C, and 126Y, an input image control unit 127K, 127M, 127C, and 127Y, and line memories 128K, 128M, and 128C. , 128Y.

  Further, the K color writing control unit 126K of the reference color includes a writing image processing unit 131K, a misregistration correction pattern generation unit 132K, and an LD data output unit 133K. The other M, C, and Y color write controllers 126M, 126C, and 126Y have write image processing units 131M, 131C, and 131Y that have the same configuration as the K color, and misregistration correction pattern generation units 132M, 132C, and In addition to the 132Y and LD data output units 133M, 133C, and 133Y, skew correction processing units 135M, 135C, and 135Y are provided.

  In FIG. 4, in order to simplify the description, the main scanning gate signals (K, M, C, Y) _IPLGATE_N and the sub-scanning gate signals (K, M, C, Y) of each color described in FIG. ) _IPFGATE_N and the three image signals (K, M, C, Y) _IPDATA_N associated with these synchronization signals are combined and written as a write control signal (K, M, C, Y) _IPDATA [7: 0] _N. .

  The write image processing units 131K, 131M, 131C, and 131Y perform various image processes using image data stored in the line memories 128K, 128M, 128C, and 128Y.

  The misregistration correction pattern generation units 132K, 132M, 132C, and 132Y image data of the correction pattern 14 transferred to the transfer belt 3 in order to calculate correction values for correcting the color misregistration of each color on the transfer belt 3. Is for generating.

  The LD data output units 133K, 133M, 133C, and 133Y send correction write commands (LDDATA) to the LD control units 114K, 114M, 114C, and 114Y according to the main / sub resist correction amounts calculated by the CPU 122, and laser light irradiation The control for correcting the deviation of the write timing due to is performed. Also, the LD data output units 133K, 133M, 133C, and 133Y send an image frequency change command (LDDATA) corresponding to the main scanning magnification correction amount calculated by the CPU 122 to the LD control units 114K, 114M, 114C, and 114Y. The correction control of the magnification error in the main scanning direction is performed. Further, the LD data output units 133K, 133M, 133C, and 133Y provide a command (LDDATA) for forming a correction pattern 14 obtained from the misregistration correction pattern generation units 132K, 132M, 132C, and 132Y on the transfer belt 3 by LD control. This is sent to the sections 114K, 114M, 114C, 114Y. The LD data output units 133K, 133M, 133C, and 133Y are provided with devices that can set the output frequency very finely, for example, a clock generator that uses a voltage controlled oscillator (VCO) for each color. .

  The input image control units 127K, 127M, 127C, and 127Y receive the image data transferred from the image processing unit 124, store the received image data in the line memories 128K, 128M, 128C, and 128Y, and store the stored image data for each color. Are transferred to the write controllers 126K, 126M, 126C, and 126Y. The input image control units 127K, 127M, 127C, and 127Y store each color in the line memories 128K, 128M, 128C, and 128Y based on the distortion line amounts calculated by the CPU 122. The input image control units 127K, 127M, 127C, and 127Y according to the present embodiment receive 1-bit binary image data from the image processing unit 124, and write the received image data to the write control units 126K, 126M, and 126C. , 126Y. In the present embodiment, the image data of the binary image is transferred to the write controllers 126K, 126M, 126C, and 126Y. However, the present invention is not limited to this. For example, image data of a binary image is converted into image data having a 4-bit density value (0 (= white pixel) to 15 (= black pixel)) and transferred to the write control units 126K, 126M, 126C, and 126Y. May be.

  The line memories 128K, 128M, 128C, and 128Y are memories for sequentially storing the image data transferred from the image processing unit 124.

  The skew correction processing units 135M, 135C, and 135Y perform skew correction of image data using K color as a reference. In the present embodiment, image data (image) in the sub-scanning direction is divided into a plurality of lines with the main scanning direction as one line, and is stored in the line memories 128M, 128C, and 128Y to represent an image of one line in the main scanning direction. The divided pixels are divided, and the divided pixels are shifted in the sub-scanning direction opposite to the skew and output. As a result, the skew generated when the toner image is formed is corrected. Hereinafter, the image writing process by the writing control units 126K, 126M, 126C, and 126Y will be described in detail.

  First, the image writing process for the K color in FIG. 4 will be described. First, the image signal K_IPDATA [7: 0] _N is transmitted from the image processing unit 124 to the input image control unit 127K. The input image control unit 127K transmits the image signal to the writing control unit 126K while temporarily storing the image signal in the line memory 128K. Inside the write control unit 126K, the write image processing unit 131K transmits the image signal transmitted from the input image control unit 127K to the LD data output unit 133K. The LD data output unit 133K generates a K color writing image signal K_LDDATA and transmits it to the LD control unit 114K.

  Next, the image writing process for the M, C, and Y colors in FIG. 4 will be described. First, the image signal (M, C, Y) _IPDATA [7: 0] _N is transmitted from the image processing unit 124 to the input image control units 127M, 127C, 127Y. Next, the input image control units 127M, 127C, and 127Y temporarily store image signals in the line memories 128M, 128C, and 128Y, respectively, in order to perform skew amount correction based on the skew correction amounts stored in the RAM 123. The skew correction processing units 135M, 135C, and 135Y perform a skew amount correction process based on the skew correction amount on the temporarily stored image signals, and then transmit the respective image signals to the write image processing units 131M, 131C, and 131Y. Similarly to the operation of the K color, the LD data output units 133M, 133C, and 133Y for the respective colors that have received the image signals from the written image processing units 131M, 131C, and 131Y for the respective colors receive the written image signals (M, C, and Y). ) _LDDATA is generated and transmitted to the LD control units 114M, 114C, and 114Y for the respective colors. Details of the skew correction amount will be described later.

  When the misregistration correction pattern 14 is output, the misregistration correction pattern generation units 132K, 132M, 132C, and 132Y to the K, M, C, and Y color pattern image signals correspond to the respective color LD data output units 133K and 133M. , 133C, 133Y. Thereafter, the same operation as described above is performed.

  As described above, in order to form a color image, the K, M, C, and Y colors must be aligned. Accordingly, the positional deviation correction operation processing will be described with reference to the flowchart of FIG. In the following misregistration correction processing, a case where the reference color is K color will be described. The reference color is a color that serves as a reference for correction, and corrects misalignment between the colors by matching other colors with the reference color.

  When the misregistration correction process is started, first, in step S11, misregistration generated by the misregistration correction pattern generation units 132K, 132M, 132C, and 132Y in the write control units 126K, 126M, 126C, and 126Y for each color in FIG. A correction pattern is formed on the transfer belt 3. In step S12, the detection patterns 15 and 16 detect the correction pattern 14 formed on the transfer belt 3.

  Thereafter, in step S13, after the correction pattern 14 detected by the pattern detection unit 121 is converted into digital data, the CPU 122 corrects the main scanning magnification correction amount for the reference color (K color) from the digital data correction pattern. And a main resist correction amount and a sub resist correction amount are calculated. At the same time, in step S14, the skew correction amount of each color with respect to the reference color (K color) is calculated. In step S15, the division position and the correction direction (shift direction) in the main scanning direction for skew correction are calculated.

  In step S16, the calculated main scanning magnification correction amount, main registration correction amount, sub-registration correction amount, skew correction amount, main scanning division position and correction direction (shift direction) for skew correction are calculated. The included information is stored in the RAM 123 (or non-volatile memory), and the misalignment correction process ends. The correction amount stored in the RAM 123 is used as a correction amount at the time of printing until the next misalignment correction process is performed.

  As described above, the main scanning magnification correction amount, the main registration correction amount, the sub-registration correction amount, and the skew correction amount, and the division position and correction direction (shift direction) of main scanning for skew correction of each color M, C, and Y And the printing process is performed. FIG. 6 is a flowchart illustrating an example of a printing processing procedure.

  First, when a print request is received, in step S31, the writing control unit 125 sets pixel clock frequencies for K, M, C, and Y colors based on the main scanning magnification correction amount described above. In step S32, the main scanning delay amount of each color is set based on the main registration correction amount stored in the RAM 123. In step S33, the sub registration correction amount of each color is set based on the sub registration correction amount stored in the RAM 123. Sets the scanning delay amount.

  Thereafter, in step S34, the skew correction amounts of M, C, and Y colors with respect to the reference color (K color) are set based on the skew correction amount of each color and the gradation number information. In step S35, the printing operation is started while executing image correction based on the set main scanning pixel clock frequency, main scanning delay amount, sub-scanning delay amount, and skew correction amount for each color of K, M, C, and Y. Then, the printing process ends.

  The correction of the color shift in the main scanning direction is performed by correcting the main scanning magnification and the writing timing of the main scanning. The main scanning magnification correction is an image based on the magnification error amount of each color detected by the writing control unit 125. This is done by changing the frequency. However, the write control unit 125 includes a device capable of setting the frequency very finely, for example, a clock generator using a VCO. Also, the writing timing in the main scanning direction is adjusted depending on from which position of the main scanning counter that operates by using the synchronization detection signal of each color as a trigger.

  Further, color misregistration correction in the sub-scanning direction is performed by adjusting the writing start timing in the sub-scanning direction. FIG. 7 is a timing chart showing an example of writing timing correction in the sub-scanning direction by the writing control unit. In FIG. 7, the writing control unit 125 counts the number of lines with reference to the start signal STTRIG_N from the CPU 122 and outputs a sub-scan timing signal (Y, M, C, K) _FSYNC_N to the image processing unit 124. .

  Thereafter, the image processing unit 124 outputs the sub-scanning gate signal (Y, M, C, K) _IPFGATE_N to the writing control unit 125 with the reception of the sub-scanning timing signal (Y, M, C, K) _FSYNC_N as a trigger. Then, the image data (Y, M, C, K) _IPDATA [7: 0] _N is transferred. Then, each color writing control unit 126Y, 126M, 126C, 126K transmits the image signal (Y, M, C, K) _LDDATA to the LD control unit 114Y, 114M, 114C, 114K.

  When correcting the registration in the sub-scanning direction, the sub-scanning delay amount (Y, M, C, K) _mfcntld from the start signal is changed according to the detected positional deviation amount. Normally, the amount of misregistration with respect to K color is reflected in the sub-scan delay amount of color (M, C, Y), and the timing of (Y, M, C, K) _FSYNC_N is changed to change the sub-scan direction. Perform position alignment.

  Next, detection of misregistration amount and correction processing in a color copying machine will be described. FIG. 8A is a diagram illustrating an example of a misregistration correction pattern formed on the transfer belt. The correction pattern 14 formed on the transfer belt 3 is detected by the detection sensors 15 and 16, and the obtained signal is converted from analog data to digital data by the pattern detection unit 121, and the data is sampled and sampled. Data is stored in the RAM 123. After the detection of the correction pattern 14 is completed, the CPU 122 calculates various color misregistration amounts (main scanning magnification, main resist misregistration amount, sub resist misregistration amount, skew) using the data stored in the RAM 123. For this reason, a correction amount of each shift component is calculated from the color shift amount.

  Here, the skew correction will be described. FIG. 8B is a diagram illustrating an example of a skew amount calculation method. Here, an example of a method for calculating the distortion amount of each color when the K color is the reference color is shown. For skew correction, first, the skew amount of each color (M, C, Y) with respect to the K color as the reference color is obtained. For example, a case where the right side of the C color image is shifted below the normal position as shown in FIG. 8B will be described as an example. The left detection sensor 15 is formed on the left side of the transfer belt 3. The positions of the patterns K11 and C11 are detected, and the left side distance KC_L between the K color and the C color is calculated from the positional relationship. On the other hand, the right detection sensor 16 detects the positions of the right patterns K21 and C21 formed on the transfer belt 3, and calculates the right distance KC_R between the K color and the C color from the positional relationship. As described above, the skew amount KC_Skew for the C color and the K color is obtained as in the following equation (1).

KC_Skew = KC_R−KC_L (1)
Similarly, for the M and Y colors, the respective skew amounts KM_Skew and KY_Skew are calculated from the following equations (2) and (3) by pattern detection.

KM_Skew = KM_R−KM_L (2)
KY_Skew = KY_R−KY_L (3)
As described above, the skew amounts KC_Skew, KM_Skew, and KY_Skew of the C, M, and Y colors based on the K color are calculated.

  Here, the details of the relationship between the skew amount and the skew correction amount will be described. FIG. 9A is a diagram illustrating an example of the skew amount of each color based on the K color when the resolution in the sub-scanning direction is 600 dpi. It is assumed that the skew amount of each color is obtained as shown in FIG. 9-1 using the above equations (1) to (3). That is, it is assumed that the skew amount of each color is M: −110 [μm], C: −130 [μm], and Y: 30 [μm] on the K color basis. Here, since the resolution in the sub-scanning direction is 600 dpi, shifting by one line moves 25,400 [μm] /600=42.3 [μm]. Therefore, the skew correction amount can be obtained by dividing each deviation amount (skew amount) by the movement amount per line, rounding off the decimal point to a value in an integer unit, and inverting the sign. FIG. 9B is a diagram illustrating an example of the skew correction amount in the case of FIG. As shown in FIG. 9-2, the skew correction amounts in the case of the skew amount shown in FIG. 9-1 are M: +3 lines, C: +3 lines, and Y: −1 lines, respectively.

  10A to 10D are diagrams illustrating an example of a skew correction method (a skew correction amount calculation method). FIG. 10A is a diagram illustrating an input image diagram, in which an input image for eight lines is illustrated. Here, one line indicates an image stored in one line memory. FIG. 10B is a diagram illustrating a state in which the input image diagram of FIG. 10A is directly output as LD data without correcting the skew. As shown in this figure, when the input image diagram is output as LD data as it is due to the skew of the scanning beam, the amount corresponding to 3 lines in the upper direction on the paper is higher than that in FIG. 10-1. (Ie, the number of skew lines is 3).

  As described above, when the right image is shifted upward by 3 lines, as shown in FIG. 10C, the main scanning direction is divided into {(number of lines of skew amount) +1}, that is, divided into 4 equal parts. . Here, a position (division position) divided in the main scanning direction on the line is referred to as a shift position, and an area in the main scanning direction on the line divided by the shift position is referred to as a shift area.

  Then, as shown in FIG. 10-4, if the image is output by shifting downward by one line every time the right shift area is moved, the left and right images are displayed on the sheet as shown in FIG. 10-5. The position can be made parallel. That is, a part of the image is accumulated in the line memory, read while switching the reading position, and an image (shift region) divided in the main scanning direction is shifted in the sub-scanning direction to correct the skew of each color. Shifting in the sub-scanning direction is called shifting.

  11-1 to 11-6 are diagrams illustrating other examples of the skew correction method (skew correction amount calculation method). The image on the right side on the sheet when the skew correction is not performed with respect to the input image diagram. Shows a skew correction method in a case where is shifted by one line downward from the normal position. Also in this case, the skew can be corrected by performing the same process as in the case of FIGS. 10-1 to 10-5 described above. In other words, the image is divided into two equal parts in the main scanning direction, and the shift area is shifted upward by one line each time the shift area is shifted to the right, so that the left and right image positions are parallel on the sheet as shown in FIG. become.

  In actual skew correction, input image data is sequentially stored in a line memory for skew correction, and the data in which line memory 128M, 128C, 128Y is divided in each divided area by the skew correction processing units 135M, 135C, 135Y. By switching whether to read or not, the image of FIG. 10-4 or FIG. 11-4 is output. Therefore, as shown in FIGS. 10-6 and 11-6, the address of the shift position in the main scanning direction for each color and information on whether the shift position shifts in the + or − direction in the sub-scanning direction. You should ask for. Information including the address of the shift position in the main scanning direction and the shift direction in the + or − direction of the sub-scanning direction at each shift position is referred to as shift correction information.

  For example, when the number of pixels in the scanning direction is 4,800 pixels as shown in FIG. 10-3, the right end is shifted up by 3 lines at the right end, so that the number is divided into four in the main scanning direction. Therefore, it is divided into shift areas of 1 to 1,200 pixels, 1,201 to 2,400 pixels, 2,401 to 3,600 pixels, and 3,601 to 4,800 pixels, and each shift area is the first block. It is assumed that the second block, the third block, and the fourth block.

  As a result, as shown in FIG. 10-4, the first block of image data in the first line memory is output from 0 to 1,200 pixels in the first line, and white pixels are output from 1,201 to 4,800 pixels. Is output. Also, the first block of image data in the second line memory is output from 0 to 1,200 pixels in the second line, and two blocks of image data in the first line memory are output in 1,201 to 2,400 pixels. Eyes are output, and white pixels are output from 2,401 to 4,800 pixels. By repeatedly executing such image data output processing, the output image can be corrected so that the left and right image positions are parallel as shown in FIG. 10-5.

  12A and 12B are timing charts at the time of skew correction of the line memory. Here, the K color is a reference color and is not divided, and the M color and the C color are 4 division correction because the skew correction amount is 3 dots, and the Y color is 1 dot because the skew correction amount is 1 dot. It is assumed that the correction is divided into two.

  FIG. 13 is a schematic diagram showing storage areas of the line memories 128K, 128M, 128Y, and 128C. In the present embodiment, as shown in FIG. 13, it is assumed that there is a line memory of 2 lines for K color and 5 lines for color colors (M, C, Y). Here, the description will be made assuming that a line memory having 2 lines for K color, 5 lines for C color, M color, and Y color is used, but the reference color may be a color other than K color, and the remaining colors are the same. The number of lines need not be, and if the amount of skew generation tends to differ depending on the color, the number of color line memories that are likely to cause skew may be increased.

  In this embodiment, the individual line memories 128K, 128M, 128Y, and 128C are provided independently for each color. However, as shown in FIG. 14, only one line memory 128 is provided for each color. A common configuration may be used.

  In FIG. 4, the write control unit 101 requires line memories 128K, 128M, 128Y, and 128C for each color in order to perform speed conversion between the input image and the write image, and skew in the color colors (M, C, and Y). In order to perform correction, more line memories are required than for the K color (reference color). In the configuration shown in FIG. 13, for K color, input image data (K_IPDATA) is sequentially stored in a two-line line memory, and an output image is generated by sequentially reading from a line memory that is not being written, The data is output to the LD control unit 106 as LD image data (K_LDDATA). As shown in FIG. 13, if there is a line memory of 2 lines for K color, 5 lines for M color, C color, and Y color, 2 lines are necessary to correct the inclination of 1 line, and 1 line. Is used for image input, the skew amount that can be corrected is a maximum of ± 3 lines.

  12A to 12B, the input image control units 127K, 127M, 127C, and 127Y perform printing operations at timings based on the sub-scan delay amount (K, M, C, Y) _mfcntld from the start signal from the CPU 122. To start. When the printing operation starts, images are stored in the line memories K-1, M-1, C-1, and Y-1.

  Next, the images are stored in the line memories K-2, M-2, C-2, and Y-2, and at the same time, the images are read from the line memories K-1, M-1, C-1, and Y-1. . Then, all pixels are output to the K-color write signal K_LDDATA through the write control unit 126K, and the first block pixel divided into four is output to the M-color write signal M_LDDATA through the write control unit 126M, and then through the write control unit 126C. The pixel of the first block divided into four is output to the C color write signal C_LDDATA, and the pixel of the first block divided into two is output to the Y color write signal Y_LDDATA through the write control unit 126Y.

  Thereafter, the images are stored in the line memories K-1, M-3, C-3, and Y-3, and at the same time, the line memories K-2, M-1, M-2, C-1, C-2, Images are read from Y-1, Y-2. Then, all pixels are output to the K-color write signal K_LDDATA via the write control unit 126K, and the pixels in the second block of the four divisions of the line memory M-1 and the line memory M to the M-color write signal M_LDDATA via the write control unit 126M. The pixel of the first block divided by 4 is output. Further, through the write control unit 126C, the pixel of the second block of the four divisions of the line memory C-1 and the pixel of the first block of the four divisions of the line memory C-2 are output to the C color write signal C_LDDATA, and the write control unit Through 126Y, the pixel in the second divided block of the line memory Y-1 and the pixel in the first divided block of the line memory Y-2 are output to the Y color write signal Y_LDDATA.

  Then, the images are stored in the line memories K-2, M-4, C-4, Y-1, and at the same time, the line memories K-1, M-1, M-2, M-3, C-1, Images are read from C-2, C-3, Y-2, and Y-3. Then, all the pixels are output to the K color write signal K_LDDATA via the write control unit 126K, and the pixels in the third block of the four divisions of the line memory M-1 and the line memory M to the M color write signal M_LDDATA via the write control unit 126M. -2 divided into the second block pixel and the line memory M-3 divided into the first block pixel. Further, after passing through the write control unit 126C, the C color write signal C_LDDATA is supplied to the fourth block pixel of the line memory C-1, the second block pixel of the line memory C-2, and the line memory C-3. The pixel of the first block divided into four is output, and after passing through the write control unit 126Y, the Y color write signal Y_LDDATA is supplied to the second divided pixel of the line memory Y-2 and one block of the line memory Y-3 divided into two. The eye pixel is output.

  Thereafter, the images are stored in the line memories K-1, M-5, C-5, and Y-2, and at the same time, the line memories K-2, M-1, M-2, M-3, M-4, Images are read from C-1, C-2, C-3, C-4, Y-1, and Y-3. All pixels are output to the K color write signal K_LDDATA via the write control unit 126K, and the fourth block pixel of the line memory M-1 and the line memory M-2 to the M color write signal M_LDDATA via the write control unit 126M. The four-divided third block pixel, the four-divided second block pixel of the line memory M-3, and the four-divided first block pixel of the line memory M-4 are output. Further, after passing through the write control unit 126C, the C color write signal C_LDDATA is supplied to the fourth block pixel of the line memory C-1, the third block pixel of the line memory C-2, and the third block pixel of the line memory C-3. The pixel of the second block divided into four and the pixel of the first block divided into four lines of the line memory C-4 are output, and the two blocks of the line memory Y-3 divided into Y color write signal Y_LDDATA via the write control unit 126Y The pixel of the first block divided into two of the eye pixel and the line memory Y-1 is output. The above processing is executed repeatedly, and image data that has undergone skew correction is printed.

  In the image processing method according to the present embodiment, a part of the image is accumulated in the line memory, read while switching the reading position, and the image divided in the main scanning direction is shifted in the sub scanning direction. The color shift of is reduced. Shifting in the sub-scanning direction is called image shift, and the position divided in the main scanning direction is called image shift position. However, in the method of performing correction by image processing according to the present embodiment, the pixel adjacency changes at the image shift position. In this case, a local density change occurs at the image shift position. This local density change is called density deviation. This density shift becomes particularly remarkable for an image expressed by pseudo gradation processing such as dithering. When the dither process is performed, a local density change periodically occurs in the sub-scanning direction at the image shift position, so that a density shift is remarkably confirmed.

  FIG. 15 is a schematic diagram illustrating an example of an image subjected to dither processing. In electrophotographic recording, since the laser beam is usually larger than the pixel size, when the image image shown in FIG. 15A is output, the toner adhesion area expands beyond the pixel size (FIG. 15B). reference).

  When the image shift process is performed on the dither image shown in FIG. 15, the pixel adjacency changes at the image shift position as shown in FIG. When the adjacency changes as in the image image of FIG. 16A, the actual output image is as shown in FIG. 16B, and the toner adhesion area in the pixels before and after the image shift position is as shown in FIG. It changes with respect to the image before the image shift shown (refer FIG.16 (b)).

  Next, a change in the toner adhesion area depending on the image shift position will be specifically described. FIG. 17 is a schematic diagram for explaining a change in the toner adhesion area depending on the image shift position. As shown in FIG. 17, the unit area of 1 dot is 1, and the toner adhesion area at the time of output of 1 dot of 100% density is 1.57. At this time, when the image image having the density subjected to the dither processing shown in FIGS. 18-1 (a) and 18-2 (a) is shifted downward (FIG. 18-1 (b), FIG. 18- 2 (b)), the pixel adjacency changes at the image shift position, and the area of the white pixel is 0.285 with respect to the actual output images of FIGS. 18-1 (a) and 18-2 (a). To increase. At the time of actual output, the area of white pixels increases, so the density is locally reduced at the image shift position. Since this density change periodically appears in the sub-scanning direction due to the periodicity of the dither matrix, a white streak-like noise image is generated as shown in FIGS.

  On the other hand, when the area of the white pixel decreases, the density locally increases at the shift position, so that a black streak noise image is generated as shown in FIG. 18-2 (c). The adjacent relationship of the pixels changes at the image shift position, and the area of the white pixel is reduced by 0.285 with respect to the actual output image of FIG. At the time of actual output, the area of the white pixel is reduced, so that the local density is increased at the image shift position. Since this density change appears periodically in the sub-scanning direction due to the periodicity of the dither matrix, a black streak noise image is generated as shown in FIG.

  As described above, the change in the adjacency relationship between the pixels at the image shift position due to the image shift causes a change in the toner adhesion area, thereby causing a local density shift. In the image subjected to the dither processing, density deviation periodically occurs in the sub-scanning direction due to the periodicity of the dither matrix at the image shift position, thereby causing a streak-like noise image. In particular, streak-like noise appears remarkably in a high-density halftone image that has been subjected to dither processing, and the image is significantly degraded.

  In the present embodiment, the density correction of pixels near the image shift position is particularly effective for an image in which a streak-like noise image appears conspicuously due to a local density shift caused by a change in the adjacent relationship of pixels at the image shift position. I do. This compensates for the difference (increment) in the toner adhesion area in the vicinity of the image shift position and reduces the local density deviation close to the adhesion area before the image shift, thereby reducing image quality due to the generation of streak noise images in the sub-scanning direction. And an optimal image correction in which traces due to density correction are not noticeable.

  Next, the skew correction of this embodiment will be described. FIG. 21 is a block diagram showing a functional configuration of the skew correction processing unit 135 in FIG. FIG. 21 shows the functional configuration of the skew correction processing unit 135M for magenta (M) color, but the same applies to the skew correction processing units 135K, 135Y, and 135C for other colors.

  The skew correction processing unit 135M includes a data selector 161, a skew block control unit 171, and a dither correction unit 181.

  The data selector 161 performs skew correction on the image data transferred from the input image control unit 127 and transfers the image data to the dither correction unit 181. The dither correction unit 181 performs density correction on the input image data and outputs the corrected image data to the writing image processing unit 131M.

  Here, details of the dither correction unit 181 will be described. FIG. 22 is a block diagram illustrating a functional configuration of the dither correction unit 181. As illustrated in FIG. 22, the dither correction unit 181 includes a dither correction input unit 201, a pattern matching unit 211, and a dither correction output unit 221.

  The dither correction process is roughly classified into an input process executed by the dither correction input unit 201, a determination process executed by the pattern matching unit 211, and a correction process executed by the dither correction output unit 221. FIG. 23 is a schematic diagram for explaining the contents of the dither correction processing.

  In the input processing, when the input of image data subjected to skew correction output from the data selector 161 is received, it is determined whether or not the target pixel of the image data is located at the image shift position (division position). By detecting the density value of the pixel near the image shift position, a density shift between the target pixel and the neighboring pixel is detected, and each density of the target pixel and the neighboring pixel is converted into a 3 × 8 input pixel pattern. It inputs (refer Fig.23 (a)).

  In the determination process, the input pixel pattern set by the input process and the shift direction are compared with the matching pattern in which the density is set in advance to determine whether or not they match (see FIG. 23B). Matching patterns are set in advance for each shift direction. Here, the shift direction is determined by the SHIFDIR signal (FIG. 22) in the SKLINESEL signal (see FIG. 21) generated by the skew block control unit 171 from the SKSHIFT signal (see FIG. 21) and input to the dither correction unit 181. To do.

  In the correction process, when the input pixel pattern and the matching pattern match in the determination process, the density value of the pixel in the density correction region is corrected by dither correction, and the dither-corrected image data, that is, the image in which the density value is corrected. Data is output to the writing image processing unit 131 (see FIG. 23C). Through the above processing, optimum density correction is performed.

  FIG. 24 is a flowchart showing the procedure of the dither correction process. In the dither correction process, first, the dither correction input unit 201 determines whether or not the target pixel is located at the image shift position in the main scanning direction set by the image processing unit (step S51). If the target pixel is not located at the image shift position in the main scanning direction (step S51: No), the process is terminated without performing the subsequent dither correction process.

  In step S51, when the target pixel is located at the image shift position in the main scanning direction (step S51: Yes), the dither correction input unit 201 executes density deviation detection processing (step S52), and then the pattern matching unit. 211 performs a pattern matching process (step S53). The details of the density deviation detection process will be described later.

  If the input pixel pattern detected by the density shift detection matches the matching pattern (step S52: Yes), the dither correction output unit 221 performs dither correction processing (density value correction) on any pixel in the density correction pixel area. Correction process) is performed (step S54), and the process is terminated.

  On the other hand, if the input pixel pattern detected by the density deviation detection and the matching pattern do not match in step S52 (step S52: No), the process ends without performing the dither correction process (density value correction process).

  In the present embodiment, the image shift position determination process in step S51 and the pattern matching process in step S53 may be executed in parallel.

  Next, FIG. 25 for explaining the details of the density deviation detection process in step S52 is a flowchart showing the procedure of the density deviation detection process.

  In the density deviation detection execution process, first, the dither correction input unit 201 inputs the shift direction (SHIFTDIR) set by the image processing unit 124 (step S71), and further mirror information indicating the presence or absence of a preset mirror process. (MIRROR signal in the SKLINESEL signal (see FIG. 21)) is input (step S72). Then, the dither correction input unit 201 inputs the target pixel and the density values of the surrounding pixels of the target pixel into the input pixel pattern (step S73), and ends the density shift detection process.

  Here, the detection item of density deviation detection may be a selection type, or the order of density deviation detection may be switched. Note that in the case of an image forming apparatus using an image process configuration that does not require mirror processing, mirror information is not required, and therefore it is possible to adopt a configuration in which mirror information is not held in advance.

  FIG. 26 is a diagram illustrating an example of a matching pattern used in the pattern matching process and dither correction.

  The shift direction detected by the density shift detection process, the input pixel pattern, and a preset matching pattern are compared, and if they match, a dither correction process (density value correction process) is performed. At this time, the matching pattern is set for each preset shift direction. In the dither correction process, as shown in FIG. 26, density target pixels are set by a matching pattern in which density values are determined in advance, and density correction is performed by performing an increase / decrease process on the original density value. In this way, the density correction is uniquely determined by the matching pattern. In dither correction (correction of density value), the density value may be replaced with a density value previously provided for each matching pattern, or correction may be performed by calculation using the original density value and surrounding density values.

  The dither correction output unit 221 can also be configured to perform density correction based on the density value of the image that has been skew-corrected in the dither correction processing. That is, an increase amount and a decrease amount are set in advance with respect to the pixel density value of the skew-corrected image, and the density correction is performed by adding the increase amount or the decrease amount to the pixel density value. Can do. In this case, the increase amount and the decrease amount may be configured to be individually settable.

  Next, the matching pattern used in the pattern matching process will be described. Usually, the printer expresses the smoothness of gradation using various dither matrices. As shown in FIGS. 27 and 28, dither matrices having different sizes and screen angles are normally set individually for each color, each bit number, each line number, and each application.

  Different types (types) of dither matrices are set depending on the number of gradations. For example, in the case of a photograph of a binary image (1 bit image), in the case of a dither matrix of dot concentration type, in the case of a photograph of a multi-value image (2/4 bit image), in the case of a line type dither matrix (see FIG. 29), Distributed dither matrices (FIG. 30) are individually set.

  In this way, dither processing is performed in the printer controller 111 in which these various dither matrices are set, and when the image data is shifted with respect to the image data subjected to the dither processing, the noticeability of the streak noise image is greatly different. .

  Further, when the image is shifted, the noticeable streak noise image differs depending on the division position. If the matching pattern is determined for all the places where the density deviation occurs, the trace caused by the correction becomes conspicuous and deteriorates the image.

  For this reason, in the present embodiment, it is preferable that the matching pattern is set only for a portion where the streak noise image is conspicuous on the basis of the dither matrix.

  Therefore, the matching pattern can be set differently depending on the dither matrix with different colors, the number of bits, the size, the screen angle, the number of lines, and the number of gradations, or different depending on the type of the dither matrix. To do. For example, a matching pattern can be set finer for a halftone density as compared to a density other than a halftone. Alternatively, in the case of a density other than halftone or in the case of a reference color for skew correction, a matching pattern may not be set, i.e., density deviation detection by pattern matching may not be performed.

  Further, for example, a case will be described in which the image data is expressed with respect to the image data expressed by the distributed dither matrix shown in FIG. In this case, local density deviation occurs. When a matching pattern is set for all areas where density deviation occurs and density correction is performed, correction processing is applied to all the pixels of interest, and only the corrected portion becomes a completely different image, and correction processing is performed. In fact, the traces stand out.

  The image expressed by the distributed dither matrix causes local density deviations, but because the matrix structure is fine, the streak noise is not so noticeable even if the image is shifted. It is better not to set a matching pattern for such a distributed dither matrix.

  In this way, if a matching pattern is set only from the viewpoint of density deviation, traces of density correction become conspicuous depending on the type of dither matrix, so optimal correction processing can be performed by setting a matching pattern based on density deviation and dither matrix. Is possible. For the line-type dither matrix as shown in FIG. 29, the noticeable stripe noise varies depending on the shift position, shift direction, and screen angle.

  Note that the setting of the matching pattern may be switched according to the dither matrix type, screen angle, number of lines, color, and number of gradations.

  In the image image after the image shift shown in FIG. 16, when the target pixel is located at the image shift position as shown in FIG. 31, the shift direction is checked, and the density value of the pixel near the target pixel is input to the input pixel pattern. . Next, the input pixel pattern is compared with the matching pattern. When the matching pattern is the example shown in FIG. 30, the input pixel pattern and the pattern D match as shown in FIG. Density correction is performed by information on density correction target pixels included in the pattern D and density value calculation. In the case of the pattern D, density correction is performed by outputting the density value of the target pixel density value + D to the density correction target pixel.

  Next, the input pixel pattern will be described. FIG. 41 is a schematic diagram illustrating an example of an input pixel pattern. When the main scanning size of the input pixel pattern is determined based on a preset dither matrix, a more optimal region can be determined. When a line-type dither matrix having a relatively gentle screen angle is set in advance instead of the line shown in FIG. 42A, a line-type matrix having a relatively steep screen angle shown in FIG. The period in the main scanning direction is increased. When the main scanning size of the matching pattern is determined based on the period of the dither matrix with a gentle screen angle in the main scanning direction, the optimum density deviation can be detected without erroneously detecting the image represented by the dither matrix with the gentle screen angle. Detection can be performed. In order to determine the shift direction, the sub-scan size of the input pixel pattern requires a minimum of three pixels in total, that is, the target pixel and the upper and lower pixels.

  FIG. 33B and FIG. 34B show the relationship shown in FIG. 17, that is, the dither correction unit 181 of the present embodiment when the toner adhesion area is 1.57 when outputting 100% pixel 1 dot. Then, density correction (dither correction) was performed with a density value such that the toner adhesion area per pixel at the time of output was 0.283 + 0.285 = 0.568 (including the erosion of the peripheral pixels of 0.285). It is an example. By the density correction of the dither correction processing, the white pixel area locally increased at the image shift position can be reduced from 0.285 to 0.002, and the white pixel area can be brought close to the state before the image shift.

  In FIG. 34C, in the case of the relationship shown in FIG. 17, the dither correction unit 181 of the present embodiment results in the toner adhesion area per pixel at the time of output being 1.285−0.285 = 1. This is an example in which density correction is performed with a density value that takes into account the erosion of peripheral pixels of 0.285 and the erosion of target pixels. By the density correction of the dither correction processing, the area of the white pixel that is locally decreased at the image shift position is increased from −0.285 to almost 0, and the area of the white pixel can be brought close to the state before the image shift.

  Next, an example of a correction result by density correction in the dither correction process will be described. 35, 36, and 37 are schematic diagrams illustrating output image examples of image data that has been subjected to dither correction (density correction). As shown in FIGS. 35, 36, and 37, it is possible to prevent the generation of a streak-like noise image by bringing the pixel at the image shift position closer to the toner adhesion area before the image shift by the density correction of the dither correction process. it can.

  38, 39, and 40 are schematic diagrams illustrating examples of states when the image data illustrated in FIGS. 35, 36, and 37 are viewed. As shown in FIGS. 38, 39, and 40, it can be seen that the corrected image is approximated to the original image by the dither correction processing of the present embodiment.

  As described above, in the present embodiment, it is determined whether or not the pixel of interest is positioned at the image shift position in the image data that has been subjected to skew correction by the dither correction unit 181 of the skew correction processing unit 135. In this case, the density shift of the pixel near the image shift position is detected, input to the 3 × 8 input pixel pattern, pattern matching is performed using the input pixel pattern and the pixel shift direction, and a preset matching pattern, When the input pixel pattern matches the matching pattern, the density value of the pixel in the density correction area is corrected by dither correction. Thereby, when performing skew correction, it is possible to detect only the conspicuous streak noise and correct the density value of the pixels near the image shift position. Therefore, according to the present embodiment, it is possible to prevent generation of a streak noise image due to image shift and realize high-quality image correction in which traces due to density correction are not conspicuous.

  The image forming program executed by the image forming apparatus according to the above embodiment is provided by being incorporated in advance in a ROM or the like.

  The image forming program executed by the image forming apparatus of the above embodiment is a file in an installable or executable format, such as a CD-ROM, a flexible disk (FD), a CD-R, a DVD (Digital Versatile Disk), or the like. It may be configured to be recorded on a computer-readable recording medium.

  Furthermore, the image forming program executed by the image forming apparatus of the above-described embodiment may be provided by being stored on a computer connected to a network such as the Internet and downloaded via the network. Further, the image forming program executed by the image forming apparatus according to the above-described embodiment may be provided or distributed via a network such as the Internet.

  The image forming program executed by the image forming apparatus of the above embodiment has a module configuration including the above-described units (dither correction input unit, pattern matching unit, dither correction output unit, image processing unit, etc.). As the hardware of the CPU, a CPU (processor) reads the program from the ROM and executes it to load the above-described units onto the main storage device. Etc. are generated on the main memory.

15, 16 Detection sensor 111 Printer controller 112 Scanner 113 Engine control unit 114 LD control unit 121 Pattern detection unit 122 CPU
123 RAM
124 Image processing units 125 and 126 Write control unit 127 Input image control unit 128 Line memory 131 Write image processing unit 132 Misalignment correction pattern generation unit 133 LD data output unit 135 Skew correction processing unit 161 Data selector 171 Skew block control unit 181 Dither correction unit 201 Dither correction input unit 211 Pattern matching unit 221 Dither correction output unit

Japanese Patent No. 3715349 (Japanese Patent Laid-Open No. 09-039294)

Claims (11)

In an image forming apparatus that divides image data of one line in the main scanning direction and performs skew correction of image data,
Based on the pixels of the image data subjected to skew correction, when the target pixel to be corrected is at a division position in the main scanning direction, the density of the target pixel and pixels around the target pixel is calculated, and the calculated density Detecting means for detecting the occurrence of density deviation based on a shift direction by skew correction and a predetermined pattern;
A correction unit that performs density correction that compensates for differences in output areas of a plurality of pixels in the vicinity of the division position based on the detected density shift when occurrence of density shift is detected;
An image forming apparatus comprising:   The detecting means determines whether or not the densities of the pixel of interest and the surrounding pixels match a predetermined pattern of the dither matrix, and detects the occurrence of a density shift if they match. The image forming apparatus according to claim 1. The detection means further determines that there is no density shift when it is determined that the densities of the target pixel and the surrounding pixels do not match any of the predetermined patterns,
The image forming apparatus according to claim 1, wherein the correction unit does not perform the density correction when it is determined that there is no density deviation. The pattern varies depending on the shift direction,
The image forming apparatus according to claim 1, wherein the detection unit detects the occurrence of the density shift by using the pattern corresponding to the shift direction.   The image forming apparatus according to claim 1, wherein the pattern is set based on a predetermined dither matrix.   The image forming apparatus according to claim 5, wherein the pattern is set based on a type of the dither matrix.   The image forming apparatus according to claim 5, wherein the pattern is set based on a screen angle of the dither matrix.   5. The correction unit performs density correction on a pixel in the density correction target region, with a region including the previous pixel of the target pixel at the division position as a density correction target region. The image forming apparatus according to any one of the above.   The image forming apparatus according to claim 5, wherein the correction unit performs density correction on a pixel in the density correction target area based on a predetermined second pattern. An image forming method for correcting image data skew by dividing one line of image data in the main scanning direction,
Based on the pixels of the image data subjected to skew correction, when the target pixel to be corrected is at a division position in the main scanning direction, the density of the target pixel and pixels around the target pixel is calculated, and the calculated density And a detection step for detecting the occurrence of density deviation based on a shift direction by skew correction and a predetermined pattern;
When the occurrence of density deviation is detected, a correction step for performing density correction that compensates for differences in output areas of a plurality of pixels in the vicinity of the division position based on the detected density deviation;
An image forming method comprising:   A program for causing a computer to execute the image forming method according to claim 10.

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