JP2006159451A - Image forming apparatus and its control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technology for forming a good image by determining the read-out coordinate position of image data to be recorded based on information indicating color shift amount in the exposure scanning direction on a image carrier at each image forming section and correcting color shift at first, and then suppressing occurrence of moire due to color shift correction by performing halftoning and suppressing occurrence of jaggy also for the edge of a character line image. <P>SOLUTION: The image forming apparatus comprises sections 403C, M, Y, K for storing actual color shift amount from an ideal scanning direction, sections 407C, M, Y, K for calculating color shift correction amount of each color component based on the actual color shift amount, sections 408C, M, Y, K for converting the coordinate at the time of reading out the image data from bit map memories 406C, M, Y, K based on the color shift correction amount thus calculated, sections 411C, M, Y, K performing halftoning if remarked image data is not located at the edge of a character line image, or the like, and exception processing sections 409C, M, Y, K performing processing for suppressing occurrence of jaggy following to color shift correction if the remarked image data is located at the edge. Consequently, occurrence of moire due to color shift correction is suppressed. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a technique for forming a color image by juxtaposing a plurality of image carriers and transferring a recording material to be a color component image developed by each image carrier onto a transported storage medium. is there.

  2. Description of the Related Art Conventionally, as a color image forming apparatus using an electrophotographic system, one that performs development with each color component using a plurality of developing units for one photoconductor is known. This device repeats the “exposure-development-transfer” process by the number of color components, and in each case, forms a color image on a single transfer paper and fixes it to obtain a full-color image. It is.

  According to this method, in order to obtain one printed image, it is necessary to repeat the image forming process 3 to 4 times (when black is used), and it takes time to complete the image formation. was there.

  As a method for compensating for this drawback, a technique is known in which a plurality of photoconductors are used, and a visible image obtained for each color is sequentially superposed on a transfer paper to obtain a full color print by one pass. Yes.

  According to this method, the throughput can be greatly shortened, but on the other hand, the color due to the positional deviation of each color on the transfer paper due to the positional accuracy and diameter deviation of each photoconductor, the optical system positional precision deviation, etc. There arises a problem of misalignment, which makes it difficult to obtain a high-quality full-color image.

  As a method for preventing this color misregistration, for example, a test toner image is formed on a transfer belt or a conveyance belt forming a part of transfer means, and this is detected, and each optical system is detected based on this result. There is known a technique for correcting the optical path of each color and correcting the image writing position of each color (Patent Document 1).

In addition, the output coordinate position of the image data for each color is automatically converted into an output coordinate position in which the registration shift is corrected, and the position of the light beam modulated by the correcting means based on the converted image data of each color is color-coded. There is also known a technique for correcting with an amount smaller than the minimum dot unit of a signal (Patent Document 2).
Japanese Patent Application Laid-Open No. 64-40956 JP-A-8-85237

  However, the following problem remains in Patent Document 1 described above.

  First, in order to correct the optical path of the optical system, it is necessary to mechanically operate a correction optical system including a light source and an f-θ lens, a mirror in the optical path, etc., and align the position of the test toner image. . That is, a highly accurate movable member is required, and the cost must be increased. Furthermore, since it takes time to complete the correction, it is impossible to perform correction frequently. Further, the optical path length deviation may change with time due to the temperature rise of the machine. In such a case, it is difficult to prevent the color deviation by correcting the optical path of the optical system.

  Second, by correcting the image writing position, it is possible to correct the displacement of the left end and upper left, but it is possible to correct the tilt of the optical system and the magnification shift due to the deviation of the optical path length. Absent.

  Further, as disclosed in Patent Document 2, by correcting the output coordinate position of image data for each color with respect to an image subjected to intermediate gradation processing, the reproducibility of halftone dots of an intermediate gradation image is deteriorated. There is a problem that color unevenness may occur and moire may become apparent.

  An example is shown in FIG. The input image 101 is an image having a constant density value. When an image 102 having undergone a certain color misregistration correction is actually printed on the input image 101, the relationship between the image density value and the toner density with respect to the image density value is not linear. Regardless of the image having the density value, when the image after color misregistration correction is printed, an image having a non-constant density value is printed. Therefore, when such non-uniform density values are periodically repeated, moire becomes obvious and a good color image cannot be obtained.

  The present invention is made in order to solve the above-described problems, and is based on deviation amount information indicating deviation amounts with respect to the exposure scanning direction on the image carrier of each image forming unit, and coordinates for reading image data to be recorded. The position is determined, color misregistration is corrected first, then halftone processing is performed for recording, and moire generation due to color misregistration correction is suppressed, and jaggy is also generated at the edge of a character line image. The present invention intends to provide a technique for suppressing and forming a good image.

In order to solve this problem, for example, an image forming apparatus of the present invention has the following configuration. That is,
An image forming unit having an image bearing member, an exposure unit that scans and exposes the image bearing member, and a developing unit that visualizes the electrostatic latent image generated by the exposure with a recording material, along the conveyance direction of the recording medium. Image forming apparatuses arranged side by side,
Image data storage means for storing image data to be formed in each image forming unit;
Exposure deviation amount storage means for storing deviation amount information indicating the deviation amount with respect to the exposure scanning direction on the image carrier of each image forming unit;
Coordinate conversion means for converting the coordinates of the read address of the image data storage means based on the exposure deviation amount information stored in the exposure deviation amount storage means, and reading out the image data according to the converted address information;
Buffer means for accumulating a plurality of lines of pixel data read by the coordinate conversion means;
Determination means for determining whether or not the target pixel data is at the image edge based on the target pixel data stored in the buffer means and the surrounding pixel data group;
A first processing unit that performs halftone processing for the non-image edge on the target pixel data when the determination unit determines that the pixel is on the non-image edge;
A correction unit that corrects the gradation of the pixel-of-interest data stored in the buffer unit based on address information when the coordinate conversion unit performs conversion when the determination unit determines that the image edge exists. ,
Based on the determination result of the second processing means for performing edge processing different from the first processing means on the pixel data obtained by the correction means and the determination result of the determination means, Output means for outputting pixel data obtained by the processing means as an exposure control signal for the exposure section of the corresponding image forming section.

  According to the present invention, it is made to solve the above-described problems. Based on the shift amount information indicating the shift amount with respect to the exposure scanning direction on the image carrier of each image forming unit, the image data to be recorded is recorded. The read coordinate position is obtained, color misregistration is corrected first, and then halftone processing is performed and recorded, moire generation due to color misregistration correction is suppressed, and the edge of a character line image is also jaggy. It is possible to suppress the occurrence and form a good image.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments according to the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 2 is a structural cross-sectional view of the image forming apparatus according to the present embodiment.

  As shown in the figure, this embodiment has a structure of a four-drum type color laser beam printer.

  In this image forming apparatus, a transfer material cassette 53 is mounted on the lower right side of the figure. The recording medium (recording paper, transmission sheet, etc.) set in the transfer material cassette 53 is taken out one by one by the paper feed roller 54 and fed to the image forming unit by a pair of transport rollers 55-a and 55-b. . In the image forming unit, a transfer conveyance belt 10 that conveys a recording medium is flattened in a recording medium conveyance direction (from right to left in FIG. 1) by a plurality of rotating rollers. Is electrostatically attracted to the conveyor belt 10. Further, four photosensitive drums 14-C, Y, M, and K serving as drum-shaped image bearing members are arranged in a straight line so as to face the belt conveyance surface to constitute an image forming unit (here. C represents cyan, Y represents yellow, M represents magenta, and K represents black.)

  Since the image forming unit for each color component is different only in the color of the toner to be mounted and there is no structural difference, the color component C will be described.

  The C-color image forming unit stores a charger 50-C for uniformly charging the surface of the photosensitive drum 14-C, C-color toner, and an electrostatic latent image generated on the photosensitive drum 14-C. It has a developing device 52-C for developing (developing) a visible image and an exposure unit 51-C. A predetermined gap is provided between the developing unit 52-C and the charger 50-C. On the photosensitive drum 14-C whose surface is uniformly charged by the charger 50-C, the laser beam from the exposure unit 51-C made of a laser scanner is scanned and exposed in the direction perpendicular to the drawing through the gap. Thus, a charged state different from the non-exposed portion, that is, an electrostatic latent image is generated in the scanned and exposed portion. The developing device 52-C transfers the toner to the electrostatic latent image to make a visible image (toner image; development).

  A transfer portion 57 -C is arranged across the conveyance surface of the transfer conveyance belt 10. The toner image formed (developed) on the peripheral surface of the photosensitive drum 14-C is charged and adsorbed onto the conveyed recording medium by the transfer electric field formed by the transfer unit 57 corresponding to the toner image. Transferred onto the medium surface.

  By performing the above-described processing for the other color components Y, M, and K in the same manner, the C, M, Y, and K color toners are successively transferred to the recording medium. Thereafter, each color toner on the recording medium is melted and fixed by the fixing device 58, and is discharged out of the apparatus by a pair of paper discharge rollers 59-a and 59-b.

  The above is an example of transferring a toner image of each color component onto a recording medium. However, when the toner images of the respective color components are transferred onto the transfer / conveyance belt, the toner image generated on the transfer / conveyance belt may be transferred again to the recording medium (secondary transfer). The transfer belt in this case is called an intermediate transfer belt.

  FIG. 3 is an image diagram for explaining the shift of the main scanning line scanned on the photosensitive drum 14-C (which may be M, Y, or K) which is an image carrier. The horizontal direction in the figure indicates the scanning direction of the laser beam, and the vertical direction indicates the rotation direction of the photosensitive drum (also the conveyance direction of the recording medium).

  301 in the figure shows an ideal main scanning line. Reference numeral 302 denotes an actual main scanning line in which the positional accuracy and the diameter of the photosensitive drum 14 are shifted and the inclination is increased due to the positional accuracy of the optical system in each color exposure unit 51-C, and the curvature is generated. An example is shown.

  When such an inclination or curvature of the main scanning line exists in any color image forming unit, color misregistration occurs when a plurality of color toner images are collectively transferred to the transfer medium.

  In the present embodiment, in the main scanning direction (X direction), an ideal main scanning line at a plurality of points (point B, point C, point D) with the point A serving as the scanning start position of the print area as a reference point. The amount of deviation in the sub-scanning direction between 301 and the actual main scanning line 302 is measured, and a plurality of regions (region 1 between Pa-Pb, region 2 between Pb-Pc, region Pc- The area between Pd is divided into areas 3), and the inclination of the main scanning line in each area is approximated by straight lines (Lab, Lbc, Lcd) connecting the points. Therefore, when the difference in the amount of deviation between points (m1 in area 1, m2-m1 in area 2, m3-m2 in area 3) is a positive value, the main scanning line of the corresponding area has a slope that rises to the right. In the case of a negative value, it indicates that it has a downward slope. In the embodiment, the number of regions is three, but this is for convenience and is not limited to this number.

  FIG. 4 is a block diagram for explaining the operation of the color misregistration correction processing for correcting the color misregistration caused by the inclination and curvature of the scanning line performed in the present embodiment.

  In the figure, 401 is a printer engine, which actually performs printing processing based on image bitmap information generated by the controller 402. The controller 402 is accommodated in the substrate and is electrically connected to the printer engine 401 when accommodated in the apparatus.

  Reference numerals 403C, M, Y, and K denote color misregistration amount storage units for each color, and the above misregistration amount information for each image forming unit for each color is written and held in the apparatus manufacturing stage. As an example, it may be realized by a writable non-volatile memory such as an EEPROM. In the drawing, a color misregistration amount storage unit is provided for each color component. However, since the amount of information to be stored is sufficiently small, the color misregistration amounts of all color components may be stored in one memory element. .

  The color misregistration amount storage units 403C, M, Y, and K in the present embodiment are the actual main scanning line 302 measured at a plurality of points and the ideal main scanning line 301 in the sub-scanning direction described with reference to FIG. The color shift amount information is stored as information indicating the shift amount and the inclination and curvature of the main scanning line.

  FIG. 5 shows an example of information stored in the color misregistration amount storage unit 403C (M, Y, K is the same, but the stored information varies depending on individual differences), and L1 to L3, and m1 to m3 have the same meaning as in FIG.

  In this embodiment, the color misregistration amount storage means stores the deviation amount between the ideal main scanning line and the actual main scanning line, but the actual main scanning line inclination and curvature characteristics are stored. If is identifiable information, it is not limited to this. Further, as described above, the information stored in the color misregistration amount storage units 403C, M, Y, and K measures the misregistration amount in the manufacturing process and stores it in advance as information unique to the apparatus. . However, a detection mechanism for detecting the amount of deviation is prepared in the apparatus itself, a predetermined pattern for measuring the deviation is formed for each color image carrier, and the amount of deviation detected by the detection mechanism is stored. Such a configuration may be used.

  The controller 402 performs the printing process by correcting the image data for each color component so as to cancel out the shift amounts of the main scanning lines stored in the color shift amount storage units 403C, M, Y, and K. The following is a description of the controller 402 in the embodiment.

  The image generation unit 404 generates raster image data that can be printed from print data (PDL data, image data, etc.) received from an external device (not shown) (for example, a computer device) or the like, and generates RGB data (8 bits each). (256 gradations) for each dot. Since this process is a well-known process, detailed description is omitted.

  The color conversion unit 405 converts the RGB data into CMYK space data (8 bits each) that can be processed by the printer engine 402 (implemented by LOG conversion and UCR processing), and a bit for each recording color component in the subsequent stage. Store in the map memory 406C, M, Y, K. The bitmap memory 406C (same for M, Y, and K) temporarily stores raster image data for performing printing processing, and has a page memory for storing image data for one page. However, any of band memories that store data for a plurality of lines may be used. In order to simplify the description, it is assumed here that the data has a capacity for storing C, M, Y, K bitmap data for one page.

  The color misregistration correction amount calculation units 407C, M, Y, and K correspond to the coordinate information in the main scanning direction based on the main scanning line misalignment information accumulated in the color misregistration amount storage units 403C, M, Y, and K. The color misregistration correction amount in the sub-scanning direction is calculated, and the result is output to the color misregistration correction units 408C, M, Y, and K, and set.

  When the coordinate data in the main scanning direction is x (dots) and the color shift amount in the sub-scanning direction is y (dots), the calculation formulas for each region based on FIG. 3 are as follows. Note that the recording resolution in the embodiment is 600 dpi.

Region 1: y = x * (m1 / L1)
Region 2: y = m1 * 23.622 + (x-L1 * 23.622) * ((m2-m1) / (L2-L1))
Region 3: y = m2 * 23.622 + (x-L2 * 23.622) * ((m3-m2) / (L3-L2)) ... (1)
Here, L1, L2, and L3 are distances (unit: mm) in the main scanning direction from the print start position to the right ends of the areas 1, 2, and 3. m1, m2, and m3 are deviation amounts between the ideal main scanning line 301 and the actual main scanning line 302 at the right end of the region 1, the region 2, and the region 3, respectively.

  The color misregistration correction units 408C, M, Y, and K respectively perform dot misregistration calculation units 407C, M, Y, and K to correct color misregistration due to the inclination and distortion of the main scanning line in the above formula (1). Based on the color misregistration correction amount calculated every time, the output timing of the bitmap data stored in the bitmap memory 406 and the exposure amount for each pixel are adjusted, and the toner image of each color is transferred to the transfer medium. This prevents color misregistration (registration misalignment) when transferred.

  The color misregistration correction units 408C, M, Y, and K are processes corresponding to any one of the halftone processing units 409C, M, Y, and K located in the subsequent stage or the exception processing units 411C, M, Y, and K. And a control signal is generated for the selector 412C, M, Y, K to select the processed result. The selected data is output to the PWM processing units 410C, M, Y, and K, where a known P pulse width modulation signal is generated, and scanning exposure is performed in each exposure unit 51-C, M, Y, and K. Done.

  The color misregistration correction units 408C, M, Y, and K have the same configuration although they have different correction amounts. Therefore, the C component color misregistration correction unit 408C will be described here as well. FIG. 8 is a block diagram of the color misregistration correction unit 408C in the embodiment.

  As illustrated, the color misregistration correction unit 408C according to the embodiment includes a coordinate counter 801, a coordinate conversion unit 802, a line buffer unit 803, an edge pattern storage unit 805, an edge detection unit 806, and a gradation correction unit 807.

  The coordinate counter 801 outputs information necessary for generating coordinates in the main scanning direction and the sub-scanning direction for performing color misregistration correction processing to the coordinate conversion unit 802 based on the equation (1) described above, and Information indicating the degree of deviation in the scanning direction (a value after the decimal point as will be described later) is output to the gradation correction unit 804.

  The coordinate conversion unit 802 performs read access to the bitmap memory 406C using the coordinate position data (X address) in the main scanning direction and the coordinate position data (Y address) in the sub-scanning direction from the coordinate counter 801. . As a result, the read data (here, C component data) is output to the line buffer unit 803.

  As shown in the figure, the line buffer unit 803 includes three line buffers 803a, 803b, and 803c, and the edge detection unit 806 is a 3 × 3 window including pixel data of interest (data obtained by coordinate conversion). Is output.

  The edge detection unit 806 compares the input 3 × 3 window data with the pattern stored in the edge pattern storage unit 805 to determine whether the pixel of interest belongs to an edge portion such as a character line drawing. If it is determined that the pixel is in the edge portion, the pixel of interest Pn (x) (line buffer 803b storing the image data of the nth line) and the main scanning direction of the (n + 1) th line are the same coordinates for gradation correction. The pixel data Pn + 1 (x) (line buffer 803a) at the position is output to the gradation correction unit 807 to perform gradation correction, and then exception processing is performed by the exception processing unit 409C.

  On the other hand, when it is determined that it is not the edge of the character line image, that is, when it is determined that the target pixel belongs to a gradation image such as a photographic image, the gradation correction is skipped and the halftone processing unit Cause halftone processing.

  At this time, a signal indicating whether an edge is detected by the edge detection unit 806, that is, whether there is a matching pattern in the edge pattern storage unit 802 is output to the selector 412C, and one of them is selected.

  The processing of the color misregistration correction unit 408C is as described above. Specific examples of the image counter 801 and the coordinate conversion unit 802 are shown in FIG.

  First, as a premise, the color misregistration correction amount calculation unit 407C is based on the distances L1, L2, and L3 in mm stored in the color misregistration correction amount storage unit 403c, and corresponds to L1, L2, and L3 in the horizontal direction (ideal Pixel positions L1 ′, L2 ′, and L3 ′ in the scanning direction are calculated. In addition, the color misregistration correction amount calculation unit 407c calculates the slope of a straight line connecting the amount of misregistration of each region. Since the inclination referred to here is an inclination in pixel units, it is expressed as Δy.

In the example of FIG. 5, the slope Δy is
Region 1: Δy1 = m1 / L1
Region 2: Δy2 = (m2-m1) / (L2-L1)
Region 3: Δy3 = (m3-m2) / (L3-L2)
It becomes.

  In FIG. 13, the register 82 stores L1 ', L2', and L3 ', and the register 84 stores Δy1, Δy2, and Δy3 (with positive and negative signs) of each region.

  The X address generator 81 is reset when generating correction data for one scan of the laser beam, and generates a horizontal read address for the bitmap memory 406C, that is, an X address by adding the pixel clock clk. To do. As a result, every time the pixel clock clk is input, the X address increases as 0, 1, 2,.

  The comparator 83 compares the value of the X address from the X address generator 83 with the registers L1 ′, L2 ′, and L3 ′, respectively, so that the current X address is any of the areas 1, 2, and 3 in FIG. Judge whether it is within the range and output the result. Since three states are possible, two bits are sufficient for the output signal.

  The selector 85 selects and outputs one of the slopes Δy1, Δy2, Δy3 stored in the register 84 in accordance with the signal from the comparator. In short, if the current X address is within the range of region 1 (X ≦ L1 ′), Δy1 is selected and output. Further, when L1 <X ≦ L2 ′, Δy2 is selectively output, and when L2 ′ <X, Δy1 is selectively output.

  The counter 86 is reset prior to one scan, cumulatively adds the slope Δy output from the counter 86 to the internal register 86a, and holds the value. Since the slope Δy includes a decimal point, this register 86a also has an appropriate number of bits. Further, the counter 86 outputs the integer part of the register 86 held by itself to the Y address generator 87 and outputs a value after the decimal point to the gradation correction unit 807.

  Prior to one scan, the Y address generator 87 sets the reference Y address in the bitmap memory 406C, adds the reference Y address and the integer value from the counter 86, and reads the result to the bitmap memory 406C. Occurs as a Y address.

  As a result, it is possible to generate integer X and Y addresses in the above-described equation (1) and read the C component data at the corresponding position into the line buffer unit 803.

  Here, a more specific example will be described. Assume that the reference Y address is “100”. That is, it is a case where data for performing the 100th scan is generated. It is assumed that the value stored in the register 86a in the counter 86 is “0.1”.

  At this time, ideally, the pixel data in which the Y coordinate of the bitmap memory 406C is at the position of “100.1” may be read. However, since the pixel position of the bitmap memory 406C is represented by an integer, the Y coordinate “ “100.1” does not exist. Changing the way of viewing the coordinates “100.1” is between the addresses “100” and “101”, and 90% of the calculated pixel value (pixel value after gradation correction) is the pixel value of the address “100”. It can be considered that the remaining 10% is affected by the pixel value of the address “101”. That is, the value after gradation correction may be calculated using a weighting coefficient that depends on the value indicated by the decimal point. This is expressed by the following formula.

_{Hx, y = C x, y} * β + C x, y + 1 * α (2)
Here, when the value of the decimal part outputted from the counter 86 is expressed by γ,
β = 1−γ
α = γ
Are in a relationship.

The gradation correction unit 807 in FIG. 8 performs the above processing. The gradation correction unit 807 receives a value γ after the decimal point output from the counter 86. The gradation correction unit 807 also has the same main scanning direction as the C component data P _n (x) of the target pixel output from the line buffer 803b that stores the data of the target line, and the n + 1-th line. The C component data P _{n + 1} (x) data stored in the line buffer 803a is input. Then, the correction coefficients α and β determined by the value γ are obtained, the above-mentioned weighted average value is calculated, and this is output as data H _{x, y} after gradation correction.

When the target pixel is not at the edge of the character / line image, the tone correction is not performed, and the C component data P _n (x) of interest is output to the halftone processing unit 411C as it is.

  The point to be noted here is that in the case of the embodiment, the reference Y address increases by “1” every time it is scanned, but the color misregistration correction amount for the reference Y address, that is, the offset amount is the same. It is a point to become. Therefore, the decimal point from the coordinate counter 801 (counter 86) is the same every time if the coordinates in the main scanning direction are the same. Therefore, no problem occurs even if the line position focused on by the coordinate counter 801 is different from the line position where the pixel of interest subjected to gradation interpolation exists.

  The configuration and operation of the color misregistration correction unit 408C in the embodiment are as described above, and will be described in more detail with reference to FIG.

  In the drawing, reference numeral 60 indicates a color shift curve plotted based on information stored in the color shift amount information storage unit 403C. The slope of region 1 is Δy1, and the slope of region 2 is Δy2.

  Reference numeral 61 indicates the data storage status in the bitmap memory 406C, and reference numeral 62 in the figure indicates the output tone correction data for one scan. Reference numeral 63 denotes an exposure image obtained by exposing the image carrier to image data that has been subjected to color shift correction in pixel units. Further, the positive direction of sub-scanning of the bitmap memory 406C indicates the downward direction with respect to the drawing as indicated by reference numeral 61.

  As shown in the figure, Δy1 is accumulated and added sequentially while the X address is being updated. However, since no carry occurs in the integer digits before the address Xa, the Y address remains on the nth line.

  When address Xa is reached, a carry to an integer digit occurs, indicating that the Y address is updated to point to the (n + 1) th line.

  The integer carry occurs when the X address is Xb, Xc, Xd. The point to be noted here is that the period in which the carry occurs is different between the area 1 and the area 2. The reason is that the slopes in the respective areas are different.

  FIG. 7 is an image diagram for explaining the operation contents for correcting the color misregistration correction less than the pixel unit performed by the gradation correcting unit 804 in the embodiment, that is, correcting the misregistration amount after the decimal point of the color misregistration correction inclination Δy. Correction of the shift amount after the decimal point is performed by adjusting the exposure ratio of dots before and after in the sub-scanning direction.

  FIG. 7A is an image of a main scanning line having a slope rising to the right. FIG. 7B is a bitmap image of a horizontal straight line before gradation correction, and FIG. 7C is a correction image for canceling the color shift due to the inclination of the main scanning line in FIG. . In order to realize the generation of the corrected image in FIG. 7C, the exposure amount adjustment of dots before and after the sub-scanning direction is performed. FIG. 7D is a table showing the relationship between the correction slope Δy for color misregistration and the correction coefficient for performing gradation correction. k is an integer part of the color misregistration correction amount Δy (the fractional part is rounded down) and represents the correction amount in the sub-scanning direction in units of pixels. β and α are correction coefficients for performing correction in the sub-scanning direction less than a pixel unit, and the relationship is as shown in the equation (2) described above. That is, α represents the preceding dot (the distribution rate for the data output from the line buffer 803a in FIG. 8 and β represents the distribution rate of the target dot output from the line buffer 803b.

  FIG. 7D is a bitmap image that has been subjected to tone correction for adjusting the exposure ratio of dots before and after the sub-scanning direction in accordance with the correction coefficient shown in FIG. FIG. 7E shows an exposure image of the bitmap-corrected bitmap image on the image carrier, in which the inclination of the main scanning line is canceled and a horizontal straight line is formed.

  Although the color misregistration correction unit 408C and the gradation correction unit 807 in the embodiment have been described above, the color misregistration correction units 408M, Y, and K of the other color components M, Y, and K are similarly performed, so Thus, the maximum color misregistration can be set to less than one pixel.

  In addition, a target for which the gradation correction unit 807 performs gradation correction is a case where it is determined that the object is in an edge portion of a character line drawing or the like.

  Next, the exception processing units 409C, M, Y, and K and the halftone processing units 411C, M, Y, and K in the embodiment will be described.

  First, consider the case where the input image is processed in the order of halftone processing → color misregistration correction, and the case where the input image is processed in the order of color misalignment correction → halftone processing.

  FIG. 9 shows an example in which the input image is processed in the order of halftone processing → color misregistration correction. Reference numeral 900 shown in the figure is an input image having a constant density of 50%. When this image is subjected to halftone processing using a 4 × 4 halftone pattern, an image 901 is obtained. If the image 901 is an image to be obtained and an image equivalent to this image can be obtained even after performing color misregistration correction, it can be said that color misregistration correction can be realized without image deterioration. Here, the image 902 is obtained when the half-tone processed image 901 is subjected to 1/2 pixel color shift correction in the upward direction (vertical direction). As can be seen from the figure, by performing color misregistration correction on the image after the halftone processing, the halftone dot halftone dot reproducibility deterioration due to the halftone processing occurs.

  On the other hand, FIG. 10 shows an example when the input image is processed in the order of color misregistration correction → halftone processing. The reference numeral 100 shown in the figure is an input image, which is an image having a constant density (50%), similar to the image 900 described above. The image 101 is obtained when the input image 100 is subjected to the ½ pixel color shift correction in the upward direction (vertical direction). By performing color misregistration correction, an image having a density of 25% is generated in the upper and lower one line portions. An image 102 is a result of performing the halftone process on the image after the color misregistration correction. In the image 102, an image having a density of 25% is generated for one line at the upper end and the lower end. Therefore, the upper and lower lines are different from the image 100, but the other parts are similar to the image 901. The halftone dots of the halftone image as seen in the image 920 are not observed.

  That is, in the case of an image having no edge, such as the image 900 and the image 100, it can be said that image degradation can be suppressed by performing halftone processing on an image subjected to color misregistration correction.

  On the other hand, in the case of an image edge portion that changes sharply with respect to surrounding density, such as characters and line drawings, the edge portion is formed according to the halftone pattern by halftone processing as shown in FIG. Since the image generated by exposure is invalidated, gaps and discontinuities occur in the edge portion as indicated by reference numeral 1100 in FIG. As a result, jaggies occur at image edge portions such as character line drawings.

  In order to prevent this, exception processing is performed on the image edge portion of a character line drawing or the like after the color misregistration correction.

  The exception processing unit 409C (the same applies to M, Y, and K) performs exception processing different from normal halftone processing on an image detected as an edge by the edge detection unit 806.

There are the following three exception processing.
-Do not perform halftone processing (through). In this case, by not performing the halftone process on the image detected as an edge by the edge detection unit 806, gaps and discontinuities due to the halftone process occurring at the edge part can be prevented.
・ Halftone processing is performed using the halftone pattern for the edge portion. As shown in FIG. 11, when a normal halftone pattern is used in the edge portion, gaps and discontinuities occur depending on the growth direction of the halftone pattern. By using a pattern, it is possible to prevent gaps and discontinuities that occur when a normal halftone pattern is used.
-Perform processing such as compensating for dots after normal halftone processing. After normal halftone processing, the gaps and discontinuities caused by normal halftone processing are compensated by supplementing dots in the gaps and discontinuities to compensate for the gaps and discontinuities generated by halftone processing. It is possible.

  On the other hand, normal halftone processing is performed on the non-edge image by the halftone processing means 411C (M, Y, K).

  A series of processing flows may be as shown in FIG.

  First, in step S121, coordinate conversion is performed using the coordinate conversion unit 802 to correct color misregistration of one line or more.

  In step S122, the converted data obtained by the coordinate conversion unit 802 is stored in the line buffer unit 803.

  In step S123, the edge detection unit 806 detects an edge portion such as a character line drawing. If it is an edge, the process proceeds to step S124, and if it is a non-edge, the process proceeds to step S125.

  In step S124, gradation correction is performed by the gradation correction unit 807 on the edge portion image, and color misregistration correction of less than one pixel is performed. Then, the exception process of step S126 is performed. That is, exception processing such as halftone processing using a halftone pattern different from the normal one and adding dots to discontinuous portions and gaps generated by the halftone processing are performed.

  On the other hand, if it is determined that it is a non-edge, halftone processing is performed in step S125.

  Based on the image data obtained from one of the above exception processing unit 409C and halftone processing unit 411C, the pulse width is modulated and converted into a binary laser drive signal, and then supplied to the exposure unit for exposure. It is exposed from the unit. Then, the same processing as this processing is performed on the other color components M, Y, and K in the same manner.

  In the above embodiment, the configuration of the color misregistration correction unit 408C (the same applies to other color components) has been described with reference to FIGS. 8 and 11 as an example. In the case of the configuration of FIG. 11, the amount of offset (shift) of the Y address is obtained by sequentially adding Δy to the register 86a in the counter 86, but the calculation accuracy (number of bits) after the decimal point of the register 86a is high. It is convenient. In other words, if the number of bits in the register 86a is small, a rounding error gradually occurs when Δy is cumulatively added, and the value of the register may deviate from the locus of the slopes Δy1 and Δy2 in FIG.

  Therefore, every time the X address for reading from the bitmap memory 406C is updated, the calculation may be performed in accordance with the equation (1). Since rounding error due to cumulative addition does not occur, it becomes possible to read out pixel data at a position indicated by a correct locus.

  Further, the configuration according to FIG. 4 may be realized by software (firmware). In this case, it is only necessary to realize a process in which image data flows according to the drawing, and those skilled in the art will be able to easily implement it from the description of the above embodiment.

It is a figure which shows the density nonuniformity in a prior art example. 1 is a cross-sectional structure diagram of an image forming apparatus in an embodiment. It is a figure explaining the shift | offset | difference of the main scanning line scanned by the photosensitive drum in embodiment. 2 is a block configuration diagram of a controller and an engine in the image forming apparatus of the present embodiment. FIG. It is a figure which shows the example of the information memorize | stored in the color shift amount memory | storage part. It is a figure for demonstrating the operation | movement which correct | amends the deviation | shift amount of the integer part of the color deviation correction amount in a coordinate conversion part. It is a figure which shows the operation | movement which the gradation correction | amendment part in embodiment corrects color misregistration of less than a pixel unit. It is a block diagram of a color misregistration correction unit in the embodiment. It is a figure which shows the example of the image in each process in the case of performing color misregistration correction after a halftone process. It is a figure which shows the example of the image in each process in the case of performing a halftone process after a color shift correction process. It is a figure for demonstrating the reason for not performing a normal halftone process in the edge part of the character line drawing in embodiment. It is a flowchart which shows the switching process based on the image edge determination result in embodiment. FIG. 9 is a specific block configuration diagram of a coordinate counter 801 and a coordinate conversion unit 802 in FIG. 8.

Claims (6)

An image forming unit having an image bearing member, an exposure unit that scans and exposes the image bearing member, and a developing unit that visualizes the electrostatic latent image generated by the exposure with a recording material, along the conveyance direction of the recording medium. Image forming apparatuses arranged side by side,
Image data storage means for storing image data to be formed in each image forming unit;
Exposure deviation amount storage means for storing deviation amount information indicating the deviation amount with respect to the exposure scanning direction on the image carrier of each image forming unit;
Coordinate conversion means for converting the coordinates of the read address of the image data storage means based on the exposure deviation amount information stored in the exposure deviation amount storage means, and reading out the image data according to the converted address information;
Buffer means for accumulating a plurality of lines of pixel data read by the coordinate conversion means;
Determination means for determining whether or not the target pixel data is at the image edge based on the target pixel data stored in the buffer means and the surrounding pixel data group;
A first processing unit that performs halftone processing for the non-image edge on the target pixel data when the determination unit determines that the pixel is on the non-image edge;
A correction unit that corrects the gradation of the pixel-of-interest data stored in the buffer unit based on address information when the coordinate conversion unit performs conversion when the determination unit determines that the image edge exists. ,
Based on the determination result of the second processing means for performing edge processing different from the first processing means on the pixel data obtained by the correction means and the determination result of the determination means, An image forming apparatus comprising: output means for outputting pixel data obtained by the processing means as an exposure control signal of an exposure unit of a corresponding image forming unit. The exposure deviation amount storage means includes
Information on a plurality of positions in the main scanning direction, which is an ideal scanning exposure direction of the image carrier, and information on a distance between the ideal scanning exposure and the actual exposure at each position are stored. The image forming apparatus according to claim 1. The coordinate conversion means includes
Calculation means for calculating inclination information of exposure deviation in each region indicated by each position information based on the position information stored in the exposure deviation amount storage means and the distance information;
X address generating means for generating an X address in the main scanning direction which is the exposure direction from the previous image data storage means;
Determination means for determining in which region the X address is calculated by the calculation means each time the X address is updated by the X address generation means;
Selection means for selecting relevant inclination information based on the result determined by the determination means;
Adding means for accumulatively adding the inclination information selected by the selecting means;
Y address generating means for generating a Y address using the integer part of the result of addition by the adding means as an offset value of the Y address,
The correction means generates pixel data after gradation correction based on data of two pixels continuous in the sub-scanning direction based on a value after the decimal point cumulatively added by the addition means. The image forming apparatus according to 2. An image forming unit having an image bearing member, an exposure unit that scans and exposes the image bearing member, and a developing unit that visualizes the electrostatic latent image generated by the exposure with a recording material, along the conveyance direction of the recording medium. A method for controlling image forming apparatuses arranged side by side,
Storing image data to be formed in each image forming unit in a predetermined image data storage unit;
A step of reading from a predetermined exposure deviation amount storage means storing deviation amount information indicating the deviation amount with respect to the exposure scanning direction on the image carrier of each image forming unit;
A coordinate conversion step of converting the coordinates of the read address of the image data storage means based on the read exposure deviation amount information, and reading the image data according to the converted address information;
Storing pixel data read out in the coordinate conversion step in a buffer means for accumulating a predetermined number of lines;
A determination step of determining whether the target pixel data is at the image edge based on the target pixel data stored in the buffer means and the surrounding pixel data group;
A first processing step of performing halftone processing for the non-image edge on the pixel-of-interest data when it is determined by the determination step that the image is at the non-image edge;
A correction step of correcting the gradation of the pixel-of-interest data stored in the buffer unit based on address information when converted by the coordinate conversion unit when the determination unit determines that the image edge exists. ,
Based on the second processing step for performing edge processing different from the first processing means on the pixel data obtained in the correction step and the determination result of the determination step, An output step of outputting pixel data obtained in the processing step as an exposure control signal of an exposure unit of the corresponding image forming unit. In the exposure deviation amount storage means,
Information on a plurality of positions in the main scanning direction, which is the ideal scanning exposure direction of the image carrier, and information on the distance between the ideal scanning exposure and the actual exposure at each position are stored. The method of controlling an image forming apparatus according to claim 4. The coordinate conversion step includes
A calculation step of calculating inclination information of exposure deviation in each region indicated by each position information based on the position information stored in the exposure deviation amount storage means and the distance information;
An X address generation step of generating an X address in the main scanning direction which is the exposure direction from the previous image data storage means;
A step of determining which region the X address is calculated in the calculation step every time the X address is updated by the X address generation step;
A selection step of selecting corresponding inclination information based on the determined result;
An addition step of cumulatively adding the slope information selected in the selection step;
A Y address generation step of generating a Y address using the integer part of the result of addition in the addition step as an offset value of the Y address,
The correction step generates pixel data after gradation correction based on data of two pixels continuous in the sub-scanning direction based on a value after the decimal point cumulatively added by the adding means. 6. A method for controlling an image forming apparatus according to 5.

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