JP4533222B2 - Color image forming apparatus and control method thereof - Google Patents

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 conveyed storage medium, and a control method therefor.

  2. Description of the Related Art Conventionally, in a color image forming apparatus using an electrophotographic method, laser light corresponding to an image signal of each color is exposed to one photoconductor, and an electrostatic latent image corresponding to each color is developed with a corresponding color. There is known a method of developing using an agent and transferring it to a transfer sheet. In a printer employing such a system, a multi-color image is formed on a single transfer sheet by repeating the exposure, development, and transfer processes using laser light a number of times corresponding to the number of colors to be printed. Then, the transfer sheet on which the color image is transferred in this way is fixed by a fixing device to obtain a full color image.

According to this method, in order to obtain a single printed image, it is necessary to repeat the image forming process three times in the case of Y, M, and C, and a total of four times if black is added to this. There was a disadvantage that it took. As a method for compensating for this defect, a plurality of color photoconductors are arranged along the transfer sheet conveyance path, and the images obtained for the respective colors are sequentially transferred onto the transfer paper and superimposed. There is a method of obtaining a full color print by passing paper once. According to this method, the image of the corresponding color is formed in parallel on each photoconductor, so that the throughput can be greatly reduced. However, on the other hand, there is a problem that color misregistration occurs due to misregistration of each color image on the transfer sheet due to positional accuracy and diameter deviation of each photoconductor, and optical system positional accuracy deviation. As a method for preventing this color misregistration, for example, a test toner image is formed on a transfer paper or a conveyance belt forming a part of transfer means, the position of the toner image is detected, and the detection result is obtained. Based on this, a method of correcting the optical path of the optical system corresponding to each color or correcting the image writing position of each color is conceivable (Patent Document 1). However, this method has the following problems.
Japanese Patent Application Laid-Open No. 64-40956 JP-A-8-85237

  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. is there. For this purpose, a highly accurate movable member is required, resulting in high costs. Furthermore, since it takes time to complete the correction, the correction cannot be performed frequently. In particular, the deviation of the optical path length may change with time due to the temperature rise of the machine. In this case, it is difficult to correct the optical path of the optical system to prevent the color deviation. Second, when correcting the writing position of the image, it is possible to correct the positional deviation at the left edge and the upper left part of the image, but it is not possible to correct the inclination of the optical system or the magnification deviation due to the optical path length deviation. There are problems such as.

  In Patent Document 2, the coordinate position for outputting the image data for each color is automatically converted into the coordinates for which the registration deviation is corrected, and the modulated light beam is converted based on the image data for each color obtained by converting the position. A configuration is disclosed in which the position is corrected by an amount smaller than the minimum dot unit of the color signal. However, in this case, the position where the image data for each color is output is corrected with respect to the image subjected to halftone processing, and the halftone dot reproducibility of the halftone image when dither processing is performed. Will deteriorate. As a result, color unevenness may occur and moire may become apparent. That is, when image data having a constant density value is input, the following problem may occur when the image data is printed with the above-described color misregistration correction. Generally, the relationship between the density value of image data and the toner density with respect to the density value is not linear. For this reason, although the input image data is an image having a constant density value, the density of the image after correction is not constant by correcting it with an amount smaller than the minimum dot unit. When such a non-uniform density portion is periodically repeated, moire appears and a good color image cannot be obtained.

  The present invention is to solve the above-mentioned drawbacks of the prior art.

Further, the present invention is characterized in that the positional deviation correction is performed in units of pixels based on the color misregistration correction amount calculated by the color misregistration correction amount calculating means. Further, from the density value obtained by the density confirmation means, it is selected whether or not to perform color misregistration correction less than a pixel unit, and moire is reduced by not performing color misregistration correction less than a pixel unit for a thin pixel. It is an object of the present invention to provide a color image forming apparatus capable of obtaining a good color image.

A color image forming apparatus according to an aspect of the present invention has the following configuration. That is,
A shift amount storage means for storing a shift amount of an ideal main scanning line of the laser with respect to the photosensitive member;
Based on the shift amount obtained from the shift amount storage unit, a shift correction unit that performs shift correction in dot units of a raster image based on print data; and
When the density of the raster image to be processed is lower than the threshold value, when the density of the raster image to be processed is equal to or higher than the threshold value without performing the correction of less than a dot unit based on the shift amount obtained from the shift amount storage unit. Correcting means for correcting a shift of less than a dot unit based on the shift amount obtained from the shift amount storage means;
And image forming means for forming an image corresponding to the dot corrected by the correcting means with the laser .

A color image forming apparatus control method according to an aspect of the present invention includes the following steps. That is,
A method of controlling a color image forming apparatus for forming an image by irradiating a photoconductor with a laser modulated by an image signal,
A shift correction step for correcting a shift in dot units of a raster image based on print data based on a shift amount obtained from a shift amount storage unit that stores a shift amount of the laser with respect to an ideal main scanning line of the laser with respect to the photosensitive member;
When the density of the raster image to be processed is lower than the threshold value, when the density of the raster image to be processed is equal to or higher than the threshold value without performing shift correction less than a dot unit based on the shift amount obtained from the shift amount storage unit. A correction step for correcting a shift of less than a dot unit based on the shift amount obtained from the shift amount storage means;
An image forming step of forming an image corresponding to the dot corrected in the correction step by the laser.

  The outline of the present invention does not enumerate all necessary features, and therefore, a sub-combination of these feature groups can also be an invention.

  According to the present invention, it is possible to form a good color image by eliminating the misalignment of images formed at each image station and suppressing the occurrence of moire due to non-uniform density.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments do not limit the invention according to the claims, and all the combinations of features described in the embodiments are not necessarily essential to the solution means of the invention. .

  FIG. 1 is a schematic cross-sectional view illustrating the configuration of an image forming unit of a color image forming apparatus according to an embodiment of the present invention, and shows a case of a four-drum type color laser beam printer.

  In this color image forming apparatus, a sheet cassette 53 is mounted on the lower right side of the main body apparatus. The transfer sheets accommodated in the sheet cassette 53 are taken out one by one by the rotation of the paper feed roller 54, and are fed to an image forming unit in which a plurality of photosensitive drums are arranged by a pair of conveyance rollers 55a and 55b. In this image forming unit, a conveyance belt 10 that conveys a transfer sheet is stretched flat in the conveyance direction of the transfer sheet by a plurality of rotating rollers, and the transfer sheet is electrostatically adsorbed to the conveyance belt 10 in the most upstream portion. The Further, four photosensitive drums 14 as image bearing members in the form of drums are arranged in a straight line so as to face the conveying surface of the conveying belt 10 to constitute an image forming unit.

  The developing units 52 (52C, 52Y, 52M, 52K) corresponding to the respective colors which are image forming units respectively correspond to the corresponding photosensitive drums 14 (14C, 14Y, 14M, 14K), C (cyan), Y (yellow). ), M (magenta), and K (black) toners, a charger, and a developer. A predetermined gap is provided between the charger and the developer in the housing of each developing unit 52, and laser light from an exposure unit 51 (51C, 51Y, 51M, 51K) having a laser scanner is irradiated in this gap. The As a result, the peripheral surface of each photosensitive drum 14 whose surface is uniformly charged is exposed by the charger in accordance with the image signal of the corresponding color to form an electrostatic latent image. Then, the developing device transfers the toner to the low potential portion of the electrostatic latent image to develop a toner image (development).

  Further, transfer members 57 (57C, 57Y, 57M, and 57K) are arranged across the conveyance surface of the conveyance belt 10. The toner images formed (developed) on the peripheral surfaces of the respective photosensitive drums 14 are absorbed by the transferred transfer sheet and transferred to the transfer sheet surface by the transfer electric field formed by the transfer member 57 corresponding thereto. Is done. The transfer sheet on which the toner image has been transferred in this manner is fixed by the fixing device 58 and then discharged out of the apparatus by the rotation of the pair of discharge rollers 59a and 59b. The transport belt 10 may be an intermediate transfer belt configured to temporarily transfer C, Y, M, and K color toners and then secondary transfer them onto a transfer sheet.

  FIG. 2 is an image diagram for explaining a shift of a main scanning line scanned by each photosensitive drum 14 (for example, a cyan photosensitive drum 14C) which is an image carrier. Since the same applies to the photosensitive drums corresponding to other colors, the description thereof is omitted.

  Reference numeral 201 denotes an image of an ideal main scanning line, and scanning is performed perpendicularly to the rotational direction of the photosensitive drum 14C (longitudinal direction of the drum). Reference numeral 202 denotes a tilt and curvature that rises to the right due to actual laser scanning, which occurs due to the positional accuracy and diameter shift of the photosensitive drum 14C, and the positional accuracy shift of the optical system in the cyan exposure unit 51C. The image of the main scanning line is shown. When such inclination or curvature of the main scanning line exists in any color image station, color misregistration occurs when a plurality of color toner images are collectively transferred to the transfer sheet.

  In the present embodiment, in the main scanning direction (x direction: longitudinal direction of the drum), a point A serving as a scanning start position of the printing area is used as a reference point, and a plurality of points (point B, point C, point D) The amount of deviation in the sub-scanning direction between the ideal main scanning line 201 and the actual main scanning line 202 is measured. The measured shift amount 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) for each measured point. Then, the inclination of the main scanning line in each region is approximated by a straight line (Lab, Lbc, Lcd) connecting these points Pa, Pb, Pc, Pd. Therefore, if the difference in deviation amount at each point Pa, Pb, Pc, Pd (m1 in region 1, (m2−m1) in region 2, and (m3−m2) in region 3) is a positive value, the corresponding region This main scanning line has an upward slope, and a negative value indicates a downward slope.

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

  A printer engine 301 includes the image forming unit illustrated in FIG. 1, and performs printing processing based on bitmap image data generated by the controller 302. Each of 303C, 303Y, 303M, and 303K is a color misregistration amount for each color of cyan, yellow, magenta, and black (this is a misregistration amount of an image to be formed. The amount of misregistration of the main scanning line for each region is stored for each color. In the present embodiment, based on the actual positions of the main scanning lines 202 measured at a plurality of points described with reference to FIG. Is stored in the color misregistration amount storage unit 303 as information indicating the inclination and curvature of the image.

  FIG. 4 is a diagram showing an example of data stored in the color misregistration amount storage unit 303 (303C, 303Y, 303M, 303K).

  In FIG. 4, for each region shown in FIG. 2, the length (L 1, L 2, L 3) in the main scanning direction from the reference point to the point actually measured on the main scanning line 202 and on the main scanning line 202 are shown. The shift amount (m1, m2, m3) between the point (Pb, Pc, Pd) and the ideal main scanning line 201 is associated with each other and stored in mm. Each of L1, L2, and L3 represents the length from the reference point (point A) to the end of region 1, region 2, and region 3. Each of m 1, m 2, and m 3 is a deviation amount between the ideal main scanning line 201 and the actual main scanning line 202 at each end of the region 1, region 2, and region 3.

  Next, an operation in the controller 302 for performing the printing process by correcting the image data so as to cancel out the main scanning line misregistration amount stored in the color misregistration amount storage unit 303 will be described.

  The image generation unit 304 generates raster image data that can be printed based on print data received from an external device (not shown) such as a computer device, and outputs the raster image data for each dot as RGB data. The color conversion unit 305 converts the RGB data into CMYK color space data that can be processed by the printer engine 301. Each of the halftone processing units 309C to 309K uses a predetermined halftone screen pattern to reduce the number of bits of the input dot data, and the gradation expression from the dot unit gradation expression to the halftone screen area unit. Convert to data. The converted data is stored in the bitmap memory 406 for each color. The bitmap memory 306 temporarily stores raster image data to be printed. At least one of a page memory that stores image data for one page and a band memory that stores data for a plurality of lines is stored. You may have.

  Each of 307C, 307Y, 307M, and 307K is a color misregistration correction amount calculation unit that calculates a color misregistration correction amount corresponding to each color data, and the main scanning line stored in the color misregistration amount storage unit 303 corresponding to each color. Based on the information indicating the amount of misregistration, color misregistration correction in the sub-scanning direction corresponding to coordinate information in the main scanning direction instructed from a color misregistration correction unit 308 (308C, 308Y, 308M, 308K) described later for each dot. The amount is calculated and output to each color misregistration correction unit 308.

  Now, assuming that the coordinate in the main scanning direction for a certain dot is x (dot), the sub-scanning direction is y-line, and the color misregistration correction amount in the sub-scanning direction is Δyi (dot) (i represents a region) An arithmetic expression of the color misregistration correction amount Δyi in the sub-scanning direction in each region based on 2 is shown below (here, the resolution is 600 dpi).

Region 1: Δy1 = xx (m1 / L1) (1)
Region 2: Δy2 = m1 × 23.622 + (x−L1 × 23.622) × ((m2−m1) / (L2−L1)) (2)
Region 3: Δy3 = m2 × 23.622 + (x−L2 × 23.622) × ((m3−m2) / (L3−L2)) Equation (3)
Each of the color misregistration correction units 308C, 308Y, 308M, and 308K corrects color misregistration due to the inclination and distortion of the main scanning line. Specifically, based on the color misregistration correction amount calculated for each dot by each of the color misregistration correction amount calculation units 307C, 307Y, 307M, and 307K, the output timing of the bitmap data stored in the bitmap memory 306 is determined. Adjustment and exposure amount adjustment for each dot are performed. This prevents color misregistration (registration misalignment) when each color toner image is transferred to a transfer sheet.

  Next, the color misregistration correction unit 308 (308C, 308Y, 308M, 308K) according to the present embodiment will be described with reference to the block diagram shown in FIG. Here, the description will be given in the case of the color misregistration correction unit 308C for cyan, but the configuration and operation of the color misregistration correction unit for other colors are the same, and thus the description thereof is omitted.

  The color misregistration correction unit 308C includes a coordinate counter 801, a coordinate conversion unit 802, a line buffer 803, and a gradation correction unit 804. The coordinate counter 801 outputs the coordinate data in the main scanning direction and the sub-scanning direction of the dots to be subjected to color misregistration correction processing to the coordinate conversion unit 802. At the same time, the coordinate data of the dot in the main scanning direction is output to the color misregistration amount calculation unit 307C and the gradation correction unit 804. The coordinate conversion unit 802 reads line data to be processed from the bitmap memory 306 based on the coordinate data in the main scanning direction and the sub-scanning direction input from the coordinate counter 801. The gradation correction unit 804 performs a correction process based on the integer part of the shift correction amount Δy based on the shift correction amount Δy obtained from the color shift correction amount calculation unit 307C, that is, a reconstruction process in the sub-scanning direction in dot units. Do. Also, the gradation correction unit 804 performs correction processing based on a value after the decimal point of the deviation correction amount Δy based on the coordinate data in the main scanning direction from the coordinate counter 801 and the deviation correction amount Δy, that is, correction in less than a dot unit. This is done by adjusting the exposure ratio of dots before and after in the sub-scanning direction. The tone correction unit 804 uses a line buffer 803 for referring to dots before and after in the sub-scanning direction.

  The operation based on the above configuration will be described below.

  The coordinate conversion unit 802 converts the address in the sub-scanning direction of the coordinate value input from the coordinate counter 801, and reads bitmap data of the corresponding line from the bitmap memory 306. The line buffer 803 includes a register 805 that stores dot data to be processed, and a FIFO buffer 806 that stores dot data for one preceding line. The tone correction unit 804 refers to dot data in the previous and subsequent lines stored in the line buffer 803 in order to generate correction data. The dot data accumulated in the register 805 is output to the gradation correction unit 804 and used to generate correction data for the next line. The gradation correction unit 804 inputs the n-line coordinate x in the main scanning direction input from the coordinate counter 801, the x-th dot data Pn (x) input from the register 805, and the preceding input from the FIFO buffer 806. X-th dot data Pn-1 (x) of the line to be input is input. Then, in order to generate correction data P ″ n (x), the following arithmetic processing is performed.

P′n (x) = Pn (x) × β (x) + Pn−1 (x) × α (x)
The density confirmation unit 804b inputs the dot data Pn (x) to be processed and confirms the density of the dot. When the density of the dot Pn (x) is lower than a predetermined density value (μ), the original dot data Pn (x) is output as P ″ n (x), and the density is lower than the predetermined density value (μ). When it is high, the corrected dot data P′n (x) is selected and output. This selection is performed by the selector 804a. In this way, bitmap data in which a color misregistration amount less than a dot unit in the sub-scanning direction is corrected is output.

  The dot data in which the shift amount is corrected in this way is converted into a pulse-width modulated signal by the PWM circuits 310C to 310K, and this signal is sent to the corresponding exposure units 51C to 51K, respectively, to drive each semiconductor laser. .

  FIG. 6 is an image diagram for explaining an operation in which the coordinate conversion unit 802 according to the present embodiment corrects the shift amount based on the integer part of the color shift correction amount Δy.

  The coordinate conversion unit 802, as indicated by 600, is stored in the bitmap memory 306 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. The coordinates of the dot data in the sub-scanning direction (Y direction) are offset. For example, as shown in 601, when the coordinate in the sub-scanning direction from the coordinate counter 801 is n (line) and the coordinate in the main scanning direction is x, in the region of (1) in the x-coordinate in the main scanning direction, The color misregistration correction amount Δy is 0 or more and less than 1. Therefore, in this case, when reconstructing the nth line data, the nth line dot data 610 is read from the bitmap memory 306 (offset = 0). In the area (2), since the color misregistration correction amount Δy is 1 or more and less than 2, when reconstructing the n-th line data, the (n + 1) -line data 611 with the sub-scan line number offset by 1 is read. A coordinate conversion process is performed. Similarly, in the area (3), since the color misregistration correction amount Δy is 2 or more and less than 3, coordinate conversion processing for reading the data 612 of the (n + 2) line is executed. Similarly, in the area (4), coordinate conversion processing is performed to read data on the (n + 3) line, and in the area (5), coordinate conversion processing is performed to read data on the (n + 4) line. .

  With the above method, reconstruction processing is performed in units of lines in the sub-scanning direction, that is, in units of dots, according to the value of the integer part of the deviation correction amount. Reference numeral 602 denotes an exposure image when the photosensitive drum is exposed based on the data 601 obtained by correcting the color misregistration in dot units by the coordinate conversion unit 802.

  FIGS. 7A to 7F are image diagrams for explaining the operation of correcting the color misregistration performed by the gradation correcting unit 804 in less than a dot unit, that is, correcting the color misregistration correction amount Δy after the decimal point. is there. Correction of the shift amount after the decimal point is performed by adjusting the exposure ratio of dots before and after in the sub-scanning direction.

  FIG. 7A is an image of a main scanning line having an upward slope. FIG. 7B shows a bitmap image before this deviation correction, and is a straight bitmap image horizontal in the main scanning direction. FIG. 7C is a correction image obtained by correcting the bitmap image of FIG. 7B in order to cancel the color shift due to the inclination of the main scanning line shown in FIG.

  Here, in order to realize the correction image shown in FIG. 7C, the exposure amount of dots in the front and rear lines positioned in the sub-scanning direction is adjusted.

FIG. 7D shows the relationship between the color misregistration correction amount Δy and the correction coefficient for performing gradation correction. k is the 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 dot units described with reference to FIG. β and α are correction coefficients for correcting in the sub-scanning direction less than a dot unit (size smaller than one dot size), and dots before and after the sub-scanning direction are determined from information below the decimal point of the color misregistration correction amount Δy. Represents the distribution ratio of the exposure amount. Each of these correction factors α, β is
α = Δy−k
β = 1−α
It is calculated by. Here, α represents the distribution ratio of dots to the (n + k) line, and β represents the distribution ratio of dots to the (n + k−1) line. That is, when k = 0, α represents the dot distribution ratio to the n line, and β represents the dot distribution ratio to the (n−1) line. When k = 1, α represents the dot distribution ratio to the (n + 1) line, and β represents the dot distribution ratio to the n line.

  Here, with reference to FIGS. 7C to 7D, the dot 700 is originally located because k = 0, α = 0, and β = 1 in FIG. 7D. It is formed by moving to the (n-1) line immediately before the n line. In FIG. 7D, since the dot 701 is α = 0.25 and β = 0.75, 3/4 of the dot is formed in the (n−1) line immediately before the n line. In the current line (n), 1/4 dots are formed. Similarly, in FIG. 7D, the dot 702 has α = 0.5 and β = 0.5, so that the half of the dot is (n−1) lines before the n line. A half dot is formed in the current line (n). Similarly, the dot 703 is (n-1) lines before the n line, and 1/4 of the dot is formed, and 3/4 dots are formed in the current line (n). The dot 704 is formed at the position of the n line where it is originally located because k = 1, α = 0, and β = 1. Since k = 1 in the dots 705 to 707, α represents the distribution ratio of dots to the (n + 1) line, and β represents the distribution ratio of dots to the n line. Furthermore, since k = 2 in the dot 708, α represents the dot distribution ratio to the (n + 2) line, and β represents the dot distribution ratio to the (n + 1) line.

  FIG. 7E shows a pulse signal based on a bitmap image in which the exposure ratio of dots before and after the sub-scanning direction is adjusted in accordance with the correction coefficient shown in FIG. Here, a pulse width modulated signal corresponding to each dot data is shown.

  FIG. 7F shows an image developed on the photosensitive drum when the laser exposure is performed with the tone-corrected pulse width as shown in FIG. Thereby, the inclination of the main scanning line is canceled and a horizontal straight line is formed.

  In the above description, correction processing by hardware has been described, but processing by software is also possible by providing the controller 302 with a CPU.

  FIG. 8 is a block diagram showing an example in which the controller 302 shown in FIG. 3 is configured by a CPU and a memory. The parts common to those in FIG. 3 are indicated by the same symbols, and the description thereof is omitted.

  The printer engine 301 has the same configuration as that shown in FIG. 3, and the exposure unit 51 and the photosensitive drum 14 are omitted here. Each of the color misregistration amount storage units 303C to 303K stores the color misregistration amount in each of the photosensitive drums 14C to 14K corresponding to each color, as described above. The controller 302 includes a CPU 1000, a ROM 1001 that stores programs executed by the CPU 1000 and various data, and a RAM 1002 that is used as a work area during control processing by the CPU 1000 and temporarily stores various data. In the RAM 1002, the color shift corresponding to each color acquired from the bitmap memory 306 storing the bitmap image data of cyan, yellow, magenta, and black and the color shift amount storage units 303C to 303K of the printer engine 301 is stored. An area 1010 for storing data is provided.

  FIG. 9 and FIG. 10 are flowcharts for explaining the image forming process executed by the CPU 1000 of the controller 302 according to the present embodiment. A program for executing this process is stored in the ROM 1001, and is controlled under the control of the CPU 1000. Executed.

  First, in step S <b> 1, the color misregistration amount for each color stored in the color misregistration amount storage units 303 </ b> C to 303 </ b> K of the printer engine 301 is read and stored in the area 1010 of the RAM 1002. In step S 2, print data is input and color conversion is performed. Then, the image data is converted into bitmap image data for each page of cyan, yellow, magenta, and black, and stored in the bitmap memory 306. In step S3, a variable n for counting the number of lines is initialized to “1”, and a variable x for counting the dot position (x coordinate) is initialized to “0”. Both of these variables are stored in the RAM 1002.

  In step S4, x-th dot data is first read out on the n-th line of cyan bitmap data. In step S5, an area including the dot (for example, any one of areas 1 to 3 in FIG. 2) is determined. In step S6, based on the area determined in step S5 and the dot position (x), a correction amount Δy in the sub-scanning direction for forming the dot is calculated. This is calculated | required by either of Formula (1)-(3) mentioned above. In step S7, it is determined whether or not the integer part of the correction amount Δy obtained in step S6 is “0”. If it is “0”, it is not necessary to perform correction in units of lines, and the process proceeds to step S10a. If it is not “0”, the process proceeds to step S8 to determine whether the integer part is positive or negative. If it is positive, the process proceeds to step S9, where the xth dot data of the (n + s) line is acquired and used as the dot data of the current line (see FIG. 6). On the other hand, if negative in step S8, the process proceeds to step S10, where the xth dot data of the (ns) line is acquired and used as the dot data of the current line (see FIG. 6). Here, s indicates the absolute value of the integer part. When step S9 or S10 is executed in this way, the process proceeds to step S10a. In step S10a, it is determined whether the density of the dot data (multi-value data) that is the processing target is smaller than a predetermined density (μ). This corresponds to the configuration of the gradation checking unit 804b of FIG. If the density value is smaller than the threshold value (μ), it is determined that the correction using the coefficients α and β described above is unnecessary, and the process proceeds to step S12. However, the density value is smaller than the threshold value (μ). If it is larger, the dot is conspicuous, and it is determined that the above-described deviation correction is necessary, and the process proceeds to step S11.

  In step S11, a process for the numerical value after the decimal point of the correction amount Δy is executed. Here, the distribution of the same x-th dot data in the current line (n line) and (n + 1) line or (n-1) line is determined according to the value after the decimal point. Here, as described above with reference to FIG. 7, the dot data is exchanged or exchanged with the dot data of the adjacent line according to the numerical value after the decimal point of S. When the x-th dot data of the current line (n line) is updated in this way, the bitmap data is updated in step S12. Next, in step S13, the variable x is incremented by 1, and then in step S14, it is determined whether or not the value of the variable x has become larger than the total number of dots in one line. Execute the process.

  If the value of the variable x becomes larger than the total number of dots in one line in step S14, the process proceeds to step S15, and the variable n for counting the number of lines is incremented by one. In step S16, it is determined whether or not the value of the variable n has exceeded the number of lines on one page. If not, the process proceeds to step S17, the variable x is returned to “0”, and the process proceeds to step S4. Execute the process. On the other hand, if the value of the variable n exceeds the number of lines on one page in step S16, the process proceeds to step S18, where it is checked whether or not the processing for cyan, yellow, magenta, and black bitmap data has been completed. The process proceeds to step S3 to execute the above-described process. When the process is completed, the process proceeds to step S19 to start the image forming process.

  In step S19, the transfer sheet is picked up from the cassette 53 and started to be transported. While being placed on the transport belt 10 and transported, the dot data is PWM-modulated, and each semiconductor laser is driven by the PWM signal to generate cyan. Then, toner images are sequentially formed in the order of yellow, magenta, and black (step S20), and sequentially transferred onto the conveyed transfer sheet. When the transfer is thus completed, the image is fixed to the transfer sheet in step S22, and when the fixing is completed, the fixed transfer sheet is discharged in step S23.

  Note that the threshold value (μ) described above may be set for each color or for each beam. For example, in the case of a color whose density difference is difficult to be discerned by human eyes, such as yellow, the threshold value μ is set larger than in the case of other colors. Accordingly, it is possible to eliminate moire by reducing the ratio of executing the gradation correction as compared with other colors.

  As described above, according to the color image forming apparatus of the present embodiment, it is possible to correct both color misregistration in dot units and color misregistration that is less than dot units based on the color misregistration amount in each photosensitive drum. . As a result, it is possible to prevent color misregistration in each color image due to the inclination or curvature of the scanning line for scanning exposure of each photosensitive drum, and obtain a good color image.

  In the present invention, a software program that realizes the functions of the above-described embodiments is supplied directly or remotely to a system or apparatus, and the computer of the system or apparatus reads and executes the supplied program code. In some cases, it can be achieved by In that case, as long as it has the function of a program, the form does not need to be a program. Accordingly, since the functions of the present invention are implemented by computer, the program code installed in the computer also implements the present invention. That is, the present invention includes a computer program itself for realizing the functional processing of the present invention. In this case, the program may be in any form as long as it has a program function, such as an object code, a program executed by an interpreter, or script data supplied to the OS.

  As a storage medium for supplying the program, for example, floppy (registered trademark) disk, hard disk, optical disk, magneto-optical disk, MO, CD-ROM, CD-R, CD-RW, magnetic tape, nonvolatile memory card ROM, DVD (DVD-ROM, DVD-R) and the like. As another program supply method, a client computer browser is used to connect to an Internet homepage, and the computer program of the present invention itself or a compressed file including an automatic installation function is downloaded from the homepage to a storage medium such as a hard disk. Can also be supplied. It can also be realized by dividing the program code constituting the program of the present invention into a plurality of files and downloading each file from a different homepage. That is, a WWW server that allows a plurality of users to download a program file for realizing the functional processing of the present invention on a computer is also included in the claims of the present invention.

  In addition, the program of the present invention is encrypted, stored in a storage medium such as a CD-ROM, distributed to users, and key information for decryption is downloaded from a homepage via the Internet to users who satisfy predetermined conditions. It is also possible to execute the encrypted program by using the key information and install the program on a computer.

  In addition to the functions of the above-described embodiments being realized by the computer executing the read program, the OS running on the computer based on the instruction of the program may be part of the actual processing or The functions of the above-described embodiment can also be realized by performing all the processing and performing the processing.

  Furthermore, after the program read from the recording medium is written in a memory provided in a function expansion board inserted into the computer or a function expansion unit connected to the computer, the function expansion board or The CPU or the like provided in the function expansion unit performs part or all of the actual processing, and the functions of the above-described embodiments are realized by the processing.

1 is a schematic cross-sectional view illustrating a configuration of an image forming unit of a color image forming apparatus according to an embodiment of the present invention. FIG. 4 is an image diagram for explaining a shift of a main scanning line scanned by each photosensitive drum of the color image forming apparatus according to the embodiment. FIG. 5 is a block diagram for explaining color misregistration correction processing for correcting color misregistration caused by inclination and curvature of a scanning line performed in the present embodiment. It is a figure which shows the example of data memorize | stored in the color misregistration amount memory | storage part which concerns on this Embodiment. It is a block diagram which shows the structure of the color shift correction | amendment part which concerns on this Embodiment. It is an image figure for demonstrating the operation | movement content which the coordinate transformation part which concerns on this Embodiment correct | amends the shift | offset | difference amount (color shift of a line unit) of the integer part of color shift correction amount (DELTA) y. It is an image diagram for explaining the operation content for correcting the color misregistration correction less than the dot unit, that is, correcting the misregistration amount of the color misregistration correction amount Δy after the decimal point performed by the gradation correcting unit according to the present embodiment. FIG. 4 is a block diagram illustrating an example in which the controller illustrated in FIG. 3 is configured by a CPU and a memory. , It is a flowchart explaining the image formation process performed by CPU of the controller which concerns on this Embodiment.

Claims (5)

A shift amount storage means for storing a shift amount of an ideal main scanning line of the laser with respect to the photosensitive member;
Based on the shift amount obtained from the shift amount storage unit, a shift correction unit that performs shift correction in dot units of a raster image based on print data; and
When the density of the raster image to be processed is lower than the threshold, when the density of the raster image to be processed is equal to or higher than the threshold without performing shift correction less than a dot unit based on the shift amount obtained from the shift amount storage unit Correcting means for correcting a shift of less than a dot unit based on the shift amount obtained from the shift amount storage means;
Image forming means for forming an image corresponding to the dot corrected by the correcting means with the laser; and
A color image forming apparatus comprising: 2. The color image forming apparatus according to claim 1, wherein the laser and the photoconductor are provided for each color component of CMYK, and the threshold value is set for each color component . A method of controlling a color image forming apparatus for forming an image by irradiating a photoconductor with a laser modulated by an image signal,
A shift correction step for correcting a shift in dot units of a raster image based on print data based on a shift amount obtained from a shift amount storage unit that stores a shift amount of the laser with respect to the ideal main scanning line of the laser with respect to the photosensitive member;
When the density of the raster image to be processed is lower than the threshold value, when the density of the raster image to be processed is equal to or higher than the threshold value without performing the dot correction based on the shift amount obtained from the shift amount storage unit. A correction step for correcting a shift of less than a dot unit based on the shift amount obtained from the shift amount storage means;
An image forming step of forming an image corresponding to the dot corrected in the correction step with the laser; and
A control method for a color image forming apparatus, comprising: 4. The method of controlling a color image forming apparatus according to claim 3, wherein the laser and the photoconductor are provided for each color component of CMYK, and the threshold is set for each color component. A program for causing a computer to execute the control method according to claim 3 or 4.

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