JP2007322682A - Color image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive color image forming apparatus which correctly corrects a tilt, distortion, and a writing-out position in a sub-scanning direction without using an expensive engineering component and not through a precise adjusting stage, and excellently put images one over the other. <P>SOLUTION: Exception processing means 2 411C, 411M, 411Y, and 411K perform exception processing 1 different from normal half-tone processing for an image detected as an edge by an edge detecting means. For an image bit map after the exception processing 1, exception processing 2 for determining a pulse growing direction while overrides a pulse growing direction determined based upon a reference position in a half-tone pattern is performed. In the exception processing 2, computation etc., based upon pulse width values and pulse growing directions of peripheral pixels all allowing center growth is carried out. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a color image forming apparatus that forms a color image by sequentially superimposing and transferring a color image obtained by juxtaposing a plurality of image carriers onto a transported storage medium.

  Conventionally, as a color image forming apparatus using an electrophotographic system, a single photoconductor is developed with each color using a plurality of developing devices, and the exposure-development-transfer process is repeated a plurality of times. A color image is superimposed on the transfer paper. A method of obtaining a full-color image by fixing this is generally used.

  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 there is a drawback that it takes time.

  As a method for compensating for this defect, there is a method of using a plurality of photoconductors and sequentially superimposing the visualized images obtained for the respective colors on the transfer paper to obtain a full color print by one paper passing. According to this method, the throughput can be significantly shortened. 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. Due to the problem of displacement, it was difficult to obtain a high-quality full-color image.

  As a method for preventing this color misregistration, for example, a method of forming a test toner image on a transfer paper or a conveying belt forming a part of a transfer unit and detecting the test toner image can be considered. Based on this result, the optical path of each optical system is corrected, and the image writing position of each color is corrected (for example, see Patent Document 1).

Japanese Patent Application Laid-Open No. 64-40956 JP-A-8-85237

However, this method has the following problems.
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. . However, this requires a highly accurate movable member, which leads to an increase in cost. Furthermore, it takes time to complete the correction. Therefore, although it is impossible to correct frequently, the deviation of the optical path length may change with time due to the temperature rise of the machine. In such a case, the color of the optical system is corrected by correcting the optical path. It is difficult to prevent the deviation.

  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. There are problems such as not.

  In the configuration disclosed in Patent Document 2, the output coordinate position of the image data for each color is automatically converted to an output coordinate position in which registration deviation is corrected. Then, based on the converted image data of each color, the correcting means corrects the position of the modulated light beam by an amount smaller than the minimum dot unit of the color signal.

  However, by correcting the output coordinate position of the image data for each color with respect to the image that has been subjected to the intermediate gradation processing, the reproducibility of the halftone dots of the intermediate gradation image is deteriorated, resulting in color unevenness and moire. There is a problem that it 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 that has undergone a certain color misregistration correction on the input image 101 is actually printed, the image after the color misregistration correction is printed regardless of whether the input image 101 is an image having a constant density value. Then, an image whose density value is not constant is printed. This is because the relationship between the image density value and the toner density with respect to the image density value is not linear. When such a non-uniform density value is periodically repeated, moire becomes obvious, and a good color image cannot be obtained.

  Therefore, the object of the present invention is to correct the writing position in the tilt, distortion, and sub-scanning direction without using expensive engineering parts or through a precise adjustment process, and has excellent image overlay. An object is to provide an inexpensive color image forming apparatus.

  The color image forming apparatus of the present invention includes a photosensitive member, an exposure unit that irradiates the photosensitive member with a light beam modulated by each color signal to form an electrostatic latent image, and the exposure unit that is formed on the photosensitive member. A plurality of image stations each having a developing unit that visualizes the electrostatic latent image formed, and a transfer unit that transfers each color image visualized by the developing unit to a transfer paper; In a color image forming apparatus for forming a color image by sequentially transferring the color image formed by the transfer material conveyed by the conveying means, the color deviation amount storage means for storing the color deviation amount for each image station; Color misregistration correction amount calculating means for calculating a color misregistration correction amount based on the color misregistration amount obtained from the color misregistration amount storage means, and color misregistration in pixel units based on the calculation result of the color misregistration correction amount calculating means Seat to be corrected A conversion unit; an edge detection unit that detects an edge portion of the image; a gradation correction unit that corrects color misregistration of less than a pixel unit with respect to the detected edge portion; The means for performing exception processing 1 that is not tone processing and the pulse growth direction added to the image data after the exception processing 1 are determined ignoring the pulse growth direction determined based on the reference position in the halftone pattern. Means for performing exception processing 2, means for performing halftone processing on the non-edge portion, and pulse for determining a pulse growth direction to be added to the image data after the halftone processing based on a reference position in the halftone pattern A growth direction control means is provided in each image station, and each light beam corrected by each correction means is applied to each photoconductor by each exposure means in each image station. And characterized in that the respective exposure.

As described above, according to the present invention, in the tandem type color image forming apparatus having a plurality of image forming units, the inclination and the curvature of the scanning line which is held in the color misregistration amount storage unit and scans the image carrier itself. The color misregistration correction amount calculation means calculates the color misregistration correction amount from the color misregistration amount due to the distortion. Based on the color misregistration correction amount, the color misregistration correction unit performs color misregistration correction in pixel units, detects the edge of the image after the misregistration correction, and the detected edge portion has a color less than the pixel unit. The image bitmap is reconstructed by performing shift correction. Accordingly, it is possible to prevent color misregistration due to the inclination and curvature of the scanning line for exposing the image carrier. Further, exception processing 1 that is not normal halftone processing is performed on the image bitmap that has been subjected to color misregistration correction, and halftone processing is performed on non-edge portions without performing color misregistration correction less than a pixel unit. Do. As a result, moire that may occur due to color misregistration correction can be eliminated, and jaggy that can occur in the image edge portion can be prevented. Further, with respect to the image bitmap after the exception process 1, the pulse growth direction is determined by the exception process 2 while ignoring the pulse growth direction originally determined based on the reference position in the halftone pattern. Then, the pulse growth direction is determined based on the reference position in the halftone pattern for the image bitmap after the halftone processing. As a result, it is possible to obtain a good color image by matching the difference between the pixel not subjected to the halftone process and the pulse growth direction.
Therefore, an inexpensive color image forming apparatus that corrects the writing position in the tilt, distortion, and sub-scanning direction correctly without using expensive engineering parts or passing through a precise adjustment process, and has excellent image overlay. Can be provided.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments to which the invention is applied will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a schematic cross-sectional view illustrating the configuration of a color image forming apparatus according to an embodiment of the present invention, and corresponds to, for example, a four-drum type color laser beam printer. In this color image forming apparatus, a transfer material cassette 53 is mounted at the lower right side of the main body apparatus. The transfer material 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 the pair of transport rollers 55-a and 55-b. In the image forming unit, a transfer conveyance belt 10 that conveys a transfer material is stretched flat in a transfer material conveyance direction (from the right to the left in FIG. 1) by a plurality of rotating rollers. It is electrostatically attracted to the conveyor belt 10. In addition, four photosensitive drums 14 as image bearing members in the form of drums are arranged in a straight line so as to face the belt conveying surface to constitute an image forming unit.

  The developing unit 52 serving as an image forming unit includes the photosensitive drum 14, C (CYAN), Y (YELLOW), M (MAGENTA), and K (BLACK) toners, a charger, and a developer. Yes. A predetermined gap is provided between the charger and the developer in the housing of each developing unit 52 described above, and the peripheral surface of the photosensitive drum 14 is exposed to a predetermined charge from the exposure means 51 formed of a laser scanner through the gap. Charge uniformly. The exposure means 51 exposes the peripheral surface of the charged photosensitive drum 14 in accordance with image information to form an electrostatic latent image, and the developing device transfers the toner to the blackout position of the electrostatic latent image. Toner image (development).

  A transfer member 57 is disposed across the conveyance surface of the transfer conveyance belt 10. The toner images formed (developed) on the peripheral surfaces of the respective photosensitive drums 14 are absorbed and transferred by the charge generated in the transferred transfer material by the transfer electric field formed by the transfer member 57 corresponding thereto. It is transferred to the material surface. The transfer material to which the toner image has been transferred is discharged out of the apparatus by a pair of discharge rollers 59-a and 59-b. The transfer / conveying belt 10 may be an intermediate transfer belt having a configuration in which toner of each color of C (CYAN), Y (YELLOW), M (MAGENTA), and K (BLACK) is temporarily transferred and then secondarily transferred to a transfer material. Absent.

  FIG. 2 is an image diagram for explaining a shift of the main scanning line scanned on the photosensitive drum 14 which is an image carrier.

  301 is an image of an ideal main scanning line, and scanning is performed perpendicular to the rotation direction of the photosensitive drum 14. Reference numeral 302 denotes an image of an actual main scanning line in which the positional accuracy and the diameter of the photosensitive drum 14 are shifted, and the inclination and the rightward rising due to the positional accuracy of the optical system in the exposure unit 51 of each color are generated. is there. When such an inclination or curvature of the main scanning line exists in an image station of any color, color misregistration occurs when a plurality of color toner images are collectively transferred to a 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 between 301 and the actual main scanning line 302 in the sub-scanning direction is measured. Divide into multiple regions (region 1 between Pa-Pb, region 2 between Pb-Pc and region 3 between Pc-Pd) for each point where the amount of deviation is measured, and connect the points. The inclination of the main scanning line in each region is approximated by a straight line (Lab, Lbc, Lcd). Accordingly, when the difference in the amount of deviation between points (m1 in the region 1, m2-m1 in the region 2, m3-m2 in the region 3) is a positive value, the main scanning line in the corresponding region has an upward slope. In the case of a negative value, it indicates that it has a downward slope.

  FIG. 3 is a block diagram for explaining the operation of the color misregistration correction process for correcting the color misregistration caused by the inclination and curvature of the scanning line performed in this embodiment.

  Reference numeral 401 denotes a printer engine, which actually performs print processing based on image bitmap information generated by the controller 402.

  403C, 403M, 403Y, and 403K are color misregistration amount storage means for each color, and store the above-described main scan line misregistration amount for each region for each color. In the present embodiment, the amount of deviation in the sub-scanning direction between the actual main scanning line 302 and the ideal main scanning line 301 measured at a plurality of points described in FIG. 2 indicates the inclination and curvature of the main scanning line. Information is stored in the color misregistration amount storage means 403 as information.

  FIG. 4 is a diagram illustrating an example of information stored in the color misregistration amount storage unit 403. In this embodiment, the color misregistration amount storage means stores the deviation amount between the ideal main scanning line 301 and the actual main scanning line 302. However, the actual main scanning line inclination and curvature are stored. However, the present invention is not limited to this as long as the characteristics of the information can be identified. The information stored in the color misregistration amount storage unit 403 may be configured to measure the misregistration amount in the manufacturing process of the apparatus and store it in advance as information unique to the apparatus. Alternatively, a detection mechanism for detecting the shift amount is prepared in the apparatus itself, a predetermined pattern for measuring the shift is formed for each color image carrier, and the shift amount detected by the detection mechanism is stored. Such a configuration may be used.

  Next, a description will be given of an operation in which the controller 402 performs the printing process by correcting the image data so as to cancel out the main scanning line shift amount stored in the color shift amount storage unit 403.

  The image generation unit 404 generates raster image data that can be printed from print data received from a computer device (not shown), and outputs it as RGB data for each dot. Reference numeral 405 denotes color conversion means for converting the RGB data into CMYK space data that can be processed by the printer engine 401 and storing the data in the bitmap memory 406 described later for each color. The bitmap memory 406 temporarily stores raster image data to be printed, and is a page memory for storing image data for one page or a band memory for storing data for a plurality of lines.

  Reference numerals 407C, 407M, 407Y, and 407K denote color misregistration correction amount calculation means. Based on the information on the amount of misalignment of the main scanning line accumulated in the color misregistration amount storage unit 403, for each dot, the sub-scanning direction corresponding to the coordinate information in the main scanning direction designated from the color misregistration correction unit 408 described later is provided. A color misregistration correction amount is calculated. The calculated color misregistration correction amount is output to the color misregistration correction means 408, respectively.

The coordinate data in the main scanning direction is x (dots), and the amount of color misregistration correction in the sub-scanning direction is Δy (dots)
In this case, the calculation formula of each area based on FIG. 2 is shown below (printing density is 600 dpi).
Region 1: Δy1 = x * (m1 / L1)
Region 2: Δy2 = m1 / 23.622 + (x − (L1 / 23.622)) * ((m2 − m1) / (L2 − L1
))
Region 3: Δy3 = m2 / 23.622 + (x − (L2 / 23.622)) * ((m3 − m2) / (L3 − L2
))
L1, L2, and L3 are distances (unit: mm) in the main scanning direction from the print start position to the left ends of the regions 1, 2, and 3. m 1, m 2, and m 3 are deviation amounts between the ideal main scanning line 301 and the actual main scanning line 302 at the left ends of the regions 1, 2, and 3.

  Reference numerals 408C, 408M, 408Y, and 408K are color misregistration correction units that correct color misregistration due to inclination and distortion of the main scanning line, and perform color misregistration (registration misregistration) when the toner image of each color is transferred to a transfer medium. It is something to prevent. Therefore, based on the color misregistration correction amount calculated for each dot by each color misregistration correction amount calculation means 407, the adjustment of the output timing of the bitmap data stored in the bitmap memory 406 and the adjustment of the exposure amount for each pixel are performed. I do.

Next, the color misregistration correction unit 408 will be described together with the configuration diagram of the color misregistration correction unit shown in FIG. The color misregistration correction unit includes a coordinate counter 801, a coordinate conversion unit 802, a line buffer 803, an edge detection unit 806, and a gradation correction unit 807. The coordinate counter 801 outputs the coordinate position data in the main scanning direction and the sub-scanning direction for performing color misregistration correction processing to the coordinate conversion means 802, and simultaneously outputs the coordinate position data in the main scanning direction to the color misregistration correction amount calculation means 407. , Output to the line buffer 803 and the tone correction means 807. The coordinate conversion means 802 corrects the integer part of the correction amount Δy based on the coordinate position data in the main scanning direction and the sub-scanning direction from the coordinate counter 801 and the correction amount Δy obtained from the color misregistration correction amount calculation means 407. That is, reconstruction processing is performed in the sub-scanning direction in units of pixels.

  The edge detection unit 806 performs edge detection of an image to be detected by comparing the n × m window data 804 obtained from the line buffer 803 with the edge pattern stored in the edge pattern storage unit 805. As an edge detection result, whether it is edge part image data or non-edge part image data is shown in the subsequent stage based on attribute data.

The gradation correction unit 807 corrects Δy below the decimal point of Δy based on the coordinate position data in the main scanning direction from the coordinate counter 801 and the correction amount Δy for the image detected by the edge detection unit 806 as an edge. Process. That is, the correction processing after the decimal point of Δy is to perform correction by adjusting the exposure ratio of dots before and after the sub-scanning direction in less than a pixel unit. The tone correction unit 807 uses a line buffer 803 for referring to dots before and after in the sub-scanning direction. For an image detected by the edge detection unit 806 as not an edge, it is determined that correction is not necessary for color misregistration less than a pixel unit, and gradation correction by the gradation correction unit is not performed, thus simplifying the process. Turn into.

FIG. 5 is an image diagram for explaining the operation content in which the coordinate conversion means 802 corrects the shift amount of the integer part of the color shift correction amount Δy. The coordinate conversion means 802 is stored in the bitmap memory 406 in accordance with the value of the integer part of the color misregistration correction amount Δy obtained from the color misregistration information of the main scanning line approximated by a straight line as shown in FIG. The coordinates in the sub-scanning direction (Y direction) of the image data are offset.

For example, as shown in FIG. 5B, when the coordinate position in the sub-scanning direction from the coordinate counter 801 is n, if the coordinate position in the main scanning direction is X, (1) In this region, the color misregistration correction amount Δy is 0 or more and less than 1. Therefore, when reconstructing the nth line data, the nth line data is read from the bitmap memory.

In the area (2), when the color misregistration correction amount Δy is 1 or more and less than 2 and the data of the nth line is reconstructed, the image bit map at the position where the number of one sub-scanning line is offset, that is, from the bitmap memory. A coordinate conversion process for reading data on the (n + 1) th line is performed.

  Similarly, coordinate conversion processing is performed in order to read data of the (n + 2) th line in the area (3) and data in the (n + 3) th line in the area (4). With the above method, reconstruction processing in the sub-scanning direction is performed in units of pixels. FIG. 5C shows an exposure image obtained by exposing the image carrier to the image data that has been subjected to color shift correction in pixel units by the coordinate conversion unit 802.

FIG. 6 shows color misregistration correction less than a pixel unit performed by the gradation correcting unit 807, that is, a color misregistration correction amount Δ.
It is an image figure for demonstrating the operation | movement content which correct | amends the deviation | shift amount after the decimal point of y. 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. 6A is an image of a main scanning line having a slope rising to the right. 6B is a bitmap image of a horizontal straight line before coordinate conversion, FIG. 6C is a bitmap image before gradation correction, and FIG. 6D is an inclination of the main scanning line in FIG. 5B is a correction image of (b) for canceling the color misregistration due to. In order to realize the correction image of FIG. 6D, the exposure amount adjustment of dots before and after in the sub-scanning direction is performed. FIG. 6E is a table showing the relationship between the color misregistration correction amount Δy 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. α and β are correction coefficients for correcting in the sub-scanning direction less than a pixel unit. From the information below the decimal point of the color misregistration correction amount Δy,
Represents the distribution rate of the exposure amount of dots before and after in the sub-scanning direction,
α = Δy−k
β = 1−α
Is calculated by α represents the distribution rate of preceding dots, and β represents the distribution rate of subsequent dots. FIG. 6F 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 of FIG. FIG. 6G shows an exposure image of an image carrier of a bitmap image whose gradation has been corrected. The inclination of the main scanning line is canceled out and a horizontal straight line is formed.

  FIG. 7 is a block diagram for explaining a method of creating a correction bitmap by the gradation correction process.

  The coordinate conversion unit 802 transfers the image bitmap data reconfigured so as to correct the color shift in units of pixels from the bitmap memory 406 to the line buffer 803.

The gradation correction unit 807 uses the line buffer 803 to refer to pixel values before and after in the sub-scanning direction in order to generate correction data. The gradation correction unit 807 sets correction data when the coordinate in the main scanning direction is x (dot), the pixel data at the current position is Pn (x), and the pixel data one line before the same x is Pn-1 (x). The following arithmetic processing is performed to generate
P′n (x) = Pn (x) * β (x) + Pn−1 (x) * α (x)

  By the above calculation, an image bitmap in which color misregistration less than a pixel unit in the sub-scanning direction is corrected is output.

  Next, exception processing means 1 409C to 409K, HT processing means 410C to 410K, exception processing means 2 411C to 411K, and pulse growth direction control means 412C to 412K will be described.

  Here, an example in which processing is performed in the order of halftone processing → color misregistration correction on the input image and in a case where processing is performed in order of color misregistration correction → halftone processing on the input image is shown. FIG. 8 shows an example when the input image is processed in the order of halftone processing → color misregistration correction. FIG. 8A shows an input image having a constant density of 50%. In contrast to FIG. 8A, when halftone processing is performed using a 4 × 4 halftone pattern, the image of FIG. 8B is obtained.

  If this image is a desired image 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, FIG. 8C shows an image obtained when ½ pixel color shift correction is performed in the upward direction (vertical direction) on the image after the halftone process. As can be seen from the figure, by performing color misregistration correction on the image after the halftone process, the halftone dot halftone reproducibility deterioration due to the halftone process occurs.

  On the other hand, FIG. 9 shows an example in which processing is performed in the order of color misregistration correction → halftone processing on the input image. FIG. 9A shows an input image, which is an image having a constant density (50%) as in FIG. 8A described above. FIG. 9B shows an image obtained when the half-pixel color misregistration correction is performed on the input image 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. FIG. 9C shows a result of performing the halftone process on the image after the color misregistration correction. In FIG. 9B, since an image having a density of 25% is generated for one line in the upper and lower directions, the image in the upper and lower lines is different from that in FIG. However, with respect to the other portions, an image similar to that shown in FIG. 8B is obtained, and the halftone dot of the halftone image as shown in FIG. 8C is not deteriorated.

  However, when attention is paid to the edge portion of the image, as shown in FIG. 10, since the edge portion is formed in accordance with the halftone pattern by the halftone process, the gradation correction is invalidated, and a gap is formed in the edge portion. Or discontinuity. As a result, an image with jaggy is formed. In order to prevent this, it is necessary to perform exception processing 1 on the image after the color misregistration correction for the edge portion. Therefore, the exception processing means 1 409C, 409M, 409Y, and 409K perform exception processing 1 different from normal halftone processing on the image detected as an edge by the edge detection means.

  As exception processing 1, halftone processing is not performed, halftone processing is performed using a halftone pattern for an edge portion, and processing such as supplementing dots after normal halftone processing is performed. On the other hand, normal halftone processing is performed by the halftone processing means 410C, 410M, 410Y, and 410K for an image detected as not being an edge.

  However, in this processing flow, pixels that perform halftone processing and pixels that perform exception processing 1 are mixed in the same image. Consider a case where a pulse width value and a pulse growth direction are determined based on a 4 × 4 halftone pattern as shown in FIG. 11 and a reference position number in the halftone pattern. In this case, if the pulse width value is determined by the exception process 1 which is not a normal halftone process, and the pulse growth direction is determined based on the reference position number in the halftone pattern, the wrinkles will not match. The following processing procedure is assumed here.

(1) Based on the reference number, a gradation value of 256 gradations and 8 bits is converted into a pulse width value of 16 gradations and 4 bits according to a halftone table as shown in FIG. 12 (halftone processing).
(2) A pulse growth direction is added according to a pulse growth direction control table as shown in FIG. 13 (pulse growth direction control).

  The above-described pulse growth direction control table designates a pulse growth direction for each reference position number in the halftone pattern, assuming a pixel subjected to halftone processing.

  Therefore, the exception processing means 2 411C, 411M, 411Y, and 411K perform exception processing 1 different from normal halftone processing on the image detected as an edge by the edge detection means. An exception process 2 is performed on the image bitmap after the exception process 1 to ignore the pulse growth direction determined based on the reference position in the halftone pattern and determine the pulse growth direction.

  As the exception process 2, processes such as making all the centers grow and calculating based on the pulse width value and the pulse growth direction of the peripheral pixels are performed.

  On the other hand, normal halftone processing is performed by the halftone processing means 410C, 410M, 410Y, and 410K for an image detected as not being an edge. For the image bitmap after the halftone processing, the pulse growth direction control means 412C, 412M, 412Y and 412K determine the pulse growth direction based on the reference position in the halftone pattern.

A series of processing flows is shown in FIG. Hereinafter, each step will be described.
Step S121: Coordinate conversion is performed using the coordinate conversion means to correct for color misregistration of one line or more. Next, the process proceeds to step S122.
Step S122: The data whose coordinates have been converted in step S121 are stored in the live buffer. Next, the process proceeds to step S123.
Step S123: The edge detection means detects an edge portion in the image data. If it is an edge, the process proceeds to step S124. If it is not an edge, the process proceeds to step S125.
Step S124: Tone correction is performed on the image of the edge portion by the tone correcting means, and color misregistration correction for less than a pixel is performed. Next, the process proceeds to step S126.
Step S125: Halftone processing is performed on an image that is not an edge, and the pulse growth direction is determined based on the reference position in the halftone pattern.
Step S126: Halftone processing is not performed on the image of the edge portion. Halftone processing is performed with a halftone pattern different from the normal. Dots are added to discontinuous portions and gaps generated by the halftone processing. Exception processing 1 is performed. The pulse growth direction is determined by performing exceptional processing 2 such as making all the growth center and calculating based on the pulse width and the pulse growth direction of the peripheral pixels.

  Image data is obtained from exception processing means 1 409C to 409K, HT processing means 410C to 410K, exception processing means 2 411C to 411K and pulse growth direction control means 412C to 412K. Based on this, pulse width modulation is performed in PWM 413C, 413M, 413Y, and 413K and converted into a binary laser drive signal, which is then supplied to the exposure unit and exposed from the exposure unit.

  As described above, if the correction amount for correcting the shift amount in the sub-scanning direction at each main scanning position is calculated from the image bitmap and reconstructed as a corrected image bitmap, the main scanning line can be obtained. It is possible to create an image in which color misregistration due to inclination and distortion of the image is corrected.

  Another object of the present invention is to supply a storage medium storing software program codes for realizing the functions of the above-described embodiments to a system or apparatus, and a computer such as the system reads and executes the program codes from the storage medium. Is also achieved.

  In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiments, and the program code itself and the storage medium storing the program code constitute the present invention.

  As a storage medium for supplying the program code, for example, a flexible disk, a hard disk, an optical disk, a magneto-optical disk, a CD-ROM, a CD-R, a magnetic tape, a nonvolatile memory card, a ROM, or the like can be used.

  In addition, the case where the functions of the above-described embodiment are realized by performing part or all of the actual processing by an OS or the like running on the computer based on the instruction of the program code read by the computer. It is.

  Further, after the program code read from the storage medium is written in a memory provided in a function expansion unit connected to the computer, the CPU or the like performs actual processing based on the instruction of the program code, and the above-described processing is performed. The case where the functions of the embodiment are realized is also included.

1 is a block diagram illustrating a configuration of a color image forming apparatus according to an embodiment of the present invention. It is a figure for demonstrating the shift | offset | difference of the main scanning line scanned to a photosensitive drum. It is a block diagram which shows the structure of this embodiment. It is a figure which shows the example of information memorize | stored in a color shift amount memory | storage means. It is a figure for demonstrating the operation | movement which a coordinate conversion means correct | amends the color shift of a pixel unit. It is a figure for demonstrating the operation | movement which a gradation correction | amendment means corrects color misregistration of less than a pixel unit. It is a block diagram which shows the structure of a color shift correction | amendment means. It is a figure which shows the image in each process of color misregistration correction and a halftone process. It is a figure which shows the image in each process of color misregistration correction and a halftone process. It is a figure which shows the image at the time of performing a halftone process to an edge image. It is a figure which shows the reference position number in a 4x4 halftone pattern and a halftone pattern. It is a figure which shows the pulse width value determination table (halftone table) for converting the input gradation value of 256 gradation 8 bits into the pulse width value of 16 gradation 4 bits. It is a figure which shows the pulse growth direction control table for designating the pulse growth direction for every reference position number in a halftone pattern with respect to the pixel by which the halftone process was carried out. It is a flowchart which shows the processing process from an edge detection part. It is a figure which shows the density nonuniformity in a prior art example.

Explanation of symbols

401 Printer Engine 402 Printer Controller 403 Color Shift Amount Storage Unit 404 Image Generation Unit 405 Color Conversion Unit 406 Bitmap Memory 407 Color Shift Correction Amount Calculation Unit 408 Color Shift Correction Unit 409 Exception Processing Unit 1
410 Halftone processing means 411 Exception processing means 2
412 Pulse growth direction control means 413 PWM
801 Coordinate counter 802 Coordinate conversion unit 803 Line buffer 804 Window data 805 Edge pattern storage unit 806 Edge detection unit 807 Tone correction unit

Claims (4)

A photosensitive member, exposure means for irradiating the photosensitive member with a light beam modulated by each color signal to form an electrostatic latent image, and developing the electrostatic latent image formed on the photosensitive member by the exposure means; A plurality of image stations each having a developing means for converting the image forming means and a transfer means for transferring each color image visualized by the developing means to transfer paper, and sequentially conveying the color images formed by the image stations. In a color image forming apparatus for forming a color image by transferring to a transfer material conveyed by means,
Color misregistration amount storage means for storing the color misregistration amount for each image station;
Color misregistration correction amount calculating means for calculating a color misregistration correction amount based on the color misregistration amount obtained from the color misregistration amount storage means;
Based on the calculation result of the color misregistration correction amount calculation means, coordinate conversion means for correcting color misregistration in pixel units,
Edge detection means for detecting an edge portion of the image;
Gradation correction means for correcting a color shift of less than a pixel unit with respect to the detected edge portion;
Means for performing exceptional processing 1 that is not normal halftone processing on the image data after gradation correction;
Means for performing exception processing 2 for determining a pulse growth direction to be added to the image data after the exception processing 1 ignoring a pulse growth direction determined based on a reference position in a halftone pattern;
Means for halftoning the non-edge portion;
Each image station is provided with pulse growth direction control means for determining a pulse growth direction to be added to the image data after the halftone processing based on a reference position in the halftone pattern,
A color image forming apparatus, wherein each exposure means of each image station exposes each light beam corrected by each correction means onto each photoconductor.   Each of the light beams is corrected by means for correcting the light amount of the light beam that is provided in each of the image stations and is pulse-width modulated based on the image data corrected by the correction means. The color image forming apparatus according to claim 1.   The correction means performs processing in the order of coordinate transformation, gradation correction, and exception processing 1 for the edge portion of the image, and performs processing in the order of coordinate transformation and halftone processing for the non-edge portion of the image. The color image forming apparatus according to claim 1, wherein the color image forming apparatus is performed.   The correction means performs processing in the order of coordinate transformation, gradation correction, exception processing 1 and exception processing 2 for the edge portion of the image, and coordinate transformation, halftone processing for the non-edge portion of the image, 2. The color image forming apparatus according to claim 1, wherein processing is performed in the order of pulse growth direction control.

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