JP5142636B2 - Color image forming apparatus - Google Patents

Description

  The present invention relates to a color image forming apparatus that forms a color image by sequentially superimposing and transferring a plurality of color images obtained by a plurality of juxtaposed image carriers onto a transferred transfer material.

  2. Description of the Related Art Conventionally, a color image forming apparatus using an electrophotographic system includes a single photoconductor and a plurality of developing devices, and repeats an image forming process such as exposure, development, and transfer a plurality of times on a single transfer sheet. A method of obtaining a full-color print image by superimposing images is generally used. However, this method has a drawback in that it takes time for the image forming process to be repeated 3 to 4 times (when black is used) in order to obtain one full-color print image. In order to compensate for this drawback, there is a method in which a plurality of photoreceptors and a plurality of developing units are used to sequentially superimpose the obtained images for each color on a transfer sheet to obtain a full-color print image by one pass. is there. According to this method, the throughput can be greatly shortened.

  On the other hand, the problem of color misregistration due to misregistration on the transfer paper of each color arises due to misregistration of each photoconductor and the diameter, misregistration of the optical system, etc., and a high-quality full-color print image is obtained. It was difficult. As a method of preventing this color misregistration, a test toner image is formed on a transfer paper or a conveyance belt forming a part of transfer means, and based on this, the optical path of each optical system is corrected, and the image writing position of each color is set. A correction method has been proposed (see, for example, Patent Document 1).

Further, the output coordinate position of the image data of each color is converted into an output coordinate position in which the registration shift is corrected in pixel units, and the converted image data of each color is corrected by an amount smaller than the minimum dot unit. Is disclosed (for example, see Patent Document 2).
Japanese Patent Application Laid-Open No. 64-40956 Japanese Patent Application Laid-Open No. 08-85237

  However, the method disclosed in Patent Document 1 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, and the position of the test toner image. However, for this purpose, a highly accurate movable member is required, resulting in an increase in cost. Also, since it takes time to complete the correction, it is impossible to make corrections frequently, but the optical path length deviation may change with time due to the temperature rise of the machine. It is difficult to prevent color misregistration by correcting the optical path of the optical system.

  Secondly, when correcting the image writing position, it is possible to correct the positional deviation at the left end and the upper left part, but it is possible to correct the inclination of the optical system or the magnification deviation due to the optical path length deviation. I can't.

  Also, pixel units for pixel data of black dots (exposed dots) that exist in isolation in the white area (non-exposed areas) and pixel data of white dots (non-exposed dots) that exist in isolation in the black area (exposed areas) Less than color misregistration correction processing is performed. As a result, the print density when an isolated dot is reproduced with one dot and the print density when the dot is reproduced with two adjacent dots on the upper and lower lines are not uniform, and the density cannot be accurately reproduced. There's a problem. An example is shown in FIGS. 18 (a) to 18 (c) and FIGS. 19 (d) to 19 (f).

  FIG. 18A is a diagram illustrating an example of image data before color misregistration correction. The input image shown in FIG. 18A is image data including black dots (exposed dots) 101a to 101e that are scattered in isolation in a white area (non-exposed area).

  FIGS. 18B and 18C are image data obtained by performing a coordinate position correction process for canceling the inclination of the scanning line on the input image shown in FIG. FIG. 18B shows image data that has been subjected to coordinate conversion processing for correcting the coordinate position in the sub-scanning direction in pixel units. FIG. 18C shows a gradation correction process for correcting the coordinate position in the sub-scanning direction in less than a pixel unit with respect to the image data subjected to the coordinate conversion process in the pixel unit in FIG. It is the performed image data. As described above, the black dots 101b, 101c, and 101d of one pixel in the input image shown in FIG. 18A are reproduced by being divided into two adjacent dots on the upper and lower lines like 103b, 103c, and 103d. become. As shown in FIG. 19F, the calculation formula for the density value of each isolated dot in the gradation correction processing when the scanning line is inclined at a rate of 1 dot in the sub-scanning direction with respect to 8 dots in the main scanning direction. Shown below.

Density value of isolated dot 103a = density value of 102a * 8/8
Density value of isolated dot 103b = (density value of upper pixel) + (density value of lower pixel)
= Density value of 102b * 2/8 + density value of 102b * 6/8
Density value of isolated dot 103c = (density value of upper pixel) + (density value of lower pixel)
= Concentration value of 102c * 4/8 + Concentration value of 102c * 4/8
Density value of isolated dot 103d = (density value of upper pixel) + (density value of lower pixel)
= Density value of 102d * 6/8 + density value of 102d * 2/8
The density value of the isolated dot 103e = the density value of 102e * 8/8
In the exposure image shown in FIG. 19D, the image data after the color misregistration correction shown in FIG. 18B is scanned on the photosensitive drum by the scanning line 110 inclined in the sub-scanning direction shown in FIG. It represents the exposure result. Further, 104b, 104c, and 104d shown in FIG. 19D are images of isolated dots that are reproduced by combining two adjacent dots in the upper and lower lines.

  The toner densities 105a to 105e shown in FIG. 19 (e) are the toner densities of the isolated dots 104a to 104e of the print result of the exposure image shown in FIG. 18 (c), respectively, and the isolated dots 104a and 104e reproduced by one pixel. Relative to the standard (100%).

  As shown in FIG. 19 (e), when the toner density of the isolated dots 104b to 104d is not constant, and when such a non-uniform density value is periodically repeated, color unevenness becomes obvious. Therefore, there arises a problem that a good color image cannot be obtained.

  The present invention has been made in view of the above problems, and provides a color image forming apparatus capable of making the toner density after printing of isolated dots uniform and obtaining a good color image. Objective.

To achieve the above object, a color image forming apparatus according to claim 1, wherein the exposure for forming the photoreceptor for each color, an electrostatic latent image by irradiating the modulated light beam to said photosensitive member in each color signal And developing means for developing the electrostatic latent image formed on the photosensitive member by the exposure means; and transfer means for transferring each color image visualized by the developing means to a transfer material; comprising an image stations having, before the color image forming apparatus for forming a color image is transferred to a transfer material conveyed by sequentially conveying means a color image formed by Kiga image stations, printing is possible from the print data Image generation means for outputting correct image data, color misregistration amount storage means for memorizing the amount of color misregistration caused by the inclination and curvature of the scanning line for scanning the photoconductor for each color, and stored in the color misregistration amount storage means. The Color misregistration correction amount calculating means for calculating a color misregistration correction amount based on the misregistration amount, coordinate conversion means for correcting color misregistration in pixel units from a calculation result by the color misregistration correction amount calculating means, A pattern detection unit for detecting a dot pattern; a tone correction unit for correcting a color shift of less than a pixel unit with respect to the isolated dot pattern detected by the pattern detection unit; and a tone correction by the tone correction unit An exception processing unit that performs exception processing other than normal halftone processing on subsequent image data, and a halftone processing unit that performs halftone processing on pixels that are not detected as the isolated dot pattern And

According to the present invention, the color misregistration correction amount is calculated based on the color misregistration amount generated by the inclination and curvature of the scanning line for scanning the photoconductor for each color obtained from the color misregistration amount storage means, and the color misregistration correction amount calculation is performed. The color shift in units of pixels is corrected from the calculation result by the means. On the other hand, an isolated dot pattern in the image data is detected, and a color shift of less than a pixel unit is corrected with respect to the detected isolated dot pattern. Then, the image data after gradation correction by the gradation correcting means is subjected to exception processing that is not normal halftone processing, while halftone processing is performed on pixels that are not detected as isolated dot patterns. As a result, by performing density correction processing according to the division ratio on the pixel data of the isolated dots that are reproduced by being divided into two adjacent upper and lower dots in the gradation correction processing, after the isolated dots are printed The toner density can be made uniform, and a good color image can be obtained.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a longitudinal sectional view showing a schematic configuration of a color image forming apparatus according to an embodiment of the present invention.

  The color image forming apparatus 1 according to the embodiment of the present invention is composed of, for example, a 4-drum type color laser beam printer. The color image forming apparatus 1 includes a transfer material cassette 53 that stores a plurality of transfer materials (such as recording paper). The transfer materials stored in the transfer material cassette 53 are taken out one by one by a paper feed roller 54 and fed to an image forming unit (image station) by a pair of conveyance rollers 55a and 55b.

The image station includes photosensitive drums (photosensitive members) 14K, 14M, 14Y, and 14C that are drum-shaped image carriers, exposure means 51K, 51M, 51Y, and 51C including a laser scanner, and transfer means. . Color image forming apparatus 1 is transferred to a transfer material conveyed by sequentially conveying means a color image formed by images station to form a color image.

  The exposure means 51K, 51M, 51Y, and 51C form an electrostatic latent image by irradiating the photosensitive drums 14K, 14M, 14Y, and 14C with light beams modulated by the respective color signals.

  The developing units 52K, 52M, 52Y, and 52C include photosensitive drums 14K, 14M, 14Y, and 14C, C (CYAN), Y (YELLOW), M (MAGENTA), and K (BLACK) color toners, chargers, and developing units. Device (developing means). A predetermined gap is provided between the charger and the developing device in the housing of each developing unit 52K, 52M, 52Y, 52C. Each charger uniformly charges each peripheral surface of the photosensitive drums 14K, 14M, 14Y, and 14C with a predetermined charge.

  The exposure means 51K, 51M, 51Y, and 51C expose the peripheral surfaces of the charged photosensitive drums 14K, 14M, 14Y, and 14C according to image information to form an electrostatic latent image. Then, the developing device transfers the toner to a charged portion of the electrostatic latent image formed on the photoconductive drums 14K, 14M, 14Y, and 14C to make a visible image.

  The transfer unit transfers each color image visualized on the photosensitive member by the developing unit to a transfer material. The transfer means includes a transfer material conveyance belt 10 (conveyance means) for conveying the transfer material, a plurality of rotating rollers (conveyance means), and transfer members 57K disposed across the conveyance surface of the transfer material conveyance belt 10. 57M, 57Y, 57C. The transfer material conveyance belt 10 is stretched flat by a plurality of rotating rollers in the transfer material conveyance direction (from right to left in FIG. 1). The photosensitive drums 14K, 14M, 14Y, and 14C are arranged in a straight line so as to face the conveyance surface of the transfer material conveyance belt 10.

  The toner images formed (developed) on the respective circumferential surfaces of the photosensitive drums 14K, 14M, 14Y, and 14C have been conveyed by transfer electric fields formed by the transfer members 57K, 57M, 57Y, and 57C corresponding thereto. It is absorbed by the charge generated on the transfer material and transferred to the transfer material surface.

  The transfer material onto which the toner image has been transferred is discharged out of the apparatus by a pair of discharge rollers 59a and 59b. Note that the transfer material conveyance belt 10 may be an intermediate transfer belt having a configuration in which C, Y, M, and K color toners are once transferred and then secondarily transferred to the transfer material.

  FIG. 2 is a diagram for explaining the deviation of the scanning lines scanned on the photosensitive drums 14K, 14M, 14Y, and 14C shown in FIG.

  In FIG. 2, reference numeral 301 denotes an ideal scanning line scanned on the photosensitive drums 14K, 14M, 14Y, and 14C. The scanning line 301 is scanned perpendicularly to the rotation direction of the photosensitive drums 14K, 14M, 14Y, and 14C.

On the other hand, 302 is an actual position where the photoconductor drums 14K, 14M, 14Y, and 14C have a positional accuracy and a deviation in diameter, and an upward tilt and curvature are caused by a positional accuracy deviation of the optical system in the exposure means 51 of each color. This is a scanning line. Such inclination and curvature of the scan line, if present, upon collectively transferred toner images of a plurality of colors onto the transfer material (or the intermediate transfer belt), the color shift occurs.

  In the present embodiment, in the main scanning direction (X direction in the figure), an ideal scanning line 301 and an actual scanning line 301 are actually formed at a plurality of points (points B to D) with a point A serving as a scanning start position of the printing area as a reference point. The amount of deviation of the scanning line 302 in the sub-scanning direction (Y direction in the figure) is measured. Each point where the amount of deviation is measured is divided into a plurality of regions (region 1 between Pa and Pb, region 2 between Pb and Pc, and region 3 between Pc and Pd), and the points are connected. The slope of the scanning line in each region is approximated by a straight line (Lab, Lbc, Lcd). If the difference in the amount of deviation between points (m1 for area 1, m2-m1 for area 2, m3-m2 for area 3) is a positive value, the scanning line of the corresponding area has an upward slope. When it is a negative value, it indicates that it has a downward slope.

  Next, an operation of color misregistration correction processing for correcting color misregistration caused by the inclination and curvature of the scanning line will be described.

  FIG. 3 is a block diagram showing a schematic configuration of the printer engine and controller in the color image forming apparatus 1 of FIG.

  The color image forming apparatus 1 includes a printer engine 401 and a controller 402. The printer engine 401 includes the above-described photosensitive drums 14K, 14M, 14Y, and 14C, exposure means 51K, 51M, 51Y, and 51C, the transfer material conveyance belt 10, a plurality of rotating rollers, and a color misregistration amount storage means 403C, 403M, 403Y, 403K. The printer engine 401 actually performs a printing process based on the image bitmap information generated by the controller 402.

  In the color misregistration amount storage means 403C, 403M, 403Y, and 403K (hereinafter simply referred to as “color misregistration amount storage means 403”), the shift amount of the scanning line for each region described above is stored for each color. In this embodiment, the amount of deviation in the sub-scanning direction between the actual scanning line 301 and the ideal scanning line measured at a plurality of points described in FIG. 2 is used as information indicating the inclination and curvature of the scanning line. It is stored in the quantity storage means 403. An example of information stored in the color misregistration amount storage unit 403 is shown in FIG.

  In FIG. 4, the information indicating the inclination and curvature of the scanning line is composed of the width and inclination for each defined area. In this embodiment, the widths L1, L2, L3 (mm) and the inclination m1 for areas 1 to 3 are used. , M2, m3 (mm). Details of L1, L2, L3 and m1, m2, m3 will be described later.

  In this embodiment, the color misregistration amount storage means 403 stores the deviation amount between the ideal scan line and the actual scan line. However, the actual scan line inclination and curvature characteristics are identified. Information that is possible is not limited to width and inclination.

  Further, the information stored in the color misregistration amount storage unit 403 is measured in advance in the manufacturing process of the image forming apparatus 1, and the misregistration amount between the ideal scan line and the actual scan line is measured and stored in advance as information unique to the apparatus. May be. Further, the apparatus itself is equipped with a detection mechanism for detecting the shift amount, and a predetermined pattern for measuring the shift is formed for each color photosensitive drum, and the shift amount detected by the detection mechanism is stored. Any configuration may be used.

  The controller 402 includes an image generation unit 404, a color conversion unit 405, and a bitmap memory 406. Further, the controller 402 includes color misregistration correction amount calculation means 407C, 407M, 407Y, and 407K (hereinafter simply referred to as “color misregistration correction amount calculation means 407”) connected to the color misregistration amount storage means 403. Also, color misregistration correction means 408C, 408M, 408Y, 408K (hereinafter simply referred to as “color misregistration correction means 408”) and exception processing means 409C, 409M, 409Y, 409K (hereinafter simply referred to as “exception processing means 409”). Abbreviated). The controller 402 includes halftone processing means 410C, 410M, 410Y, and 410K (hereinafter simply referred to as “halftone processing means 410”). Furthermore, PWM (Pulse Width Modulation) 411C, 411M, 411Y, and 411K (hereinafter simply referred to as “PWM411”) for correcting the light amount of the light beam that is pulse-width modulated based on the halftone processed image data are provided.

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

  The image generation unit 404 generates raster image data that can be printed from print data received from an information processing apparatus such as a computer (not shown), and outputs the raster image data as RGB data to the color conversion unit 405 for each dot.

  The color conversion unit 405 converts RGB data into CMYK space data that can be processed by the printer engine 401 and stores the data in the bitmap memory 406 for each color. The bitmap memory 406 temporarily stores raster image data (hereinafter referred to as “image data”) to be printed, and stores a page memory for storing image data for one page or data for a plurality of lines. This is a band memory.

  The color misregistration correction amount calculation unit 407 corresponds to the coordinate information in the main scanning direction designated by the color misregistration correction unit 408 for each dot based on the information on the amount of misalignment of the scanning line stored in the color misregistration amount storage unit 403. The color misregistration correction amount in the sub-scanning direction is calculated. Then, the calculated color misregistration correction amount is output to the color misregistration correction means 408, respectively.

  When the coordinate data in the main scanning direction is x (dots) and the color misregistration correction amount in the sub-scanning direction is Δy (dots), the calculation formula of Δy for each region shown in FIG. 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 (mm) in the main scanning direction from the printing start position to the left end of the region 1, the region 2, and the region 3, respectively. m1, m2, and m3 are deviation amounts of the ideal scanning line 301 and the actual scanning line 302 at the right end of the region 1, the region 2, and the region 3, respectively.

  Based on the color misregistration correction amount calculated for each dot by the color misregistration correction amount calculation unit 407, the color misregistration correction unit 408 converts the coordinate data of the bitmap data stored in the bitmap memory 406 and the exposure amount for each pixel. Make adjustments. This is to prevent color misregistration (registration misregistration) due to the inclination or curvature of the scanning line when the toner image of each color is transferred to the transfer medium.

  Next, the color misregistration correction unit 408 will be described with reference to FIGS.

  FIG. 5 is a diagram showing a schematic configuration of the color misregistration correction unit 408.

  In FIG. 5, the color misregistration correction unit 408 includes a coordinate counter 801, a coordinate conversion unit 802, a line buffer 803, window data 804, a pattern storage unit 805, a pattern 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, which are targets of color misregistration correction processing, to the coordinate conversion unit 802. At the same time, the coordinate position data in the main scanning direction is output to the color misregistration amount calculation means 408.

  The coordinate conversion unit 802 corrects the integer part of Δy based on the coordinate position data in the main scanning direction and the sub-scanning direction from the coordinate counter 801 and the color misregistration correction amount Δy obtained from the color misregistration correction amount calculating unit 407. I do. That is, color misregistration correction processing (reconstruction processing) in the sub-scanning direction in pixel units is performed.

  The pattern detection means 806 specifies the n × m (3 × 3 in this embodiment) window data 804 obtained from the line buffer 803 and the gradation correction processing stored in advance in the pattern storage unit 805. Compare the patterns. Thus, pattern detection in the image data is performed, and whether or not the pixel data of the specific pattern to be subjected to the gradation correction processing is shown in the subsequent stage.

  The gradation correction unit 807 applies the Δy of the pixel detected by the pattern detection unit 806 as a pattern to be subjected to the gradation correction process based on the color shift correction amount Δy from the color shift amount calculation unit 407. Performs correction processing after the decimal point. That is, the exposure ratio of the adjacent upper and lower dots in the sub-scanning direction is adjusted, and the positional deviation correction is performed in less than a pixel unit.

  Next, the gradation correction unit 807 will be described with reference to FIG.

  FIG. 6 is a diagram showing a schematic configuration of the gradation correction unit 807.

  In FIG. 6, the gradation correction unit 807 includes multipliers 810 and 811 and an adder 812, and inputs and receives pixel data of the (n−1) th and nth lines having the same main scanning position. The following arithmetic processing is performed with correction coefficients α and β based on the color misregistration correction amount Δy.

P′n (x) = α * Pn−1 (x) + β * Pn (x)
P′n (x): Pixel data after correction of the main scanning position x of the nth line Pn−1 (x): Pixel data before correction of the main scanning position x of the n−1th line Pn1 (x): n Pixel data before correction of the main scanning position x of the line α: Decimal part of color misregistration correction amount Δy β: 1-α
In addition, for image data detected by the pattern detection unit 806 as a pattern that should not be subjected to gradation correction processing, it is determined that correction processing is not necessary for color shifts less than pixels, and the gradation by the gradation correction unit is determined. No correction will be made.

  FIGS. 7A to 7C are diagrams for explaining the contents of the shift amount correction of the integer part of the color shift correction amount Δy by the coordinate conversion unit 802.

  As shown in FIG. 7A, the coordinate conversion unit 802 stores in the bitmap memory 406 according to the value of the integer part of the color misregistration correction amount Δy obtained from the color misregistration information of the scanning line approximated by a straight line. The coordinates in the sub-scanning direction (Y direction in the figure) of the image data that has been set are offset.

  For example, as shown in FIG. 7B, 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 ), The color misregistration correction amount Δy is 0 or more and less than 1. 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 n-th line is reconstructed, n + 1 from the image bitmap at the position where the number of one sub-scan line is offset, that is, from the bitmap memory. A coordinate conversion process for reading the data of the 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. 7C is a diagram showing a state where image data that has been subjected only to color shift correction in pixel units by the coordinate conversion unit 802 is exposed on the photosensitive drum.

  FIGS. 8A to 8C and FIGS. 9D to 9F show the color misregistration correction performed by the gradation correcting unit 807 in less than a pixel unit, that is, the decimal part of the color misregistration correction amount Δy. It is a figure for demonstrating the content of deviation | shift amount correction. Correction of the amount of shift after the decimal point is performed by adjusting the ratio of the exposure amount (exposure ratio) between two adjacent upper and lower dots in the sub-scanning direction.

  FIG. 8A is a diagram illustrating an example of a scanning line having an upward slope. FIG. 8B is a diagram showing a bitmap image of a horizontal straight line before gradation correction, and FIG. 8C is a diagram for canceling the inclination of the scanning line in FIG. It is a figure which shows the correction | amendment bitmap image by which the bitmap image of this was correct | amended. In order to realize the corrected bitmap image of FIG. 8C, the exposure amount adjustment of two adjacent upper and lower dots in the sub-scanning direction is performed. FIG. 9D is a diagram showing an example of a table showing the relationship between the color misregistration correction amount Δy and the correction coefficient for performing gradation correction.

  In FIG. 9D, k is an integer part of the color misregistration correction amount Δy (rounded down after the decimal point) 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, and the ratio of the exposure amount of two adjacent upper and lower dots in the sub-scanning direction based on information below the decimal point of the color misregistration correction amount Δy. And is calculated by the following equation.

α = Δy−k (α: ratio of preceding dots)
β = 1−α (β: ratio of following dots)
FIG. 9E is a diagram showing a bitmap image that has been subjected to tone correction for adjusting the exposure ratio of two adjacent upper and lower dots in the sub-scanning direction in accordance with the correction coefficient of FIG. 9D. . FIG. 9F is a diagram showing an exposure image on the photosensitive drum of the gradation-corrected bitmap image of FIG. 9E, in which the inclination of the scanning line is canceled and a horizontal straight line is formed. It will be.

  Next, the exception processing unit 409 and the halftone processing unit 410 in FIG. 3 will be described.

  The exception processing unit 409 performs exception processing different from the halftone processing by the halftone processing unit 410 on the pixel data after the gradation correction processing of the specific pattern detected by the pattern detection unit 806. The exception processing unit 409 performs optimal processing according to a specific pattern detected by the pattern detection unit 806. On the other hand, normal halftone processing (pseudo halftone processing such as dithering and error diffusion) is performed by the halftone processing unit 410 for pixel data that is not a specific pattern. A series of these processes is shown in FIG. Hereinafter, each step will be described.

  Step S121: Coordinate conversion is performed using the coordinate conversion means 802 to correct color misregistration in units of pixels. Proceed to step S122.

  Step S122: The image data coordinate-converted in step S121 is stored in the line buffer 803. Proceed to step S123.

  Step S123: The pattern detection means 806 detects a specific pattern in the image data. When a specific pattern registered in advance in the pattern storage unit 805 is detected, the process proceeds to step S124, and when a specific pattern is not detected, the process proceeds to step S125.

  Step S124: The gradation correction unit 807 performs gradation correction on the detected pixel data of the specific pattern, and performs color misregistration correction less than a pixel unit. Proceed to step S126.

  Step S125: Halftone processing is performed on pixel data that is not a specific pattern.

  Step S126: Exception processing is performed on pixel data of a specific pattern that has been corrected for color misregistration less than a pixel unit.

  The pixel data obtained from the exception processing unit 409 and the halftone processing unit 410 is appropriately selected based on the detection result of the pattern detection unit 806, is pulse width modulated by the PWM unit 411, and is output as a binary laser drive signal to the printer engine 401. Is output.

  Next, gradation correction processing and exception processing when an isolated dot pattern is registered as a specific pattern in the pattern storage unit 805 will be described.

  In the tone correction process, the exception processing unit 409 makes the toner density after printing of the isolated dots divided and reproduced in two adjacent upper and lower dots equal to the toner density of the isolated dots reproduced by one dot. Thus, the density correction process is performed. That is, the density correction processing is performed on the pixel data subjected to the gradation correction processing according to the correction coefficient value at the time of the gradation correction processing.

  FIG. 11 is a flowchart illustrating color misregistration correction processing for pixel data of an isolated dot pattern.

  Step S131: The pattern detection means 806 detects the pattern of isolated dots in the image data to be detected. If the pixel is an isolated dot pattern, the process proceeds to step S133. If the pixel is not an isolated dot pattern, the process proceeds to step S132. Here, the pattern detection means 806 is a black dot (exposed dot) that exists in isolation in the white area (non-exposed area) or a white dot (non-exposed) that exists in isolation in the black area (exposed area). Exposure dot) is detected and output to the exception processing means 409 at the subsequent stage.

  Step S132: Halftone processing is performed on pixel data that is not an isolated dot pattern.

  Step S133: The gradation correction unit 807 performs gradation correction on pixel data that is a pattern of isolated dots, performs color misregistration correction less than a pixel unit, and has large correction coefficients α and β at the time of gradation correction processing. Is added to the exception processing means 409 at the subsequent stage. Proceed to step S134.

  Step S134: If the isolated dot is a black dot existing in the white area, the process proceeds to step S135. If the isolated dot is a white dot existing in the black area, the process proceeds to step S140.

  Step S135: Steps S136 to S139 are performed according to the value of the correction coefficient.

  Step S136: When the value of the correction coefficient is 1.0, the density correction process is not performed because the isolated dot is reproduced by one pixel.

  Step S137: When the value of the correction coefficient is 0.8 or more and less than 1.0, the isolated dot is reproduced by two adjacent upper and lower pixels, so 1.1 times the pixel data after gradation correction Density correction processing is performed.

  Step S138: When the value of the correction coefficient is 0.6 or more and less than 0.8, the isolated dot is reproduced by two adjacent upper and lower pixels, so that the pixel data after gradation correction is 1.2 times Density correction processing is performed.

  Step S139: If the value of the correction coefficient is less than 0.6, the isolated dot is reproduced by two adjacent upper and lower pixels, so that the density correction process is performed 1.3 times on the pixel data after gradation correction. .

  Step S140: Steps S141 to S144 are performed according to the value of the correction coefficient.

  Step S141: When the correction coefficient value is 1.0, the density correction processing is not performed because the isolated dot is reproduced by one pixel.

  Step S142: If the value of the correction coefficient is 0.8 or more and less than 1.0, the isolated dot is reproduced by two adjacent upper and lower pixels, so 0.9 times the pixel data after gradation correction Density correction processing is performed.

  Step S143: If the value of the correction coefficient is 0.6 or more and less than 0.8, the isolated dot is reproduced by two adjacent upper and lower pixels, so that the pixel data after gradation correction is 0.8 times Density correction processing is performed.

  Step S144: When the value of the correction coefficient is less than 0.6, the isolated dot is reproduced by two adjacent upper and lower pixels, so that the density correction process is performed 0.7 times on the pixel data after gradation correction. .

  Next, the flow of color misregistration correction processing for black level isolated dots in the white region by the above processing will be described with reference to FIGS. 12 (a) to 14 (f).

  The input image shown in FIG. 12A is image data including black dots (exposure dots) 901a to 901e that are isolated and scattered in a white area (non-exposure area).

  In FIG. 12B, the color shift correction amount Δy in the sub-scanning direction at each pixel position in the main scanning direction, the coordinate conversion amount k for each pixel in the sub-scanning direction, and the correction coefficients α and β at the time of gradation correction processing are shown. Show.

  The image data shown in FIGS. 13C and 13D is subjected to a coordinate position correction process for canceling the inclination of the scanning line in the sub-scanning direction with respect to the input image shown in FIG. It is the performed image data. The image data shown in FIG. 13C is image data that has been subjected to coordinate conversion processing for correcting the coordinate position in the sub-scanning direction in pixel units.

  The image data shown in FIG. 13D is a level for correcting the coordinate position in the sub-scanning direction in less than a pixel unit with respect to the image data subjected to the coordinate conversion process in the pixel unit in FIG. This is image data that has undergone tone correction processing. With this processing, the black dots 901b, 901c, and 901d of one pixel in the input image data shown in FIG. 12A are reproduced by being divided into two adjacent dots on the upper and lower lines as in 903b, 903c, and 903d. Will be. The calculation formula of the density value of each isolated dot in the gradation correction processing based on the color misregistration correction amount of (b) is shown below.

Density value of isolated dot 903a = density value of 902a * 1.0
Density value of isolated dot 903b = (density value of upper pixel) + (density value of lower pixel)
= Density value of 902b * 0.25 + density value of 902b * 0.75
Density value of isolated dot 903c = (density value of upper pixel) + (density value of lower pixel)
= Density value of 902c * 0.5 + density value of 902c * 0.5
Density value of isolated dot 903d = (density value of upper pixel) + (density value of lower pixel)
= Density value of 902d * 0.75 + density value of 902d * 0.25
Density value of isolated dot 903e = density value of 902e * 1.0
The image data shown in FIG. 14E is image data after exception processing of isolated dots, and is obtained by performing density correction processing according to the larger value of the correction coefficients α and β at the time of gradation correction processing. is there. An arithmetic expression for density correction of each isolated dot is shown below.

In case of isolated dot 904a (correction coefficient 1.0): density value of 903a * 1.0
In the case of an isolated dot 904b (correction coefficient 0.75): (density value of upper pixel) + (density value of lower pixel) = density value of 903b1 * 1.2 + density value of 903b2 * 1.2
In the case of an isolated dot 904c (correction coefficient 0.5): (density value of upper pixel) + (density value of lower pixel) = density value of 903c1 * 1.3 + density value of 903c2 * 1.3
For isolated dot 904d (correction coefficient 0.75): (density value of upper pixel) + (density value of lower pixel) = density value of 903d1 * 1.2 + density value of 903d2 * 1.2
In the case of isolated dot 904e (correction coefficient 1.0): density value of 903e * 1.0
The exposure images 905a to 905e in FIG. 14 (f) are pixel data 904a to 904e of the isolated dot pattern after the exception processing in FIG. 14 (e) by the scanning lines 910 inclined in the sub-scanning direction in FIG. 12 (b). 2 shows the exposure result of scanning on the photosensitive drum.

  In FIG. 14F, reference numerals 905b, 905c, and 905d denote exposure images of isolated dots that are reproduced by combining two upper and lower two dots. Reference numerals 905b2, 905c2, and 905d2 indicate the sizes of isolated dots when no exceptional processing is performed, that is, when 903b, 903c, and 903d in FIG. 13D are exposed. For isolated dots that are reproduced by dividing them into two adjacent upper and lower dots, exception processing is performed according to the correction coefficient so that the toner density after printing is equivalent to the toner density of the isolated dots that are reproduced with one dot. By correcting the density value in the dark direction, the size of the exposure dot is corrected.

  As described above, when the gradation correction processing is performed on the black level isolated dots (exposed dots) existing in the white region (non-exposed region) detected by the pattern detecting unit 806, the following exceptional processing is performed. That is, for isolated dots that are reproduced by being divided into two upper and lower dots adjacent in the sub-scanning direction, an exception process is performed in which the density value is corrected in the dark direction according to the correction coefficient during the tone correction process. . As a result, it is possible to achieve a toner density level equivalent to that of an isolated dot reproduced with one dot.

  Next, the flow of color misregistration correction processing for white level isolated dots (non-exposed dots) in the black area (exposed area) will be described with reference to FIGS. 15 (a) to 17 (f).

As in the case of FIGS. 12A to 14F, the color misregistration correction processing for each pixel shown in FIG. 16C and the color misregistration correction processing for less than the pixel shown in FIG. 16D are performed. . Here, in the exceptional process shown in FIG. 17E, the density correction process is performed according to the larger value of the correction coefficients α and β at the time of the gradation correction process. An arithmetic expression for density correction of each isolated dot is shown below.
In the case of an isolated dot 1004a (correction coefficient 1.0): density value of 1003a * 1.0
In the case of an isolated dot 1004b (correction coefficient 0.75): (density value of upper pixel) + (density value of lower pixel) = density value of 1003b1 * 0.8 + density value of 1003b2 * 0.8
In the case of an isolated dot 1004c (correction coefficient 0.5): (density value of upper pixel) + (density value of lower pixel) = density value of 1003c1 * 0.7 + density value of 1003c2 * 0.7
In the case of isolated dot 1004d (correction coefficient 0.75): (density value of upper pixel) + (density value of lower pixel) = density value of 1003d1 * 0.8 + density value of 1003d2 * 0.8
In the case of isolated dot 1004e (correction coefficient 1.0): density value of 903e * 1.0
The exposure images 1005a to 1005e in FIG. 17 (f) are the pixel data 1004a to 1004e of the isolated dot pattern after the exception processing in FIG. 17 (e) on the scanning line 910 inclined in the sub-scanning direction in FIG. 15 (b). It represents the exposure result scanned on the photosensitive drum.

  In FIG. 17F, reference numerals 1005b, 1005c, and 1005d are isolated dots that are reproduced by combining two upper and lower lines with different division ratios. Then, the size of the exposure dot is corrected by correcting the density value in the thin direction by the exception processing unit 409 according to the division ratio so as to be equal to the toner density of the isolated dots 1005a and 1005e reproduced by one dot. Is done.

  As described above, when the gradation correction processing is performed on the white level isolated dots (non-exposed dots) existing in the black region (exposure region) detected by the pattern detection unit 806, the following exceptional processing is performed. In other words, for isolated dots that are reproduced by being divided into two upper and lower dots adjacent in the sub-scanning direction, an exception process is performed in which the density value is corrected in a thin direction according to the correction coefficient at the time of the gradation correction process. . As a result, it is possible to achieve a toner density level equivalent to that of an isolated dot reproduced with one dot.

According to the above embodiment, the color misregistration correction amount is calculated based on the color misregistration amount generated by the inclination and curvature of the scanning line for scanning the photoconductor for each color obtained from the color misregistration amount storage means, and the color misregistration correction is performed. The color shift in units of pixels is corrected from the calculation result by the quantity calculation means. Then, isolated dot pattern detection is performed on the image data after the color misregistration correction, and tone correction processing is performed on the detected isolated dot pixel data to correct a positional shift in the sub-scanning direction less than a pixel unit. In the gradation correction processing, density correction processing corresponding to the division ratio is performed on pixel data of isolated dots that are reproduced by being divided into two adjacent upper and lower dots. Thereby, the toner density after printing of isolated dots can be made uniform, and a good color image can be obtained.

1 is a longitudinal sectional view showing a schematic configuration of a color image forming apparatus according to an embodiment of the present invention. FIG. 2 is a diagram for explaining a shift of a scanning line scanned on the photosensitive drums 14K, 14M, 14Y, and 14C illustrated in FIG. FIG. 2 is a block diagram illustrating a schematic configuration of a printer engine and a controller in the color image forming apparatus 1 of FIG. 1. It is a figure which shows an example of the information memorize | stored in the color shift amount memory | storage means 403C, 403M, 403Y, 403K shown in FIG. FIG. 4 is a diagram showing a schematic configuration of a color misregistration amount correction unit 408. 3 is a diagram showing a schematic configuration of a gradation correction unit 807. FIG. (A)-(c) is a figure for demonstrating the operation | movement which correct | amends the shift | offset | difference amount of the integer part of color shift correction amount (DELTA) y by the coordinate conversion means 802. FIG. It is a figure for demonstrating the content of the color misregistration correction less than the pixel unit which the gradation correction | amendment means 807 performs, (a) shows an example of the scanning line which has a right-up inclination, (b) is before gradation correction. (C) shows a corrected bitmap image. It is a figure for demonstrating the content of the color misregistration correction less than the pixel unit which the gradation correction means 807 performs, (d) represented the relationship between color misregistration correction amount (DELTA) y and the correction coefficient for performing tone correction. FIG. 8E shows an example of a table, and FIG. 8E shows 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. Indicates an exposure image on a photosensitive drum of a bitmap image whose gradation has been corrected. 5 is a flowchart illustrating an example of color misregistration correction processing in a color misregistration correction unit 408. It is a flowchart which shows the color shift correction process of the pixel data of an isolated dot pattern. It is a figure which shows the flow of the color misregistration correction process of the isolated dot of the black level in a white area, (a) shows the image data before color misregistration correction, (b) shows an example of the color misregistration correction amount. It is a figure which shows the flow of the color misregistration correction process of the isolated dot of the black level in a white area, (c) is image data after color misregistration correction (pixel unit), (d) is after color misregistration correction (less than pixel unit) ) Shows an example of image data. It is a figure which shows the flow of the color shift correction process of the isolated dot of the black level in a white area, (e) is image data after exception processing (density correction), (f) shows an example of an exposure image. It is a figure which shows the flow of the color misregistration correction process of the isolated dot of the white level in a black area, (a) is the image data before color misregistration correction, (b) shows an example of a color misregistration correction amount. It is a figure which shows the flow of the color misregistration correction process of the white level isolated dot in a black area, (c) is image data after color misregistration correction (pixel unit), (d) is after color misregistration correction (less than pixel unit) ) Shows an example of image data. It is a figure which shows the flow of the color shift correction process of the isolated dot of the white level in a black area, (e) is image data after exception processing (density correction), (f) shows an example of an exposure image. (A)-(c) is a figure for demonstrating the density nonuniformity of the isolated dot in a prior art example. (D)-(f) is a figure for demonstrating the density nonuniformity of the isolated dot in a prior art example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 401 Printer engine 402 Controller 403C, 403M, 403Y, 403K Color shift amount storage means 404 Image generation means 405 Color conversion means 406 Bitmap memory 407C, 407M, 407Y, 407K Color shift correction amount calculation means 408C, 408M, 408Y, 408K Color misregistration correction means 409C, 409M, 409Y, 409K Exception processing means 410C, 410M, 410Y, 410K Halftone processing means 801 Coordinate counter 802 Coordinate conversion means 803 Line buffer 805 Pattern storage unit 806 Pattern detection means 807 Tone correction Means 810, 811 Multiplier 812 Adder

Claims (5)

A photoconductor for each color; an exposure unit that irradiates the photoconductor with a light beam modulated by each color signal; and an electrostatic latent image formed on the photoconductor by the exposure unit. a developing unit that visualizes includes an image stations and a transfer means for transferring the transfer material of each color image is visualized by the developing unit, a color image formed by the front Kiga image station In a color image forming apparatus that forms a color image by transferring to a transfer material that is sequentially conveyed by a conveying means,
Image generation means for outputting image data that can be printed from print data, color misregistration amount storage means for storing the amount of color misregistration caused by the inclination and curvature of a scanning line that scans the photoconductor for each color, and
A color misregistration correction amount calculation unit that calculates a color misregistration correction amount based on the color misregistration amount stored in the color misregistration amount storage unit;
Coordinate conversion means for correcting color misregistration in pixel units from the calculation result by the color misregistration correction amount computing means;
Pattern detection means for detecting a pattern of isolated dots in the image data;
Gradation correction means for correcting a color shift of less than a pixel unit with respect to the pattern of isolated dots detected by the pattern detection means;
Exception processing means for performing exception processing other than normal halftone processing on the image data after gradation correction by the gradation correction means;
A color image forming apparatus comprising: a halftone processing unit that performs a halftone process on a pixel that is not detected as the isolated dot pattern.   2. The color image forming apparatus according to claim 1, wherein the pattern detecting unit detects a pattern of exposure dots that exist in isolation in the non-exposure area or a pattern of non-exposure dots that exist in isolation in the exposure area. .   In the exception processing unit, the pattern detection unit detects an exposure dot pattern that exists in isolation in the non-exposure region, and the gradation correction unit divides the isolated dot into two adjacent dots in the sub-scanning direction. The color image forming apparatus according to claim 2, wherein when reproduced, the density value of the isolated dot is corrected in a dark direction.   In the exception processing unit, the pattern detection unit detects a pattern of non-exposed dots isolated in the exposure area, and the gradation correction unit divides the isolated dot into two adjacent dots in the sub-scanning direction. The color image forming apparatus according to claim 2, wherein when reproduced, the density value of the isolated dot is corrected in a thin direction. The exception processing unit performs density correction processing according to a correction coefficient when the isolated dot is reproduced by being divided into two dots adjacent in the sub-scanning direction by the gradation correction unit. Item 5. The color image forming apparatus according to any one of Items 1 to 4.

Images